Annular Air Gap Area Impact on Flame Regime Transition and Combustion in Low-chamber-pressure Air Heater

This study combines high-speed shadowgraph imaging with numerical simulations to systematically examine the effect of the annular air gap area on the spray combustion characteristics of an alcohol-liquid-oxygen-air tripropellant coaxial direct-flow injector in an air heater operating under a low chamber pressure of 1.2 MPa. The underlying mechanisms of ignition, flame structure, injector atomization, and combustion stability are analyzed in detail. Results show that the annular air gap area has a significant impact on flame morphology and combustion performance. When the air gap area is relatively large (corresponding to an annular gap spacing of 1.95 mm), an elongated attached flame forms, and ignition is completed within 19 ms. Although the short ignition time and favorable flame stability are advantageous, the combustion efficiency is relatively low (91%), and the nozzle and throat are prone to ablation. When the air gap area is moderate (1.41 mm spacing), a conical flame develops, exhibiting the longest ignition time (997.4 ms) and a stratified structure consisting of fuel-rich combustion at the core and fuel-lean combustion at the periphery. This configuration demonstrates good stability. When the air gap area is small (1.10 mm spacing), a lifted flame forms. Although mixing and ignition occur relatively quickly (around 386.4 ms), stability is poor, with large chamber pressure fluctuations and a high risk of extinction once the air velocity exceeds the critical threshold. Reducing the air gap area effectively shortens the liquid oxygen atomization distance by 50% and significantly improves evaporation efficiency; however, excessive reduction promotes ignition-quenching-reignition cycles and worsens flame instability. Further analysis indicates that flame stability is primarily governed by the ratio of injection velocity to flame propagation velocity. When this ratio exceeds a critical value, shear-layer instability arises, increasing the amplitude of chamber pressure fluctuations by up to 200%. This research provides a theoretical foundation for optimizing injector design and improving combustion stability control in air heaters. The insights gained are essential for enhancing ignition reliability and thermal protection in hypersonic applications.