S. F. Gimelshein

Transient heat transfer and gas flow in a MEMS-based thruster

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.

Vibrational relaxation rates in the direct simulation Monte Carlo method

Exact relationship is developed that connects the vibrational relaxation number, ZvDSMC, used in the direct simulation Monte Carlo method and that employed in continuum simulations. An approximate expression for ZvDSMC is also derived that is cost-effective and applicable when translational temperature is larger than vibrational temperature.

Numerical Modeling of Axisymmetric and Three-Dimensional Flows in Microelectromechanical Systems Nozzles

A numerical study of three-dimensional effects on the performance of a micronozzle fabricated from e at silicon wafers is performed by use of both continuum and kinetic approaches. The nozzle operates in a low-Reynoldsnumber regime, and viscous effects dominate the gas expansion. Thrust losses occur because the shear on the wall is greater in a e at nozzle cone guration than in an axisymmetric conical nozzle. Therefore, the prediction of the micronozzle performance based on axisymmetric or two-dimensional modeling can lead to signie cant design errors.Comparisonofsimulationwithrecentdatashowsgood agreementintermsofthrustpredictionsforcold-gas thrusters at Reynolds numbers of approximately 2 ££ 102.

Simulation of gas dynamics and radiation in volcanic plumes on Io

Abstract Modeling results of volcanic plumes on Jupiter’s moon Io are presented. Two types of low density axisymmetric SO2 plume flows are modeled using the direct simulation Monte Carlo (DSMC) method. Thermal radiation from all three vibrational bands and overall rotational lines of SO2 molecules is modeled. A high resolution computation of the flow in the vicinity of the vent was obtained by multidomain sequential calculation to improve the modeling of the radiation signature. The radiation features are examined both by calculating infrared emission spectra along different lines-of-sight through the plume and with the DSMC modeled emission images of the whole flow field. It is found that most of the radiation originates in the vicinity of the vent, and non-LTE (non-local-thermodynamic equilibrium) cooling by SO2 rotation lines exceeds cooling in the v2 vibrational band at high altitude. In addition to the general shape of the plumes, the calculated average SO2 column density (∼1016 cm−2) over a Pele-type plume and the related frost-deposition ring structure (at R ∼ 500 km from the vent) are in agreement with observations. These comparisons partially validate the modeling. It is suggested that an observation with spatial resolution of less than 30 km is needed to measure the large spatial variation of SO2 near a Pele-type plume center. It is also found that an influx of 1.1 × 1029 SO2 s−1 (or 1.1 × 104 kg s−1) is sufficient to reproduce the observed SO2 column density at Pele. The simulation results also show some interesting features such as a multiple bounce shock structure around Prometheus-type plumes and the frost depletion by plume-induced erosion on the sunlit side of Io. The model predicts the existence of a canopy shock, a ballistic region inside the Pele-type plume, and the negligible effect of surface heating by plume emission.

Experimental and numerical determination of micropropulsion device efficiencies at low Reynolds numbers

Abstract : The need for low thrust propulsion systems for maneuvers on micro- and nano-spacecraft 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 nozzles performance. At low Reynolds numbers, improvements to the specific impulse on the order of 4 to 8% were achieved through a combination of decreasing the nozzle length and increasing the nozzle expansion angle relative to the nominal experimental geometry.

Performance Analysis of Microthrusters Based on Coupled Thermal-Fluid Modeling and Simulation

Gas flow and performance characteristics of a high-temperature micro-electronically machined systems (MEMS)-based thruster are studied using a coupled thermal-fluid analysis. The material thermal response governed by the transient-heat-conduction equation is obtained by the finite element method. The low-Reynolds number gas flow in the microthruster is modeled by the direct simulation Monte Carlo approach. The effects of Reynolds number, thermal boundary conditions, and micronozzle height are considered in detail. The predicted thrust and mass-discharge coefficient of the three-dimensional microthruster under different flow conditions decrease with time as the viscous losses increase for higher wall temperatures.

Numerical modeling of axisymmetric and three-dimensional flows in MEMS nozzles

A numerical study of three-dimensional effects on the performance of a micronozzle fabricated from flat silicon wafer is performed by both continuum and kinetic approaches. The nozzle operates in a low Reynolds numbers regime and viscous effects dominate the gas expansion. Thrust losses occur because the shear on the wall is greater in the nozzle of a flat configuration compared to an axisymmetric conical nozzle. Therefore, the prediction of the micronozzle performance based on axisymmetric or two-dimension al modeling can lead to significant design errors.

Numerical Simulation of High-Temperature Gas Flows in a Millimeter-Scale Thruster

High-temperature nozzle flows at low Reynolds numbers are studied numerically by the direct simulation Monte Carlo method. Modeling results are compared with the experimental data on the specific impulse efficiency of a heated nitrogen flow at Re = 1.78 X 10 2 -4.09 x 102. Good agreement between modeling and data was observed for nonadiabatic wall conditions. The relative influence of three major thrust loss factors-flow divergence, surface friction, and heat transfer in axisymmetric and three-dimensional nozzles-is estimated for stagnation temperatures of 300, 1000, and 2000 K and Re = 2.05 x 10 2 . For a stagnation temperature of 1000 K, the specific impulse is 50% larger than in the cold gas case (300 K), whereas the efficiency is 10% lower as a result of heat-transfer losses of the same magnitude as friction losses. Axisymmetric conical nozzle thrust performance was studied for a hydrogen-air propellant over a range of Re=2 ( 10 2 -2 x 10 3 . It is found that vibrational relaxation could be a significant factor in the simulation of such flows.

Numerical Analysis of Shock Wave Reflection Transition in Steady Flows

Different aspects of the transition between regular and Mach reflections of strong shock waves in steady flows are numerically studied. Two approaches-kinetic (the direct simulation Monte Carlo method) and continuum (Euler equations)-are used to investigate the hysteresis phenomenon in the flow about two symmetrical wedges in two- and three-dimensional statements. The dependence of the final shock wave configuration on initial conditions, the transition from regular to Mach reflection by means of flow perturbations, and three-dimensional effects are examined. The three-dimensionality of the flow is shown to increase the angles of transition from regular to Mach reflection and back and to decrease the Mach stem height

Measurements and computations of mass flow and momentum flux through short tubes in rarefied gases

Gas flows through orifices and short tubes have been extensively studied from the 1960s through the 1980s for both fundamental and practical reasons. These flows are a basic and often important element of various modern gas driven instruments. Recent advances in micro- and nanoscale technologies have paved the way for a generation of miniaturized devices in various application areas, from clinical analyses to biochemical detection to aerospace propulsion. The latter is the main area of interest of this study, where rarefied gas flow into a vacuum through short tubes with thickness-to-diameter ratios varying from 0.015 to 1.2 is investigated both experimentally and numerically with kinetic and continuum approaches. Helium and nitrogen gases are used in the range of Reynolds numbers from 0.02 to 770 (based on the tube diameter), corresponding to Knudsen numbers from 40 down to about 0.001. Propulsion properties of relatively thin and thick tubes are examined. Good agreement between experimental and numerica...