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.

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关于闪蒸液氮喷雾的形态

由于当前和未来太空推进中的环境和安全问题，用于上层级或卫星火箭发动机的典型推进剂（如有毒肼）将被绿色推进剂所替代，例如液态氧，氢或甲烷的组合。将这种低温流体注入太空的真空气氛会导致过热状态，这会由于闪蒸而导致突然的喷发雾化。为了对过热的低温流体进行详细的实验研究，在DLR Lampoldshausen建立了带有温度控制注入系统的新型低温测试台M3.3。选择液氮（LN2）作为测试液是因为它的物理性质与液氧相似。依靠喷射参数和导出的无量纲量，借助高速阴影照相法确定了闪速沸腾的LN2喷雾的特征形态，例如破裂模式和喷雾角度。与可存储的流体类似，过热程度的提高导致LN2射流破裂时闪蒸沸腾的优势日益增加。因此，有关可储存流体的射流破裂状态的相关性也表明它们也适用于低温氮。另外，提出了另一种特征性闪蒸方案。对于高度过热的喷雾，我们观察到由于膨胀和汽化导致的温度下降而导致的氮滴凝固。