Fluid dynamics in starting and terminating transients of zero-secondary flow ejector

Starting transient and plume blowback at diffuser breakdown of a straight cylindrical supersonic exhaust diffuser with no externally supplied secondary flow are numerically investigated. Fluctuating pressures occurring in the transitional periods are measured by a small-scale cold-gas simulator for high altitudes. The flow-fields evolving in the diffuser-type ejector are solved by a preconditioned Favre-averaged Navier–Stokes equations with a low-Reynolds number k–ε turbulence model edited for comprehending compressibility effects. In the steady operation regime, strong momentum transfer by a high-speed jet at diffuser inlet is balanced while it forms a recirculation zone at the diffuser inlet. Flow separation occurs at the diffuser downstream during the starting transient of the diffuser. During the starting transient and diffuser breakdown, abrupt changes in internal shock structures including a transition from Mach to regular shock wave reflection occur. Diffuser choking is advanced by anchoring the sonic line from the nozzle onto the wall of diffuser inlet before the reversed flow from the downstream subsonic region reaches the upstream of the diffuser. Immediately after the diffuser breakdown, higher ambient pressure acts on the upstream flows and the exhaust gases from the nozzle surge into the vacuum chamber. Confronted with the plume blowback, the exhaust flows are blocked and non-monotonic pressure recovery occurs.