Numerical Analysis of Advanced Fighter Auxiliary Power Unit Exhaust Impingement

A three-dimensional, viscous, Computational Fluid Dynamics (CFD) analysis of the exhaust flow associated with the auxiliary power unit (APU) of an advanced fighter aircraft was conducted. Calculations were performed to investigate the level of surface heating on the fuselage adjacent to the APU exhaust port. The CFD model was constructed using the Chimera domain decomposition technique. A wingless (unclassified) fighter configuration consisting of five grids was used to provide the background flowfield for the APU investigation. Comparisons with wind tunnel test data were conducted to validate the fighter model before the APU geometry was added to the calculation. Seventeen additional Chimera grids were necessary to adequately model the APU. Two different APU configurations were analyzed to assess the effect of the position of a small surge line (used to discharge a supersonic flow bled from the APU inlet) on the APU exhaust flow. Results predicted that by relocating the APU surge line from its original position directly downstream of the APU exhaust port to a position directly inboard of the APU exhaust port, a significant reduction in the overall heat transfer to the fuselage could be achieved. Introduction The function of a jet aircraft's APU is to provide emergency power in the event of a main engine failure. In the present case, the APU consists of a small turbine engine located inside the upper surface of the fuselage (Figures 1 and 2). During a main engine flameout, two small doors (which are normally flush with the fuselage surface) open to initiate the APU's operation. Attached to the first door is a scoop (Figure 3) which diverts a flow of air down through the fuselage and into the APU plenum. After compression and combustion, the high temperature effluents are exhausted upward into the external flowfield through the APU exhaust port. Power generated by the APU is then used to restart the main engines. The present analysis investigates APU exhaust impingement onto a region of composite structure inboard of the APU exhaust port. The material in this * Aerospace Engineer, CFD Research Branch, Aeromechanics Division. This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. region was not designed to withstand extreme temperatures and is susceptible to heating related damage during periods of prolonged APU operation (e.g., during a return flight after a main engine flameout and successful restart). It was proposed that the inboard heat transfer to the fuselage could be minimized by relocating a small high pressure surge line from its original position directly downstream of the APU exhaust port to a position directly inboard of the APU exhaust port (Figures 3 and 4). In this way, the flow discharged by the surge line could be used to redirect the hot effluents downstream and away from the vulnerable area. The current numerical study was designed to investigate the impact of the surge line relocation on the APU exhaust flow. Numerical Procedure The three-dimensional, compressible, Reynolds averaged Navier-Stokes equations were solved to predict the flow about the two APU configurations. The computer code, FDL3DI, which was developed in the CFD Research Branch of the Air Force Research Laboratory, was used in all calculations. The numerical algorithm employed by the code, which incorporates a two equation (K— e) turbulence model, is outlined below. Using the freestream density (p^), velocity molecular viscosity (u,,,) and wing root chord (c) to nondimensionalize the variables, the governing equations may be written in conservative form as Here t is time and (£,T|,Q are the computational coordinates. The state vector (Q) and inviscid fluxes (F,G,H) are given by p pu pv pw PE PK pej PU