| At present,the internal combustion engine is still the main power unit of transportation.Diesel engine has become the main power equipment for vehicle,engineering machinery,agricultural machinery and other equipment with its strong power performance.However,the more prominent disadvantage of diesel engines is its high exhaust noise,and the most direct and effective way to reduce the exhaust noise of diesel engines is to install exhaust muffler.The performance of a diesel engine is directly related to the performance of the muffler.Therefore,the research on the performance of the muffler is particularly important.The performance of the muffler mainly includes aerodynamic performance,airflow regenerative noise performance,and acoustic performance.These properties are closely related to its structural parameters.As a result,the study of accurately predicting its performance through the structural parameters of the muffler is of great significance.In order to study the influence of the structural parameters of the split-stream-rushing exhaust muffler unit on its overall performance,the pressure loss model,airflow regeneration noise model,and transmission loss model were studied in this paper based on the structure parameters of the muffler unit and airflow velocity.At the same time,the structural optimizations of the multi-objective split-stream-rushing exhaust muffler unit based on the performance model were studied,and on the basis of this,its comprehensive performance was studied.The main results achieved are as follows:(1)Taking the CG25 single-cylinder diesel engine as the prototype,the volume of the split-stream-rushing exhaust muffler unit is determined by using the empirical formulas of the muffler design.According to the empirical formulas of muffler design and the optimal coupling relationship between each subunit,the main structural parameters of the split-stream-rushing exhaust muffler unit are determined.(2)The influence of structural parameters on the pressure loss and airflow regeneration noise of the split-stream-rushing exhaust muffler unit was studied by single factor test,five structural parameters include inner cavity diameter,the shape of rushing holes,the center distance of rushing holes,the cone angle of inner cavity split unit,and the number of rushing holes.The influence law of structural parameters of muffler unit on the pressure loss and the total sound pressure level of the airflow regeneration noise is found,which provides a reference for the study of the performance model of the muffler unit based on the Box-Behnken experiment.(3)The test design scheme of the pressure loss,the total sound pressure level of airflow regeneration noise and the transmission loss for the split-stream-rushing exhaust muffler unit were designed by design-expert.Taking pressure loss,total sound pressure level of airflow regeneration noise and average transmission loss of 0?1000Hz as response values,the mathematical regression models between pressure loss,total sound pressure level of airflow regeneration noise,transmission loss,and test factors(inner cavity diameter A,the shape of the rushing holes B,the center distance C of the rushing holes,the cone angle of inner cavity split unit,the number of rushing holes E,the airflow velocity F)are established according to regression analysis,and the accuracy of the regression models are verified by experiments.(4)The influence of the second-order interaction between the test factors on the pressure loss,the total sound pressure level of the airflow regenerative noise and the transmission loss were analyzed by three-dimensional surface chart.The results show that the airflow speed,inner cavity diameter and the number of rushing holes are the main factors affecting pressure loss and airflow regeneration noise.Inner cavity diameter and the center distance of rushing holes are the main factors affecting transmission loss.As the air velocity increases,the pressure loss and airflow regeneration noise increase significantly.As the diameter of the cavity increases,the pressure loss and airflow regeneration noise slowly decrease and then increase rapidly,while the transmission loss increases.As the number of rushing holes increases,the pressure loss and airflow regeneration noise decrease significantly.When the diameter of internal cavity is 70mm? 75mm,the transmission loss increases first and then decreases as the number of rushing holes increases,and when the diameter of internal cavity is 75mm ? 90mm,the transmission loss increases as the number of rushing holes increases.When the shape of the rushing hole is rectangular,the pressure loss and airflow regeneration noise are small,the shape of the rushing holes has little impact on the transmission loss.The center distance of rushing holes and cone angle of the inner cavity split unit have little impact on the pressure loss and airflow regeneration noise.As the cone angle of the internal cavity split unit increases,the transmission loss decreases.When the diameter of internal cavity is between 70mm and 80mm,the center distance of the rushing holes changes from "Smin" to "Smax",the transmission loss increases first and then decreases.When the diameter of internal cavity is between 80mm and 90mm,the center distance of rushing holes changes from "Smin" to "Smax",the transmission loss decrease.(5)Using the pressure loss,total sound pressure level of airflow regeneration noise,and average transmission loss as optimization indicators,single-objective optimizations on the model of pressure loss,the model of total sound pressure level of airflow regeneration noise,and the model of transmission loss were performed respectively.The muffler unit models before and after the single objective optimization were built.Taking the inlet air velocity of 40m/s as an example,a comparative study was performed through numerical simulation.The results show that the pressure loss of the optimized muffler unit are reduced by 55.2%,the total sound pressure level of air flow regeneration noise is decreased by 14.04%,and the transmission loss is increased by 69.41%.(6)Based on the one-dimensional plane wave theory,the transfer matrix mathematical model of the split-stream-rushing exhaust muffler unit is derived.Through the mathematical model,the specific acoustic performance and relationship with its structured parameters of the muffler unit can be revealed theoretically.(7)The regression models,numerical simulations,and tests were performed to study the pressure loss and the airflow regeneration noise of the single-objective optimized muffler units.The results show that the relative error of the regression models between calculated values,numerical simulation values and experimental values are small,which further verifies the accuracy of the pressure loss regression model and the regression model of airflow regeneration noise.(8)The cross process,mutation process and elite retention strategy of the traditional fast non-dominated sort genetic algorithm(NSGA-?)are improved,an improved fast non-dominated sort genetic algorithm(Improved NSGA-?)is proposed.The Improved NSGA-II algorithm is applied to the multi-objective structure optimization of the split-stream-rushing exhaust muffler unit based on the models of pressure loss,the models of airflow regeneration noise,and the models of transmission loss.The pareto optimal solution set is obtained,which has avoided the influence of artificial selection of weight value on the optimization result in the traditional solutions.(9)Taking the multi-objective structure optimizations of the split-stream-rushing exhaust muffler units as the research object,the pressure loss,the turbulent kinetic energy,the sound power,the total sound pressure level of airflow regeneration noise and insertion loss were compared by numerical simulations and experiments.The results show that the pressure loss,turbulent kinetic energy,sound power,total sound pressure level,transmission loss and insertion loss of the optimized muffler unit are better than those before optimization.(10)The airflow regenerative noise spectrums at selected measurement points of the muffler units were analyzed.The results show that when the frequency is lower than 2000Hz,as the frequency increases,the sound pressure level of the airflow regeneration noise at each measuring point inside the muffler unit gradually decreases;when the frequency is higher than 2000Hz,the airflow regeneration noise belongs to wideband noise.When the frequency of external noise measurement points of the muffler unit are 0?1200Hz,1200?2400Hz,2400?3200Hz and 3600?4800Hz,the noise sound pressure level increases first and then decreases with the increase of frequency.The noise energy mainly concentrates in the frequency band within 0?1200Hz.Taking the inlet airflow velocity of 40m/s as an example,the sound pressure level spectrum of airflow regeneration noise at the measurement points of the muffler unit before and after optimization is compared and analyzed.It is found that the spectrum trend of the measurement points of the muffler unit before and after optimization is basically the same,but the sound pressure level of the muffler unit after optimization is lower than the sound pressure level before optimization,and he peak value of sound pressure level is significantly reduced.(11)The model of pressure loss,the model of airflow regeneration noise,and the model of transmission loss of the split-stream-rushing exhaust muffler unit based on structural parameters and airflow velocity can provide a basis for the design and theoretical optimization of such muffler units. |
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