Thermomechanical Fatigue Analysis of Water-Cooled Exhaust Manifold Using CFD and FEA Based Modelling Approach for Better Correlation with Experimental Data

The water-cooled exhaust manifold is one of the critical components of an engine exhaust system used in marine applications. Thermo-mechanical fatigue (TMF) analysis of water-cooled exhaust manifold using Finite Element Analysis (FEA) is more complicated as the water jacket interconnects with multiple parts, due to interaction of various thermal boundary conditions and the results are more sensitive to the metal temperatures predicted on exhaust manifold. Difference between experimental and FEA temperature could be significant due to assumptions like steady state averaged exhaust gas flow CFD from all cylinders instead of transient flow CFD in the simulation approach and variation in radiation from the surrounding hot components to exhaust manifold in different areas. Furthermore, this being a liquid cooled system, brings in the possibility of boiling and correct modelling of it is vital to the simulation outcome. This leads to inaccuracy in TMF analysis results. Therefore, thermal distribution on water cooled exhaust manifold plays a very important role in the TMF life prediction of manifold. In this study, there is a significant difference between measured and FEA temperatures in uncalibrated thermal model which eventually leads to incorrect prediction of TMF hotspots compared to crack locations. Therefore, thermal calibration is performed using precise CFD results followed by TMF analysis that well predicted the actual crack locations observed in the endurance test. In summary, water-cooled manifolds has lower temperatures than air-cooled manifolds, however TMF issues still can exist due to temperature variations at local regions which leads to differential expansion of manifold. To get uniform temperature distribution, Conjugate CFD analysis with Gas, Coolant and Solids are performed considering boiling effect. In addition to this, FEA thermal calibration by tweaking CFD results helped to get temperature profile closer to test data. This helped to calibrate TMF analysis and made decision on manifold design. Keywords: Thermo-mechanical Fatigue (TMF), Finite Element Analysis (FEA), Water Cooled Exhaust Manifold, Thermal Calibration.