Effects of mixing schemes on kerosene combustion in a supersonic airstream

A study of kerosene combustion in a supersonic vitiated airflow at Mach 4.75 night enthalpy was conducted in direct-connect tests at Mach 1.8 at a stagnation temperature of 1000 K. The effects of shock-and vortex-enhanced mixing mechanisms on the combustion efficiency were evaluated. Also included in this study were the effects of fuel heating and jet penetration. The experimental conditions corresponded to the low end of the hypersonic night regime, The following geometric configurations were employed: 1) a generic, rearward-facing step, 2) a modified rearward-facing step with beveled edges to facilitate vortex-enhanced mixing, and 3) a rearward-facing wedge (15 or 30 deg) placed downstream of the rearward-facing step to induce shock-enhanced mixing, In all configurations, a gaseous hydrogen-pilot jet was injected parallel to the main flow from the base of the rearward-facing step and the liquid kerosene was injected normal to the main flow at three or five step heights downstream of the step (the step height was 10 mm). Stable kerosene combustion was obtained for a maximum injected kerosene equivalence ratio of 0.86. For efficiency evaluation, the pilot-hydrogen equivalence ratio was selected between 0.02-0.04, while the kerosene equivalence ratio was maintained at 0.325. In all experiments, locally rich stratified kerosene combustion took place in a layer close to the injection wall. The wedge flameholder contributed to an increased kerosene combustion efficiency by the generation of shock-jet interactions. The beveled-edge step improved far-field mixing, thereby reducing the local kerosene equivalence ratio, resulting in the highest kerosene combustion efficiency among all configurations tested. Fuel heating below levels required for flash vaporization (one-third of the flash vaporization energy, in this case) did not contribute to increased combustion efficiency. On the contrary, this level of heating reduced the fuel density with adverse effects on penetration and mixing.