On the backpressure phenomenon in modeled scramjets

Although a large number of studies has been carried out on the backpressure phenomenon and unstart in scramjet models, including the development of pseudo-shock location control algorithms, the distinction between mechanical and thermal causes of backpressure and unstart has not received enough attention. Moreover, a large proportion of these aforementioned studies have focused on artificially blocking the flow downstream of the combustor. In the present work, backpressures in scramjet engines have been generated using two different approaches that do not involve the use of artificial blocking of flows. Moreover, the two phenomena respectively result from pure mechanical and thermal processes. Both the general and detailed aspects of the two backpressure phenomena are significantly different for the two processes, and are described in this paper. For example, the mechanically-generated back pressure, which is accomplished in this work via specified fuel injection conditions in a pure species-mixing mode, shows an aggressive backpressure phenomenon, with upstream shock speeds of over 300 m/s, depending on the injection pressure, and with shock disgorging in a few of the cases. However, the thermally-generated backpressure, which is obtained by using multiple fuels and keeping the injection conditions of the fuels identical and leveraging on the differential flammability of the fuels; is due mainly to the choking phenomenon. The latter gives modest upstream shock propagation speeds and leading shocks that aren’t disgorged but stagnated just downstream of the isolator exit.