Coupled Analysis of Thermal Flow and Thermal Stress of an Engine Exhaust Manifold

Using the temperature distribution of solid structures obtained by a CFD (computational fluid dynamics) analysis, thermal-stress analyses can be conducted. In the paper, an FVM (finite volume method) based CFD software is used to solve for the temperature and flow field. Making complex mesh model, it uses a hybrid mesh consists of tetrahedron, hexahedron, prism and pyramid elements. And all variables are defined on nodes of elements. Because the hybrid mesh and variables on nodes are compatible between structural analysis FEM (finite element method) elements and node based CFD FVM elements, not only the temperature distribution of the solid but also the mesh can be directly applied to the thermal-stress analysis. The following three approaches are investigated for the best possible solution of FVM/FEM coupled problems. 1) The mesh and the temperature distribution of CFD are applied in a thermal-stress analysis directly, taking full advantage of the mesh compatibility. This is the simplest and most direct approach. 2) Flow and temperature field is solved using CFD software and resulting film coefficient between solid and fluid and the ambient temperature is interpolated to the corresponding mesh of the solid surfaces generated for thermal-stress analysis. After that, the temperature distribution within the solid is calculated and the thermal-stress analysis is carried out. With this method, a fluid region and a solid region are calculated separately. Therefore, this method is suitable for the analysis of large-scale models. 3) Flow and temperature field is solved using CFD software and resulted temperature field is interpolated to the nodes of the FEM based mesh for thermal-stress analysis. Here the resulting temperature of hybrid mesh nodes in the solid part can be mapped into the node load temperature of a new shape element for one's structure analysis.

Above three methods are compared in this paper. The third approach is found to be the easiest and the most accurate. Finally, a complex engine exhaust manifold is analyzed using the third approach to demonstrate the accuracy, flexibility and efficiency of the coupled analysis of FVM fluid and FEM thermal stress.