Abstract
Combustion is an essential process for generating propulsion for most existing aircraft. The combustion processes, such as ignition, flame propagation, and flame stability, are comprehensively discussed in this review for subsonic and supersonic burning conditions. Due to high maturity and commercialization reasons, subsonic combustion research in the open literature is quite low when compared to supersonic combustion. The combustion processes take place in annular, can, or can-annular flame holders for typical subsonic combustors. Spark discharge with a flame kernel igniter is typically used for subsonic ignition. The flame is ignited in a single igniter and propagates and stabilizes inside the whole combustion chamber. High-speed aircraft such as scramjets (supersonic combustion) have simpler flame holder designs, but the ignition, flame propagation, and stability are still challenging and less mature due to low residential time. Furthermore, the pressure drop reaches twenty times higher than the low-speed combustion resulting in inefficient propulsion. Thus, various flame holders were proposed to have successful flame propagation and stability. Cavity, strut, and combined cavity-strut holders are numerously investigated. Supersonic combustion could be self-ignited as shock waves, and high-speed particles are generated at high thermal conditions. Detonation is triggered in such holders. Cavity-based holders show less pressure drop, lower drag, and more stability than strut ones. Strut-based combustors use mostly hydrogen and mixtures of ethylene with methane, whereas cavity-based ones use ethylene and kerosene. The combined cavity-strut holder showed better fuel/air mixing, flame stability, and efficiency. Several innovative designs of scramjet engines, such as shcramjet engines, pulse detonation engines, and rotating detonation engines, have the advantages of better fuel efficiency, higher impulse, shorter and lighter design, improved thermal efficiency, and better combustion stability.