ABOUT THE MORPHOLOGY OF FLASH BOILING LIQUID NITROGEN SPRAYS

Abstract

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 fluid into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to flash boiling. For a detailed experimental investigation of superheated cryogenic fluids the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. Liquid nitrogen was chosen as the test fluid because of its similar physical properties to liquid oxygen. The characteristic morphologies like breakup patterns and spray angles of the flash boiling liquid nitrogen sprays were determined by means of high-speed shadowgraphy in dependence on the injection parameters and derived dimensionless quantities. Similar to storable fluids, increasing degrees of superheat lead to a growing dominance of flash boiling on the breakup of the liquid nitrogen jet. Hence, correlations about jet breakup regimes derived for storable fluids showed their suitability for cryogenic nitrogen as well. In addition, a further characteristic flash boiling regime is presented. For highly superheated sprays, we observed the solidification of nitrogen droplets due to the temperature drop as a consequence of expansion and vaporization. Besides new insights into the flash boiling process of cryogenic liquids, the experimental data of flash boiling liquid nitrogen generated within this study provide a comprehensive database for the validation of numerical models and further numerical investigations.