Study On Airflow Velocity Of Split-stream-rushing Exhaust Muffler Unit For Diesel Engine

It is the most simple and effective way to reduce the exhaust noise of diesel engine by using an exhaust muffler.Because of the contradiction between the noise elimination and the exhaust back pressure,the performance of traditional exhaust muffler for diesel engine is poor.Therefore,it is of great significance to develop a new muffler with good performance and low exhaust back pressure.It was found that the airflow velocity is a key factor to determine the performance of the muffler.For this reason,in this paper,a novel principle of reducing the velocity of airflow in the muffler by split-stream-rushing was presented,and base on the principle,the muffler unit was developed.In this dissertation,aiming at the split-stream-rushing muffler unit,the variation of the internal airflow velocity was analyzed by using the methods of theoretical calculation,numerical simulation and experimental analysis.At first,a special muffler test system was designed,on which the importance of the effect of airflow velocity on the performance of reactive muffler was verified;The velocity variation of the split-stream-rushing muffler unit in airflow rushing process and the nexus between velocity reduction from rushing and internal flow field were analyzed;The influence of the structured parameters on the airflow velocity was researched,and the optimum structured parameters of the muffler unit was determined by orthogonal test method;In the end,the performance of the split-stream-muffler unit was studied synthetically.The following main conclusions have been drawn from the study:(1)A mathematical model for the longitudinal velocity variation of micro-volume gas in airflow rushing process was established,and by using Matlab software the velocity variation of micro-volume gas was analysed in the process of airflow rushing,the results show that the longitudinal velocity of airflow decreased to zero rapidly near the rushing surface when two airflow rushing at a certain speed.(2)The velocity variation of airflow in the muffler unit during airflow rushing process was summarized.It was found that the main flow was changed from longitudinal flow to horizontal flow after two split stream rushing.At the same time,some low velocity vortexes were formed near the rushing surface,and the overall velocity was attenuated further;The longitudinal airflow velocity was reduced to the minimum at the center of rushing area,with an average reduction of 65%.In the central line of two rushing areas,the transverse airflow velocity was almost zero because of a second time rushing.(3)The experimental study on the nexus between the airflow rushing and pressure field indicated that some pressure loss was caused by two split stream rushing,which accounts for about 50%of the pressure loss of the whole muffler unit.The average resistance coefficient of the fluid is about 0.91 during the rushing process.(4)The influence of four kinds of structural parameters,such as the diameter of the inner cavity,the shape of rushing hole,the center distance of rushing hole as well as the transition circular arc radii of the tail pipe,on the airflow average velocity was analyzed by orthogonal test method.The better structured combination of the muffler unit with the 70mm inner cavity diameter,rectangular structure of the rushing hole,the 'Save'(the length of the inner tube is 1/3)center distance of the rushing hole,and the 5mm transition circular arc radii of the tail pipe was acquired.The sequence of influence of four kinds of structures on the muffler unit airflow velocity as followed:the diameter of the inner cavity,the shape of rushing hole,the transition circular arc radii of the tail pipe,and the center distance of rushing hole.(5)The performance of the split-stream-rushing muffler unit was researched comprehensively,and compared with the original exhaust muffler of CG25 single cylinder diesel engine.Results show that in medium and low frequency,the average transmission loss of the split-stream-rushing muffler unit is improved by 30.8%,and the average insertion loss is increased by 30%;When the inlet velocity is 30m/s,the pressure loss of the split-stream-rushing muffler unit is reduced by 16.8%;The distribution range of turbulent kinetic energy of the split-stream-rushing exhaust muffler unit is less than that of the original muffler at the same inlet velocity condition,and the maximum kinetic energy of the two is equal,to a certain extent,it is indicated that the regenerative noise of the split-stream-rushing exhaust muffler unit is less than that of the original muffler.