Experimental Investigation of the Effect of Pre-evacuation on the Starting Performance of a Vacuum Simulator Diffuser with Conical Nozzles of Different Expansion Ratios

Document Type : Article

Authors

1 1Space Transportation Research Institute, Iranian Space Research Center, Tehran, Iran.

2 Department of Aerospace Engineering, Sharif University of Technology, Tehran, Iran

3 Department of Aerospace Engineering, Sharif University of Technology, Tehran, Iran.

Abstract

The starting time of a high-altitude exhaust diffuser is a key factor in evaluating an engine’s unsteady performance. One common approach to reduce this time is to pre‑evacuate the vacuum chamber and part or all of the diffuser. This study examines how pre-evacuation influences diffuser performance using four diffuser inlet to nozzle outlet area ratios 1.27, 1.91, 4.1, and 7.81, tested using compressed air and rapid nozzle pressurization. These ratios correspond to four conical nozzles with expansion ratios of 45, 30, 15, and 7.5. Wall pressures were measured at 13 points along the diffuser and vacuum chamber, both with and without pre-evacuation. The pre-evacuation process used a vacuum pump. Results showed that at area ratios of 1.91 and higher, harmonic pressure oscillations develop in the diffuser and vacuum chamber. Pre-evacuation did not eliminate these oscillations but shortened their onset and the diffuser starting time, especially at an area ratio of 1.27, where the narrow annular gap delays starting. Mass flow rate analysis revealed alternating filling and emptying of the vacuum chamber during oscillations. Fourier analysis indicated that oscillation frequency increases with area ratio, while amplitude decreases.

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References

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