Exhaust gases thermal impact simulation on helicopter rear structure using CFD

Abstract

Exhaust gases ejected by helicopters engines can be assimilated to jets in crossflow as they interact with a complex flow field composed of rotor downwash, relative wind and ground effect for some flight cases. This jet in crossflow configuration is characterized by a strong temperature gradient between the jet and the crossflow as exhaust gases are ejected at temperatures close to 600°C to 700°C. Their interaction with the external flow field can lead to thermal issues such as the overheating of structural parts or external equipment items and the re-ingestion of hot gases through engine and avionics bays air intakes. Hence, being able to predict exhaust gases trajectory and their turbulent mixing with the surrounding flow is critical for design purpose. The present work investigates the capability of aerothermal CFD simulations to correctly reproduce trajectory and mixing of such flows in order to support design, through two representative configurations. Different Steady RANS and SAS/LES approaches are first compared on a dedicated hot jet in crossflow database collected at ONERA. It is shown that even advanced RANS modelling cannot reproduce the jet mixing with a high level accuracy, contrary to unsteady methods. The suggested unsteady methodology is then applied on a real helicopter wind tunnel experiment and shows a very satisfactory reproduction of the near wall temperature field in the helicopter region of interest.