Robert J. Collins

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.

Examination of theory for bow shock ultraviolet rocket experiments—II

In this article the radiation model, NEQAIR, used to calculate the mid-uv radiance for Bow Shock 1 and 2 flight conditions, is examined in detail. An approximate equation for the upper state NO population is derived that explains the density dependence of the calculations and sensitivity to changes in specific rates. Estimated neutral collisional excitation rates are replaced with rates based on experiment. For conditions corresponding to the two flights, the radiation obtained with an explicit treatment of the electronic excitation process was similar to that obtained from the original model. For the first time, we also test the validity of the quasi-steadystate assumption in these rarefied flows. We find that the quasi-steady-state distribution of electronic states of NO is valid for these flow conditions.

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.

Measurements of ultraviolet radiation from a 5-km/s bow shock

Ultraviolet emission from a 5.1-km/s re-entry bow shock was measured in a sounding rocket experiment launched from the Barking Sands Research Range (Kauai, Hawaii) in February 1991 at 14:30 GMT. Optical data were obtained on the downleg portion of the flight as the pay load descended from 115 to 62 km in a very shallow trajectory at a nearly constant speed. The intensity of the ultraviolet spectrum (A200-400 nm), and the vacuum ultraviolet resonance radiation emitted by atomic oxygen and hydrogen at A130.4 nm and A121.5 nm, respectively, were measured. Data from optical instruments in the 200-400-nm spectral region is presented here. Langmuir probe measurements provided data on the total plasma density and electron temperature in the boundary layer over a limited altitude range.

Comparison of theory with experiment for the bow shock ultraviolet rocket flight

Comparison is made between the results obtained from a state-of-the-art thermochemical nonequilibrium flowfield and radiation code and data obtained from a recent experiment. The experiment obtained the first measurements of ultraviolet radiation from the shock-heated gas in the nose region of a 0.1016-m nose radius vehicle traveling at about 3.5 km/s at altitudes between 37-75 km. The preflight computations agree at low altitudes but underpredict the data at high altitudes. Postflight flowfield and radiation sensitivity studies suggest improvements for the models at high altitudes. Specifically, excitation mechanisms that contribute to production of NO gamma-band emission need to be revised. Altitude dependence of the radiation observed from the OH radical can be understood in terms of nonequilibrium chemistry in the flow.

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.

Flight measurements of low-velocity bow shock ultraviolet radiation

The ultraviolet spectrum, atomic oxygen 130.4-nm radiation intensity, total plasma density, and electron temperature of a Mach 12 bow shock were obtained by a sounding rocket experiment launched from the Wallops Flight Facility (WFF) on April 25, 1990 at 12:32 a.m. Eastern Standard Time (EST). A two-stage, Terrier Malamute rocket which attained an apogee of 720 km was used in this experiment. Optical data in the 200400-nm wavelength range were obtained from 37 to 75 km at a vehicle velocity of 3.5 km/s at various locations on the 0.1016-m radius hemispherical dome. Electron probe and VUV OI 130.4-nm measurements were obtained near nose cone ejection at 37 km. This article presents a discussion of the instruments used and the key data obtained.

Modeling of glow radiation in the rarefied flow about an orbiting spacecraft

An approach for modeling visible glow radiation about a spacecraft in low Earth orbit has been examined. A new technique for simulation of surface chemical reactions based on the direct simulation Monte Carlo method is used. The study focuses on the sensitivity of glow radiation to the gas-phase reaction model and surface reaction cross sections in the altitude range from 140 to 200 km. Comparison of predictions for different gas reaction cross sections and surface parameters is given with the Atmospheric Explorer data. It is shown that although the radiance is increased by a factor of two when a quasi-classical model is used, the altitude dependence of the predicted radiation is the same as that obtained using the total collisional energy model. Furthermore, it is found that the ine uence of the freestream NO concentration on the total radiation is small for altitudes up to 200 km. The main contribution is the formation of NO in bow-shock gas-phase reactions. Nomenclature A = preexponential factor in the Arrhenius expression B = temperature exponent in the Arrhenius expression Ea = activation energy Fnum = ratio of molecules to simulated particles fi = number e ux of the ith species Hs = heat of absorption I = radiation intensity k = Boltzmann constant, chemical rate constant kb = desorption rate M = chemical species Ni = number of collisions of species i with species j n = number density nT = total number of surface sites P = probability S = vacant surface site So = sticking coefe cient T = temperature v = relative velocity, gas velocity Wk = weighting factor of the kth species Zij = collision frequency of species i with j D t = time step r f 3, f 4 = cross sections for glow production r T = total collision cross section s l = radiation lifetime Subscript S = surface adsorbed species Superscript ¤ = electronically excited state

Examination of OH ultraviolet radiation from shock-heated air

Two recent sounding rocket experiments obtained spectral data at wavelengths of 200-400 nm from the shock-heated air surrounding a vehicle under flight conditions of 3.5 km/s at altitudes of 40-70 km and 5 km/s for altitudes of 110-65 km. Previous analyses of the data have emphasized modeling the radiation from the NO molecular system. The chemical kinetics of OH, a trace species in the flow, and the electronic state excitation mechanisms are simpler than those for NO. Hence, the comparison between modeling and data potentially provides a clearer assessment of the modeling of the flow thermochemical processes. This article discusses the OH flowfield and radiation models that we have developed, comparisons with data, and the implications of this work to ongoing NO flow and radiation modeling. Additional data from the side-viewing spectrometers from the 5-km/s flight will also be presented and analyzed. The angular dependence of the photometer data was compared with a solution obtained from a viscous threedimensional flowfield calculation with reacting water chemistry.