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Experimental Investigation of Pressure Oscillations Phenomenon in Test Chamber of High-Altitude Simulator

Nojoumi, Amir Ali | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56869 (45)
  4. University: Sharif University of Technology
  5. Department: Aerospace Engineering
  6. Advisor(s): Farahani, Mohammad; Fouladi, Nematollah
  7. Abstract:
  8. Depending on the geometry and nozzle pressure, increasing the ratio of the diffuser inlet area to the nozzle outlet area can lead to instability in the high-altitude simulator vacuum chamber startup process. This instability results in the formation of harmonic pressure oscillations throughout the diffuser and the vacuum chamber. In this study, a scaled down second throat diffuser with compressed air has been utilized. The performance of the diffuser has been investigated using four conical nozzles with different expansion ratios to simulate instability at area ratios of 1.27, 1.91, 4.1, and 7.81 with rapid nozzle pressure rise. Experiments were conducted by measuring wall pressures along the diffuser and vacuum chamber, both with and without pre-evacuation. The results show that instability at an area ratio greater than 1.27 initiates and expands with an increase in area ratio and nozzle pressure. It was also observed that pre-evacuation inside the diffuser and vacuum chamber has no effect on the formation of oscillations but reducing startup time and the beginning time of oscillations. The frequency characteristics of the oscillations along the diffuser have been extracted using frequency analysis. By calculating the cross-spectrum, time delay, and coherence between oscillations at different frequencies, the direction of perturbations propagation during oscillations has been determined. Pressure oscillations in the diffuser consist of two modes: non-harmonic pressure oscillations with low amplitude and high frequency in regions with flow separation, and pulsating mode with high amplitude and low frequency throughout the diffuser and vacuum chamber, during which the diffuser oscillates between start and unstart condition. Diffuser instability begins with non-harmonic mode, causing shock waves to move upstream to the second throat inlet, until suddenly a shock wave is propagating to the diffuser inlet, resulting pulsating mode. Some flow characteristics during the formation of these two modes, such as oscillation amplitude and location of separation, have also been investigated
  9. Keywords:
  10. High Altitude Test Stand ; Vacuum Chamber ; Supersonic Exhaust Diffuser ; Second Throat Exhaust Diffuser ; Pressure Oscillation

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