Gaseous hydrogen-oxygen combustion: Fundamental mixing phenomena and vibrational nonequilibrium in rocket engine flows

Low thrust rocket engines encompass flowfield conditions ranging from subsonic turbulent reacting flowfields to rarified gas nozzle expansion flows. In an effort to improve understanding of internal rocket flows, investigations were initiated into turbulent mixing and nozzle flow phenomena. The cooling film layer within small rocket engines forms a mixing layer which develops as the flow progresses downstream. A modeling effort is described which attempts to capture the essential elements of a highly turbulent flowfield. The method is developed from nonreacting and reacting mixing layer experiments performed at NASA Lewis. The flowfield regime studied is the range of high subsonic Mach numbers, conditions pertinent to internal rocket engine flows as well as jet engines. Favorable comparison with experimental data shows that a relatively quick and simple desktop computer calculation is capable of simulating the basic flow structure in the reacting and nonreacting shear layers. Success of the model is attributed to simulation of large scale turbulent transport of fluid.The possible presence of thermal nonequilibrium in the exit flow of an oxidizer-rich rocket injector impacts vibrational temperature measurements. Oxygen vibrational relaxation phenomena among the combustion product bath molecules are studied and found to be rapid relative to relaxation of a pure oxygen system. A computational method is developed to simulate gas nozzle flows expanding in vibrational nonequilibrium. Comparisons of the effect of nonequilibrium on flowfield variables are made between available experimental and computational data and results obtained with the current formulation. Significant changes in the properties of flowfields with a large degree of nonequilibrium are obtained, including decreased temperature and pressure. Nonequilibrium flowfield effects are computed for the rocket engines of interest and shown to be minor in terms of performance losses. The analytical prediction of oxygen vibrational relaxation rate is validated in order of magnitude through spontaneous Raman spectroscopy. Chamber and low area ratio nozzle vibrational temperature measurements are shown to be valid.The rocket chamber flowfield is shown to mix and react throughout the measurement range. Oxygen number densities follow closely ideal gas expansion profiles. Temperatures increase through the converging region, throat, and initial expansion region.