Transient hot shut down, CFD simulation technique for Underhood thermal management 2020-28-0032
During design and development stages of vehicles, thermal protection is of prime importance. Numerical simulations play key role in identifying critical thermal issues for different systems. Hot shut down is one such case where thermal soak phenomenon plays vital role from thermal robustness point of view and there is a need to address this phenomenon using Computational Fluid Dynamics (CFD), which in turn will reduce the development time / testing efforts considerably. This condition is of utmost importance especially when vehicle is moving at higher gradients (uphill sections). In these critical conditions, hot engine compartment starves for cooling airflow despite the fact that fan is operating at maximum speed. The sudden stoppage of vehicle after this high thermal load is known as hot shut down. Maximum temperatures on critical components in engine compartment during continuous running condition are lower as compared to temperatures during hot shut down. This rise in temperatures are critical in designing and optimizing critical components in the proximity of engine and exhaust system for peak thermal loads.
This paper describes methodology, which developed to replicate real time hot shut down test condition through transient CFD simulation technique in ANSYS fluent. For this study, transient simulations are carried out for worst test conditions (grade / road load). Results of low vehicle speed condition is simulated and it is taken further for hot shut down for last 15 minutes.
All the transient simulations require a converged steady state CFD simulation as a starting point and therefore steady state thermal simulation is carried out by using constant temperature input on all the major heat sources. This steady state simulation replicates the vehicle driving condition with peak thermal loads (like fan running at full speeds, with radiator working at its higher capacity). To capture real world scenario, hot soak condition is simulated in stepwise manner. In a typical hot soak condition, initially flow is driven by buoyancy and gravity due to absence of convection heat transfer. This is because radiator fan stops working. In this step, all the critical parts are monitored until they attain constant temperatures. The temperature variation in heat sources captured by modeling their thermal inertia. In the initial period, temperatures on critical components shoot up since radiation is dominant and subsequently reduced with time.
Behavior of component temperatures in each stage is observed and qualitative correlation study with actual test condition and temperature trends are matching. This method helps in optimizing the cooling requirements, fan logic, thermal protection / insulation strategies, which in turn results in increase in life of critical parts from thermal failures.
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