Droplet velocity and diameter distributions in fash boiling liquid nitrogen jets by means of phase Doppler diagnostics

Due to current and future environmental and safety issues in space propulsion, typical propellants for upper stage or satellite rocket engines such as the toxic hydrazine are going to be replaced by green propellants like the combination of liquid oxygen and hydrogen or methane. The injection of that kind of cryogenic fuids into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to fash boiling. For a detailed experimental investigation of superheated cryogenic fuids, the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. After a frst test campaign with high-speed shadowgraphy of fash boiling liquid nitrogen sprays, a laser-based Phase Doppler system was set-up to determine the spatial distributions of droplet velocities and diameters in highly superheated sprays. The spatial distributions revealed a core region with high mean velocities close to the injector orifce. With increasing distance from the injector orifce, the sprays develop a more and more monodisperse pattern. These distributions also showed that atomization due to fash boiling generates fner sprays with growing degrees of superheat. In certain spray regions, two droplet populations varying in their direction of motion, velocity and diameter due to possible recirculation zones were observed. The experimental data of fash boiling liquid nitrogen generated within this study provide a comprehensive data base for the validation of numerical models and further numerical investigations