Infrared Radiation From Turbojet Exhaust Plume

Abstract

In recent years, there was an increasing interest in aircraft IR signature as an effective passive means of detecting and locking on to an aircraft. There are two main sources of the aircraft IR emission: engine-fuselage layout and gaseous exhaust plume. The size of aircraft plume is several times more than the size of aircraft. The present work focuses on the exhaust plume. The relatively high temperature plume ejected from the nozzle is a mixture of several species that are products of hydrocarbon fuel combustion under excess air condition: CO2, H2O, CO, O2, and N2. Experimental investigation of IR radiation from exhaust gases of a small turbojet engine was conducted in a micro jet engine SR-30 by Turbine Technologies, Ltd. The compact engine features a centrifugal flow compressor, reverse flow annular combustor and an axial flow turbine stage. The SR-30 follows the fundamental Brayton gas turbine cycle. The engine is operated at different regimes with various flow rates of air and kerosene. Measurements were obtained with engine operating at global equivalence ratio of about 0.23. The IR radiation images of the exhaust flow were obtained by a thermocamera equipped with a narrow bandpass filter that falls on the CO2 fundamental band emission. Temperature profiles were measured by a thermocouple in the exhaust flow. An in-house computer program was developed to calculate the IR emission from optically thick gas object and it takes into account the self-absorption of the IR radiation along a line-of-sight. An essential feature of the calculation is the fact that the optical path is non-isothermal. The typical IR spectrum of a turbo-jet engine exhaust flow was simulated using the developed computer program. The calculations allow prediction of “apparent” plume temperatures measured by the IR camera. The calculated results were compared with the experimental measurements and a good agreement was found.