Spray interaction in adjacent GCSC injector elements: role of droplet collision and secondary droplet breakup

Abstract

This study investigates the evolution of spray characteristics in adjacent gas-centered swirl coaxial (GCSC) injectors, which finds application in liquid propellant rocket engines. The main objectives here are to measure the axial evolution of droplet characteristics in the spray interaction zone and understand the fundamental physics governing the spray interaction process. Experiments were conducted using air and water as the working fluids under atmospheric conditions. Utilizing the high-speed shadow imaging technique, the droplet images were captured at different axial and radial measurement stations for gas-to-liquid momentum flux ratio (M) ranging from 30 to 70. The images were processed to obtain droplet size, axial/radial components of droplet velocity, and droplet mass flux. The Mie-scattering images of the spray were acquired by laser sheet imaging to visualize the spray structure and spatial distribution of the droplets. Droplet measurements were also obtained by operating the injectors individually. Comparative analysis between the interacting and individual sprays highlighted the significant reduction in characteristic droplet size and an increase in the mean droplet velocity and local mass flux due to spray interaction. To elucidate the physical mechanisms behind the above observations, further analysis was carried out by evaluating the droplet collision, secondary atomization, and droplet dispersion in the interaction zone. Interestingly, the results highlight that, despite the intuitive notion that droplet collisions are the primary driver of the spray interaction process in the intersecting sprays, the improved secondary droplet atomization due to modification of airflow characteristics serves as the dominant factor in altering the droplet characteristics.