Experimental and Numerical Studies of Transcritical LOX Droplets

Abstract

A 137 atm pressure vessel has been developed for the study of transcritical liquid oxygen and liquid nitrogen droplets and jets. At the same time, numerical modeling of the same phenomena is being pursued using scaled and unscaled molecular dynamics. Experimental and numerical results both show the disappearance of surface tension as the droplet passes through its critical point and the effectof various environment mixtures on the critical point. a combustion chamber which is operating above the critical pressure and temperature of the oxygen. The oxygen, initially above its critical pressure, is heated upon injection into the chamber to a value above its critical temperature, thus undergoing a transition from subcritical to supercritical conditions during the atomization and vaporization processes. There are few previous experimental studies of transcritical LOX evaporation due to the difficulty of creating and maintaining liquid oxygen droplets in a high pressure environment. At the same time, CFD studies are hindered by assumptions concerning the solution geometry, equations of state, mixture rules and the need for extensive tables of material properties, some ofwhich change dramatically near the critical point. EXPERIMENTAL STUDIES -Experimental studies of transcritidal oxygen droplets wdre performed using the droplet generator illustrated in Fig 1. Gaseous oxygen at room temperature was introduced into a central tube at the pressure selected for study (oxygen is actually a supercritical fluid under these conditions). The oxygen was then cooled and densified (condensed) by a surrounding bath of liquid nitrogen at atmospheric pressure. A piezoelectric crystal (not shown) was mounted in the tube to provide acoustic excitation. Droplets were then formed in the throat of a converginrgdiverging section surrounded by a flow of gaseous helium (actually also a supercritical fluid) which has also been chilled by liquid nitrogen. The chilled helium served three purposes. First, it helped keep the drop generator tip cold, preventing heat transfer and/or boiling from interfering with drop production. Second, the aerodynamic interaction with the emerging densifled oxygen produced a stripping action which, in addition to piezoelectric excitation, is also a proven method for producing monodisperse streams of droplets, at least for subcritical flows. The droplets described below were in fact produced using this method; piezoelectric excitation was not used. The third and most important function of the helium was to provide surface tension for forming the droplets. Previous experience showed that it was not possible to create transcritical oxygen droplets in a gas of similar molecular weight such as oxygen or nitrogen, presumably because the reduction in surfacetension as the critical pressure was approached did not allow droplets to be formed. Helium is hypothesized to create helium/oxygen mixtures at the interface which have critical mixing pressures much higher than pure oxygen. Thus the interface becomes subcritical and surface tension can exist, even though the pressure is higher than the critical pressure of pure oxygen. After droplet formation, the helium and droplets flowed into a pressure vessel (13.7 MPa maximum pressure), where the helium formed a buoyant layer at the top and droplets fell into a test gas having controlled composition and at a temperature slightly below room temperature. For safety reasons, nitrogen was used as the test gas in this study. Earlier experience indicated nitrogen/oxygen mixtures exhibited transcritical behavior similar to oxygen/oxygen mixtures. The flows werevisualized using backlit shadowgraphy using two 180 degree opposed 133.4 mmn sapphire windows. An additional set of oblong quartz windows oriented at 90 degrees to the sapphire windows allows introduction of laser sheets for spontaneous Raman imaging. However, Raman results will not be reported here.