The influences of length to depth ratio of cavity as well as its depth on the mixing effect of fuel and air and the combustion efficiency in supersonic flow field with/without reactions were numerically investigated. It is found that although increasing length to depth ratio could bring about fuel entrainment into cavity

delay the mixing time in low speed high-temperature recirculation zone and enhance the mixing and combustion efficiency. Yet

excessive length to depth ratio no longer improves combustion

but enhances the attached shock

increasing total pressure loss and drag between front and back walls. Increasing depth of cavity produces more loss of available energy induced by shock wave

but increases mass exchange rate between mainstream and cavity

enlarging cavity volume remarkably and increasing combustion efficiency.