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Exhaust system is mainly composed of Catalytic Converter, corrugated pipe, mufflers, flanges and connecting pipes, it is an important part of emission and noise reduction of automobile, and also has an influence on automobile dynamics and fuel economy. In the past, the design of exhaust system was usually based on plane wave theory, only considered the travel of sound. Recent research indicates that the flow field of exhaust system could also produce some flow noises which worsen the system acoustic performance. With the widespread applications of CFD (Computational Fluid Dynamics) softwares, more and more companies and institutions simulate the flow field of exhaust system with three dimensional CFD softwares. However, the majority of CFD simulations calculate system flow field by using air to approximate exhaust gas, it will cause big errors. This paper, using exhaust gas which calculated from simulation, analyzed three dimensional flow field and calculated sound power level of exhaust system base on broadband noise source model by Fluent software, unified the sound and flow field, and performed relative verification tests.First, this paper established four-cylinder gasoline engine model in GT-Power software on the basis of certain engine structure and performance, it got the relative engine indicated performance index and effective performance index to verify the correction of model. Meanwhile, it analyzed and compared system and local back pressure of three muffler designs. Project one has the highest system back pressure, project two is the secondary, project three has lowest of that. From which, we discovered that, the more complicated the structure of muffler, the higher back pressure of system, the more sound energy dissipation, the greater silence capacity of muffler. It provided input conditions for further CFD analysis by extracting exhaust gas parameters from relative locations of exhaust systems.Second, this paper modeled three dimensional flow field solid in UG software according to the structure of exhaust system, imported the solid to Gambit software for meshing finite volume discrete grids and set the flow boundaries, got the pressure, temperature, velocity, turbulent kinetic energy and sound power level distribution of exhaust system from CFD analysis by Fluent software, explained the results using the theory of hydrodynamics, heat transfer theory and aeroacoustics. We found that, the major flow energy dissipative areas are inflows and outflows around holes of mufflers, inflows of inner pipes and impinging jets and so on. These locations commonly relate to flows which are contracted and expanded abruptly or flows of turned and bent, all these will produce intense local turbulent areas which are usually with phenomena such as the vortex paired, separated, shed and perturbed, make more airflow noises ultimately. This paper chose project one as the optimum project after synthetizing back pressure, silence capacity and rustiness, etc.Finally, this paper tested back pressure and tail pipe noise of three design projects coupled with engine, verified the correction of simulations and provided references for further analysis and optimal design of mufflers.
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