LES of kerosene spray combustion with pilot flame in a model dual mode ramjet chamber

Large-eddy simulations (LES) have been performed for kerosene spray combustion with methane pilot flame in a model dual mode ramjet chamber with a cavity flame holder under the condition of incoming Mach number of 2 and inlet static temperature of 520K. Three cases are compared with a constant equivalence ratio of methane pilot flame of 0.08 but different equivalence ratios of kerosene of 0.10, 0.25 and 0.6 respectively. It is found that the kerosene spray interacts strongly with the pilot flame in the near wall region and the depth of the spray penetration plays an important part in deciding the characteristics of the combustion process, its efficiency and the overall heat addition. When kerosene injection mass is small, spray droplets are close to the wall and are surrounded with the hot air generated by the pilot flame, the kerosene spray combustion can achieve higher efficiency due to nearly complete combustion in the separated boundary layer. However the resulting flame flashback in the boundary layer could induce engine inlet unstarting. When spray droplets penetrate further into the freestream but still interact intermittently with the pilot flame, the spray evaporation is enhanced by the pilot flame front. The heat consumption during this process results in rich fuel mixtures accumulating near the wall. Thus the combustion processes of both kerosene vapor and pilot gas have been suppressed, resulting the lowest heat releasing. As for the case of droplets fully penetrating into the freestream core, small amount of evaporated kerosene gradually diffuses and burns further downstream of the pilot flame, accompanied with the lowest combustion efficiency due to low evaporation rate but the relative higher additional heat releasing. Considering inlet unstarting is undesirable for ramjet engine under a relative low flight Mach number, unless the boundary layer separation can be controlled, the deep spray penetration is to be used to achieve a better overall kerosene combustion performance. However the spray combustion efficiency still needs to be improved.