Investigation of Second Throat Supersonic Exhaust Diffusers for the Altitude Testing of Rocket Motors

Experimental and numerical investigations of a second throat supersonic exhaust diffuser (STED) system used for the simulated altitude testing of upper stage rocket motors are presented in this paper. The performance of STED in a high-altitude test (HAT) facility is governed by its geometrical parameters. An extensive and systematic experimental and numerical investigation has been carried out using various STED configurations without any ejector, to evaluate the influence of these geometrical parameters on STED starting characteristics and pressure recovery characteristics for a motor nozzle of 70 and 100 area ratios. The geometrical parameters considered in the present study are: annular gap between nozzle and diffuser (yg), second throat contraction ratio (Ad/Ast) and axial distance from nozzle exit to the start of diffuser contraction (Xc). The experimental and numerical results show that the diffuser starting pressure increases with increase in annular gap, even if diffuser area ratio is maintained constant. Based on the minimum starting pressure required, the near optimum diffuser contraction ratio (Ad/Ast) is 2.9 and 2.5 for the STED configurations with diffuser area ratios of 100 and 134.5, respectively. The diffuser starting pressure increases with increase in the axial distance of nozzle from the diffuser contraction (Xc). Also, it is observed from the results that the location of the terminal normal shock inside the second throat is highly influenced by Xc.