Transient Heat Transfer Analysis Of An Engine Exhaust Manifold Based On Fluid-Solid Coupling

The vehicle engine exhaust manifold acts as an important component of the exhaust system and is directly connected to the engine cylinders.The high temperature exhaust gas directly contacts the inner wall surface of the exhaust manifold,resulting in a large temperature gradient and thermal stress.The performance of the exhaust manifold can have a large impact on engine performance and can even have a major impact on the normal running of the vehicle.The thermodynamic performance and flow performance of the engine exhaust manifold directly affect the economic,dynamic and emission characteristics of the engine,so the demand for its research is increasing.If it causes problems such as air leakage or crack under the action of thermal load and thermal stress during the working process,it will affect the working state and efficiency of the engine.Therefore,the calculation and analysis of the temperature field and thermal stress thermal strain of the exhaust manifold are Critical.At present,most studies use steady-state methods for calculation.The theoretical knowledge of transient analysis is less and the analysis error is larger.The model studied in this paper is a gasoline engine type 4-1 exhaust manifold.Based on the theory of computational fluid dynamics,a transient fluid-solid coupling heat transfer model is established.Firstly,the engine was modeled in one dimension using GT-POWER software,and the model was verified by error.The error between the simulation result of the torque and the experimental result is 2.67%;the error between the simulation result of the power and the experimental result is 3.5%.Coupling it with STAR-CCM+ software,the intake mass flow rate is used as the boundary condition of CFD calculation,which solves the problem of setting dynamic boundary conditions in transient analysis.The transient simulation is used to investigate the gas flow state inside the exhaust manifold at different moments of engine exhaust,the steady-state pressure loss unevenness of each cylinder and the flow velocity distribution at the outlet end.The research shows that the 4-cylinder recirculation is obvious when 1 cylinder is exhausted,and there is almost no return gas in 2 and 3 cylinders;the pressure loss unevenness of each cylinder is11.5%,19.4%,4.8% and 10.2%,and the airflow velocity at the outlet end face is relatively uniform.The transient fluid-solid coupling heat transfer calculation of the exhaust manifold is carried out,and the distribution trend of the temperature of the inner and outerwall surfaces of the exhaust manifold is explored.In order to verify the rationality of the number of grids,grid-independent verification was performed,and the results showed that the number of grids of 1.12 million met the requirements.The effect of heat dissipation on the thermal temperature field of the exhaust manifold is studied.The results show that the temperature value of the joint is negatively correlated with the heat transfer coefficient of the outer wall surface,and the maximum temperature drop is smaller than the minimum temperature drop.Therefore,the heat dissipation is judged.The impact of the field is significant.The thermal stress and thermal deformation under transient temperature field are calculated and the original model is improved.The thermal fatigue life of the exhaust manifold was estimated and a general qualitative analysis was given.The results show that the thermal stress at the exit end of the exhaust manifold and the flange is the highest,but the cracking failure does not occur due to the concentration of large thermal stress.The thermal stress and thermal deformation of the improved model are reduced compared to the original model,and the fatigue life is increased.The maximum thermal stress is reduced by 21 MPa,the decrease is 8.5%;the maximum thermal strain is reduced by 0.07 mm,accounting for 4%,and the fatigue life is increased by about 1100 times.This study has carried out a comprehensive analysis of the exhaust manifold through a set of processes,and provides an effective method for the design and improvement of the exhaust manifold,which has certain reference significance.