Oxygen droplet combustion in hydrogen under microgravity conditions

The combustion of single liquid oxygen droplets in gaseous hydrogen is investigated experimentally under microgravity conditions to shed light on spray combustion processes in rocket engines. Using a drop tower apparatus, experiments are performed varying the ambient pressure between 0.1–5.7 MPa, which corresponds to a reduced pressure of oxygen p_r between 0.02–1.12. The combustion is investigated using high-speed shadowgraph imaging to track the droplet shape and OH-chemiluminescence to identify the flame zone. At low pressures (p_r<0.15), the droplet shape is found to change significantly during combustion likely due to the formation of a water ice layer around the droplet. Small jets of oxygen appear to break out of this ice layer, leading to an observed increase in linear and angular momentum of the droplet. At higher pressures, the visible effect of ice formation near the droplet surface decreases. The combustion process at different pressures in the subcritical and the supercritical regime is compared and discussed. The pressure has a limited influence on the flame standoff ratio, whereas it influences the burning rate constant substantially. Specifically, the experimental data suggest a maximum of the burning rate constant near the critical pressure, which is consistent with several experiments on hydrocarbon droplet combustion.