CFD Modelling of Gas Turbine Combustor Post-Shutdown: Heat Soak-Back Phenomenon

Heat soak-back to engine components on shutdown, due to the thermal inertia of heated turbine parts, has the potential to cause deposits to build up in fuel injectors which can over time block the injectors. Blocked or partially blocked injectors must then be removed from the engine, inspected and sent for cleaning. The potential for injector blockage on helicopter engines in particular is elevated due to the generally cramped engine compartments which trap heat, and also to the helicopter engine's unique operating characteristics that allow for a very short time interval between maximum power settings and shutdown. Field Inspection intervals for helicopter engine injectors thus tend to be the shortest of any engine component, and such inspections can prove to be a significant driver of engine maintenance costs. Thus, an understanding of the physics of the heat soak-back processes involved would provide for possible design improvements, changes in engine operation and more proactive maintenance actions all aiming at reducing operating costs. A 3D CFD simulation is carried out in order to predict the thermal behaviour and magnitude of heat soak-back and its potential consequence on the fuel delivery system. The ANSYS FLUENT™ commercial code is used for the numerical calculations of the shutdown and post shutdown thermal conditions in a basic can combustor. Additional thermal masses with typical turbine weight/surface ratio are added downstream of the combustor in order to compensate for the lack of turbine blades/confinement rings, which acquire most of the heat during gas turbine steady operation. A full 360° model is used to develop a detailed mesh. A steady state combustion simulation is performed initially and followed by a transient simulation, to allow thermal build-up (at steady state) and transient cooling of solid parts (thermal inertia) afterwards. The numerical model is to be validated by experiment.