Aerothermal Simulations and Testing of an Air-Cooled APU Bay

Abstract An Auxiliary Power Unit (APU) is an additional gas turbine engine located in a bay on an aircraft or a helicopter. The APU may be used to start the main engines, provide pressurized air to the cabin and generate electricity. Some of the equipment of the APU, such as the fuel metering pump or the ignitor, have certified temperature limits that must not be exceeded. In order to control the thermal environment around the APU, where these equipment are located, the bay needs to be cooled down when the APU is in operation. In this study, bay cooling is achieved with an ejector system. Numerical simulations using ANSYS Fluent with a RANS (Reynolds Averaged Navier-Stokes) approach and a k-ω SST turbulence model have been performed to study the thermal environment of the APU in the bay. The volume inside the compartment, including many details such as oil and fuel ducts, was meshed using a complex wrapping process. The air-oil heat exchanger located at the inlet of the bay was modeled as a porous zone. Radiative, conductive and convective heat transfers were taken into account. The cooling mass flow through the bay and the surface temperature of all components were calculated. These calculations have been compared to the results of a test campaign performed in a dedicated facility. During the tests, the APU was installed in a bay representative of the installation on the helicopter. The inlet of the bay was equipped with a calibrated bell mouth and thermal probes were installed on most of the equipment to monitor their temperature. Static and total pressure probes were also installed in the bay to evaluate the total pressure loss of the cooling flow. The comparisons between numerical and test results showed good correlation regarding the surface temperatures.