Effects of oxidizer reactivity and shear-induced breakup in stabilizing LNH₃[jls-end-space/]/O₂ bipropellant coaxial flames

Liquid ammonia (LNH3[jls-end-space/]) is a promising propellant for liquid rocket engines due to its high heat sink and system safety. However, the combustion characteristics of LNH3[jls-end-space/]/O2 flames remain poorly understood. This study experimentally investigates LNH3 shear-coaxial jet flames using synchronous NH-PLIF/Mie scattering, PIV, and axial thermocouple measurements. By varying oxidizer type (air and O2[jls-end-space/]) and equivalence ratio, the different but coupled roles of oxidizer reactivity and shear-induced breakup are examined. Results show that oxidizer composition more strongly affects the compactness and continuity of the reaction zone, whereas higher oxidizer velocity mainly promotes liquid-core breakup, evaporation, and entrainment. In the air cases, stronger shear alone accelerates breakup and evaporation but also enhances flame fragmentation under relatively weak chemistry, resulting in Island-dominated reaction zones. In contrast, in the O2 cases, the stronger chemical reactivity is the primary factor enabling the formation of a thin and continuous Ring-dominated reaction layer, while favorable breakup and entrainment further shorten the spray-to-reaction spacing. Front-spacing analysis based on DMie−NH confirms that O2 markedly reduces the spatial separation between the spray boundary and the NH-marked reaction front, especially in case O0.8. Although the direct geometric overlap between the spray region and the reaction region can be modulated by oxidizer behavior, it remains limited ((Formula presented)) under all tested conditions. This phenomenon is associated with the persistent dense liquid core and the accompanying low-temperature spray region of LNH3 flames. This work provides the first experimental data on LNH3[jls-end-space/]/O2 coaxial flame dynamics.Novelty and significance statement: This study presents the first experimental investigation of LNH3[jls-end-space/]/O2 shear-coaxial jet flames for liquid rocket bipropellant combustion. Through synchronous NH-PLIF/Mie scattering, PIV, and temperature measurements, this work reveals the distinct yet coupled roles of oxidizer reactivity and shear-induced breakup in LNH3 coaxial jet flames, and identifies the formation of a thin and continuous reaction layer as primarily chemistry-controlled. Using O2 as the oxidizer demonstrates that LNH3[jls-end-space/]/O2 coaxial combustion can sustain spatially compact and stabilized flames despite relatively weaker atomization, thereby supporting the feasibility of LNH3[jls-end-space/]/O2 as a rocket-relevant bipropellant configuration. Meanwhile, the direct geometric overlap between the spray region and the reaction region remains limited in LNH3[jls-end-space/]/O2 flames. These findings provide a mechanistic basis for optimizing injector design and flame stabilization in LNH3[jls-end-space/]/O2 rocket engines, offering pathways toward more reliable propulsion systems.