Gas Density Effects on Dual-Bell Transition Behavior

An experimental test campaign was conducted to study the dual-bell transition behavior inside a high-altitude test facility under different back pressure environments. The high-altitude chamber was evacuated to low ambient pressure and the test nozzle driving pressure, P0, was reduced from 30bar to 5.3bar in separate test campaigns. As the nozzle driving pressure was reduced gradually, the dual-bell transition was observed to get delayed and the transition NPR increased by as much as 20% for the lowest driving pressure tested. The visible intensity of condensation shock was also observed to decrease gradually and for P0 / 22bar, no condensation shock was visible. The combined effect of drop in gas density and reduced mass flow seems to delay the movement of separation point in the vicinity of wall inflection, resulting in an increase in the transition NPR. This emphasizes that the results of dual-bell tests inside a high-altitude test facility should be carefully interpreted especially those caused due to reduction in test gas density