Flow separation studies during the transition modes of a sub-scale dual-bell nozzle

An experimental investigation has been carried out to study the flow characteristics of a dual-bell nozzle during its transition modes. Cold gas tests using gaseous nitrogen were carried out in (i) a horizontal test rig with nozzle exhausting into atmospheric conditions and, (ii) also inside a high altitude simulation chamber with nozzle operation under self-evacuation mode. Transient tests indicate that increasing δP₀/δt reduces the transition time as well as the amplitude of pressure fluctuations of separation shock at wall inflection point. Sea-level tests show 15-17% decrease in transition NPR during subsequent tests in a single run primarily due to frost formation in the nozzle extension up to the wall inflection location. This reduces the wall inflection angle and hence, the transition NPR. However, tests inside the altitude chamber show nearly constant NPR value during subsequent runs primarily due to decrease in back temperature with decrease in back pressure that prevents any frost formation.