Local supersonic and subsonic combustion mode transition in a supersonic jet flame

An experimental and computational study has been carried out for a supersonic jet flame by using OH chemiluminescence imaging, shadowgraph visualization, temperature measurement by TDLAS, pressure measurement by transducers, and large eddy simulation (LES) together with a skeletal reaction mechanism involving 13 species and 41 steps. Agreements have been found between experimental data and LES results, which are subsequently used to analyze the flow, mixing, combustion, and heat release processes involved. A systematic method is adopted to qualitatively as well as quantitatively investigate different combustion modes and their contributions to heat release in the combustor. Influences of airstream temperature and pressure on combustion mode and heat release are also discussed by comparing four different cases. Results show that the heat is released from a combination of supersonic combustion mode and subsonic combustion mode even when the main flow is at supersonic speed. Local mode transition occurs as the jet flame propagates and interacts with shocks that enhance mixing because of baroclinic effects and induce subsonic combustion due to deceleration effects. It is also observed that subsonic combustion releases more than 50% of heat at the base of the jet flame because of recirculation zones behind the strut. Supersonic combustion mode gradually becomes prominent in the turbulent far field with small values of heat release rate. The overall dominant combustion mode is dependent on not only inflow conditions but also combustion intensity.