A high altitude test facility of liquid rocket engine makes use of ejector diffuser systems to evaluate the performance of the high expansion nozzle of the liquid engine. The low pressure environment of the operating rocket motor is simulated at ground level with the help of ejector diffuser and also to recover the pressure of the exhaust gases before letting out to the atmosphere at ground level. The basic function of the ejector diffuser is pressure recovery and sustaining the vacuum condition inside the test chamber. When the diffuser attains "started" condition, the momentum of the rocket motor exhaust creates sufficient suction to maintain low vacuum level in the test chamber and also the shock cells occurring in the diffuser duct seal the vacuum environment against backflow. The starting condition of the diffuser itself depends on the operational and geometric parameters of the diffuser and motor. In this paper a straight diffuser with a conical nozzle-area ratio 21.77- is numerically simulated by solving compressible Navier-Stokes equations for different back pressures. Mach contours are obtained for various operating conditions. The flow is supersonic at the exit of the nozzle and the exit plane pressure is below atmospheric pressure. In order to recover the pressure and to discharge the gases into the atmosphere, the flow is decelerated in the constant area duct and the gases are let out to the atmosphere with subsonic velocity. The portion, where the supersonic flow turns into sonic velocity, is called the second throat. The satisfactory performance of the facility needs proper pressure recovery under stable flow inside the diffuser. This analysis will be of help to design the diffuser for the high altitude facility.
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