Investigation of the unsteadiness during starting transient operation of a zero secondary flow ejector.

Zero secondary flow ejectors/ vacuum ejectors are typically used on the downstream section of a high altitude test (HAT) facility to maintain low back pressure required to test high area ratio nozzle in supersonic and hypersonic regime at ground level conditions. Experiments were carried out to investigate the unsteadiness present in the secondary region of the vacuum ejector during its starting transient operation. Unsteady pressure measurement was acquired on the fore-wall of the secondary region. The primary jet chamber pressure (P0) was increased from ambient to 4bar to study the starting transient operation of the vacuum ejector. The evolution of pressure in the secondary region indicated the presence of perturbations during starting transient as well as started mode. The root mean squared (RMS) amplitude of the fluctuating secondary pressure signal was higher during transient startup compared to started mode. Power spectral density of the secondary pressure showed frequency peaks at 247, 542, 1004, 2520 and 4022 Hz for various primary jet chamber pressures during unstarted mode of operation (P0< 3.8bar). At started mode of operation (P0=3.8bar), only a 247 Hz frequency peak was observed. The unsteadiness and frequencies present in the secondary pressure were studied for different transient startup durations ranging from 6.5 to 1.5s. Continuous wavelet transform of the secondary pressure for different cases of startup duration showed the presence of similar frequencies for all the cases. The RMS amplitude of the secondary pressure signal decreased with decreasing startup duration which confirms that unsteadiness decreases with shorter startup duration.