Studies Of Boundary Element Methods For Acoustic Performance Prediction Of Marine Engine Exhaust Silencers And Experiments

Silencer is one of the efficient methods to reduce the exhaust noise of marine diesel engines. In view of the complexity of sound field inside the silencers, the three-dimensional numerical methods should be used to predict the acoustic characteristics of silencers. The boundary element method (BEM) is an advanced and efficient numerical method which has been widely used to predict the acoustic performance of ducts and silencers. However, the huge computations and memory requirements of the conventional boundary element method (CBEM) limit its application to solve the large-scale or high frequency acoustic problems. In addition, the CBEM can not be used to predict the sound propagation problems in higher Mach number subsonic flow. In order to solve these problems, the fast multipole boundary element method (FMBEM) and the dual reciprocity boundary element method (DRBEM) are developed in this paper.For solving the internal acoustic problems in the static medium, a kind of FMBEM is developed. The basic principle and the numerical procedure of FMBEM were introduced, and applied successfully to predict the acoustic performance of silencers. Some special rules were studied and analyzed when applying FMBEM to predict the acoustic performance of silencers. Numerical results showed that the proposed method has high accuracy and can save much memory requirements, and the computation time may de reduced greatly in a certain frequency range.For the internal acoustic problems in the higher Mach number subsonic complex flow field, the DRBEM is developed, the numerical procedure of the method was introduced, and has been successfully applied to predict the acoustic performance of ducts and silencers with complex flow. Compared with CBEM, DRBEM takes into account the second order terms of Mach number in the acoustic governing equation, so it is suitable for the cases with higher Mach number subsonic complex flow. Numerical results showed that the present method is valid. The computation of DRBEM involves three matrix multiplications and one inversion, thus the computation time cost of DRBEM is 13 times of CBEM at least, which restrict the application and development of DRBEM. Another aspect, the application of CBEM to the complex silencers will generate the hyper-singular integral problems, which make the numerical operations very tediously and lead to bigger computation errors. The proposed substructure DRBEM solved both problems efficiently. The basic ideas is that: an acoustic system being analyzed is divided into a number of substructures first and then DRBEM is applied to each substructures to obtain their impedance matrices, the entire acoustic system can be solved by using the continuity conditions at interfaces between substructures. Numerical results verified the validity and efficiency of the method. Influence of flow on the acoustic performance of complex silencer was examined briefly, and the results showed that the effect of higher Mach number subsonic flow on the acoustic performance of complex silencer is not negligible.In order to verify the validity of acoustic performance predictions of exhaust silencers with the proposed BEM and serve the design and performance evaluation of the practical exhaust silencers of marine diesel engines, it is necessary to study and develop the relative experimental test techniques. A test bed for performance study of marine diesel engine exhaust silencers at low and high temperatures was designed and built successfully, which is the first one in China. Two high pressure centrifugal fans are used to generate air flow, and an aero-turbine combustor serves as a burning gas generator which can meet the requirements of the test bed on the flow and pressure loss. Temperature and flow of the exhaust gas are controlled by two types of control systems including automatic one and manual one. There are also two responsible control programs. One is the room temperature gas program and the other is the high temperature gas program. There is only one variable including flow in the room temperature gas program, so it is easy to realize. There are two variables including flow and temperature in the high temperature gas program which are also deep coupling. So the high temperature gas program is more difficult to realize. The measuring system is reasonable which can get parameters of the gas in the duct and of the silencers accurately. It can satisfy the requirements of the experimental analysis and the control system for measuring accuracy. A new type of acoustic enclosure was designed, and the noise radiated from the test bed has been reduced to a required level. The experimental results show that the test bed can be used to evaluate exhaust silencers of engines with power less than 2000 kW. The gas volumetric flow range is 0～8 m~3/s with error in the range of±5%. The gas temperature range is 30～600℃with error in the range of±10℃. The adjusting time for change of working situations is less than 5 minutes.The method for measuring acoustic performance of ducts and silencers was studied, and has been applied to measure acoustic performance of ducts and silencers with flow and without flow on the test bed. Measurement results agree well with the BEM predictions, which demonstrated that the developed measurement method and numerical prediction methods and the proposed numerical method are correct.Finally, effects of the number of inter-connecting tubes, positions of inlet and outlet, annuluses and complex flow on the acoustic performance of double expansion chamber silencers were predicted and analyzed by using the present developed BEM. Some useful conclusions were obtained according to these results.