Gas-Density Effects on Dual-Bell Transition Behavior

An experimental investigation was conducted to study the dual-bell transition behavior inside a high-altitude test\udfacility under different back pressure environments. The high-altitude chamber was evacuated using an ejector\udnozzle and the dual-bell nozzle driving pressure, P\ud0N, was varied from 30 to 5.3 bar in subsequent test campaigns. As P0N was decreased, the dual-bell transition was observed to get delayed, and the transition nozzle pressure ratio increased by as much as 20% for the lowest driving pressure tested. Similar results were also observed for the dual bell retransition nozzle pressure ratio. For very low values of P0N (of 3.5 bar), transition did not occur at all. The delay in the dual-bell transition process to higher nozzle pressure ratio is primarily attributed to the increase in the width of inflection region with decrease in Reynolds number, which seems to control the dual-bell transition behavior. During these tests, it was also observed that the visible intensity of condensation shock decreased gradually with decreasing P0N and finally, for P0N 17:4 bar, no condensation shock was observed. The decrease in gas density/reduced mass flow is also accompanied by a significant decrease in the amplitude of wall pressure fluctuations in the region of separation due to decrease in shock strength