Thermal Calibration of Exhaust Manifold FEA Model to Enhance TMF Life Prediction

Exhaust manifold Thermo-mechanical Fatigue (TMF) assessment using Finite Element Analysis (FEA) is complex and the results are sensitive to predicted metal temperatures on manifold. Hence it is recommended to calibrate thermal FEA model with actual test data for accurate prediction of TMF life. This helps to design reliable products. Typically, TMF life of exhaust manifold is validated using transient thermal cycle. Due to few assumptions made in simulation approach, it is observed that the difference between experimental and FEA temperature could be significant. In current approach, exhaust manifold temperature at different manifold locations are calibrated with the measured thermocouple data by varying heat transfer coefficient (HTC) at outer and inner surface of manifold. This approach is capable to provide good correlation at peak temperature and minimum temperature during the thermal cycle. However, there is no degree of freedom to calibrate transient thermal response in between maximum and minimum temperature. Due to this, a difference of ~40-50 degree C is observed in the transient part of the thermal cycle. As, transient thermal behavior plays a very important role in the TMF life prediction of exhaust manifold this resulted in multiple false hotspots. As part of new approach, in addition to HTC few other parameters were investigated such as bulk exhaust gas temperatures and ramp time of thermal boundary conditions. A combination of scaling bulk temperatures along with HTC and varying ramp time of thermal boundary conditions gave good correlation of test vs FEA temperature within +/- 25 degree C over the entire thermal cycle including maximum and minimum temperatures. Using this temperature field for further fatigue analysis gave TMF results that are better correlated with experimental observations. Keywords: Thermo-mechanical Fatigue (TMF), Finite Element Analysis (FEA), Heat Transfer Coefficient (HTC), Exhaust Manifold, Thermal Calibration