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

An experimental study was performed to characterize the effects of vitiation due to combustion-air preheating on dual-mode scramjet combustion. Major combustion vitiation species (H(2)O and CO(2)) were added to the freestream of an electrical-resistance-heated, direct-connect facility simulating Mach 5 flight enthalpy. With clean, 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 dual-mode transition region between 0.22 and 0.26, as the mode of combustion was dependent on whether the fuel 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. Vitiation also caused transition between supersonic and subsonic combustion to occur at a higher fuel equivalence ratio than with clean air. This work represents the first direct evaluation of the effect of test-medium vitiation on dual-mode scramjet combustion at Mach 5 enthalpy simulation in the same facility. The results indicate the importance of accounting for test-medium vitiation when extrapolating from ground-testing to flight, particularly in the dual-mode transition region between subsonic and supersonic combustion regimes.