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Numerical study of a 5900-lb solid propellant thruster plume at an altitude of 118 km is performed using a combined multistep continuum/kinetic approach. The Navier--Stokes equations are solved for the flow inside the nozzle and the first several meters into the plume. The direct simulation Monte Carlo method is used to simulate the remaining plume and the plume-free stream interaction. A Monte Carlo based radiation code is applied in an overlay mode to calculate ultraviolet radiation in the near and far field of the plume. The computations take into account both alumina particles and soot. The effect of alumina particle emissivities and size distribution, as well as soot concentration, on ultraviolet radiation is clarified. Comparison of numerical results with available ultraviolet measurements is conducted. DOI: 10.2514/1.44067

A multistep continuum-kinetic approach is used to model a steady state plume flow from a Star 27 motor at an altitude of 118 km. Two-way coupled Navier--Stokes equations and the direct simulation Monte Carlo method are used to predict the interaction between plume and atmospheric gases and micron-sized alumina particles from the thruster. A Monte Carlo radiation code that accounts for photon scattering on particles is used to calculate ultraviolet radiation based on the obtained flowfield solutions. Comparison of computed spectral and integral radiant intensity with available flight data is performed. Photon scattering by submicron particles in the 200 to 400 nm range was found to be a dominant process in the far-field ultraviolet emission.

A particle simulation method to model water condensation process in a supersonic rocket plume is proposed and developed. Classic nucleation theory is used to predict nucleation, condensation, and the evaporation rates for water clusters. Microscopic kinetic models are developed to simulate collision processes between water clusters and monomers, between water clusters and foreign molecules, and evaporation of monomers from water clusters. These models are integrated into the direct simulation Monte Carlo method to simulate an axisymmetric mullispecies gas expansion coupled with condensation. The developed computational scheme, first verified by empirical scaling laws of condensation in supersonic microjets, is then applied to predict the spatial distributions of water cluster number density, size, and temperature in a rocket exhaust plume. An empirical equation is used to correct classical nucleation rate, condensation results are compared between the original and corrected nucleation rate, and the impact of the nucleation rate on a flow with condensation is discussed in detail.

The need for low-thrust propulsion systems for maneuvers on micro- and nanospacecraft is growing. Low-thrust characteristics generally lead to low-Reynolds-number flows from propulsive devices that utilize nozzle expansions. Low-Reynolds-number flows of helium and nitrogen through a small conical nozzle and a thin-walled orifice have been investigated both numerically, using the direct simulation Monte Carlo technique, and experimentally, using a nano-Newton thrust stand. For throat Reynolds number less than 100, the nozzle-to-orifice thrust ratio is less than unity; however, the corresponding ratio of specific impulse remains greater than one for the Reynolds number range from 0.02 to 200. Once the direct simulation Monte Carlo model results were verified using experimental thrust and mass flow data, the model was used to investigate the effects of geometrical variations on the conical nozzle's performance. At low Reynolds numbers, improvements to the specific impulse on the order of 4-8% were achieved through a combination of decreasing the nozzle length and increasing the nozzle expansion angle relative to the nominal experimental geometry.

A numerical study of three-dimensional effects on the performance of a micronozzle fabricated from flat silicon wafers is performed by use of both continuum and kinetic approaches. The nozzle operates in a low-Reynolds-number regime, and viscous effects dominate the gas expansion. Thrust losses occur because the shear on the wall is greater in a flat nozzle configuration than in an axisymmetric conical nozzle. Therefore, the prediction of the micronozzle performance based on axisymmetric or two-dimensional modeling can lead to significant design errors. Comparison of simulation with recent data shows good agreement in terms of thrust predictions for cold-gas thrusters at Reynolds numbers of approximately 2 x [10.sub.2].

A particle simulation method to model water condensation process in a supersonic rocket plume is proposed and developed. Classic nucleation theory is used to predict nucleation, condensation, and the evaporation rates for water clusters. Microscopic kinetic models are developed to simulate collision processes between water clusters and monomers, between water clusters and foreign molecules, and evaporation of monomers from water clusters. These models are integrated into the direct simulation Monte Carlo method to simulate an axisymmetric multispecies gas expansion coupled with condensation, The developed computational scheme, first verified by empirical scaling laws of condensation in supersonic microjets, is then applied to predict the spatial distributions of water cluster number density, size, and temperature in a rocket exhaust plume. An empirical equation is used to correct classical nucleation rate, condensation results are compared between the original and corrected nucleation rate, and the impact of the nucleation rate on a flow with condensation is discussed in detail. [ABSTRACT FROM AUTHOR]