Parametric investigation on the essential flow factors commanding steady operations of the second throat exhaust diffuser

The constant area exhaust diffuser (CAED) demands a higher starting pressure than the second throat exhaust diffuser (STED). In the design process, one dimensional (1D) normal shock theory was explored to obtain the optimum starting pressure of STED. Experiments were conducted with a small scale cold gas simulator using nitrogen gas as injectant. The application of 1D normal shock theory for STED design was examined with 20-25% difference in each accuracy in comparison to numerical and experimental evidences. The relation between evacuation quality and essential geometrical parameters such as diffuser-to-nozzle throat area ratio (A_d/A_t), diffuser-to-the second throat area ratio (A_d/A_s), and the nozzle expansion ratio (ε) is presented for the optimization of STED. The optimum starting pressure increases in proportion to A_d/A_t and the optimized Ad/As was predicted in the range of 2.2-2.5. After STED starting, the evacuation quality is the same whether the expansion ratio of the nozzle is large or not. However, the range of the transition regime varied according to the nozzle expansion ratio.