Title of Invention	A SILENCER
Abstract	The present invention relates to a device (a silencer) for silencing a flow of gas such as an exhaust gas, comprises at least one acoustic chamber (1,2,3) through-flowed by gas, at least one inlet pipe (6) for leading gas into the device and at least one pipe or passage (4,12,13) interconnecting two chambers or a chamber and an exterior environment or chamber and is designed with such features, including such cross-sectional area transitions between pipes or passages (4,6,12,13) and the chambers (1,2,3) that the sound attenuation achieved by the device is high while the pressure drop across the silencer is low and that high attenuation at low characteristic frequencies of flow systems comprising the device are obtained. One or more diffusers (10a,12a,13a) for diffusing at least part of the gas flow, e.g., at the inlet to the chambers may be comprised in the device. One or more monolithic bodies or catalyzers may be comprised in the device. Devices with curved or helical passages (Fig. 5) allowing a low natural frequency and embodiments having resonance chambers (FIGs 10a,b) attenuating at selected frequencies are disclosed. The device may be used for vehicles, including ships or boats and for stationary installations, such as power plants or stationary engines. A method for designing and/or dimensioning the device for a given desired sound attenuation over a given frequency spectrum at given spatial restrictions is disclosed.

Title of Invention

A SILENCER

Abstract

The present invention relates to a device (a silencer) for silencing a flow of gas such as an exhaust gas, comprises at least one acoustic chamber (1,2,3) through-flowed by gas, at least one inlet pipe (6) for leading gas into the device and at least one pipe or passage (4,12,13) interconnecting two chambers or a chamber and an exterior environment or chamber and is designed with such features, including such cross-sectional area transitions between pipes or passages (4,6,12,13) and the chambers (1,2,3) that the sound attenuation achieved by the device is high while the pressure drop across the silencer is low and that high attenuation at low characteristic frequencies of flow systems comprising the device are obtained. One or more diffusers (10a,12a,13a) for diffusing at least part of the gas flow, e.g., at the inlet to the chambers may be comprised in the device. One or more monolithic bodies or catalyzers may be comprised in the device. Devices with curved or helical passages (Fig. 5) allowing a low natural frequency and embodiments having resonance chambers (FIGs 10a,b) attenuating at selected frequencies are disclosed. The device may be used for vehicles, including ships or boats and for stationary installations, such as power plants or stationary engines. A method for designing and/or dimensioning the device for a given desired sound attenuation over a given frequency spectrum at given spatial restrictions is disclosed.

Full Text	The present invention relates to a method tor designing and/or dimensioning a device for silencing a flow of gas such as exhaust gasses originating from a combustion device, a method for silencing such a flow and a number of devices for silencing such gasses, a vehicle comprising one or more such devices and a stationary power generating installation comprising one or more such devices. It is well known within the art to silence such a flow by directing the flow into an inlet passage to a container, through one or more chambers in said container intercommuni¬cating by means of passages, through a diffuser associated with one of said passages and into an outlet passage from said container. The design and/or dimensioning of such known devices has been based on experience, empirical iterations, partial applica¬tion of acoustic theory, and traditional solutions. This method of design and/or dimensioning has traditionally resulted in attenuation of the sound intensity of the exhaust flow to a degree that has been acceptable in the past. How¬ever, this known method has not consistently been able to provide sound intensity attenuation in general, and in par¬ticular for special applications, that complies with the increasing low acceptance of noise in modern society. For applications in the mass production industry of for instance gasoline and diesel engines, the great number of modifications and design changes based on experiments and empirical methods allowed by the economical resources avail¬able therefor has resulted in some relatively acceptable silencing devices. However, as the method is to a great extent based on trial and error, it has not been possible to consistently translate the success in one case to a general principle for achieving success in apparently similar cases, not to speak of rather different cases. In the case of tailor-made solutions for one-off instal¬lations or very small production series, application of the traditional method has not been able to provide optimal solutions except in exceptional cases where the element of luck has been a factor. This is owing to the fact that the economical and practical possibilities for carrying out experiments and consequent design and/or dimensioning modifi¬cations and changes are not to hand. Furthermore, the large number of parameters and considerations having implications for the sound attenuation in a silencing device have in the past prevented those skilled in the art from designing and dimensioning such a device simply and reliably in such a manner that a desired sound attenuation with an acceptable loss of pressure through the device and acceptable overall dimension consistently were achieved. A main object of the invention is to provide a method for simply and reliably designing and/or dimensioning a device comprising certain elements and for silencing a flow of exhaust gasses originating from a combustion device wherein the same general mathematical expressions are applied in connection with the particular given parameters regarding at least the combustion means, the space constraints, the acceptable pressure loss across the device, the sound spectra to be attenuated and the desired attenuation of the sound spectra. This object is achieved by the method being applied to a device of the type comprising one or more passages leading the flow into and/or out of one or more chambers of the device and one or more diffusers diffusing at least a part of the gas flow through one or more of the passages, the geome¬tric configuration and arrangement and the relative dimen¬sions of the one or more chambers and the one or more pas¬sages being designed and/or dimensioned mainly on the basis of the number of changes in the cross sectional area of the gas flow, the values of the individual changes in cross sectional area, the volume of each of the one or more cham¬bers and the length of each of said one or more passages. Hereby, a consistent compliance with the desired attenuation of the sound spectra has been achieved for the said given particular parameters while the overall dimensions of the device are minimized. Preferably, the device is applied for silencing a flow of exhaust gasses originating from a combustion means, - the device being of the type comprising a first container with one or more compartments or chambers each having one or more inlet passages and one or more outlet passages, and at least one diffuser associated with one or more inlet passages and/or one or more outlet passages, - the flow of exhaust gasses being directed into said one or more inlet passages and out of said one or more outlet pas¬ sages and at least partly through said at least one diffuser, - the method comprising applying the following expressions for designing and/or dimensioning the geometric configuration and arrangement and the relative dimensions of the one or more chambers, the one or more inlet passages, the one or more outlet passages and the at least one diffuser: where feis the local natural frequency of a system of two vol¬umes interconnected by a passage, c is the sound velocity, a is the representative cross sectional area of the pas¬sage, L is the length of the passage, V-^ is one volume, V2 is the other voliune, A Db is the total attenuation of the sound spectrum, k is a constant, and A is the representative cross sectional area of a chamber upstream or downstream relative to the respective passage with respect to the flow direction. The invention also relates to a method for silencing a flow of gasses, the method comprising: directing the flow through a device comprising one or more passages leading the flow into and/or out of one or more chambers of the device and - diffusing at least a part of the flow through one or more diffusers, - the geometric configuration and arrangement and the rela¬tive dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned mainly on the basis of the number of changes in the cross sectional area of the gas flow, the values of the individual changes in cross sectional area, the volume of each of the one or more cham¬bers and the length of each of said one or more passages. The invention further relates to a device for silencing a gas flow directed therethrough - the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, - the geometric configuration and arrangement and the rela¬ tive dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned mainly on the basis of the number of changes in the cross sectional area of the gas flow, the values of the individual changes in cross sectional area, the volume of each of the one or more cham¬ bers and the length of each of said one or more passages. The combustion device/means mentioned in the present text may¬be an internal combustion engine, such as a diesel, petrol, gasoline or gas engine, e.g. a two or four stroke piston engine, a Wankel engine comprised, gas turbine connected to a boiler or any other appropriate combustion or energy extracting device, e.g., the combustion system of a stationary power generating installation, such as power station. According to a further aspect of the invention, a device for silencing a gas flow directed therethrough is provided, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, wherein at least one pipe or passage is annular, constituted by an inner cylinder and by an outer cylinder. The annular pipes or passages may be provided with means, such as e.g. walls, for I segmentating the annular passage into a number of sub-passages having a rectangular or circular cross sectional outline or any other cross sectional outline. Thereby, rotating stall phenomena may be eliminated or at least reduced. At least one of the at least one pipe or passage which is annular may be a passage connecting two chambers. The annular passage may diffuse at least part of the gas flow directed therethrough. The at least pipe one passage being annular may thus constitute a flow cross sectional area increase in the flow direction. By applying an annular passage constituting a cross sectional area increase it is possible to achieve a relatively large cross sectional area increase over a relatively short longitudinal distance while avoiding flow separation in the passage. Thus, a relatively large pressure recovery may be achieved over a relative short distance which is important, e.g., for applications where the available space is limited, e.g., in vehicles such as trucks. The annular passage may comprise a constant flow area part and an outlet diffuser part. The constant flow area part contributes to the length of The inner cylinder may extend into said first chamber by a cylinder of substantially the same diameter as said inner cylinder, and said outer cylinder may be connected to a flow-guiding body with a curvature, so as to obtain optimal flow conditions through the device. Sound absorptive material is preferably contained within said cylinder and/or within a continuation cylinder extending into said first chamber and/or within a continuation cylinder extending into said second chamber. Obviously, one aim of providing sound absorptive material is to reduce the sound level of the gas flow. Though in preferred embodiments, the sound absorptive material is comprised within said cylinders, it may additionally/alternatively be comprised at the outer periphery of the surrounding casing. Preferably, at least some of the sound absorptive material communicates with the gas flow, e.g., through a perforated wall. Thus, at least part of the continuation cylinder may be perforated. It is preferred to apply said cylinders which at least partly separates the sound absorptive material from the gas flow in order to avoid that the sound absorptive material is being damaged by the gas flow. At locations of cross-sectional increase or decrease or in the vicinity of such locations, the walls are preferably non-perforated so as to avoid damaging of the sound absorptive material and/or so as to avoid un-desired flow perturbations which may increase pressure loss or generate turbulence. In a preferred embodiment, the outflow from said connecting passage passes into an annular passage inside said second chamber, said annular passage being made up of at least a perforated portion of an inner cylinder and an outer, perforated cylinder, both said cylinders separating sound absorptive materials from gas flow within said second chamber. The outflow from the connecting passage may pass directly into an annular passage. In order to obtain optimal flow conditions in the device, unstable flow conditions in the devices according to the invention should be avoided. Thus, for example, the distance between the inlet to the first chamber and the inlet to the annular passage should be so large that essentially no unstable flow occurs in the first chamber. With the aim of preventing unstable flow in the first chamber and/or allowing for a rather long passage, the distance may be at least 2% larger than the distance below which unstable flow would occur. Preferably, said distance should be at least 5% larger than the distance below which unstable flow would occur, normally at least 10% larger. Usually, it is not desired that the distance is more than 50% larger than the distance below which unstable flow would occur, however for some applications the distance may exceed said 50%. In a further aspect, the present invention relates to a device for silencing a gas flow directed therethrough, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or gas flow through one or more of the passages, wherein at least a part of at least one passage connecting two chambers is wound in a peripheral direction. At least the wound part of the passage may extend in a longitudinal direction, the generatrix of the wound part of the passage thus being substantially helical. Alternatively, the generatrix of the wound part of the passage may extend in a single plane, the plane being either perpendicular to the main flow direction or being inclined in relation thereto. By winding the connecting passage, i.e. by utilizing the third spatial dimension, the length of the passage may be significantly increased, the natural frequency of the silencing device thus being decreased, cf, equation (1). The flow the passage may constitute a flow cross sectional area increase in the flow direction. Thus, a diffusing effect may be obtained for static pressure recovery. The cross sectional area increase may be two- or three-dimensional. The passage may have any cross sectional shape, such as rectangular, circular, ellipsoidal or any other shape. The wound part of the passage may extend radially over an angle between 0° and 90°, or over an angle between 90° and 180°, or over an angle between 180° and 270°, or over an angle between 270° and 360°, or over an angle between 360° and 720°, or over an angle of 720° or more. The geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages of the devices according to the invention may preferably be designed and/or dimensioned according to a method according to the invention. The devices according to the invention may be suited for being mounted to the exhaust system of a vehicle comprising an internal displacement engine and/or a turbo machine or they may suited for being mounted to the exhaust system of a stationary power generating installation comprising an internal displacement engine and/or a turbo machine. The above mentioned vehicle may be any vehicle, such as e.g., a diesel engine powered truck, a bus, a car or a railway locomotive, a petrol, a gasoline or a gas engine powered truck, bus, car or any other moveable engine driven device. The vehicle may also be any ship or boat having a combustion device. The stationary power generating installation may be a power station having one or more gas turbines driven by flow originating from suitable combustion means, such as, e.g., one or more boiler, fuel engines or other combustion means. One major benefit of a device according to the invention is a significant reduction of pressure loss over the device compared to known devices. The reduction of pressure loss over the device reduces the fuel consumption of the combustion device and increases the power generated by the combustion device at a given fuel consumption. The pressure may be expressed as the dimensionless parameter being defined as the ratio between the pressure loss over the device and the dynamic pressure at an appropriate location in the device or adjacent to the device, i.e: where: Ap is the pressure loss over the device, p is the density of the gas at said location, u is a velocity of the gas at said location, preferably the mean gas velocity. An appropriate location could be, e.g., the inlet pipe, the outlet pipe, a location upstream of the inlet pipe, a location downstream of the outlet pipe, or any appropriate position inside the device where the flow velocity corresponds to the gas flow rate origination from the combustion device. As will illustrated in the example below the invention provides a device for silencing a gas now, the device having a substantially lower f-value. In preferred embodiments of a device according to the invention, the value of f is less than 10. In more preferred embodiments, the value of f is less than 7.5, preferably less than 6, more preferably less than 5, normally less 4, such as less than 3.5, preferably less than 3, more preferably less than 2.5, such as less than 2.2 or less than or equal to 2. Obviously, the value of f cannot be lower than 0, unless supplying energy to the gas flow. Typical values of f for devices according to the invention when applied to exhaust systems of commercially available diesel engine trucks are 0.5 In the devices according to the invention curvatures, preventing flow separation, are preferably applied to at least part of the contour of the outlet and/or inlet of a pipe or passage of the device, said pipe or passage being the inlet pipe or its extension into the apparatus and/or the outlet pipe or its extension into the apparatus and/or a passage connecting two chambers. Thereby vena contracta phenomena may be eliminated or at least reduced, thereby reducing the pressure loss over the device. According to a further aspect of the invention, a vehicle comprising an internal displacement engine and/or a turbo machine and a device according to the invention is provided, the device being comprised in the exhaust system of the vehicle. The invention further relates to a stationary power generating installation comprising an internal displacement engine and/or a turbo machine and a device according to the invention, the device being comprised in the exhaust system of the power generating installation. In the following, embodiments of the method according to the invention for designing and/or dimensioning an embodiment of a silencing device according to the invention will be explained solely by way of example with reference to the drawings where Fig. 1 is a diagrammatic representation in longitudinal section and cross sections along line I-I and II-II of a silencer comprising the elements associated with the method for designing and/or dimensioning according to the invention, Fig. 2 is a diagrammatic representation in longitudinal section of an embodiment of a device according to the invention, and Depending on the particular circumstances, various combina¬tions of parameters may be given as the basis of the design and dimensioning of a silencer. The following combination is often the case for a sub¬stantially cylindrical silencer for the exhaust gasses from a piston engine (see Figure 1): SDB (dB) = Total sound attenuation (damping) requirement. The silencer 5 illustrated in Figure 1 is substantially cylindrical and comprises an inlet pipe 6 leading exhaust gasses from a piston engine (not shown) into the silencer 5, an outer casing 7 and an outlet pipe 4 leading the silenced gasses out of the silencer 5 to the atmosphere. The outer casing 7 is subdivided into three chambers 1, 2 and 3 having axial lengths L^^, L2 and L3, respectively, by means of partitions 8 and 9. A radial diffuser 10 with outlet 10a is arranged as the outlet of the inlet passage (pipe) 11 to the first chamber 1, An axial diffuser 12 consisting of a series of pipes 12 having an axial length Lj^2 approx. equal to 0.5 times L2 and outlets 12a with increasing diameter in the flow direction constitutes the passage from chamber 1 to chamber 2. A radial diffuser 13 with outlet 13a is arranged as the outlet of the passage (pipe) 14 having a length L23 approx. equal to 0.5 times L3 and leading from chamber 2 to chamber 3. Sound damping material B such as mineral wool is arranged in the chambers adjacent to the pipes 11, 12 and 13. SDB is often arrived at by means of a separate, conventional acoustical calculation based on the measured unattenuated noise at a certain distance from the outlet from the exhaust system correlated with a desired maximum noise level at another point in space. f^gjj is determined by the rpm of the engine, the number of cylinders and the type of engine process (two-stroke or four-stroke) . If the engine is coupled to the general power grid by means of a generator, the rpm will be given thereby. During start-up of such engines or in propulsion engines for ships, road vehicles and so on, the rpm is variable, and therefore the value of fign in such cases must be estimated suitably low based on a rough estimate or on more detailed considerations, for instance based on known acoustical-statistical calculations. In some cases in connection with V-type cylinder arrange¬ments, a frequency around half the value of the ignition frequency may be preponderant which may motivate utilizing this frequency as a basic parameter for the dimensioning and design according to the invention of the respective silencer according to the invention. Q and c can be calculated from the given mass flow and tem¬perature of the exhaust gasses. SDP and 0D are typically "semi-fixed" parameters. Often it is very desirable to limit each of them to a maximum value, but if SDB already is fixed then SDP and 0D may not be determined freely. The smaller 0D is chosen, the larger SDP will be. Therefore, it will often be a question of combining the design and dimensioning of the silencer with considerations regarding the interrelationship between SDP and 0D including individually adapting the silencer structure to the geometrical constraints given by erection requirements, available space etc. A typical procedure when carrying out the method of designing and/or dimensioning according to the invention is: The number of chambers in the silencer is determined as Hereafter, the types of intermediate pipes (passages) and of diffusers are decided. A combination of different types may be chosen for maximum repression of chamber resonances. So as to further hinder or avoid resonances, rather different chamber lengths L^^-Lg may be chosen. The outlets 10a, 12a and 13a of the diffusers 10, 12 and 13, respectively, are located at or near the axial centre of the respective chambers and at or near 2/3 of the radius corresponding to the pressure nodes of the respective chambers. The relation A/a of the cross sectional areas may be tentatively chosen to be 10. Typically, different chamber lengths are chosen, for instance: wherefrom L3 is determined. L^^ and L2 are determined herefrom so that the total length of the silencer 5 is determined. The other cross sectional areas of the connecting tubes 12 and 11 are determined such that the relations between the corresponding areas A and a also are approx. 10, a for the pipes 12 being the total cross sectional area of the pipes 12. Now the total sound attenuation SDB may be calculated. The total pressure loss SDP may now be calculated by using a combination of known elementary formulas and a detailed knowledge of the efficiency of different diffuser types, consideration being given to the inlet flow profiles to the diffusers, the chosen detailed geometry and so on. If one or more of the calculated values for fg, SDB or SDP differ from the desired values then one or more of 0D, 0d or L are adjusted and the calculations indicated above are re¬peated. In cases where maximum compliance of a silencer with the given requirements is desired, the dimensioning indicated above is supplemented by an adjustment so that the "peaks" and "troughs" are adapted to the requirements. This is done by varying the dimensions (chamber lengths etc) and calcula¬ting the damping spectrum by means of impedance analysis and constants involved herein, the constants being determined by separate theoretical and empirical investigations. Referring now to Figs. 2-7, a practical example of designing and dimensioning a silencer for a six-cylinder, four stroke engine is illustrated. The length of the drum (outer casing) is ideally desired to be 2600 mm but may be slightly larger if necessary. The stud diameter minus stud thickness gives the internal diameter of the passage tubes and thereby the areas a. The drum diameter minus the drum thickness gives the area A. In the initial calculation step, Fig. 3, the number of cham bers is chosen to be three and the chamber lengths are chosen equal as are the pipe diameters (areas a). The tail pipe (outlet from the drum) is also involved in the dimensioning, the volume of the "chamber" constituted by the atmosphere being infinite. The length of the tail pipe is initially equal to the lengths of the other passage pipes. The diffusers are all radial and the pressiire drop through 20 the silencer is calculated by inter alia taking into con sideration the diffuser data indicated, a detailed explanation of said data not being important for the understanding of the invention. The total pressure drop is calculated as 151.66 mmWG, i.e. 25 well below the maximum back pressure of 475 mmWG allowable for the engine. The total sound attenuation is 26.13 dB, i.e. too small. The local natural frequency of the chamber system 1-2 and the chamber system 2-3 is too high as it should be much nearer half the firing frequency, i.e. 45 Hz. In Fig. 4 the diameters of the tubes between chambers 1 and 2 and between chamber 2 and 3 are reduced by the same amount so as to increase the sound attenuation and lower the natural frequency of the two said chamber systems in accordance with the expressions for A dB and fe. Now the local frequencies are close enough to 45 Hz to give a satisfactory attenuation of the firing frequency of the engine. Even though further optimization will be achievable such further improvement will be relatively small and without much practical value in the actual situation. The example given above of an embodiment of a method according to the invention for designing and dimensioning a device for silencing a flow of exhaust gasses is directed to relatively simple and uncomplicated situations where the silencing, pressure loss and space constraints are not strict. In case one or more of such constraints are strict and/or the situation is complicated by other factors, more complex and far-reaching embodiments may be employed wherein for instance optimising calculation procedures may be employed. Furthermore, constructive measures may be employed to enhance the silencing, pressure maintainment and optimal local natural frequencies. The passages between the chambers may be prolonged by several means either alone or in combination. The pipes may be prolonged backwards into the upstream chamber and forwards into the downstream chamber, or the pipes may be prolonged by adopting a helical design for same. If the pipes either alone or combined with a diffuser are prolonged both upstream and downstream, the outlet of the upstream pipe in a chamber may be downstream of the inlet of the downstream pipe thereby twice reversing the direction of the main flow in said chamber. A diffuser having an umbrella-like shape with the convex surface thereof facing downstream will also have the effect of prolonging the passage and reversing the flow. Various types and shapes of baffle plates and guiding plates and bodies may be employed. The design and dimensions of the diffusers are important for optimising the pressure recuperation thereof and thereby minimising the pressure loss through the silencer. Each diffuser may be a radial diffuser or an axial diffuser or a circular conical diffuser or an annular diffuser or a multi¬plicity of conical diffusers arranged on a cylindrical sur¬face or a diffuser for reversing the direction of flow or a double diversion diffuser or any other kind of diffuser. All inlets of the passages should be suitably rounded so as to avoid vena contracta flow with associated vortices giving rise to pressure loss and noise that may be amplified in the passage by resonance. The outlets of the diffusers should, if possible, be located at the centre of the longitudinal direction of the chamber relative to the main flow direction and/or at the pressure node of a transverse oscillation in the chamber. Hereby, the basic resonance of the chamber in the two directions is repressed. The arrangement of any sound absorbing material in the silencer is important. Particularly in large silencers it should not be too thick or compact so as to not decrease the acoustic volume of the chambers. A further embodiment of the device is designed to further attenuate certain sound frequencies in addition to the attenuation achieved by the device as described in the foregoing. This device comprises at least one second container with or without a therein enclosed sound absorbing means and solely communicating with a respective chamber of the first container, the volume of the at least one second container and the geometric configuration of the at least one second container and the communication passage or passages therefrom to said respective chamber being designed and/or dimensioned in accordance with the frequencies of the sound vibration whereof the sound intensity is to be reduced thereby. Each second container (resonating container) and its associated chamber and communication passage or passages will interact in such a manner that a certain respective sound frequency will be further attenuated. The volume of the at least one second container may be adjustable, for instance by means of displaceable partitioning means, and said volume may be adjusted according to theoretical considerations and/or empirical considerations taking into consideration the specific characteristics of the combustion means and/or the interrelationship thereof with the device and/or with the surroundings in general. Hereby, .^ standard, adjustable second container may be used for various silencers, or a tailor-made silencer may be "tuned" for certain frequencies by applying one or more adjustable resonating containers. Naturally the second container may be a further chamber of the silencer container instead of a separate container. Fig. 8 shows a preferred embodiment of a device according to the invention. Here, two chambers 1 and 2, are contained within a casing 7 and are separated by partition walls 8a and b. An inlet passage pipe 6 passes the flow to chamber 1 via a radial diffuser 10. From chamber 2 the flow is passed to the outlet passage pipe 4 via an opening provided with a curvature 22 preventing flow separation. The two chambers are interconnected by an annular passage 12. As can be seen from the figure, the combination of the radial inlet diffuser and the annular passage effectively prevents sound waves from 'short cutting' chamber 1 in passing from the inlet passage 6 to the annular passage, even though the inlet to the annular passage is positioned at a not very long distance Dl from the inlet to the first chamber. I.e., sound energy effectively fills chamber 1. The flow path from inlet to outlet of the second chamber contains less change of direction. However, due to the bigger distance D2 the tendency for sound waves to 'shortcut' the second chamber is rather small. The inlet 12a to the annular passage 12 contains several features which contribute to make the inlet smooth, preventing pressure-loss associated vena contracta phenomena: Both cylinders 42 and 44 are extended by design parts into the first chamber, thus providing guidance for the flow accelerating into the annular channel. The inner cylinder 42 is extended to the left by a cylinder 41, which is full immediately upstream of the inlet to the annular passage, and is otherwise perforated and contains sound absorptive r~-. material Ba. The outer cylinder 44 is extended to the left hyi^^' ' a conical cylinder 21 being connected to cylinder 44 by a curvature 20. The outermost diameter of the conical cylinder is big enough to provide sufficient guidance for the flow, i.e. the flow velocity at this diameter is much smaller than the flow velocity in the annular passage. On the other hand, the distance D3 between conical cylinder 21 and the casing 7 is not unnecessesarily small, since this would tend to acoustically isolate the right-hand, annular part of chamber 1. The outlet 12d from annular diffuser 12c passes the flow into an annular passage 30 inside chamber 2, constituted by an inner, perforated cylinder 43 and an outer, likewise perforated cylinder 46. Sound absorptive materials Bd and Be are placed inside cylinder 43 and outside cylinder 46, respectively. The size D4 of passage 3 0 is chosen as a compromize between opposing demands: On the one hand, the smaller the size of D4 is the more efficient sound absorption is achieved. On the other hand, D4 should not become so small that too strong, turbulent noise is generated, or that too strong fluid-mechanical forces, tending to abstract absorptive material, are generated. Annular flow between sound absorbing walls is a per se known as an acoustically efficient configuration. The length L12 of connecting passage 12 is chosen to be long enough for the local natural frequency fe to become sufficiently low, as given by the method of the invention, cf. equation (1). When the total length of the casing is given and is comparatively short, L12 is chosen by balancing of number of demands: Distance Dl can be made rather small, say in the order of half the diameter of the casing, depending upon a number of further geometric choices, among them the size of distance D3. Shortening distance D2 will cause a somewhat deteriorated acoustic function of chamber 2, but as a gradual function of D2. In general, it is desired that the level of the noise which may be generated due to ' turbulence should not, at any frequency, exceed the level of the noise created by the engine. For a given length L12, the annular passage type, the flow-friendly features of a smooth and guiding inlet and a diffuser outlet, allow for a comparatively low natural frequency fe and rather big, effective sound-reflective flow area ratios A/a, cf. equation (2). From fluid-dynamic diffuser theory it is known that a maximum angle a of divergence, indicated in fig. 8, exists which, when exceeded, leeds to flow separation occurring inside the diffuser, i.e. a less than optimal operation which should be avoided. This angle is not very wide, so that for a large outlet to inlet flow area ratio of the diffuser, it tends to become long. However, for given sizes of inlet and outlet cross-sectional areas, an annular diffuser allows for a shorter diffuser length than does a conical diffuser. This means that, with an embodiment as shown in fig. 8, even though a low flow velocity is wanted at outlet 12d, and a high flow velocity is wanted in the constant flow area part 12b of passage 12, the diffuser length L-^jc "^^^ ^® chosen rather short, without flow separation occurring. For a given total length L12, this in turn means that the constant flow area part length L12b can be made rather long, so as to obtain a rather long L in equation (1) and so as to thereby obtain a rather low natural frequency, fe. The acoustically effective area a of the interconnecting passage 12 is a weighted mean of all cross-sectional areas occurring from inlet 12a to outlet 12d. Therefore, it is acoustically favourable that the smallest area, i.e. the cross-sectional area of constant-area part 12b, is comparatively long. The silencer embodiment shown in fig. 8 contains a rather simple, central body 40 which is securely and accurately fixed to the outer parts of the silencer via a number of flow- aligned arms or sheets 47, 48, and 49. E.g., in radial" diffuser 10 there may be four arms or sheets 49, positioned at 90 degrees' angle around the periphery. The total interface area between empty chamber volumes and volumes filled with sound absorptive material is big, providing a maximum of sound absorptive effect to assist sound reduction due to reflection at changes in cross-sectional area. In the embodiment shown in fig. 8, the flow occurring in annular passage 12 is coherent all around the periphery of the annularity, apart from the small interruptions provided by the arms or sheets 47 and 48. For manufacturing reasons it may be expedient to adopt various types of peripheral segmentations of the annular passage. Thus, the arms or sheets 47 and 48 may be substituted by deformations which may be formed be pressing operations made on inner parts 42 and 43 and / or on outer parts 44 and 45 constituting the annular passage 12, In bigger silencers, or when extreme outlet and/or inlet flow cross-sectional area ratios are wanted for the annular diffuser, it may be advisable to adopt a thorough peripheral segmentation of the annular passage, to prevent occurrence of fluid-mechanical instability of the rotating stall type, well-known from turbo machinery. This can be done by inserting radial partition walls into the annular passage. An alternative is to split the annular passage into a multiplicity of flow-parallel pipes arranged with the centerlines of all pipes situated on a cylinder with a centerline coinciding with the centerline of the silencer as such. These pipes can be circular, in which case diffusing outlet parts will be conical diffusers. Many other cross-sectional forms for pipes are also possible, e.g. squared cross-sections. Fig. 9 shows another preferred embodiment of the invention. As in fig. 8, an annular passage 12 connects two chambers 1 and 2. In fig. 9 the casing is more elongated, as is e.g. uypicai ot vertical silencers on trucks, i.e. it is rather long, and the diameter is rather small. « The following differences compared to the embodiment of fig. 8 are notable: Distance D3 has vanished, so that conical cylinder 21 provides the partition wall 8b between chambers 1 and 2, together with rounded part 2 0 and central parts of the silencer. Still, the annular spacing 32 is not 'wasted' as an acoustic volume of the silencer, since it is a part of chamber 2. Perforated cylinders 41 and 50 provide inner delimiters of annular spaces 31 and 30, respectively. Whereas in fig. 8 annular space 3 0 is delimited by both the inner and outer perforated cylinders 43 and 46, annular spaces 30 and 31 in fig. 9 outwardly are delimited by the casing 7. Arms or sheets 51, 52, and 53 helps fix cylinders 41 and 50, together with filled-in sound absorptive materials Ba and Bd. Whereas in fig. 8 the outflow from annular passage 12 passes directly into passage 30, the outflow in fig. 9 passes a short distance D5 before entering annular passage 30. Figs. 10a-lOe show a preferred embodiment of the invention in which a helical passage 12 connects two chambers 1 and 2, contained within a cylindrical casing 7 and separated by an inner, flat partition wall 8. The helical passage is delimited by casing 7, by an inner cylinder 42, and by helically formed sheets 60 and 61. Helical passage 12 is subdivided into a constant flow-area part 12b and a diffuser part 12c, in which the flow area gradually widens in the gas flow direction, as given by a gradually widening distance between sheets 60 and 61. Both chambers 1 and 2 are partly filled with sound absorption material Ba, contained behind perforated plates 41 and 43. These plates have been so formed and positioned that, together with the absorptive material they help guide the flow inside chambers 1 and 2 with low pressure drop and preventing unwanted flow swirling inside the chambers. Gas flow is led to the silencer via inlet pipe 6 and conical diffuser 10, which recovers dynamic pressure and helps further to prevent unwanted swirl inside chamber 1 by-lowering the inlet flow velocity to the chamber. Here, the flow generally turns 90 degrees before entering helical connecting passage 12 at 12a. Here, a cylindrical rod 21 has been fitted onto inner cylinder 42 to improve inlet flow conditions, preventing vena contracta phenomena and inlet pressure losses. Inside passage 12 the flow first passes constant flow area part 12b and then diffuser part 12c in which dynamic pressure is recovered. The flow leaves passage 12 at outlet 12d, entering chamber 2. Inside this second chamber the general flow direction turns 90 degrees, both in plane AA and in plane CC, before entering outlet pipe 4. From flow inlet 12a to flow outlet 12d of the helical passage 12, the flow in total turns 360 degrees inside the silencer casing. Thus, the length of the passage is approximately TT times the casing diameter, contributing to a very low acoustical natural frequency fe, constituted by passage 12 and chambers 1 and 2. In fig. 10, the length of casing 7 is only slightly in excess of the diameter. The embodiment thus demonstrates how, by adopting a helical passage between chambers according to the invention, it has become possible to achieve a much lower natural frequency than with a straight passage. Examples of particularly relevant applications of the embodiment of fig. 10 are silencers for buses of trucks where there is space for a rather big silencer volume, given by a casing of a rather big diameter but of a short length. Even though the flow turns quite a lot inside the casing, the associated pressure loss is remarkably low. In spite of the embodiment being truly three-dimensional, the essentially 1-dimensional and dimensioning method of the invention applies. Naturally, an accurate description of the fluid-flow and acoustic properties of the silencer should be three-dimensional. However, this is also the case in silencers wherein the acoustic field and the flow field is substantially two-dimensional. It should also be pointed out, that in spite of the three-dimensional flow path through the embodiment of fig. 10, it can be manufactured by rather simple members and by simple methods like sheet pressing, rolling, welding, etc. Many types of silencers with helical flow patterns inside casing are known from prior art. However, in known silencer embodiments, helical flows have been desired for reasons differing from those of the present invention. E.g., very efficient sound absorption has been achieved by adopting helical channels made by perforated cylinders in contact with absorptive material. Another reason for adopting helical internal flow in silencers has been to achieve a spark-arresting effect by increasing the residence time for exhaust gasses inside a silencer. The helical configuration of the invention allows the engineer to select the length of connecting channel 12 very freely and optimize this length according to the method of the invention. Thus, when a very low natural frequency is desired, even substantially more than 360 degrees turning inside the passage will be beneficial to select in some cases. An example of this could be a truck application, for which it is desired to attenuate infra-sound created by the engine when running at low speed at engine start-up or at hauling operation of the truck. Attenuation of infra-sound is further relevant in connection with gas turbine power stations. In other cases, less than 3 60 degrees flow turning in the helical passage can be appropriate, e.g. with higher ignition frequencies of engines, and when it becomes essential not to create too low resonant frequencies of the helical passage. To the silencer design engineer it obvious that the goals addressed by the embodiment shovra in fig. 10 can be achieved by many variations in design configuration. E.g., the flow-widening of the diffuser 12c can be achieved by varying the 5 diameter of the inner cylinder 42. A helical passage can be fitted into a cubic casing, instead of a cylindrical casing. The wall 8, separating chamber 1 from chamber 2, can be a cylinder, and chamber 2 can be arranged essentially outside chamber 1, which is favourable from a shell noise emission 10 point of view, since the sound level inside chamber 1 is higher than inside the downstream chamber 2. WE CLAIM : 1. A method designing and/or dimensioning a device for silencing a flow of exhaust gasses originating from a combustion means, - the device being of the type comprising a first container 20 with one or more compartments or chambers each having one ormore inlet passages and one or more outlet passages, and at least one diffuser associated with one or more inlet passages and/or one or more outlet passages, - the flow of exhaust gasses being directed into said one or 25 more inlet passages and out of said one or more outlet pas sages and at least partly through said at least one diffuser, - the method comprising applying the following expressions for designing and/or dimensioning the geometric configurationand arrangement and the relative dimensions of the one or 30 more chambers, the one or more inlet passages, the one or more outlet passages and the at least one diffuser: where fe is the local natural frequency of a system of two volumes interconnected by a passage, c is the sound velocity, a is the representative cross sectional area of the passage, L is the length of the passage, Vi is one volume, V2 is the other volume, A dB is the total attenuation of the sound spectrum, k is a constant, and A is the representative cross sectional area of a chamberupstream or downstream relative to the respective passagewith respect to the flow direction. 2. A method as claimed in claim 1, wherein the combustion means is an internal combustion engine. 3. A method as claimed in claim 3, wherein the internal com bustion engine is a piston engine. 4. A method as claimed in claim 4 the method comprises the steps of ensuring that the lowest value of the local natural frequency fe in the device is approximately half the firing frequency of the internal combustion engine. 5. A method as claimed in claim 3, the method comprises the steps of ensuring that the lowest value of the local natural frequency fe in the device is approximately three quarters of the firing frequency of the internal combustion engine. 6. A method according to any of the claims 1-5, wherein the chambers are substantially cylindrical and one or more outlets from said at least one diffuser ai located substantially at the axial centre of the chamber associated with said diffuser. 7. A method as claimed in claims 1-5, wherein the chambers are substantially cylindrical and the one or more outlets from said at least one diffuser are located substantially at a distance of two thirds of the radius of the chamber from the axis of same. 8. A method as claimed in claims 1 -7, wherein the value of k is in the range 5-7. 9. A method as claimed in claims 1-7, wherein the value of k is in the range 6-6.5. 10. A method as claimed in claims 1-7, wherein the value of k is approximately 6.25. 11. A device for silencing a flow of exhaust gasses from a combustion means, the device comprising a first container with one or more compartments or chambers with one or more inlet passages and one or more outlet passages, and at least one diffuser associated with one or more inlet passages and/or one or more outlet pas sages, the geometric configuration and arrangement and the relative dimensions of the one or more chambers, the one or more inlet passages, the one or more outlet passages being designed and/or dimensioned by application of the following two expressions: A device as claimed in claimll, wherein the diffuser is a radial diffuser or an axial diffuser or a circular conical diffuser or an annular diffuser or a multiplicity of conical diffusers arranged on a cylindrical surface or a diffuser for reversing the' direction of flow or a double diversion diffuser. 13. A device according to any of the claims 11-12, wherein the device comprises at least one second container with or without a therein enclosed sound absorbing means and solely communicating with a respective chamber of the first container, the volume of the at least one second container and the geometric configuration of the at least one second container and the communication passage or passages therefrom to said respective chamber being designed and/or dimensioned in accordance with the frequencies of the sound vibration whereof the sound intensity is to be reduced thereby. 14. A device as claimed in claims 11-13 wherein curvatures, preventing flow separation, are applied to at least part of the contour of the outlet and/or inlet of a pipe (4,6) or passage (12) of the device, said pipe or passage being the inlet pipe (6) or its extension into the apparatus and/or the outlet pipe (4) or its extension into the apparatus and/or a passage (12) connecting two chambers(l,2). 15. A device for silencing a gas flow directed therethrough, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, wherein at least one pipe or passage is annular, constituted by an inner cylinder (42, 43) and by an outer cylinder (44, 45). 16. A device as claimed in claim 15, wherein at least one of the at least one pipe or passage which is annular is a passage connecting two chambers (1,2). 17. A device as claimed in claim 15 or 16, wherein the at least one pipe or passage being annular constitutes a flow cross sectional area increase in the flow direction. 18. A device as claimed in claims 15-17 wherein said annular passage (12) comprises a constant flow area part (12b) and an outlet diffuser part (12c). 19. A device as claimed in claims 25-28 wherein said inner cylinder extends into said first chamber (1) by a cylinder (41) of substantially the same diameter as said inner cylinder, and said outer cylinder is connected to a flow-guiding body (21) with a curvature (20). 20. A device as claimed in claims 15-19 wherein sound absorptive material is contained within said cylinder (42, 43) and/or within a continuation cylinder (41) extending into said first chamber (1) and/or within a continuation cylinder (43) extending into said second chamber (2). 21. A device as claimed in claim 20 wherein at least part of the continuation cylinder (41) is perforated. 22. A device as claimed in claims 15-21 wherein the outflow from said connecting passage (12) passes into an annular passage (30) inside said second chamber (2), said annular passage (30) being made up of at least a perforated portion of an inner cylinder (43) and an outer, perforated cylinder (46), both said cylinders (43, 46) separating sound absorptive materials (Bd, Be) from gas flow within said second chamber. 15. A device as claimed m claim 22 wherein the outflow from said connecting passage (12) passes directly into an annular passage (30). 24. A device as claimed in claims 22-23 wherein the inner cylinder (43) has a non-perforated portion which constitutes at least part of the inner cylinder constituting an inner wall of the outlet diffusor part (12). 25. A device as claimed in claims 22-24 wherein the non-perforated portion of the inner cylinder (43) constitutes at least part of an inner wall of the chamber (2). 26. A device for silencing a gas flow directed therethrough, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, wherein at least a part of at least one passage (12) connecting two chambers (1, 2) is wound in a peripheral direction. 27. A device as claimed in claim 26, wherein at least the wound part of the passage (12) extends in a longitudinal direction, the generatrix of the wound part of the passage (12) thus being substantially helical. 28. A device as claimed in claim 26, wherein the generatrix of the wound part of the passage (12) extends in a single plane. 29. A device as claimed in claims 26-28, wherein the flow the passage (12) constitutes a flow cross sectional area increase in the flow direction. 30. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 0° and 90°. 31. A device according to any of claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 90° and 180°. 32. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 180° and 270°. 33. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 270° and 360°. 34. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 360° and 720°. 35. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle of 720° or more. 36. A device as claimed in claims 15-25 or any of claims 26-35 wherein curvatures, preventing flow separation, are applied to at least part of the contour of the outlet and/or inlet of a pipe (4,6) or passage (12) of the device, said pipe or passage being the inlet pipe (6) or its extension into the apparatus and/or the outlet pipe (4) or its extension into the apparatus and/or a passage (12) connecting two chambers (1,2). 37. A device as claimed in claims 15-25 or any of claims 26-35 or claim 36, wherein the geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned according to any of claims 1-10.

Full Text

The present invention relates to a method tor designing and/or dimensioning a device for silencing a flow of gas such as exhaust gasses originating from a combustion device, a method for silencing such a flow and a number of devices for silencing such gasses, a vehicle comprising one or more such devices and a stationary power generating installation comprising one or more such devices.
It is well known within the art to silence such a flow by directing the flow into an inlet passage to a container, through one or more chambers in said container intercommuni¬cating by means of passages, through a diffuser associated with one of said passages and into an outlet passage from said container.
The design and/or dimensioning of such known devices has been based on experience, empirical iterations, partial applica¬tion of acoustic theory, and traditional solutions.
This method of design and/or dimensioning has traditionally resulted in attenuation of the sound intensity of the exhaust flow to a degree that has been acceptable in the past. How¬ever, this known method has not consistently been able to provide sound intensity attenuation in general, and in par¬ticular for special applications, that complies with the increasing low acceptance of noise in modern society.
For applications in the mass production industry of for instance gasoline and diesel engines, the great number of modifications and design changes based on experiments and empirical methods allowed by the economical resources avail¬able therefor has resulted in some relatively acceptable silencing devices. However, as the method is to a great extent based on trial and error, it has not been possible to consistently translate the success in one case to a general principle for achieving success in apparently similar cases, not to speak of rather different cases.

In the case of tailor-made solutions for one-off instal¬lations or very small production series, application of the traditional method has not been able to provide optimal solutions except in exceptional cases where the element of luck has been a factor. This is owing to the fact that the economical and practical possibilities for carrying out experiments and consequent design and/or dimensioning modifi¬cations and changes are not to hand.
Furthermore, the large number of parameters and considerations having implications for the sound attenuation in a silencing device have in the past prevented those skilled in the art from designing and dimensioning such a device simply and reliably in such a manner that a desired sound attenuation with an acceptable loss of pressure through the device and acceptable overall dimension consistently were achieved.
A main object of the invention is to provide a method for simply and reliably designing and/or dimensioning a device comprising certain elements and for silencing a flow of exhaust gasses originating from a combustion device wherein the same general mathematical expressions are applied in connection with the particular given parameters regarding at least the combustion means, the space constraints, the acceptable pressure loss across the device, the sound spectra to be attenuated and the desired attenuation of the sound spectra.
This object is achieved by the method being applied to a device of the type comprising one or more passages leading the flow into and/or out of one or more chambers of the device and one or more diffusers diffusing at least a part of the gas flow through one or more of the passages, the geome¬tric configuration and arrangement and the relative dimen¬sions of the one or more chambers and the one or more pas¬sages being designed and/or dimensioned mainly on the basis of the number of changes in the cross sectional area of the

gas flow, the values of the individual changes in cross sectional area, the volume of each of the one or more cham¬bers and the length of each of said one or more passages.
Hereby, a consistent compliance with the desired attenuation of the sound spectra has been achieved for the said given particular parameters while the overall dimensions of the device are minimized.
Preferably, the device is applied for silencing a flow of exhaust gasses originating from a combustion means,
- the device being of the type comprising a first container
with one or more compartments or chambers each having one or
more inlet passages and one or more outlet passages, and at
least one diffuser associated with one or more inlet passages
and/or one or more outlet passages,
- the flow of exhaust gasses being directed into said one or
more inlet passages and out of said one or more outlet pas¬
sages and at least partly through said at least one diffuser,
- the method comprising applying the following expressions
for designing and/or dimensioning the geometric configuration
and arrangement and the relative dimensions of the one or
more chambers, the one or more inlet passages, the one or
more outlet passages and the at least one diffuser:

where
feis the local natural frequency of a system of two vol¬umes interconnected by a passage, c is the sound velocity,
a is the representative cross sectional area of the pas¬sage,
L is the length of the passage, V-^ is one volume, V2 is the other voliune,
A Db is the total attenuation of the sound spectrum, k is a constant, and
A is the representative cross sectional area of a chamber upstream or downstream relative to the respective passage with respect to the flow direction.
The invention also relates to a method for silencing a flow of gasses, the method comprising:
directing the flow through a device comprising one or more passages leading the flow into and/or out of one or more chambers of the device and
- diffusing at least a part of the flow through one or more diffusers,
- the geometric configuration and arrangement and the rela¬tive dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned mainly on the basis of the number of changes in the cross sectional area of the gas flow, the values of the individual changes in cross sectional area, the volume of each of the one or more cham¬bers and the length of each of said one or more passages.
The invention further relates to a device for silencing a gas flow directed therethrough

- the device comprising one or more passages for leading the
flow into and/or out of one or more chambers of the device
and one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages,
- the geometric configuration and arrangement and the rela¬
tive dimensions of the one or more chambers and the one or
more passages being designed and/or dimensioned mainly on the
basis of the number of changes in the cross sectional area of
the gas flow, the values of the individual changes in cross
sectional area, the volume of each of the one or more cham¬
bers and the length of each of said one or more passages.
The combustion device/means mentioned in the present text may¬be an internal combustion engine, such as a diesel, petrol, gasoline or gas engine, e.g. a two or four stroke piston engine, a Wankel engine comprised, gas turbine connected to a boiler or any other appropriate combustion or energy extracting device, e.g., the combustion system of a stationary power generating installation, such as power station.
According to a further aspect of the invention, a device for silencing a gas flow directed therethrough is provided, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, wherein at least one pipe or passage is annular, constituted by an inner cylinder and by an outer cylinder. The annular pipes or passages may be provided with means, such as e.g. walls, for I segmentating the annular passage into a number of sub-passages having a rectangular or circular cross sectional outline or any other cross sectional outline. Thereby, rotating stall phenomena may be eliminated or at least reduced.

At least one of the at least one pipe or passage which is annular may be a passage connecting two chambers. The annular passage may diffuse at least part of the gas flow directed therethrough. The at least pipe one passage being annular may thus constitute a flow cross sectional area increase in the flow direction. By applying an annular passage constituting a cross sectional area increase it is possible to achieve a relatively large cross sectional area increase over a relatively short longitudinal distance while avoiding flow separation in the passage. Thus, a relatively large pressure recovery may be achieved over a relative short distance which is important, e.g., for applications where the available space is limited, e.g., in vehicles such as trucks.
The annular passage may comprise a constant flow area part and an outlet diffuser part. The constant flow area part contributes to the length of
The inner cylinder may extend into said first chamber by a cylinder of substantially the same diameter as said inner cylinder, and said outer cylinder may be connected to a flow-guiding body with a curvature, so as to obtain optimal flow conditions through the device.
Sound absorptive material is preferably contained within said cylinder and/or within a continuation cylinder extending into said first chamber and/or within a continuation cylinder extending into said second chamber. Obviously, one aim of providing sound absorptive material is to reduce the sound level of the gas flow. Though in preferred embodiments, the sound absorptive material is comprised within said cylinders, it may additionally/alternatively be comprised at the outer periphery of the surrounding casing. Preferably, at least some of the sound absorptive material communicates with the gas flow, e.g., through a perforated wall. Thus, at least part of the continuation cylinder may be perforated. It is preferred to apply said cylinders which at least partly separates the sound absorptive material from the gas flow in

order to avoid that the sound absorptive material is being damaged by the gas flow. At locations of cross-sectional increase or decrease or in the vicinity of such locations, the walls are preferably non-perforated so as to avoid damaging of the sound absorptive material and/or so as to avoid un-desired flow perturbations which may increase pressure loss or generate turbulence.
In a preferred embodiment, the outflow from said connecting passage passes into an annular passage inside said second chamber, said annular passage being made up of at least a perforated portion of an inner cylinder and an outer, perforated cylinder, both said cylinders separating sound absorptive materials from gas flow within said second chamber. The outflow from the connecting passage may pass directly into an annular passage.
In order to obtain optimal flow conditions in the device, unstable flow conditions in the devices according to the invention should be avoided. Thus, for example, the distance between the inlet to the first chamber and the inlet to the annular passage should be so large that essentially no unstable flow occurs in the first chamber.
With the aim of preventing unstable flow in the first chamber and/or allowing for a rather long passage, the distance may be at least 2% larger than the distance below which unstable flow would occur. Preferably, said distance should be at least 5% larger than the distance below which unstable flow would occur, normally at least 10% larger. Usually, it is not desired that the distance is more than 50% larger than the distance below which unstable flow would occur, however for some applications the distance may exceed said 50%.
In a further aspect, the present invention relates to a device for silencing a gas flow directed therethrough, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or

gas flow through one or more of the passages, wherein at least a part of at least one passage connecting two chambers is wound in a peripheral direction. At least the wound part of the passage may extend in a longitudinal direction, the generatrix of the wound part of the passage thus being substantially helical. Alternatively, the generatrix of the wound part of the passage may extend in a single plane, the plane being either perpendicular to the main flow direction or being inclined in relation thereto. By winding the connecting passage, i.e. by utilizing the third spatial dimension, the length of the passage may be significantly increased, the natural frequency of the silencing device thus being decreased, cf, equation (1). The flow the passage may constitute a flow cross sectional area increase in the flow direction. Thus, a diffusing effect may be obtained for static pressure recovery. The cross sectional area increase may be two- or three-dimensional. The passage may have any cross sectional shape, such as rectangular, circular, ellipsoidal or any other shape.
The wound part of the passage may extend radially over an angle between 0° and 90°, or over an angle between 90° and 180°, or over an angle between 180° and 270°, or over an angle between 270° and 360°, or over an angle between 360° and 720°, or over an angle of 720° or more.
The geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages of the devices according to the invention may preferably be designed and/or dimensioned according to a method according to the invention.
The devices according to the invention may be suited for being mounted to the exhaust system of a vehicle comprising an internal displacement engine and/or a turbo machine or they may suited for being mounted to the exhaust system of a stationary power generating installation comprising an

internal displacement engine and/or a turbo machine. The above mentioned vehicle may be any vehicle, such as e.g., a diesel engine powered truck, a bus, a car or a railway locomotive, a petrol, a gasoline or a gas engine powered truck, bus, car or any other moveable engine driven device. The vehicle may also be any ship or boat having a combustion device. The stationary power generating installation may be a power station having one or more gas turbines driven by flow originating from suitable combustion means, such as, e.g., one or more boiler, fuel engines or other combustion means.
One major benefit of a device according to the invention is a significant reduction of pressure loss over the device compared to known devices. The reduction of pressure loss over the device reduces the fuel consumption of the combustion device and increases the power generated by the combustion device at a given fuel consumption. The pressure may be expressed as the dimensionless parameter being defined as the ratio between the pressure loss over the device and the dynamic pressure at an appropriate location in the device or adjacent to the device, i.e:

where:
Ap is the pressure loss over the device, p is the density of the gas at said location, u is a velocity of the gas at said location, preferably the mean gas velocity.
An appropriate location could be, e.g., the inlet pipe, the outlet pipe, a location upstream of the inlet pipe, a location downstream of the outlet pipe, or any appropriate position inside the device where the flow velocity corresponds to the gas flow rate origination from the combustion device. As will illustrated in the example below

the invention provides a device for silencing a gas now, the device having a substantially lower f-value. In preferred embodiments of a device according to the invention, the value of f is less than 10. In more preferred embodiments, the value of f is less than 7.5, preferably less than 6, more preferably less than 5, normally less 4, such as less than 3.5, preferably less than 3, more preferably less than 2.5, such as less than 2.2 or less than or equal to 2. Obviously, the value of f cannot be lower than 0, unless supplying energy to the gas flow. Typical values of f for devices according to the invention when applied to exhaust systems of commercially available diesel engine trucks are 0.5 In the devices according to the invention curvatures, preventing flow separation, are preferably applied to at least part of the contour of the outlet and/or inlet of a pipe or passage of the device, said pipe or passage being the inlet pipe or its extension into the apparatus and/or the outlet pipe or its extension into the apparatus and/or a passage connecting two chambers. Thereby vena contracta phenomena may be eliminated or at least reduced, thereby reducing the pressure loss over the device.
According to a further aspect of the invention, a vehicle comprising an internal displacement engine and/or a turbo machine and a device according to the invention is provided, the device being comprised in the exhaust system of the vehicle.
The invention further relates to a stationary power generating installation comprising an internal displacement engine and/or a turbo machine and a device according to the invention, the device being comprised in the exhaust system of the power generating installation.
In the following, embodiments of the method according to the invention for designing and/or dimensioning an embodiment of

a silencing device according to the invention will be explained solely by way of example with reference to the drawings where
Fig. 1 is a diagrammatic representation in longitudinal section and cross sections along line I-I and II-II of a silencer comprising the elements associated with the method for designing and/or dimensioning according to the invention,
Fig. 2 is a diagrammatic representation in longitudinal section of an embodiment of a device according to the invention, and

Depending on the particular circumstances, various combina¬tions of parameters may be given as the basis of the design and dimensioning of a silencer.
The following combination is often the case for a sub¬stantially cylindrical silencer for the exhaust gasses from a piston engine (see Figure 1):
SDB (dB) = Total sound attenuation (damping)
requirement.

The silencer 5 illustrated in Figure 1 is substantially cylindrical and comprises an inlet pipe 6 leading exhaust gasses from a piston engine (not shown) into the silencer 5, an outer casing 7 and an outlet pipe 4 leading the silenced gasses out of the silencer 5 to the atmosphere.
The outer casing 7 is subdivided into three chambers 1, 2 and 3 having axial lengths L^^, L2 and L3, respectively, by means of partitions 8 and 9.
A radial diffuser 10 with outlet 10a is arranged as the outlet of the inlet passage (pipe) 11 to the first chamber 1, An axial diffuser 12 consisting of a series of pipes 12 having an axial length Lj^2 approx. equal to 0.5 times L2 and outlets 12a with increasing diameter in the flow direction constitutes the passage from chamber 1 to chamber 2. A radial diffuser 13 with outlet 13a is arranged as the outlet of the passage (pipe) 14 having a length L23 approx. equal to 0.5 times L3 and leading from chamber 2 to chamber 3.
Sound damping material B such as mineral wool is arranged in the chambers adjacent to the pipes 11, 12 and 13.

SDB is often arrived at by means of a separate, conventional acoustical calculation based on the measured unattenuated noise at a certain distance from the outlet from the exhaust system correlated with a desired maximum noise level at another point in space.
f^gjj is determined by the rpm of the engine, the number of cylinders and the type of engine process (two-stroke or four-stroke) . If the engine is coupled to the general power grid by means of a generator, the rpm will be given thereby. During start-up of such engines or in propulsion engines for ships, road vehicles and so on, the rpm is variable, and therefore the value of fign in such cases must be estimated suitably low based on a rough estimate or on more detailed considerations, for instance based on known acoustical-statistical calculations.
In some cases in connection with V-type cylinder arrange¬ments, a frequency around half the value of the ignition frequency may be preponderant which may motivate utilizing this frequency as a basic parameter for the dimensioning and design according to the invention of the respective silencer according to the invention.
Q and c can be calculated from the given mass flow and tem¬perature of the exhaust gasses.
SDP and 0D are typically "semi-fixed" parameters. Often it is very desirable to limit each of them to a maximum value, but if SDB already is fixed then SDP and 0D may not be determined freely. The smaller 0D is chosen, the larger SDP will be. Therefore, it will often be a question of combining the design and dimensioning of the silencer with considerations regarding the interrelationship between SDP and 0D including individually adapting the silencer structure to the geometrical constraints given by erection requirements, available space etc.

A typical procedure when carrying out the method of designing and/or dimensioning according to the invention is:
The number of chambers in the silencer is determined as

Hereafter, the types of intermediate pipes (passages) and of diffusers are decided. A combination of different types may be chosen for maximum repression of chamber resonances. So as to further hinder or avoid resonances, rather different chamber lengths L^^-Lg may be chosen. The outlets 10a, 12a and 13a of the diffusers 10, 12 and 13, respectively, are located at or near the axial centre of the respective chambers and at or near 2/3 of the radius corresponding to the pressure nodes of the respective chambers.
The relation A/a of the cross sectional areas may be tentatively chosen to be 10.
Typically, different chamber lengths are chosen, for instance:

wherefrom L3 is determined. L^^ and L2 are determined herefrom so that the total length of the silencer 5 is determined.
The other cross sectional areas of the connecting tubes 12 and 11 are determined such that the relations between the corresponding areas A and a also are approx. 10, a for the pipes 12 being the total cross sectional area of the pipes 12.
Now the total sound attenuation SDB may be calculated.
The total pressure loss SDP may now be calculated by using a combination of known elementary formulas and a detailed knowledge of the efficiency of different diffuser types, consideration being given to the inlet flow profiles to the diffusers, the chosen detailed geometry and so on.
If one or more of the calculated values for fg, SDB or SDP differ from the desired values then one or more of 0D, 0d or L are adjusted and the calculations indicated above are re¬peated.
In cases where maximum compliance of a silencer with the given requirements is desired, the dimensioning indicated above is supplemented by an adjustment so that the "peaks" and "troughs" are adapted to the requirements. This is done by varying the dimensions (chamber lengths etc) and calcula¬ting the damping spectrum by means of impedance analysis and constants involved herein, the constants being determined by separate theoretical and empirical investigations.

Referring now to Figs. 2-7, a practical example of designing and dimensioning a silencer for a six-cylinder, four stroke engine is illustrated.

The length of the drum (outer casing) is ideally desired to be 2600 mm but may be slightly larger if necessary.
The stud diameter minus stud thickness gives the internal diameter of the passage tubes and thereby the areas a. The drum diameter minus the drum thickness gives the area A.
In the initial calculation step, Fig. 3, the number of cham bers is chosen to be three and the chamber lengths are chosen equal as are the pipe diameters (areas a). The tail pipe (outlet from the drum) is also involved in the dimensioning, the volume of the "chamber" constituted by the atmosphere being infinite. The length of the tail pipe is initially equal to the lengths of the other passage pipes.

The diffusers are all radial and the pressiire drop through 20 the silencer is calculated by inter alia taking into con sideration the diffuser data indicated, a detailed explanation of said data not being important for the understanding of the invention.
The total pressure drop is calculated as 151.66 mmWG, i.e. 25 well below the maximum back pressure of 475 mmWG allowable for the engine. The total sound attenuation is 26.13 dB, i.e. too small. The local natural frequency of the chamber system 1-2 and the chamber system 2-3 is too high as it should be much nearer half the firing frequency, i.e. 45 Hz.
In Fig. 4 the diameters of the tubes between chambers 1 and 2 and between chamber 2 and 3 are reduced by the same amount so as to increase the sound attenuation and lower the natural frequency of the two said chamber systems in accordance with the expressions for A dB and fe.

Now the local frequencies are close enough to 45 Hz to give a satisfactory attenuation of the firing frequency of the engine.
Even though further optimization will be achievable such further improvement will be relatively small and without much practical value in the actual situation.
The example given above of an embodiment of a method according to the invention for designing and dimensioning a device for silencing a flow of exhaust gasses is directed to relatively simple and uncomplicated situations where the

silencing, pressure loss and space constraints are not strict.
In case one or more of such constraints are strict and/or the situation is complicated by other factors, more complex and far-reaching embodiments may be employed wherein for instance optimising calculation procedures may be employed.
Furthermore, constructive measures may be employed to enhance the silencing, pressure maintainment and optimal local natural frequencies.
The passages between the chambers may be prolonged by several means either alone or in combination. The pipes may be prolonged backwards into the upstream chamber and forwards into the downstream chamber, or the pipes may be prolonged by adopting a helical design for same. If the pipes either alone or combined with a diffuser are prolonged both upstream and downstream, the outlet of the upstream pipe in a chamber may be downstream of the inlet of the downstream pipe thereby twice reversing the direction of the main flow in said chamber. A diffuser having an umbrella-like shape with the convex surface thereof facing downstream will also have the effect of prolonging the passage and reversing the flow. Various types and shapes of baffle plates and guiding plates and bodies may be employed.
The design and dimensions of the diffusers are important for optimising the pressure recuperation thereof and thereby minimising the pressure loss through the silencer. Each diffuser may be a radial diffuser or an axial diffuser or a circular conical diffuser or an annular diffuser or a multi¬plicity of conical diffusers arranged on a cylindrical sur¬face or a diffuser for reversing the direction of flow or a double diversion diffuser or any other kind of diffuser.
All inlets of the passages should be suitably rounded so as to avoid vena contracta flow with associated vortices giving

rise to pressure loss and noise that may be amplified in the passage by resonance.
The outlets of the diffusers should, if possible, be located at the centre of the longitudinal direction of the chamber relative to the main flow direction and/or at the pressure node of a transverse oscillation in the chamber. Hereby, the basic resonance of the chamber in the two directions is repressed.
The arrangement of any sound absorbing material in the silencer is important. Particularly in large silencers it should not be too thick or compact so as to not decrease the acoustic volume of the chambers.
A further embodiment of the device is designed to further attenuate certain sound frequencies in addition to the attenuation achieved by the device as described in the foregoing. This device comprises at least one second container with or without a therein enclosed sound absorbing means and solely communicating with a respective chamber of the first container, the volume of the at least one second container and the geometric configuration of the at least one second container and the communication passage or passages therefrom to said respective chamber being designed and/or dimensioned in accordance with the frequencies of the sound vibration whereof the sound intensity is to be reduced thereby.
Each second container (resonating container) and its associated chamber and communication passage or passages will interact in such a manner that a certain respective sound frequency will be further attenuated.
The volume of the at least one second container may be adjustable, for instance by means of displaceable partitioning means, and said volume may be adjusted according to theoretical considerations and/or empirical considerations

taking into consideration the specific characteristics of the combustion means and/or the interrelationship thereof with the device and/or with the surroundings in general. Hereby, .^ standard, adjustable second container may be used for various silencers, or a tailor-made silencer may be "tuned" for certain frequencies by applying one or more adjustable resonating containers. Naturally the second container may be a further chamber of the silencer container instead of a separate container.
Fig. 8 shows a preferred embodiment of a device according to the invention. Here, two chambers 1 and 2, are contained within a casing 7 and are separated by partition walls 8a and b. An inlet passage pipe 6 passes the flow to chamber 1 via a radial diffuser 10. From chamber 2 the flow is passed to the outlet passage pipe 4 via an opening provided with a curvature 22 preventing flow separation.
The two chambers are interconnected by an annular passage 12. As can be seen from the figure, the combination of the radial inlet diffuser and the annular passage effectively prevents sound waves from 'short cutting' chamber 1 in passing from the inlet passage 6 to the annular passage, even though the inlet to the annular passage is positioned at a not very long distance Dl from the inlet to the first chamber. I.e., sound energy effectively fills chamber 1. The flow path from inlet to outlet of the second chamber contains less change of direction. However, due to the bigger distance D2 the tendency for sound waves to 'shortcut' the second chamber is rather small.
The inlet 12a to the annular passage 12 contains several features which contribute to make the inlet smooth, preventing pressure-loss associated vena contracta phenomena: Both cylinders 42 and 44 are extended by design parts into the first chamber, thus providing guidance for the flow accelerating into the annular channel. The inner cylinder 42 is extended to the left by a cylinder 41, which is full

immediately upstream of the inlet to the annular passage, and is otherwise perforated and contains sound absorptive r~-. material Ba. The outer cylinder 44 is extended to the left hyi^^' ' a conical cylinder 21 being connected to cylinder 44 by a curvature 20. The outermost diameter of the conical cylinder is big enough to provide sufficient guidance for the flow, i.e. the flow velocity at this diameter is much smaller than the flow velocity in the annular passage. On the other hand, the distance D3 between conical cylinder 21 and the casing 7 is not unnecessesarily small, since this would tend to acoustically isolate the right-hand, annular part of chamber 1.
The outlet 12d from annular diffuser 12c passes the flow into an annular passage 30 inside chamber 2, constituted by an inner, perforated cylinder 43 and an outer, likewise perforated cylinder 46. Sound absorptive materials Bd and Be are placed inside cylinder 43 and outside cylinder 46, respectively. The size D4 of passage 3 0 is chosen as a compromize between opposing demands: On the one hand, the smaller the size of D4 is the more efficient sound absorption is achieved. On the other hand, D4 should not become so small that too strong, turbulent noise is generated, or that too strong fluid-mechanical forces, tending to abstract absorptive material, are generated. Annular flow between sound absorbing walls is a per se known as an acoustically efficient configuration.
The length L12 of connecting passage 12 is chosen to be long enough for the local natural frequency fe to become sufficiently low, as given by the method of the invention, cf. equation (1). When the total length of the casing is given and is comparatively short, L12 is chosen by balancing of number of demands: Distance Dl can be made rather small, say in the order of half the diameter of the casing, depending upon a number of further geometric choices, among them the size of distance D3. Shortening distance D2 will cause a somewhat deteriorated acoustic function of chamber 2,

but as a gradual function of D2. In general, it is desired that the level of the noise which may be generated due to ' turbulence should not, at any frequency, exceed the level of the noise created by the engine.
For a given length L12, the annular passage type, the flow-friendly features of a smooth and guiding inlet and a diffuser outlet, allow for a comparatively low natural frequency fe and rather big, effective sound-reflective flow area ratios A/a, cf. equation (2).
From fluid-dynamic diffuser theory it is known that a maximum angle a of divergence, indicated in fig. 8, exists which, when exceeded, leeds to flow separation occurring inside the diffuser, i.e. a less than optimal operation which should be avoided. This angle is not very wide, so that for a large outlet to inlet flow area ratio of the diffuser, it tends to become long. However, for given sizes of inlet and outlet cross-sectional areas, an annular diffuser allows for a shorter diffuser length than does a conical diffuser. This means that, with an embodiment as shown in fig. 8, even though a low flow velocity is wanted at outlet 12d, and a high flow velocity is wanted in the constant flow area part 12b of passage 12, the diffuser length L-^jc "^^^ ^® chosen rather short, without flow separation occurring. For a given total length L12, this in turn means that the constant flow area part length L12b can be made rather long, so as to obtain a rather long L in equation (1) and so as to thereby obtain a rather low natural frequency, fe. The acoustically effective area a of the interconnecting passage 12 is a weighted mean of all cross-sectional areas occurring from inlet 12a to outlet 12d. Therefore, it is acoustically favourable that the smallest area, i.e. the cross-sectional area of constant-area part 12b, is comparatively long.
The silencer embodiment shown in fig. 8 contains a rather simple, central body 40 which is securely and accurately fixed to the outer parts of the silencer via a number of

flow- aligned arms or sheets 47, 48, and 49. E.g., in radial" diffuser 10 there may be four arms or sheets 49, positioned at 90 degrees' angle around the periphery. The total interface area between empty chamber volumes and volumes filled with sound absorptive material is big, providing a maximum of sound absorptive effect to assist sound reduction due to reflection at changes in cross-sectional area.
In the embodiment shown in fig. 8, the flow occurring in annular passage 12 is coherent all around the periphery of the annularity, apart from the small interruptions provided by the arms or sheets 47 and 48. For manufacturing reasons it may be expedient to adopt various types of peripheral segmentations of the annular passage. Thus, the arms or sheets 47 and 48 may be substituted by deformations which may be formed be pressing operations made on inner parts 42 and 43 and / or on outer parts 44 and 45 constituting the annular passage 12,
In bigger silencers, or when extreme outlet and/or inlet flow cross-sectional area ratios are wanted for the annular diffuser, it may be advisable to adopt a thorough peripheral segmentation of the annular passage, to prevent occurrence of fluid-mechanical instability of the rotating stall type, well-known from turbo machinery. This can be done by inserting radial partition walls into the annular passage. An alternative is to split the annular passage into a multiplicity of flow-parallel pipes arranged with the centerlines of all pipes situated on a cylinder with a centerline coinciding with the centerline of the silencer as such. These pipes can be circular, in which case diffusing outlet parts will be conical diffusers. Many other cross-sectional forms for pipes are also possible, e.g. squared cross-sections.
Fig. 9 shows another preferred embodiment of the invention. As in fig. 8, an annular passage 12 connects two chambers 1 and 2. In fig. 9 the casing is more elongated, as is e.g.

uypicai ot vertical silencers on trucks, i.e. it is rather
long, and the diameter is rather small. «
The following differences compared to the embodiment of fig. 8 are notable:
Distance D3 has vanished, so that conical cylinder 21 provides the partition wall 8b between chambers 1 and 2, together with rounded part 2 0 and central parts of the silencer. Still, the annular spacing 32 is not 'wasted' as an acoustic volume of the silencer, since it is a part of chamber 2. Perforated cylinders 41 and 50 provide inner delimiters of annular spaces 31 and 30, respectively. Whereas in fig. 8 annular space 3 0 is delimited by both the inner and outer perforated cylinders 43 and 46, annular spaces 30 and 31 in fig. 9 outwardly are delimited by the casing 7. Arms or sheets 51, 52, and 53 helps fix cylinders 41 and 50, together with filled-in sound absorptive materials Ba and Bd. Whereas in fig. 8 the outflow from annular passage 12 passes directly into passage 30, the outflow in fig. 9 passes a short distance D5 before entering annular passage 30.
Figs. 10a-lOe show a preferred embodiment of the invention in which a helical passage 12 connects two chambers 1 and 2, contained within a cylindrical casing 7 and separated by an inner, flat partition wall 8. The helical passage is delimited by casing 7, by an inner cylinder 42, and by helically formed sheets 60 and 61. Helical passage 12 is subdivided into a constant flow-area part 12b and a diffuser part 12c, in which the flow area gradually widens in the gas flow direction, as given by a gradually widening distance between sheets 60 and 61.
Both chambers 1 and 2 are partly filled with sound absorption material Ba, contained behind perforated plates 41 and 43. These plates have been so formed and positioned that, together with the absorptive material they help guide the

flow inside chambers 1 and 2 with low pressure drop and preventing unwanted flow swirling inside the chambers.
Gas flow is led to the silencer via inlet pipe 6 and conical diffuser 10, which recovers dynamic pressure and helps further to prevent unwanted swirl inside chamber 1 by-lowering the inlet flow velocity to the chamber. Here, the flow generally turns 90 degrees before entering helical connecting passage 12 at 12a. Here, a cylindrical rod 21 has been fitted onto inner cylinder 42 to improve inlet flow conditions, preventing vena contracta phenomena and inlet pressure losses. Inside passage 12 the flow first passes constant flow area part 12b and then diffuser part 12c in which dynamic pressure is recovered. The flow leaves passage 12 at outlet 12d, entering chamber 2. Inside this second chamber the general flow direction turns 90 degrees, both in plane AA and in plane CC, before entering outlet pipe 4.
From flow inlet 12a to flow outlet 12d of the helical passage 12, the flow in total turns 360 degrees inside the silencer casing. Thus, the length of the passage is approximately TT times the casing diameter, contributing to a very low acoustical natural frequency fe, constituted by passage 12 and chambers 1 and 2. In fig. 10, the length of casing 7 is only slightly in excess of the diameter. The embodiment thus demonstrates how, by adopting a helical passage between chambers according to the invention, it has become possible to achieve a much lower natural frequency than with a straight passage.
Examples of particularly relevant applications of the embodiment of fig. 10 are silencers for buses of trucks where there is space for a rather big silencer volume, given by a casing of a rather big diameter but of a short length. Even though the flow turns quite a lot inside the casing, the associated pressure loss is remarkably low. In spite of the embodiment being truly three-dimensional, the essentially 1-dimensional and dimensioning method of the invention applies.

Naturally, an accurate description of the fluid-flow and acoustic properties of the silencer should be three-dimensional. However, this is also the case in silencers wherein the acoustic field and the flow field is substantially two-dimensional. It should also be pointed out, that in spite of the three-dimensional flow path through the embodiment of fig. 10, it can be manufactured by rather simple members and by simple methods like sheet pressing, rolling, welding, etc.
Many types of silencers with helical flow patterns inside casing are known from prior art. However, in known silencer embodiments, helical flows have been desired for reasons differing from those of the present invention. E.g., very efficient sound absorption has been achieved by adopting helical channels made by perforated cylinders in contact with absorptive material. Another reason for adopting helical internal flow in silencers has been to achieve a spark-arresting effect by increasing the residence time for exhaust gasses inside a silencer.
The helical configuration of the invention allows the engineer to select the length of connecting channel 12 very freely and optimize this length according to the method of the invention. Thus, when a very low natural frequency is desired, even substantially more than 360 degrees turning inside the passage will be beneficial to select in some cases. An example of this could be a truck application, for which it is desired to attenuate infra-sound created by the engine when running at low speed at engine start-up or at hauling operation of the truck. Attenuation of infra-sound is further relevant in connection with gas turbine power stations. In other cases, less than 3 60 degrees flow turning in the helical passage can be appropriate, e.g. with higher ignition frequencies of engines, and when it becomes essential not to create too low resonant frequencies of the helical passage.

To the silencer design engineer it obvious that the goals addressed by the embodiment shovra in fig. 10 can be achieved by many variations in design configuration. E.g., the flow-widening of the diffuser 12c can be achieved by varying the 5 diameter of the inner cylinder 42. A helical passage can be fitted into a cubic casing, instead of a cylindrical casing. The wall 8, separating chamber 1 from chamber 2, can be a cylinder, and chamber 2 can be arranged essentially outside chamber 1, which is favourable from a shell noise emission 10 point of view, since the sound level inside chamber 1 is higher than inside the downstream chamber 2.

WE CLAIM :
1. A method designing and/or dimensioning a device for silencing a flow of exhaust gasses originating from a combustion means,
- the device being of the type comprising a first container 20 with one or more
compartments or chambers each having one ormore inlet passages and one or more
outlet passages, and at least one diffuser associated with one or more inlet passages
and/or one or more outlet passages,
- the flow of exhaust gasses being directed into said one or 25 more inlet
passages and out of said one or more outlet pas sages and at least partly through said
at least one diffuser,
- the method comprising applying the following expressions for designing
and/or dimensioning the geometric configurationand arrangement and the relative
dimensions of the one or 30 more chambers, the one or more inlet passages, the one or
more outlet passages and the at least one diffuser:

where
fe is the local natural frequency of a system of two volumes interconnected by a passage,
c is the sound velocity,
a is the representative cross sectional area of the passage,
L is the length of the passage,
Vi is one volume,
V2 is the other volume,
A dB is the total attenuation of the sound spectrum,
k is a constant, and
A is the representative cross sectional area of a chamberupstream or downstream relative to the respective passagewith respect to the flow direction.
2. A method as claimed in claim 1, wherein the combustion means is an internal combustion engine.
3. A method as claimed in claim 3, wherein the internal com bustion engine is a piston engine.
4. A method as claimed in claim 4 the method comprises the steps of ensuring that the lowest value of the local natural frequency fe in the device is approximately half the firing frequency of the internal combustion engine.
5. A method as claimed in claim 3, the method comprises the steps of ensuring that the lowest value of the local natural frequency fe in the device is approximately three quarters of the firing frequency of the internal combustion engine.

6. A method according to any of the claims 1-5, wherein the chambers are substantially cylindrical and one or more outlets from said at least one diffuser ai located substantially at the axial centre of the chamber associated with said diffuser.
7. A method as claimed in claims 1-5, wherein the chambers are substantially cylindrical and the one or more outlets from said at least one diffuser are located substantially at a distance of two thirds of the radius of the chamber from the axis of same.
8. A method as claimed in claims 1 -7, wherein the value of k is in the range 5-7.
9. A method as claimed in claims 1-7, wherein the value of k is in the range 6-6.5.
10. A method as claimed in claims 1-7, wherein the value of k is approximately 6.25.
11. A device for silencing a flow of exhaust gasses from a combustion means, the device comprising a first container with one or more compartments or chambers with one or more inlet passages and one or more outlet passages, and at least one diffuser associated with one or more inlet passages and/or one or more outlet pas sages, the geometric configuration and arrangement and the relative dimensions of the one or more chambers, the one or more inlet passages, the one or more outlet passages being
designed and/or dimensioned by application of the following two expressions:

A device as claimed in claimll, wherein the diffuser is a radial diffuser or an axial diffuser or a circular conical diffuser or an annular diffuser or a multiplicity of conical diffusers arranged on a cylindrical surface or a diffuser for reversing the' direction of flow or a double diversion diffuser.
13. A device according to any of the claims 11-12, wherein the device comprises at least one second container with or without a therein enclosed sound absorbing means and solely communicating with a respective chamber of the first container, the volume of the at least one second container and the geometric configuration of the at least one second container and the communication passage or passages therefrom to said respective chamber being designed and/or dimensioned in accordance with the frequencies of the sound vibration whereof the sound intensity is to be reduced thereby.
14. A device as claimed in claims 11-13 wherein curvatures, preventing flow separation, are applied to at least part of the contour of the outlet and/or inlet of a pipe (4,6) or passage (12) of the device, said pipe or passage being the inlet pipe (6) or its extension into the apparatus and/or the outlet pipe (4) or its extension into the apparatus and/or a passage (12) connecting two chambers(l,2).
15. A device for silencing a gas flow directed therethrough, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, wherein at least one pipe or passage is annular, constituted by an inner cylinder (42, 43) and by an outer cylinder (44, 45).
16. A device as claimed in claim 15, wherein at least one of the at least one pipe or passage which is annular is a passage connecting two chambers (1,2).

17. A device as claimed in claim 15 or 16, wherein the at least one pipe or passage being annular constitutes a flow cross sectional area increase in the flow direction.
18. A device as claimed in claims 15-17 wherein said annular passage (12) comprises a constant flow area part (12b) and an outlet diffuser part (12c).
19. A device as claimed in claims 25-28 wherein said inner cylinder extends into said first chamber (1) by a cylinder (41) of substantially the same diameter as said inner cylinder, and said outer cylinder is connected to a flow-guiding body (21) with a curvature (20).
20. A device as claimed in claims 15-19 wherein sound absorptive material is contained within said cylinder (42, 43) and/or within a continuation cylinder (41) extending into said first chamber (1) and/or within a continuation cylinder (43) extending into said second chamber (2).
21. A device as claimed in claim 20 wherein at least part of the continuation cylinder (41) is perforated.
22. A device as claimed in claims 15-21 wherein the outflow from said connecting passage (12) passes into an annular passage (30) inside said second chamber (2), said
annular passage (30) being made up of at least a perforated portion of an inner cylinder (43) and an outer, perforated cylinder (46), both said cylinders (43, 46) separating sound absorptive materials (Bd, Be) from gas flow within said second chamber.

15. A device as claimed m claim 22 wherein the outflow from said connecting passage (12) passes directly into an annular passage (30).
24. A device as claimed in claims 22-23 wherein the inner cylinder (43) has a non-perforated portion which constitutes at least part of the inner cylinder constituting an inner wall of the outlet diffusor part (12).
25. A device as claimed in claims 22-24 wherein the non-perforated portion of the inner cylinder (43) constitutes at least part of an inner wall of the chamber (2).
26. A device for silencing a gas flow directed therethrough, the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and/or one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, wherein at least a part of at least one passage (12) connecting two chambers (1, 2) is wound in a peripheral direction.
27. A device as claimed in claim 26, wherein at least the wound part of the passage (12) extends in a longitudinal direction, the generatrix of the wound part of the passage (12) thus being substantially helical.
28. A device as claimed in claim 26, wherein the generatrix of the wound part of the passage (12) extends in a single plane.
29. A device as claimed in claims 26-28, wherein the flow the passage (12) constitutes a flow cross sectional area increase in the flow direction.

30. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 0° and 90°.
31. A device according to any of claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 90° and 180°.
32. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 180° and 270°.
33. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 270° and 360°.
34. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle between 360° and 720°.
35. A device as claimed in claims 26-29, wherein the wound part of the passage (12) extends radially over an angle of 720° or more.
36. A device as claimed in claims 15-25 or any of claims 26-35 wherein curvatures, preventing flow separation, are applied to at least part of the contour of the outlet and/or inlet of a pipe (4,6) or passage (12) of the device, said pipe or passage being the inlet pipe (6) or its extension into the apparatus and/or the outlet pipe (4) or
its extension into the apparatus and/or a passage (12) connecting two chambers (1,2).

37. A device as claimed in claims 15-25 or any of claims 26-35 or claim 36, wherein the geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned according to any of claims 1-10.

Documents:

2160-mas-1997 abstract duplicate.pdf

2160-mas-1997 abstract.pdf

2160-mas-1997 claims duplicate.pdf

2160-mas-1997 claims.pdf

2160-mas-1997 correspondence others.pdf

2160-mas-1997 correspondence po.pdf

2160-mas-1997 description (complete) duplicate.pdf

2160-mas-1997 description (complete).pdf

2160-mas-1997 drawings duplicate.pdf

2160-mas-1997 drawings.pdf

2160-mas-1997 form-1.pdf

2160-mas-1997 form-13.pdf

2160-mas-1997 form-19.pdf

2160-mas-1997 form-26.pdf

2160-mas-1997 form-29.pdf

2160-mas-1997 form-4.pdf

2160-mas-1997 form-6.pdf

2160-mas-1997 others document.pdf

2160-mas-1997 others-1.pdf

2160-mas-1997 others.pdf

2160-mas-1997 petition.pdf

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Patent Number

202120

Indian Patent Application Number

2160/MAS/1997

PG Journal Number

05/2007

Publication Date

02-Feb-2007

Grant Date

11-Sep-2006

Date of Filing

30-Sep-1997

Name of Patentee

SILENTOR HOLDING A/S

Applicant Address

BALDERSBUEN 22, DK-2640 HEDEHUSENE,

Inventors:

#	Inventor's Name	Inventor's Address
1	FREDERIKSEN, EYVIND	NEALMINDINGEN 39, DK-2860 SBORG
2	FREDERIKSEN, SVEND	MASNEDOGADE 6E 22 DK-2100 COPENHAGEN

PCT International Classification Number

F01N007/18

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	1070/96	1996-09-30	Denmark