Study on fuel equivalence ratio range for supersonic premixed combustion mode to establish in a scramjet

With the tendency of high-speed flight and economic cruise, combustion mode in a scramjet engine will transfer from diffusion combustion mode to premixed combustion mode because of the decrease of Damko??hler Number in combustor. In order to study the fuel equivalence ratio conditions for the establishment of premixed combustion mode and corresponding combustion mechanism, series of direct-connected experiments with gaseous ethylene as fuel were conducted. The total temperature of Mach 2.0 incoming flow ranged from 1200 K to 1700 K. Wall pressure of the combustor was collected to analyze related combustion mode and planar laser induced fluorescence (PLIF) was adopted to research combustion mechanism of corresponding combustion mode. Results showed that with the increment of fuel equivalence ratio ER, combustion mode transited from premixed combustion mode to partially premixed combustion mode and finally diffusion combustion mode. Bilateral injection scheme was more advantageous for premixed combustion to establish due to better fuel/air mixing and further downstream reaction position originating from diluted fuel in height position and corresponding low flame speed. Besides, total temperature of inflow had effect on the combustion characteristics in different aspects. On one hand, higher inflow total temperature brought lower lean fuel blow-off ER limit. On the other hand, the stabilization equivalence ratio range of premixed combustion mode would widen with the increment of inflow total temperature in the variation of 1200 K???1700 K which could be attributed to the coupling result of inflow temperature (affecting flame speed) and inflow velocity, indicating inevitability of appearance of premixed combustion mode in high-speed economic cruise with high inflow total temperature.