Improvement of ejector-diffuser performance by reduction of backflow.

Abstract

T use of ejector-diffusers to produce low ambient pressures is an important technique in simulated altitude testing of jet aircraft engines. As engine airflows mushroom and flight Mach numbers and altitudes increase, this technique assumes ever greater significance. Ejector-diff users used in jet-engine testing are usually of the basic straight-duct geometry, although other configurations (e.g., second throat, guided inlet) are known to be capable of better performance. The reasons for this include the higher expense of high-performance designs and the necessity of testing a given engine over a wide range of conditions (airflows, nozzle pressure ratios, and nozzle area ratios). It is more practical, if sufficient exhauster capacity exists, to size a straight diffuser to operate unstarted over the entire range of test conditions for a given engine. Simulated altitude conditions are produced and controlled by exhauster machinery, with only a relatively modest boost from the ejector-diff user. If the improvement of ejector-diff user performance is approached by focusing attention upon reduced exhaust backflow, it appears that sizeable improvements are possible in aircraft engine testing. This Note presents the results of model testing of one very simple backflow-reduction technique, consisting of an orifice plate installed at the inlet of a straight diffuser duct. This technique was observed to improve performance by a factor of about 2 under zero-pumping conditions. It could be used, for example, as a simple, lowcost way of changing existing facilities to accommodate NPR ENVELOPE OF OBTAINABLE__ PERFORMANCE USING INLET ORIFICES —— ~~~~