This study aims to reveal the influence of Local Momentum Ratio (LMR) on the combustion efficiency of an LOX/GCH4 pintle injector from the perspective of spray characteristics. Hot fire tests were conducted to establish the relationship between combustion efficiency and LMR. The spray characteristics for different LMRs were simulated by the validated volume of fluid-to-discrete phase model method, taking into account the combustion chamber wall confinement. Subsequently, the difference in combustion efficiency was analyzed by comparing the spray simulation results of backpressure conditions similar to hot fire tests. The results indicate that combustion efficiency increased initially (LMR = 1.12–1.64) and then decreased (LMR>1.64). Quantitative analysis revealed a linear correlation (R² = 0.95) between LMR and combustion efficiency within 1.12 < LMR < 1.64. As the LMR increased, the improvement in combustion efficiency was attributed to a wider spray distribution range and smaller droplet sizes. The area of the mantle recirculation zone that is detrimental to combustion decreased by approximately 38 %, and the droplet size reduced from 37 to 16 μm. This effectively enhanced both the mixing of the propellant and the evaporation process. When the LMR exceeded the critical value (1.64 in this study), the impingement of liquid oxygen on the combustion chamber wall was confirmed via overheating discoloration marks on the inner surface of combustion chamber’s cylindrical section. The impingement of liquid oxygen on the combustion chamber wall increased the transport of liquid oxygen to the wall, directly reducing the mixing quality and combustion efficiency. The outcomes of this study provide the practical guidance for design and improvement in combustion efficiency of the pintle injector thrust chamber.

The injection and atomization process of a liquid fuel jet is critical for an ignition start of a scramjet engine. Airwall-mounted crossflow injection strategy is widely used in scramjet combustors, avoiding high total pressure loss and allowing the liquid fuel to rapidly undergo atomization, mixing, and evaporation. In this review, research progress on a liquid jet in supersonic crossflow was evaluated from aspects of atomization mechanism and spray characteristics. When a liquid jet is injected into a supersonic crossflow, primary and secondary breakups occur successively. The surface instability of liquid can significantly affect the breakup process. This review discusses the current understanding of the breakup process and spray characteristics of a liquid jet in supersonic crossflow including the mechanism of atomization and the characteristics of distribution and atomization. The development of windward Rayleigh-Taylor (R-T) unstable waves is the main factor in column breakup. The development of Kelvin-Helmholtz (K-H) unstable waves along the circumferential direction of the jet or droplets is the main factor of surface and droplet breakups. The liquid–gas momentum ratio is the most important factor affecting the penetration depth. The span width of the liquid jet is affected by the windward area. Breakup and coalescence lead to a transformation of the size distribution of droplets from S- or C-shaped to I-shaped, and the velocity distribution of the droplets on the central symmetry plane has a mirrored S-shape. The droplet distribution on the spanwise cross-section retains a structure similar to an “Ω” shape. At last, some promising recommendations have been proposed, namely a theoretical predictive model which can describe the breakup mechanism of a liquid jet, the distribution characteristics and droplets size distribution of a liquid jet under a cavity combustion chamber, especially for enthalpy flows with complex wave structures.