Droplet Populations in Flash Boiling Liquid Nitrogen Sprays

Technology development for propulsion systems of upper stages and reaction control thrusters is driven by green propellants to substitute hydrazine. At high-altitude conditions prior to ignition the liquid propellants are injected into the combustor at near-vacuum. Due to the sudden pressure drop the liquid is in a superheated thermodynamic state resulting in an eruptive evaporation and fast expansion, a process called flash boiling. To know the composition related to phase and atomization is important for both to determine the probability of a successful ignition and to avoid destructive pressure peaks. Furthermore, the experimental results provide a data base for further numerical investigations about superheated cryogenic fluids. Hence, the cryogenic test bench M3.3 with a temperature-controlled injection system for an experimental investigation of cryogenic flash boiling sprays is in operation at DLR. By means of laser-based Phase Doppler diagnostics the velocity and droplet size distributions of this kind of sprays were determined and two different droplet populations were found. The analysis shows that the flashing sprays are axially symmetrical and that both droplet populations are linked to each other due to similar fluctuations in the transient injection phase