Thermal Fatigue Life of Exhaust Manifolds Predicted by Simulation

A combined computational fluid dynamics (CFD) and finite element (FE) analysis approach has been developed to simulate in the early stages of design the temperature distribution and estimate the thermal fatigue life of an engine exhaust manifold. To simulate the temperature distribution under actual operating conditions, we considered the external and internal flow fields. Digital mock-ups of the vehicle and engine were used to define the geometry of the engine compartment. External-air-flow simulation using in-house CFD code was used to predict the flow fields in the engine compartment and the heat transfer coefficients between the air and the exhaust manifold wall at various vehicle speeds. Unsteady-gas-flow calculation using the STAR-CD thermal- fluids analysis code was to predict the heat transfer coefficients between the exhaust gas and the manifold wall under various operating conditions. To shorten the calculation time, a new method that reduces the calculation time was developed and used in this simulation. The predicted heat transfer coefficients were used in thermal FE analysis to predict the detailed temperature distribution. Nonlinear thermal stress-strain FE analysis was used to predict the crack initiation points and to estimate t he thermal fatigue life under thermal condition. The number of cycles to crack generation was then predicted based on the relationship between the calculated thermal plastic strain range and the thermal fatigue strength of the manifold materials. The predicted temperature distribution and thermal fatigue life of the manifold agreed well with the experimental data. The turnaround time of this analytical approach was short enough for this approach to be used in the design of exhaust manifolds. Its use would improve the design quality of exhaust manifolds and reduce development cost and time.