Evaluation of the Effect of H2O Vitiation on Forced Ignition in a Scramjet Combustor Using a Forced Ignition Model

Abstract Reliable ignition and flame stabilization are critical challenges in scramjet combustors, particularly under high-speed flow conditions. Forced ignition techniques, such as micro-rocket torches, have been investigated as alternatives to plasma jet (PJ) torches owing to their ability to inject high-temperature gases and active radicals directly into the combustion region. However, previous studies have primarily relied on ground experiments utilizing hydrogen combustion vitiated heaters, where H2O in vitiated air significantly affects ignition characteristics. To address this, this study examined the impact of H2O vitiation on forced ignition using a micro-rocket torch in a scramjet combustor with a cavity flame holder. A numerical analysis was conducted by combining the forced ignition model (FIM) in the shear layer (FIMS) with a plug flow reactor (PFR) model to investigate ignition phenomena in the shear layer. The model was validated using previous combustion experimental data, demonstrating that the forced ignition limits could be accurately predicted based on the Damköhler number (Da). The results revealed that H2O vitiation altered the radical recombination processes and increased the ignition delay time, thereby shifting the ignition location upstream. Moreover, its impact was more significant in cavity configurations with shorter residence times, such as Type A cavities. Additionally, increasing the net input energy of the micro-rocket torch from 15 kW to 30 kW lowered the forced ignition limit temperature, mitigating the H2O vitiation effects. These findings emphasize the necessity of considering vitiation effects when applying ground test data to flight conditions.