Performance evaluation of a second throat exhaust diffuser with a thrust optimized parabolic nozzle

In the present study, the experimental analysis of a second-throat exhaust diffuser (STED) performance has been conducted for the high altitude test of a parabolic bell-type nozzle. The diffuser starting performance has been explored considering two different approaches, namely the gradual and instantaneous increase of the nozzle chamber pressure. Also, the influence of pre-evacuating the internal regions of the system on the diffuser starting performance has been studied. Numerical simulations have been carried out to have more physical insight into some test results. It is demonstrated that the non-dimensional hysteresis range in STED starting performance with a bell-type nozzle is up to 4 times larger than that with a conical nozzle. The pre-evacuation of the internal area does not affect the diffuser minimum starting pressure. However, starting the diffuser with pre-evacuation takes 50 to 70 percent less time than without pre-evacuation. Also, it is illustrated that in spite of large differences in minimum starting pressure of STED with conical and bell-type nozzles, the STED breakdown pressure (minimum operating pressure) is independent of nozzle profile. Moreover, it is shown with experimental evidences and numerical analyses that the creation of restricted shock separation (RSS) inside the bell nozzle leads to some unexpected behaviors in STED starting and breakdown behaviors. Finally, it is demonstrated that the instantaneous increase in the nozzle chamber pressure with steeper slope and pre-evacuation of the internal regions can eliminate separation pattern transition from common free shock separation to unwanted RSS. (C) 2019 Elsevier Masson SAS. All rights reserved.