W. H. Hsieh

Evaporation of LOX under supercritical and subcritical conditions

The evaporation of LOX under supercritical and subcritical conditions was studied experimentally and theoretically. In experiments, the evaporation rate and surface temperature were measured for LOX strand vaporizing in helium environments at pressures ranging from 5 to 68 atmospheres. Gas sampling and chromatography analysis were also employed to profile the gas composition above the LOX surface for the purpose of model validation. A comprehensive theoretical model was formulated and solved numerically to simulate the evaporation process of LOX at high pressures. The model was based on the conservation equations of mass, momentum, energy, and species concentrations for a multicomponent system, with consideration of gravitational body force, solubility of ambient gases in liquid, and variable thermophysical properties. Good agreement between predictions and measured oxygen mole fraction profiles was obtained. The effect of pressure on the distribution of the Lewis number, as well as the effect of variable diffusion coefficient, were further examined to elucidate the high-pressure transport behavior exhibited in the LOX vaporization process.

Ignition of RDX-based solid propellant subjected to crossflow convective heating

Crossflow convective ignition tests of RDX-based XM-39 LOVA solid-propellant grains were carried out in air and nitrogen using a shock tunnel facility. Test conditions were in the range of pressure: 1.38-3.45 MPa (200-500 psia); temperature: 1100-1500 K (1980-2700°R); and flow velocity: 60-70 m/s (197-230 ft/s). In the tests carried out in air, ignition was observed at or very near the sample surface in the region just beyond the gaseous boundary-layer separation point. No ignition was observed for the tests carried out in nitrogen. Ignition delay time was correlated to a dimensionless convective heat flux calculated from flow parameters. Post-test microscopic analysis of samples indicated the formation of an apparent liquid layer on the exposed surface which grows in thickness in the vicinity of the flow separation location. The existence of numerous near-circular, craterlike holes was detected using scanning electron microscopy. The rims of these holes exhibit evidence of surface reactions and phase changes. The physical interactions between the flow and test sample are discussed to provide insight into the complex ignition process, the ignition mechanism is believed to be gas-phase in nature rather than a surface reaction mechanism.

Modeling of Hot Fragment Conductive Ignition of Solid Propellants With Applications to Melting and Evaporation of Solids

Abstract : A comprehensive theoretical model has been formulated for studying the degree of vulnerability of various solid propellants being heated by hot spall fragments. The model stimulates the hot fragment conductive ignition (HFCI) processes caused by direct contact of hot inert particles with solid propellant samples. The model describes the heat transfer and displacement of the hot particle, the generation of the melt (or foam) layer caused by the liquefaction, pyrolysis, and decompression of the propellant, and the regression of the propellant as well as the time variation of its temperature distributions. To partially validate the theoretical model in the absence of the necessary chemical kinetic data, an ice-melting and evaporation experiment was designed and conducted. These experiments provide features of the conductive heating, melting, and evaporating process. Calculated results compare well with experimental data in temperature-time traces, spall particles-sinking velocity, and displacement.

Erosive Burning Study of Stick Propellants.

Abstract : A theoretical model was solved numerically for simulating erosive-burning processes occurring inside the center perforation of an unslotted NOSOL-363 stick propellant. Results show that the erosive-burning phenomenon is caused by the enhanced heat feedback from the gas phase to solid phase resulting from the combined effect of increased turbulent mixing and reduction in flame stand-off distance from the burning surface. The real-time X-ray radiography system was demonstrated to be a powerful and reliable tool for nonintrusive measurements of instantaneous burning rates. A model was validated by experimental data in terms of time variation of internal diameter distributions. Thermal wave structures of NOSOL-363 stick propellants under erosive- and strand-burning conditions were measured by fine-wire thermocouples.