Characterization of a LOX/natural gas liquid-centered swirl coaxial injector element in an optically accessible combustion chamber

Despite the fact that swirl coaxial injector elements are a popular choice in the design of LOX/methane liquid rocket engines, there is still a distinct lack of experimental studies optically evaluating their behavior under hot-fire conditions. Such optical data is essential for validating numerical models, for example CFD simulations. Previous optical studies of liquid-centered swirl injectors are generally limited to low subcritical combustion pressures. To fill this gap in literature, this study presents the results of a hot-fire test campaign featuring a single liquid-centered swirl coaxial injector element, designed for high thrust LOX/natural gas main stage engines in a research combustion chamber with full-length optical access windows. The chamber pressure is varied between 35 and 105 bar with mass flow rates up to 458 g/s and 133 g/s for liquid oxygen and natural gas, respectively. The change in spray behavior is assessed via high-speed shadowgraphy imagery while the flame topology is simultaneously evaluated using CH* and OH* chemiluminescence. Key findings include the dependence of liquid oxygen expansion angle on the momentum flux ratio of the injected propellants, which is found as the key factor governing the injector behavior. Following this, flame topology is also shown to be highly influenced by this parameter where for high momentum flux ratios the flame center moves closer to the injection plane while a concurrent increase in near-injector flame intensity is observed. Three discrete load points are evaluated and presented in more detail with deconvoluted flame images being superimposed on the shadowgraphy data — for simultaneous assessment of propellant breakup and flame shape. The results obtained in this study help to generate a deeper understanding of liquid centered injector elements in a representative environment as well as delivering important benchmark data for numerical models. Novelty and Significance Statement This study presents the first detailed optical characterization of a LOX/methane swirl injector element representative of those used in large-scale liquid rocket engines. It provides new experimental data, including shadowgraphy and flame imaging, at chamber pressure conditions that have not been previously reported in the literature. These results fill a critical gap in high-pressure combustion diagnostics for LOX/methane engines and offer valuable benchmarks for the development and validation of injector design methodologies and numerical simulations.