Mixing and Combustion in Supersonic Reactive Flows

Abstract

Supersonic combustion is a key issue in any future plan to develop supersonic combustion (SC) ramjets (SCRJ) and rocket based combined cycles (RBCC) vehicles. Past experience has shown that among the major fundamental problems to be solved in SC are the need for rapid (i.e., < 1 ms) mixing and combustion efficiency. In this context numerical simulations, in particular LES, can help in improving the understandi ng of these issues. Current LES subgrid models developed for subsonic and adapted to supersonic combustion do not predict well or at all experimental results such as flame anchoring, whilst past experimental results with hydrogen injected at Mach 2.5 in Mach 2 airstreams showed combustion taking pl ace in about 2 ft. In fact, theoretical analysis shows that at high Mach number mixing and combustion are driven not only by transfer of kinetic energy by vortex stretching, as in subsonic reacting flows, but also by compressibili ty and baroclinic effects. Compressibility favours combustion by increasing reaction rates, as supersonic combustion occurs locally at about constant volume. Thus, when modelling mixing and combustion at small scales using LES, all these effects must be accounted before attempti ng to reproduce experimental results and to predict performance. To this purpose, a novel sub-grid scale (SGS) model (called henceforth ISC model, or ISCM for short) including these physical effects has been developed. This model has been validated so far by means of two experimental test cases. The first is the cross-flow injection of a sonic gaseous hydrogen jet in a Mach 2 airstream in the combustor built at the University of Tokyo; the second consists in 30° oblique injection of gaseous hydrogen in a Mach 2.5 airflow, an experiment performed in the supersonic combustion facility at NASA Langley Research Centre . For brevity, only the second test case validation is reported here. LES simulations using the well known Smagorinsky-Lilly SGS closure have been also performed for comparison. Results show that the ISCM is in better agreement with experimental data. In fact, while the Smagorinsky-Lilly model predicts neither combustion nor vortex structures, ISCM predicts flame anchoring, streamwise vorticity and temperatures close to those observed in the NASA-Langley experiments.