Experimental investigation of supersonic diffuser starting performance in conical nozzle test with different area ratio

One of the most important parameters in the operation of the diffuser of the vacuum simulator is the ratio of the cross-sectional area of the diffuser inlet to the outlet cross-section of the nozzle. Increasing this parameter increases the expansion length of the supersonic flow and causes instability in the process of starting up the diffuser. In the present study, the effect of the different diffuser inlet to nozzle exit area ratio (Ad/Ae) on the starting performance of a second throat exhaust diffuser has been investigated experimentally. Vacuum simulation tests have been carried out on several conical nozzles with different exit area by a high-pressure air apparatus. Investigations show that at high Ad/Ae before starting the diffuser, oscillations in the pressure of the test chamber are created due to the instability of the flow at the diffuser inlet. By frequency analysis of the vacuum chamber pressure, it is observed that as Ad/Ae increases, the number of oscillatory modes of the vacuum chamber pressure increases and the dominant frequency of the oscillations before starting the diffuser becomes larger. It was also observed that, generally, with the increase of Ad/Ae the amount of motor pressure corresponding to the start of oscillations decreases. In addition, according to the investigations, with the increase of Ad/Ae from 1.27 to 7.81, the minimum starting pressure of second throat exhaust diffuser increases by 20.33 %.