Numerical Simulation Of Flow Field On Exhaust System Of Passenger Car

With the improvement of modernization and the development of auto industry and transportation, the number of retained urban motor vehicles has risen sharply. The air and noise pollution arising therefore has been attached more and more importance. In recent years, low noise has become a part of riding comfort and an important parameter in evaluating vehicle quality with power performance, fuel economic and emission. As the major equipment to control exhaust pollution, exhaust system requires low noise, low vibration, low emission, low power consumption (low backpressure) and low cost. Compared with traditional experimental method that consumes time and labor, numerical simulation could analyze the pressure loss, temperature loss and flow field distribution of muffler theoretically, analyze the rationality of its structure and analyze the potential problems to provide a basis for optimum design. This will shorten the development period, enhance the work efficiency and reduce the cost.This paper is based on an engineering project and the flow field distribute law of an exhaust system is analyzed by modeling and calculating and an optimum designee of the muffler is made based on the results.Firstly, the present development of numerical simulation technology, the mechanism of exhaust noise, the factors that effect exhaust noise, the design requirement of exhaust system, the application of CFD in exhaust system design and the basic flow field theory of CFD are introduced.A comparision is made among flow field results of three different mufflers to discuss the influence of structure factors on exhaust system flow field. The influence of micropore number of perforated pipe, perforated pipe length and expansion chamber on flow field are taken into account respectively. The backpressure, temperature loss and difference of flow field distribution under different operating conditions are compared and analyzed to discuss the influence of engine speed on flow field. The results show that the pressure loss increases parabolically with engine speed, the temperature loss decreases parabolically with engine speed. Under low speed condition the flow field distributes more even and swirl speed and turbulent kinetic energy loss decrease highly.Finally, according to the simulated results, an improvement scheme is provided. The depth of perforated pipe is reduced and the inlet shape of exhaust perforated pipe is changed to trumpet-shape. The comparative analysis of flow field before and after improvement shows a satisfying result.