Experimental Study of Test Medium Vitiation Effects on Dual-Mode Scramjet Mode Transition

An experimental study was performed to characterize the effect of vitiates due to combustion air preheating on dual-mode scramjet combustion. Major vitiate species (H2O and CO2) were added to the freestream of an electrically heated, direct connect facility simulating Mach 5 flight enthalpy. With dry air, the combustor operated in the supersonic mode at fuel equivalence ratios below 0.22 and in the subsonic mode for equivalence ratios above 0.26. Hysteresis was observed in the transitional region between 0.22 and 0.26, as the mode of combustion was dependent on whether the fuel flow rate was increasing or decreasing. Adding increasing amounts of water vapor and carbon dioxide to the freestream decreased combustor pressures by 10% to 30% for the same fuel equivalence ratio. Vitiates also caused the transition between supersonic to subsonic combustion to occur at a higher fuel equivalence ratio than with clean air. These results indicate the importance of accounting for vitiates when extrapolating from ground testing to flight, particularly in the transition region between subsonic and supersonic combustion regimes.