Exhaust Hot end system Transient Heat Transfer Analysis using Heat Transfer Coefficient (HTC) approach, Thermo-Mechanical Fatigue Analysis to evaluate thermal durability and physical validation through Thermo-Shock Testing

Exhaust system suppliers are continuously enhancing their processes by adopting advanced simulation techniques and expedited testing procedures for optimizing exhaust systems. This aims to reduce time to market, accelerate vehicle launches, and ensure first-time right and consistent compliance with performance and durability standards. Traditionally, the CFD team conducts Conjugate Heat Transfer (CHT) analysis on the exhaust hot end system using input conditions such as engine-out gas temperature and maximum mass flow rate. The CAE team then performs steady-state heat transfer analysis by sharing the structural CAE model with CFD for CHT skin temperature mapping. Once steady state heat transfer analysis complete, skin temperature plots are to be compared with CFD to match CHT temperature results. These results are used for steady-state thermo-mechanical analysis across all heat-up cycles and assume system under ambient temperature for all cool-down cycles to evaluate system thermal displacement and plastic strain against acceptance criteria. Given the maturity of CAE software tools and testing methodologies, OEMs now expect system suppliers to perform thermo-mechanical analysis using a transient approach. This approach replicates customer-specific thermal shock testing (TST) profiles. Customers provide TST profiles, including exhaust gas temperature and mass flow rate under rated speed, motoring, and low idle conditions over specified time periods. In this method, CFD generates skin temperature and convective heat transfer coefficient (HTC) plots for three thermal cycle conditions. The CAE team conducts transient heat transfer analysis under these conditions, considering predicted HTC from CFD for the exhaust hot end's internal flow path surface and assume outside surface exposed to ambient temperature. Once the transient heat transfer analysis is complete, the skin temperature contours are checked to ensure they match steady-state conditions. Thermo-mechanical analysis for three thermal cycles is then conducted using the transient heat transfer results. Equivalent plastic strain results for each component are verified against acceptance criteria of PEEQ <2% and plastic strain difference, ∆PEEQ <0.5% at the third thermal cycle. Additionally, thermo-mechanical fatigue analysis is performed to ensure the fatigue life meets the acceptance criteria of over 3000 cycles. Physical prototype samples are also constructed and subjected to thermal shock testing according to customer-provided TST profiles. The test is conducted for the entire duration, and the physical parts are visually inspected for structural cracks and deformations. If all parts meet the plastic strain targets and thermo-mechanical fatigue life targets in CAE simulations, and no physical damage is observed from thermal shock testing, the developed hot end system is considered to meet thermal durability requirements. A light vehicle exhaust hot end system was used as a case study to demonstrate this approach and deliver the results.