Publications

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The effect of non-equilibrium internal energy excitation on the reaction rates predicted by Bird's Quantum-Kinetic (Q-K) model for dissociation and exchange reactions is analyzed. The effect of vibrational non-equilibrium is treated explicitly by the Q-K model. The effect of rotational non-equilibrium is introduced as a perturbation to the effect of vibrational non-equilibrium in chemical reactions. For dissociation reactions, a small but measurable improvement in the rates is observed. For exchange reactions, the change is negligible. These findings are in agreement with experimental observations and theoretical predictions. The results from one-dimensional stagnation-streamline and two-dimensional axi-symmetric DSMC code implementations of the original and modified Q-K models are compared for a typical re-entry flow. The influence of rotational non-equilibrium in promoting chemical reactions is seen to be small for this type of flow. © 2012 American Institute of Physics.

The effect of non-equilibrium internal energy excitation on the reaction rates predicted by Bird's Quantum-Kinetic (Q-K) model for dissociation and exchange reactions is analyzed. The effect of vibrational non-equilibrium is treated explicitly by the Q-K model. The effect of rotational non-equilibrium is introduced as a perturbation to the effect of vibrational non-equilibrium in chemical reactions. For dissociation reactions, a small but measurable improvement in the rates is observed. For exchange reactions, the change is negligible. These findings are in agreement with experimental observations and theoretical predictions. The results from one-dimensional stagnation-streamline and two-dimensional axi-symmetric DSMC code implementations of the original and modified Q-K models are compared for a typical re-entry flow. The influence of rotational non-equilibrium in promoting chemical reactions is seen to be small for this type of flow. © 2012 American Institute of Physics.

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The Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics is used to simulate the steady flow of an ideal gas through a long thin isothermal microscale tube connecting two infinite reservoirs at different pressures. The tube wall is at the reservoir temperature, and molecules reflect from the walls according to the Maxwell model (i.e., a linear combination of specular reflections and diffuse reflections at the wall temperature). The computed mass flow rates approach the known expressions in the near-continuum and free-molecular regimes and agree reasonably with recent experimental measurements in microscale tubes and channels. Approximate closed-form expressions for the mass flow rate and the pressure profile along the tube are developed and are in reasonable agreement with the DSMC results in all regimes and for all values of the accommodation coefficient. © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

The Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics is used to simulate the steady flow of an ideal gas through a long thin isothermal microscale tube connecting two infinite reservoirs at different pressures. The tube wall is at the reservoir temperature, and molecules reflect from the walls according to the Maxwell model (i.e., a linear combination of specular reflections and diffuse reflections at the wall temperature). The computed mass flow rates approach the known expressions in the near-continuum and free-molecular regimes and agree reasonably with recent experimental measurements in microscale tubes and channels. Approximate closed-form expressions for the mass flow rate and the pressure profile along the tube are developed and are in reasonable agreement with the DSMC results in all regimes and for all values of the accommodation coefficient. © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

Moving-boundary algorithms for the Direct Simulation Monte Carlo (DSMC) method are investigated for a microbeam that moves toward and away from a parallel substrate. The simpler but analogous one-dimensional situation of a piston moving between two parallel walls is investigated using two moving-boundary algorithms. In the first, molecules are reflected rigorously from the moving piston by performing the reflections in the piston frame of reference. In the second, molecules are reflected approximately from the moving piston by moving the piston and subsequently moving all molecules and reflecting them from the moving piston at its new or old position. © 2011 American Institute of Physics.

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An experimental apparatus has been developed to determine thermal accommodation coefficients for a variety of gas-surface combinations. Results are obtained primarily through measurement of the pressure dependence of the conductive heat flux between parallel plates separated by a gas-filled gap. Measured heat-flux data are used in a formula based on Direct Simulation Monte Carlo (DSMC) simulations to determine the coefficients. The assembly also features a complementary capability for measuring the variation in gas density between the plates using electron-beam fluorescence. Surface materials examined include 304 stainless steel, gold, aluminum, platinum, silicon, silicon nitride, and polysilicon. Effects of gas composition, surface roughness, and surface contamination have been investigated with this system; the behavior of gas mixtures has also been explored. Without special cleaning procedures, thermal accommodation coefficients for most materials and surface finishes were determined to be near 0.95, 0.85, and 0.45 for argon, nitrogen, and helium, respectively. Surface cleaning by in situ argon-plasma treatment reduced coefficient values by up to 0.10 for helium and by ∼0.05 for nitrogen and argon. Results for both single-species and gas-mixture experiments compare favorably to DSMC simulations. © 2011 American Institute of Physics.

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The effect of collision-partner selection schemes on the accuracy and the efficiency of the Direct Simulation Monte Carlo (DSMC) method of Bird is investigated. Several schemes to reduce the total discretization error as a function of the mean collision separation and the mean collision time are examined. These include the historically first sub-cell scheme, the more recent nearest-neighbor scheme, and various near-neighbor schemes, which are evaluated for their effect on the thermal conductivity for Fourier flow. Their convergence characteristics as a function of spatial and temporal discretization and the number of simulators per cell are compared to the convergence characteristics of the sophisticated and standard DSMC algorithms. Improved performance is obtained if the population from which possible collision partners are selected is an appropriate fraction of the population of the cell.

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A bubble in an acoustic field experiences a net 'Bjerknes' force from the nonlinear coupling of its radial oscillations with the oscillating buoyancy force. It is typically assumed that the bubble's net terminal velocity can be found by considering a spherical bubble with the imposed 'Bjerknes stresses'. We have analyzed the motion of such a bubble using a rigorous perturbation approach and found that one must include a term involving an effective mass flux through the bubble that arises from the time average of the second-order nonlinear terms in the kinematic boundary condition. The importance of this term is governed by the dimensionless parameter {alpha} = R{sup 2} {phi}/R{sup 2} {phi} {nu}.-{nu}, where R is the bubble radius, {phi} is the driving frequency, and {nu} is the liquid kinematic viscosity. If {alpha} is large, this term is unimportant, but if {alpha} is small, this term is the dominant factor in determining the terminal velocity.

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We will present experimental and computational investigations of the thermal performance of microelectromechanical systems (MEMS) as a function of the surrounding gas pressure. Lowering the pressure in MEMS packages reduces gas damping, providing increased sensitivity for certain MEMS sensors; however, such packaging also dramatically affects their thermal performance since energy transfer to the environment is substantially reduced. High-spatial-resolution Raman thermometry was used to measure the temperature profiles on electrically heated, polycrystalline silicon bridges that are nominally 10 microns wide, 2.25 microns thick, 12 microns above the substrate, and either 200 or 400 microns long in nitrogen atmospheres with pressures ranging from 0.05 to 625 Torr. Finite element modeling of the thermal behavior of the MEMS bridges is performed and compared to the experimental results. Noncontinuum gas effects are incorporated into the continuum finite element model by imposing temperature discontinuities at gas-solid interfaces that are determined from noncontinuum simulations. The experimental and simulation results indicate that at pressures below 0.5 Torr the gas-phase heat transfer is negligible compared to heat conduction through the thermal actuator legs. As the pressure increases above 0.5 Torr, the gas-phase heat transfer becomes more significant. At ambient pressures, gas-phase heat transfer drastically impacts the thermal performance. The measured and simulated temperature profiles are in qualitative agreement in the present study. Quantitative agreement between experimental and simulated temperature profiles requires accurate knowledge of temperature-dependent thermophysical properties, the device geometry, and the thermal accommodation coefficient.

Thermal accommodation coefficients have been derived for a variety of gas-surface combinations using an experimental apparatus developed to measure the pressure dependence of the conductive heat flux between parallel plates at unequal temperature separated by a gas-filled gap. The heat flux is inferred from temperature-difference measurements across the plates in a configuration where the plate temperatures are set with two carefully controlled thermal baths. Temperature-controlled shrouds provide for environmental isolation of the opposing test plates. Since the measured temperature differences in these experiments are very small (typically 0.3 C or less over the entire pressure range), high-precision thermistors are used to acquire the requisite temperature data. High-precision components have also been utilized on the other control and measurement subsystems in this apparatus, including system pressure, gas flow rate, plate alignment, and plate positions. The apparatus also includes the capability for in situ plasma cleaning of the installed test plates. Measured heat-flux results are used in a formula based on Direct Simulation Monte Carlo (DSMC) code calculations to determine the thermal accommodation coefficients. Thermal accommodation coefficients have been determined for three different gases (argon, nitrogen, helium) in contact with various surfaces. Materials include metals and alloys such as aluminum, gold, platinum, and 304 stainless steel. A number of materials important to fabrication of Micro Electro Mechanical Systems (MEMS) devices have also been examined. For most surfaces, coefficient values are near 0.95, 0.85, and 0.45 for argon, nitrogen, and helium, respectively. Only slight differences in accommodation as a function of surface roughness have been seen. Surface contamination appears to have a more significant effect: argon plasma treatment has been observed to reduce thermal accommodation by as much as 0.10 for helium. Mixtures of argon and helium have also been examined, and the results have been compared to DSMC simulations incorporating thermal-accommodation values from single-species experiments.