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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.

Time-dependent performance of a high-temperature MEMS-based thruster is studied in detail by a coupled thermal-fluid analysis. The material thermal response governed by the transient heat conduction equation is obtained using the finite element method. The low-Reynolds number gas flow in the microthruster is modeled by the direct simulation Monte Carlo (DSMC) approach. The temporal variation of the thruster material temperature and gas flowfields are obtained as well as the thruster operational time limits for thermally insulated and convectively cooled thrusters. The predicted thrust and mass discharge coefficient of both two-dimensional (2-D) and three-dimensional (3-D) micronozzles decreases in time as the viscous losses increase for higher wall temperatures. [1046] Index Terms--Fluid flow, kinetic methods, microfluidics, semiconductor device, space vehicle propulsion.

Gaseous flow through a microchannel is treated numerically and analytically in order to assess pressure and mass flow losses due to a constriction of a finite length. Numerical modeling of two-dimensional (2-D) microchannel flow in the slip and transitional regimes is carried out using the direct simulation Monte Carlo (DSMC) method. The prediction of pressure losses and mass flow based on a simple analytic model for constriction microchannel flow are found to be in excellent agreement with DSMC simulations. Constriction has a dramatic effect on pressure loss and mass flow rate for the considered cases. The DSMC results indicate that the flow in the constriction microchannel separates in the transition section, but the separation does not significantly impact pressure distributions. [1189] Index Terms--Fluid flow, gases, losses, microfluidics, modeling, Monte Carlo methods, numerical analysis, pressure effects, separation. NOMENCLATURE a Speed of sound, m/s. m Mass flow rate, kg/s. [mu] Viscosity coefficient, kg/m-s. p Pressure, Pa. P Inlet to outlet pressure ratio. u X-component of velocity, m/s. v Y-component of velocity, m/s. R Gas constant, J/kg x K. T Temperamre, K. [rho] Density, kg/[m.sup.3]. [sigma] Accommodation coefficient. Subscripts int Interior cell. e Boundary. in Inlet. out Outlet.

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 computational approach to homogeneous nucleation is proposed based on Eulerian description of the gas phase expansion coupled with a Lagrangian approach to the cluster formation. A continuum, Euler/Navier-Stokes solver versatile advection code is used to model the gas transport, and a kinetic particle solver is developed in this work to simulate cluster nucleation and growth. Parameters in the new model were adjusted so as to match the known theoretical dimer formation equilibrium constants for the two gases under consideration, argon and water. Reasonable agreement between computed and available experimental data was found in terminal cluster size distributions for nozzle water expansions in a wide range of stagnation pressures. The proposed approach was found to be orders of magnitude faster than a comparable approach based on the direct simulation Monte Carlo method. [ABSTRACT FROM AUTHOR]

A computational approach capable of modeling homogeneous condensation in nonequilibrium environments is presented. The approach is based on the direct simulation Monte Carlo (DSMC) method, extended as appropriate to include the most important processes of cluster nucleation and evolution at the microscopic level. The approach uses a recombination-reaction energy-dependent mechanism of the DSMC method for the characterization of dimer formation, and the RRK model for the cluster evaporation. Three-step testing and validation of the model is conducted by (i) comparison of clusterization rates in an equilibrium heat bath with theoretical predictions for argon and water vapor and adjustment of the model parameters, (ii) comparison of the nonequilibrium argon cluster size distributions with experimental data, and (iii) comparison of the nonequilibrium water cluster size distributions with experimental measurements. Reasonable agreement was observed for all three parts of the validation. [ABSTRACT FROM AUTHOR]

The ability of a traveling periodic optical potential to increase the thrust and specific impulse of microthrusters is investigated. Two flow regimes, high density and low density, are considered. The thrust from a micronozzle, with a stagnation pressure of 1 atm and temperature of 300 K, can be increased by more than an order of magnitude. These conditions can be achieved for a constant velocity lattice, produced by two near counterpropagating optical fields that are focused into the nozzle throat. A propulsion system that operates in low-density regime and is driven by molecules trapped by an accelerating optical lattice is proposed. It is shown that such a system has a potential to achieve a specific impulse of thousands of seconds. [ABSTRACT FROM AUTHOR]