Thermo-Mechanical Coupling Ananysis And Optimization Of Exhaust Manifold

The exhaust manifold is an important component of an automobile engine,and is subjected to high-temperature exhaust gas shock and vibration excitation from the engine,and its fatigue durability is critical to the reliability of the engine.Therefore,this paper studies the local cracking of the exhaust manifold during the engine reliability test of a certain enterprise.According to the boundary conditions of the test,the engine exhaust manifold is simulated and analyzed by the finite element method.The main research contents and results are as follows:Firstly,an analytical model is established according to the standard process of enterprise modeling and corresponding standards.The advantages and disadvantages of the three k-? turbulence models are compared and analyzed.The realizable k-? turbulence model is determined as the flow field model.According to the boundary conditions of the mass flow and pressure and so on obtained by the experiment,the temperature field of the exhaust manifold is calculated by the fluid-solid coupling method and the heat transfer theory.It is found that the temperature at the outlet flange and the corner of the exhaust manifold is high,and the temperature is up to 800 ?.By comparison,it is found that the higher temperature of the exhaust manifold is in good agreement with the actual crack location of the exhaust manifold,which indicates that the fatigue failure may be caused by the excessive thermal stress,and that the established finite element model of the exhaust manifold is feasible.Therefore,the calculated exhaust manifold temperature field can be used as a thermal load boundary condition for subsequent modal characteristic calculation and thermodynamic analysis.Secondly,the dynamic characteristics and thermodynamic performance of the exhaust manifold under temperature influence are analyzed.The temperature field is used as the thermal load boundary condition for solving the modal characteristics of the exhaust manifold and the thermal stress and thermal strain.The free mode and constrained mode analysis and thermodynamic analysis are performed at normal temperature and high temperature.The modal analysis results show that the modal frequency at room temperature is slightly higher than the modal frequency at high temperature,because the stiffness is reduced at high temperature;but the first-order natural frequency of the modal is much higher than the engine exhaust excitation frequency regardless of the condition,so it is considered that the possibility of resonance causing fatigue damage of the exhaust manifold is small.The thermodynamic calculation results show that the first crack thermal stress value is about 342.99 MPa,the plastic deformation increment value is about 1.03%,the second crack thermal stress value is about 344.86 MPa,and the plastic deformation increment value is about 1.04%,the plastic deformation increment of both places exceeds the allowable 1% of the enterprise.Therefore,it is considered that the two cracks of the exhaust manifold are thermal fatigue caused by the excessive plastic deformation due to the excessive local thermal stress.Then the stress and strain of the exhaust manifold considering only mechanical load,thermal load and thermo-mechanical coupling load are analyzed,and the fatigue life is judged.The results of considering only the mechanical load indicate that the stress and the plastic strain of the exhaust manifold are small in this state,and the stress is less than the fatigue limit,so that it is not caused by mechanical fatigue.When only the thermal load is considered,the fatigue life at the crack 1 of the exhaust manifold is 79,103 times,and the fatigue life at the crack 2 is 75,913 times,which is a low-cycle thermal fatigue;considering thermo-mechanical coupling loading,the stress value of the first crack of the exhaust manifold is about 351.71 MPa,the plastic deformation increment is about 1.10%,the number of fatigue life is 58944 times,and the stress value of the second crack is about 348.79.MPa,the plastic deformation increment is about 1.08%,the number of fatigue life is 59,820 times,and the plastic deformation increments of both places exceed the allowable 1% of the enterprise,it is further explained that the fatigue failure of the exhaust manifold is caused by the large plastic deformation increment due to the excessive stress.At the same time,comparing the stress,plastic deformation increment and fatigue life times under three conditions,it is found that the thermal load plays a leading role in the fatigue damage of the exhaust manifold,and the reason of the exhaust manifold cracking is caused by the excessive plastic deformation due to the excessive local thermal stress.Finally,the exhaust manifold has been improved and calculated according to the analysis results.It is found that the stress value and plastic deformation increment value of the improved exhaust manifold are all reduced compared to before the improvement,and the plastic deformation of the improved exhaust manifold is less than 1% allowed by the company,and the number of fatigue life is increased,which meets the requirements of the enterprise,indicating that the improved exhaust manifold scheme is feasible.