This paper presents infra-red signature data for a small- scale, low pressure ratio turbojet engine typical of that used in unmanned air vehicle applications. The aim of the study was to test a number of different conver- gent nozzle designs concentrating on those with trail- ing edge modifications. The engine used in the tests has a single stage centrifugal compressor and radial in- flow turbine and is designed to produce approximately 150 N of thrust at 103500 rpm using liquid propane fuel. The test rig consisted of a calibrated thrust stand whilst the engine was controlled through an electronic engine control unit and laptop PC. The jet plume was visualised using an infra-red spectroradiometer which yielded qualitative data across the infra-red spectrum. Simultaneous measurements were also made of the en- gine thrust. A Pitot probe was used to take pressure readings across different sections of the exhaust flow. Analysis of the infra-red signature of the engine exhaust plume and any thrust penalty yielded a performance comparison for each of the nozzles tested. Correlation of engine thrust with engine rpm showed that, within the accuracy of the measurements, there was no signifi- cant thrust penalty associated with the notched nozzles. Infra-red imagery of the plain and 60~ notched nozzles indicated that the latter reduced the length of the hottest part of the exhaust plume by approximately 33%. The spectroradiometer data shows a significant reduction in spectral radiance for the CO2 wavelength of approxi- mately 4.3 #m when the notched nozzles are used. The 60~ notched nozzle appeared to perform best in reduc- ing the spectral radiance at this wavelength. Centreline total pressure measurements in the exhaust plume cor- related well with the infra-red imagery in that a potential core length reduction of up to 30% could be achieved using the 60~ notched nozzle. Total pressure contours recorded 20 mm (0.43D) downstream of the nozzle exit plane suggest that the notched nozzles are promoting increased mixing through radial spreading of the jet possibly associated with increased streamwise vortic- ity (although the latter could not be confirmed). There were also signs that the jet plumes being investigated were swirling.
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