Infrared Signature of Fixed and Variable Area C-D Nozzle of Aircraft Engine

The use of converging-diverging (C-D) variable area nozzle (VAN) in military aeroengines is now common, as it can give optimal expansion and control over engine back pressure, for a wide range of engine operations. At higher main combustion temperatures (desired for supercruise), an increase in the nozzle expansion ratio is needed for optimum performance. But changes in the nozzle throat and exit areas affect the visibility of engine hot parts as the diverging section of the nozzle is visible for a full range of view angle from the rear aspect. The solid angle subtended by engine hot parts varies with change in visibility, which affects the aircraft infrared (IR) signature from the rear aspect. This study compares the performances of fixed and variable area nozzles (FAN and VAN) in terms of engine thrust and IR signature of the engine exhaust system in the boresight for the same increase in combustion temperature. This study is performed for two cases: (i) variable throat area and fixed mass flow rate, (ii) fixed throat area with increased mass flow rate. Engine off-design performance is estimated by GasTurb software for the FAN and VAN, and the IR signature levels from the boresight are studied in the 1.9–2.9 μm and 3–5 μm bands for the same increase in combustion temperature.