John Robert Torczynski

Direct Simulation Monte Carlo: The Quest for Speed.

In the 50 years since its invention, the acceptance and applicability of the DSMC method have increased significantly. Extensive verification and validation efforts have led to its greater acceptance, whereas the increase in computer speed has been the main factor behind its greater applicability. As the performance of a single processor reaches its limit, massively parallel computing is expected to play an even stronger role in its future development.

Measurements of thermal accommodation coefficients.

A previously-developed experimental facility has been used to determine gas-surface thermal accommodation coefficients from the pressure dependence of the heat flux between parallel plates of similar material but different surface finish. Heat flux between the plates is inferred from measurements of temperature drop between the plate surface and an adjacent temperature-controlled water bath. Thermal accommodation measurements were determined from the pressure dependence of the heat flux for a fixed plate separation. Measurements of argon and nitrogen in contact with standard machined (lathed) or polished 304 stainless steel plates are indistinguishable within experimental uncertainty. Thus, the accommodation coefficient of 304 stainless steel with nitrogen and argon is estimated to be 0.80 {+-} 0.02 and 0.87 {+-} 0.02, respectively, independent of the surface roughness within the range likely to be encountered in engineering practice. Measurements of the accommodation of helium showed a slight variation with 304 stainless steel surface roughness: 0.36 {+-} 0.02 for a standard machine finish and 0.40 {+-} 0.02 for a polished finish. Planned tests with carbon-nanotube-coated plates will be performed when 304 stainless-steel blanks have been successfully coated.

Accuracy and Convergence of a New DSMC Algorithm

The accuracy and convergence of a new Direct Simulation Monte Carlo (DSMC) method, termed “sophisticated DSMC”, are investigated for one-dimensional combined CouetteFourier flow. An argon-like hard-sphere gas at 273.15 K and 266.644 Pa is confined between two parallel, fully accommodating walls 1 mm apart that have unequal temperatures and unequal tangential velocities. The simulations are performed using a one-dimensional implementation of the sophisticated DSMC algorithm. The primary convergence metrics studied, in harmony with previous work, are the ratios of the DSMC-calculated thermal conductivity and viscosity to their corresponding infinite-approximation Chapman-Enskog theoretical values. As discretization errors are reduced, both the traditional and the sophisticated DSMC algorithms are shown to approach the theoretical values to high precision. The convergence behavior of sophisticated DSMC is compared to that of traditional DSMC. The new algorithm is shown to significantly reduce the computational resources required for a DSMC simulation to achieve a particular level of accuracy. The advantages of using the new algorithm are quantified by comparing the two methods for two real-world applications: supersonic flow over a wedge and heat transfer around a microbeam.

Validation of Thermal Models for a Prototypical MEMS Thermal Actuator

This report documents technical work performed to complete the ASC Level 2 Milestone 2841: validation of thermal models for a prototypical MEMS thermal actuator. This effort requires completion of the following task: the comparison between calculated and measured temperature profiles of a heated stationary microbeam in air. Such heated microbeams are prototypical structures in virtually all electrically driven microscale thermal actuators. This task is divided into four major subtasks. (1) Perform validation experiments on prototypical heated stationary microbeams in which material properties such as thermal conductivity and electrical resistivity are measured if not known and temperature profiles along the beams are measured as a function of electrical power and gas pressure. (2) Develop a noncontinuum gas-phase heat-transfer model for typical MEMS situations including effects such as temperature discontinuities at gas-solid interfaces across which heat is flowing, and incorporate this model into the ASC FEM heat-conduction code Calore to enable it to simulate these effects with good accuracy. (3) Develop a noncontinuum solid-phase heat transfer model for typical MEMS situations including an effective thermal conductivity that depends on device geometry and grain size, and incorporate this model into the FEM heat-conduction code Calore to enable it to simulate these effects withmorexa0» good accuracy. (4) Perform combined gas-solid heat-transfer simulations using Calore with these models for the experimentally investigated devices, and compare simulation and experimental temperature profiles to assess model accuracy. These subtasks have been completed successfully, thereby completing the milestone task. Model and experimental temperature profiles are found to be in reasonable agreement for all cases examined. Modest systematic differences appear to be related to uncertainties in the geometric dimensions of the test structures and in the thermal conductivity of the polycrystalline silicon test structures, as well as uncontrolled nonuniform changes in this quantity over time and during operation.«xa0less

Assessment of reaction-rate predictions of a collision-energy approach for chemical reactions in atmospheric flows.

A recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates is assessed for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary non-equilibrium conditions. DSMC non-equilibrium reaction rates are compared to Park’s phenomenological nonequilibrium reaction-rate model, the predominant model for hypersonic-flow-field calculations. For near-equilibrium conditions, Park’s model is in good agreement with the DSMC-calculated reaction rates. For far-from-equilibrium conditions, corresponding to a typical shock layer, significant differences can be found. The DSMC predictions are also found to be in very good agreement with measured and calculated non-equilibrium reaction rates, offering strong evidence that this is a viable and reliable technique to predict chemical reaction rates.

Computational investigation of thermal gas separation for CO2 capture.

This report summarizes the work completed under the Laboratory Directed Research and Development (LDRD) project 09-1351, Computational Investigation of Thermal Gas Separation for CO{sub 2} Capture. Thermal gas separation for a binary mixture of carbon dioxide and nitrogen is investigated using the Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics. Molecular models for nitrogen and carbon dioxide are developed, implemented, compared to theoretical results, and compared to several experimental thermophysical properties. The molecular models include three translational modes, two fully excited rotational modes, and vibrational modes, whose degree of excitation depends on the temperature. Nitrogen has one vibrational mode, and carbon dioxide has four vibrational modes (two of which are degenerate). These models are used to perform a parameter study for mixtures of carbon dioxide and nitrogen confined between parallel walls over realistic ranges of gas temperatures and nominal concentrations of carbon dioxide. The degree of thermal separation predicted by DSMC is slightly higher than experimental values and is sensitive to the details of the molecular models.

Investigation of oil injection into brine for the Strategic Petroleum Reserve : hydrodynamics and mixing experiments with SPR liquids.

An experimental program was conducted to study a proposed approach for oil reintroduction in the Strategic Petroleum Reserve (SPR). The goal was to assess whether useful oil is rendered unusable through formation of a stable oil-brine emulsion during reintroduction of degassed oil into the brine layer in storage caverns. An earlier report (OHern et al., 2003) documented the first stage of the program, in which simulant liquids were used to characterize the buoyant plume that is produced when a jet of crude oil is injected downward into brine. This report documents the final two test series. In the first, the plume hydrodynamics experiments were completed using SPR oil, brine, and sludge. In the second, oil reinjection into brine was run for approximately 6 hours, and sampling of oil, sludge, and brine was performed over the next 3 months so that the long-term effects of oil-sludge mixing could be assessed. For both series, the experiment consisted of a large transparent vessel that is a scale model of the proposed oil-injection process at the SPR. For the plume hydrodynamics experiments, an oil layer was floated on top of a brine layer in the first test series and on top of a sludgemorexa0» layer residing above the brine in the second test series. The oil was injected downward through a tube into the brine at a prescribed depth below the oil-brine or sludge-brine interface. Flow rates were determined by scaling to match the ratio of buoyancy to momentum between the experiment and the SPR. Initially, the momentum of the flow produces a downward jet of oil below the tube end. Subsequently, the oil breaks up into droplets due to shear forces, buoyancy dominates the flow, and a plume of oil droplets rises to the interface. The interface was deflected upward by the impinging oil-brine plume. Videos of this flow were recorded for scaled flow rates that bracket the equivalent pumping rates in an SPR cavern during injection of degassed oil. Image-processing analyses were performed to quantify the penetration depth and width of the oil jet. The measured penetration depths were shallow, as predicted by penetration-depth models, in agreement with the assumption that the flow is buoyancy-dominated, rather than momentum-dominated. The turbulent penetration depth model overpredicted the measured values. Both the oil-brine and oil-sludge-brine systems produced plumes with hydrodynamic characteristics similar to the simulant liquids previously examined, except that the penetration depth was 5-10% longer for the crude oil. An unexpected observation was that centimeter-size oil bubbles (thin oil shells completely filled with brine) were produced in large quantities during oil injection. The mixing experiments also used layers of oil, sludge, and brine from the SPR. Oil was injected at a scaled flow rate corresponding to the nominal SPR oil injection rates. Injection was performed for about 6 hours and was stopped when it was evident that brine was being ingested by the oil withdrawal pump. Sampling probes located throughout the oil, sludge, and brine layers were used to withdraw samples before, during, and after the run. The data show that strong mixing caused the water content in the oil layer to increase sharply during oil injection but that the water content in the oil dropped back to less than 0.5% within 16 hours after injection was terminated. On the other hand, the sediment content in the oil indicated that the sludge and oil appeared to be well mixed. The sediment settled slowly but the oil had not returned to the baseline, as-received, sediment values after approximately 2200 hours (3 months). Ash content analysis indicated that the sediment measured during oil analysis was primarily organic.«xa0less

Direct simulation Monte Carlo investigation of hydrodynamic instabilities in gases

The Rayleigh-Taylor instability (RTI) is investigated using the Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics. Here, two-dimensional and three-dimensional DSMC RTI simulations are performed to quantify the growth of flat and single-mode-perturbed interfaces between two atmospheric-pressure monatomic gases. The DSMC simulations reproduce all qualitative features of the RTI and are in reasonable quantitative agreement with existing theoretical and empirical models in the linear, nonlinear, and self-similar regimes. At late times, the instability is seen to exhibit a self-similar behavior, in agreement with experimental observations. For the conditions simulated, diffusion can influence the initial instability growth significantly.

The Effect of Internal Energy on Chemical Reaction Rates as Predicted by Bird?s Quantum-Kinetic Model.

The effect of non-equilibrium internal energy excitation on the reaction rates predicted by Birds 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.

Steady Isothermal Gas Mass Flow Rate in a Microscale Tube from Continuum to Free-Molecular Conditions

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.