Effects of Injector Geometry on Cryogenic Shear Coaxial Jets at Supercritical Pressures

Abstract

For shear coaxial injectors, which are widely used in liquid oxygen–gaseous hydrogen rocket engines, the key parameters that characterize injector geometry are the recess length, taper angle, and wall thickness of the liquid oxygen post, which is the inner tube of the coaxial injector. In the present study, the effects of the recessed length and taper angle of the liquid oxygen post on cryogenic coaxial jets at supercritical pressures were experimentally investigated. The large-scale structure and mixing enhancement of cryogenic nitrogen and gaseous helium coaxial jets were evaluated using a high-speed backlighting technique. Instantaneous backlit images revealed that the dark core, which is believed to be dense nitrogen, exhibited sinusoidal forms induced by the presence of the outer helium jets. However, the sinusoidal oscillations were less influenced by the injector geometry. The mixing enhancement of coaxial jets was evaluated with respect to the dark-core length and the jet spread angle. The recessed liquid oxygen post enhanced the mixing of cryogenic nitrogen when the recess length was sufficiently large, which suggests that a recess of a certain length is required to improve the mixing of cryogenic coaxial jets at supercritical pressures. Because the decrease in the dark-core length with the recessed length was consistent with the increase in the jet spread angle at the injector exit, the mixing enhancement caused by the recess can be regarded as a result of an improvement in the mixing between the inner and outer jets in the recessed region. The effects of the liquid oxygen post exit taper on the mixing enhancement were more significant than those of the recess. The decrease in the dark-core length with the inner-to-outer momentum flux ratio ( number) is consistent with the relevant empirical correlation at supercritical pressures. Therefore, the mixing enhancement caused by the liquid oxygen post exit taper can be considered the result of an increase in the number, rather than the result of giving azimuthal velocity to the cryogenic nitrogen jet at the liquid oxygen post exit.