On the start condition of a second-throat ejector-diffuser

The start condition of a second-throat ejector-diffuser (STED) system has been studied by solving the axisymmetric Navier-Stokes equations with a high order conservative supra-characteristics method (CSCM). According to the conventional concept, when a STED is started, the flow passing through the second-throat contraction should be supersonic so that the normal shock swallowing condition applying for a supersonic windtunnel holds. The present numerical results, however, suggest that a STED can be started with a normal shock forming ahead of the second-throat contraction because the high speed flow in the region close to the wall of the diffuser serves to release the mass blocked by the normal shock, leading to a decrease in chamber pressure as well as the start of the system. This not only contradicts the conventional concept of the start of a STED, but also provides an explanation for the experimental discrepancy between the start conditions of a STED and a supersonic windtunnel.