Design and analysis of a second-throat exhaust diffuser for altitude simulation

In this paper, as an altitude simulator, a second-throat exhaust diffuser with no induced secondary flow has been studied. To design a startable diffuser, we first summarize a simple theoretical method based on the normal shock theory. Using the diffusers designed from the present design method, we obtain the diffuser characteristic curves from the small-scale cold-gas tests using cold nitrogen gas as a working fluid. All the experimental test cases are numerically reproduced by using a Reynolds-averaged Navier-Stokes solver, and the numerical method is properly validated with the measured pressure distributions along the diffuser wall and the pressure in the vacuum chamber. We investigate the effects of the essential geometric factors of a second-throat exhaust diffuser, such as subsonic diffuser, second throat area ratio, nozzle expansion ratio, and nozzle contour, on the starting and evacuation performance. Finally, to get a full picture of a second-throat exhaust diffuser operation, evolving diffuser flows during the starting transient and plume blowback at diffuser breakdown are also studied.