Experimental Investigation of the Pre-evacuation Effect on Starting Performance of a Vacuum Simulator Diffuser with different Expansion Ratio Conical Nozzle

Articles in Press

Document Type : Article

Authors

1 Iranian space research center

2 Department of Aerospace Engineering, Sharif University of Technology

10.24200/j40.2024.64065.1703

Abstract

Starting time of a high-altitude exhaust diffuser is one of the most important parameters in engine unsteady performance evaluations. Pre-evacuating vacuum chamber and a part or all of the inside of the diffuser is one of the frequent methods of reducing the starting time. In this study, the effect of pre-evacuation on the unsteady performance of a diffuser has been investigated using four different diffuser inlet to nozzle outlet area ratios of 1.27, 1.91, 4.1, and 7.81 with compressed air and rapid nozzle pressure rises. These area ratios have been constructed considering four conical nozzles with different expansion ratios of 45, 30, 15, and 7.5. Experiments have been conducted by measuring wall pressures along the diffuser and vacuum chamber with 13 pressure transducers, both with and without pre-evacuation. Pre-evacuation tests have been conducted using a vacuum pump and placing a removable obstruct at the end of the subsonic diffuser before the pressure loading stage. The investigation of the diffuser's starting performance under rapid pressure loading showed that at area ratio of 1.91 and above, harmonic pressure oscillations appear along the diffuser and the vacuum chamber. It was also observed that pre-evacuation of whole space of the diffuser, vacuum chamber, and nozzle does not eliminate the pressure oscillations, but reduces the beginning time of oscillations and diffuser starting time specially for ratio of 1.27. Because in the ratio of 1.27, due to small annular gap, the diffuser starting time is relatively long without pre-evacuation process. Calculating the mass flow rate of the vacuum chamber also showed that during pressure oscillations, it alternately fills and empties. According to the frequency analysis of oscillations using fast Fourier transform, the pressure oscillation frequency remains constant along the diffuser and elevates with an increase in the area ratio, while the amplitude of the oscillations decreases.

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Sharif Journal of Mechanical Engineering

Articles in Press, Accepted Manuscript
Available Online from 20 December 2024

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