High Speed Visualization of LOX/GH2 Rocket Injector Flowfield: Hot-Fire and Cold-Flow Experiments

Abstract

The primary atomization and combustion characteristics of a liquid oxygen (LOX) / gaseous hydrogen (GH2) shear coaxial injector element were experimentally investigated. High speed movies using a shadowgraph imaging technique to visualize the LOX core were recorded for both hot-fire (LOX/GH2) and cold-flow (LOX/gaseous oxygen (GO2)) conditions with the same injector and chamber. Flow conditions were set to approximate realistic rocket conditions. For the hot-fire tests (LOX/GH2), chamber pressures were 600, 730, and 920 psia, with momentum flux ratios (annulus flow/post flow) of 2.7, 2.0 and 1.6 respectively. The rocket assembly utilized a preburner to provide a background flow (M≈0.1) of hot gaseous nitrogen (GN2)/GH2/water (H2O) gas with 25% volumetric concentration of hydrogen. For the cold-flow tests (LOX/GO2 with GO2 background flow), chamber pressures were 650 and 830 psia, thus above and below the critical pressure of oxygen (731.6 psia), with momentum flux ratios (annulus flow/post flow) of 2.2 and 1.8 respectively. The high speed visualizations under hot-fire conditions show a long sinuous LOX core region that breaks into large dense-oxygen structures, which are then quickly consumed. These results do not agree with the classical phenomenological breakup model that suggests a liquid core that is rapidly sheared into a drop cloud. Rather, a large-scale fragmentation model may be better suited to describe the primary atomization behavior in combusting flow from a LOX/GH2 shear coaxial injector element at realistic rocket conditions. Unlike the hot-fire case, cold-flow LOX visualization movies show a clear difference between the two chamber pressures, with the higher pressure (supercritical) case resembling behavior indicative of gaseous mixing compared to the typically two phase mixing appearance of the lower pressure (subcritical) case. Time-resolved measurements of the intact-core length are presented, along with size and frequency distributions of separating large dense-oxygen structures under hot-fire conditions.