Non premixed Supersonic flames: Combustion models

The aim of this work is to investigate the validity of combustion models that were developed for low-speed combustion and then traditionally extended to high-speed combustion. In fact, the assumption of fast chemistry, as well as the flamelet chemistry model, must be validated in supersonic flows, where compressibility may affect the flame structure. LES of the HyShot test case, showed that the interactions between the airstream entering the combustor and the H2 sonic jet produce an average vorticity of order 105 Hz. The interaction between the hydrogen transverse jets and the supersonic air flow leads to bow shock formation and, accordingly, to boundary layer separation. This separation allows H2 to be convected upstream through the spanwise recirculation vortices created by the baroclinic effect. Once created, the vortices are tilted, stretched, compressed and expanded according to the vorticity transport equation. These vortices are the key structures responsible for the observed fast fuel air mixing. In this context, an analysis of the flame structure is of theoretical and numerical interest. In fact, depending on this structure, a appropriate kinetic and chemical/turbulence model can be chosen to correctly predict experimental results. The flame structure has been analyzed by means of the Burke and Schumann theory.