Study On Resistance Muffler Based On CFD Simulation And Test

The method of computational fluid dynamics (CFD) and test has been used to study the fluid dynamics and acoustic performances of resistance muffler. The main contents of this dissertation are shown as following: the changing regularities of pressure loss effected by the structure parameters and the boundary conditions for resistance muffler; validity certification of pressure loss computation with the method of CFD with pressure loss results comparison of CFD and test; muffling performance study with sound signal spectrum analysis of noise exhausted out of the diesel, sound radiated from the shell of the diesel and the noise exhausted out of mufflers. The main object of this dissertation is to explore a method for resistance muffler design based on CFD simulation and tests to improve the design quality integrated the performances of fluid dynamics and acoustics.Resistance muffler is the main device to attenuate the exhausted noise of internal-combustion engine presently. Noise pollution has become one of the important problems of environment pollution with the applications of automobile and construction machinery driven by diesel, and the exhausted noise has been paid attention to improve the integrated performance of diesel for a long time. The main performances of resistance muffler include insert loss, pressure loss, processing property of manufacture and operational life span. Insert loss represents the sound attenuation degree of muffler, which is the difference of sound pressure levels of exhaust noise at the positioned place with the muffler installed or not. Pressure loss reflects the power loss ratio of internal-combustion engine with the difference of the mean total pressure in the double pipes of input and output. The dimensions of muffler design should be limited in the demand of assembly with simple structure, and the operational life span is another factor with the muffler being one non-vulnerable part.Resistance muffler research relates with the fields of acoustics, fluid dynamics, heat transfer and mechanism design. Acoustic performance has been the most important property for muffler design depicted in literatures at presents. The method of transmission matrix based on 1-demensionsl plane wave theory is the traditional way to study the acoustical performance of resistance muffler, which has become mature for simple structure muffler with the deficiencies of low decision for complex muffler design as for perforated muffler. With the fast development of software and hardware of computer and the corresponding arithmetic improvment, acoustics simulation of resistance muffler has been becoming more and more actual. The acoustics simulations have experienced the developments based on 1-dimension, 2-dimension and 3-dimension, with the simulation results has becoming more and more approach to actual muffler.The half-experienced equations based on the theory of fluid dynamics have been used in the traditional way for pressure loss computation, which is not suitable for pressure loss computation of resistance muffler with the structure of pyknic type and fast fluid velocity in resistance muffler. The computational fluid dynamics (CFD) affords a new method for fluid dynamic simulation of resistance muffler which can be used to compute the fluid dynamics performances with the method of finite volume based on basic theory and the pressure loss can be calculated with the corresponding post data-processing. The main object of this dissertation is to explore an effective method for pressure loss computation during the procedure of muffler design, which can be used to study the performances of resistance muffler combined with the acoustic performance.The method of pressure loss computation with CFD simulation has been used with the procedure of modeling, meshing, boundary condition confirmed, arithmetic selection, simulation analysis and data post-processing. Structured jointed with unstructured grids have been used to mesh the complex resistance muffler, which can accelerate the computing velocity with the calculation reduction under the premise of precision confirmed.The pressure loss results calculated with the methods of half-experience equations and the CFD simulation have been proved to be the same with fluid flowing at low speed with the fluid can be released adequately. The former method is unsuitable when the flow speed increasing with the fluid pushed out of the muffler nonnaturally. Temperature should be considered during CFD simulation because it relates with the properties of air as density and viscidity. The pressure loss decreases with the fluid temperature increasing of input duct of muffler with the same fluid velocity.The regularities of pressure loss changing with structure parameters and boundary conditions have been achieved for single-chamber resistance muffler, four types of which are typical muffler, muffler with interpolated pipes, muffler with input/output offset and muffler with interpolated offset. Results have been obtained as following: Pressure loss of all the four types of muffler increases with the velocity of input flow increasing; Pressure loss of typical muffler and muffler with interpolated pipes rises with the chamber-length increasing, and the interpolated pipes makes the pressure loss decrease; Interpolated pipes and the chamber length affect pressure loss little for the input/output offset muffler; Pressure loss of input/output offset muffler is much bigger than pressure loss of muffler with input/output coaxial.Two types of double-chamber muffler have been studied to explore the effect of baffle position on the pressure loss computation, and the types of which are the interpolated pipes offset muffler and the input/output coaxial muffler without interpolated pipes. The baffle position affects pressure loss little for each double-chamber muffler. The pressure loss of the each double-chamber muffler is nearly the same as the pressure loss sum of two single-chamber muffler with equivalence structure and same boundary conditions, which afford a good method for pressure loss estimation of multi-chamber muffler during the procedure of muffler design.Straight-through and cross-flow perforated single-chamber mufflers have been selected to seek the relations of pressure loss changing with the main parameters and the boundary conditions, and the results obtained as following: Pressure loss of the straight-through perforated mufflers increases with the increase of all the porosity, chamber-length and the flow velocity of input; For the cross-flow perforated mufflers, pressure loss decreases with porosity increasing, pressure loss increases abruptly when the porosity is less than the low threshold value while it changes little when the porosity is greater than the high one. The porosity should be controlled between the two thresholds for effective muffler design; The diameters of the perforated holes for the cross-flow perforated mufflers effects the pressure loss little, and it can be negligible during muffler design; Pressure loss of the cross-flow perforated mufflers is much bigger than that of the straight-through mufflers and non-perforated muffler under the same boundary conditions. The dynamical property of perforated muffler is much better than the non-perforated one.One practical resistance muffler has been designed together with another five mufflers with typical structure for performance comparison based on the conclusion acoustics and fluid dynamics which has been achieved. The boundary conditions of the composite muffler have been confirmed with the simplified working conditions according to the practical working conditions of practical muffler for an appointed model number of excavator. The dynamical properties have been obtained with CFD simulation, and the pressure loss of composite has been solved based on the corresponded standard.Velocity and pressure at the appointed positions in the input/output pipes of muffler have been measured with the corresponding apparatus equipped. CFD has been proved to be an effective method for pressure loss computation with the results comparison of the CFD and test.The test equipment for noise measure has been made up of signal collection appliance, sound level meter and computer according to the practical operational position of resistance mufflers. The spectrums of sound level for radiated noise of diesel, noise exhausted out of diesel directly and muffler have been analyzed to research the noise property muffler, the corresponding results as following: the noise signal measured beside the output of muffler has been proved to be disturbed by the noise signal radiated from the shell of diesel and the noise signal of diesel accessories. The muffling rules of muffler changing with the speed of rotation have been studied with the spectrum analysis of noise signal exhausted out of muffler with different speed of diesel. The sound pressure level of noise increases with the diesel speed of rotation increase, but the signal spectrum properties of noise changes little with different speed of diesel rotation.Conclusions have been obtained with spectrums analysis of exhausted noise together with pressure loss computation of the six practical resistance mufflers as following: Expand ratio influences the effect of sound attenuation, and the expand ratio is bigger, the muffling is better; Interpolated pipes radial is available for noise muffling of low-frequency stage; Sound attenuation of intermediate and high frequency for four-chamber muffler is much better than three-chamber one with the same outline parameters. Multi-interpolated pipes used in the muffler have better muffling effect than single-interpolated pipes for low-frequency muffling. The low porosity for perforated muffler is not only useful for high frequency noise muffling, but also for low-frequency sound attenuation. The taper interpolated pipes can improve the muffling effect of low and intermediate frequency stage instead of column ones. The resistance muffler designed for an appointed evacuator has been validated with good performances both for noise attenuation and fluid dynamics.The method for resistance muffler design based on CFD simulation and test has been studied integrated with the conclusions of both acoustical and fluid dynamical in existence. The corresponding steps has been offered as objective confirmed of design, flow parameters confirmed for fluid in muffler, preliminary design of structure, pressure loss computation, structure design inside muffler and the test validity, which has been testified to be reasonable with a practical muffler design which has good performances of both muffling and fluid dynamics design in this method.