Alina Alexeenko

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.

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.

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.

Direct measurements and numerical simulations of gas charging in microelectromechanical system capacitive switches

Gas breakdown in microelectromechanical system capacitive switches is demonstrated using high resolution current measurements and by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations. Measurements show an electric current through a 3 μm air gap increasing exponentially with voltage, starting at 60 V. PIC/MCC simulations with Fowler-Nordheim [Proc. R. Soc. London, Ser. A 119, 173 (1928)] field emission reveal self-sustained discharges with significant ion enhancement and a positive space charge. The effective ion-enhanced field emission coefficient increases with voltage up to about 0.3 with an electron avalanche occurring at 159 V. The measurements and simulations demonstrate a charging mechanism for microswitches consistent with earlier observations of gas pressure and composition effects on lifetime.

Scaling law for direct current field emission-driven microscale gas breakdown

The effects of field emission on direct current breakdown in microscale gaps filled with an ambient neutral gas are studied numerically and analytically. Fundamental numerical experiments using the particle-in-cell/Monte Carlo collisions method are used to systematically quantify microscale ionization and space-charge enhancement of field emission. The numerical experiments are then used to validate a scaling law for the modified Paschen curve that bridges field emission-driven breakdown with the macroscale Paschen law. Analytical expressions are derived for the increase in cathode electric field, total steady state current density, and the ion-enhancement coefficient including a new breakdown criterion. It also includes the effect of all key parameters such as pressure, operating gas, and field-enhancement factor providing a better predictive capability than existing microscale breakdown models. The field-enhancement factor is shown to be the most sensitive parameter with its increase leading to a signif...

Compact model of squeeze-film damping based on rarefied flow simulations

A new compact model of squeeze-film damping is developed based on the numerical solution of the Boltzmann kinetic equation. It provides a simple expression for the damping coefficient and the quality factor valid through the slip, transitional and free-molecular regimes. In this work, we have applied statistical analysis to the current model using the chi-squared test. The damping predictions are compared with both Reynolds equation-based models and experimental data. At high Knudsen numbers, the structural damping dominates the gas squeeze-film damping. When the structural damping is subtracted from the measured total damping force, good agreement is found between the model predictions and the experimental data.

Experimental and Computational Studies of Temperature Gradient–Driven Molecular Transport in Gas Flows through Nano/Microscale Channels

Studies at the University of Southern California have shown that an unconventional solid-state device, the Knudsen compressor, can be operated as a microscale pump or compressor. The critical components of Knudsen compressors are gas transport membranes, which can be formed from porous materials or densely packed parallel arrays of channels. An applied temperature gradient across a transport membrane creates a thermal creep pumping action. Experimental and computational techniques that have been developed for the investigations will be discussed. Experimental studies of membranes formed from machined aerogels, activated by radiant heating, have been used to investigate thermal creep flows. In computational studies, several approaches have been employed: the direct simulation Monte Carlo (DSMC) method and discrete ordinate solutions of the ellipsoidal statistical (ES) and Bhatnagar-Gross-Krook (BGK) kinetic models. Beyond the study of Knudsen compressor performance, techniques discussed in this article could be used to characterize the properties of gas flows in nano/microscale channels.