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During transient phase of motor burning in an altitude test simulator, the low momentum exhaust combustion gases could not establish supersonic flow in the exhaust diffuser. This leads to a reverse flow of the exhaust gases into the vacuum chamber through the annular gap between nozzle and diffuser walls. This spoils the vacuum in the chamber and it is potentially dangerous for measurement instruments inside the vacuum chamber. In this research; at the first, the physics of the flow at initial transient phase of motor burning is investigated numerically and the backflow phenomenon is illustrated. Then, the influences of backflow arrester (BFA) in variations of vacuum chamber pressure and temperature are studied at the transient starting phase. It has been found that applying BFA to decrease the backflow to the vacuum chamber has no significant effect on starting time of the diffuser but it has strong effect on peak pressure and temperature reduction of the vacuum chamber. It is although found that attaining to the steady condition in vacuum chamber is delayed with this instrument. However, using one-way backflow arrester of reasonable size, both the starting time of diffuser and the peak temperature of the vacuum chamber experience a strong reduction. Although, the vacuum chamber steady condition is accelerated with this alternative. Therefore, this device is more suitable than conventional BFA for transient starting phase of the altitude test simulators.

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In this research, the performance study of an altitude test simulator at transient phase of motor burnout is conducted by numerical approach. Using a time dependent pressure profile of a motor in burnout phase, the unsteady exhaust flow is simulated in the main sections of the altitude test simulator, i.e. high expansion ratio nozzle, second throat exhaust diffuser, and vacuum chamber. Present investigation shows that in spite of the high pressure starting condition of the altitude simulator, the supersonic flow in the diffuser tends to breakdown at relatively low combustion pressure in the motor terminating phase. At the breakdown condition, the nozzle exhaust hot gases directed into the vacuum chamber through the annular gap between nozzle and diffuser walls. Present simulation shows that the overall temperature of fluid in vacuum chamber is reached up to 2000 K after the motor burnout. Really, It is potentially dangerous for measurement instruments inside the vacuum chamber. Furthermore; in this research, the influences of backflow arrester (BFA) in variations of vacuum chamber temperature are studied at this transient terminating phase. It has been shown that, the BFA can decrease the exhaust streaming into the chamber in initial breakdown periods. Results show that utilizing a suitable BFA size, the safe temperature condition could be established in vacuum chamber during and after the transient terminating phase of motor.