Evaporation of sub-millimeter flying dinitrogen tetroxide droplet under high temperature and pressure: Experimental measurement and theoretical modeling

The evaporation behaviors are crucial for the flame location estimation in liquid rocket engines. This work, for the first time, experimentally reports the sub-millimeter droplet evaporation characteristics of the corrosive dinitrogen tetroxide (NTO, one prevailing hypergolic oxidizer) at high ambient pressure up to 4.5 MPa. An in-house corrosion-resistant droplet generator is used to generate isolated flying droplets of sub-millimeter size, which are then exposed in a gas environment with temperatures between 1010 K and 1210 K and pressures in the range between 2.0 MPa and 4.5 MPa, provided by an optical rapid compression machine. Parallelly, a theoretical model considering both the droplet ambient convection and the NTO dissociation is developed. Results indicate that firstly, the present theoretical model that considers the transient droplet-ambient convection as well as the temperature and pressure dependent rate of dissociation shows good agreement with the experimentally observed droplet lifetime. In addition, the flying droplets velocity regress gradually due to momentum exchange with the ambient, which is more prominent at higher pressure. The evaporation caused droplet size reduction is consistent with the classical D²-law prediction, in the present temperature and pressure range. Finally, higher temperature and pressure accelerate the evaporation and an empirical correlation for the temperature and pressure dependent evaporation rate constant is proposed, which shows good agreement with experiment and simulation results.