Israel Borges Sebastiao

Low-pressure gas sensor exploiting the Knudsen thermal force: DSMC modeling and experimental validation

We present performance modeling and experimental validation of a novel MEMS vacuum gas sensor based on the Knudsen thermal force. Direct simulation Monte Carlo (DSMC) modeling of thermally-driven gas flow and force measurements show a non-monotonic dependence on ambient pressure, peaking at a Knudsen number on the order of unity. Combining force dependence on pressure with the monotonically varying heat transfer rate allows both ambient pressure and species concentration to be determined if the constituents are known. The DSMC modeling also shows that thermal gradients between the shuttle and heater induce complex vortical flow structures that could be applied to control mixing/separation in gas-phase microfluidic devices.

DSMC study of oxygen shockwaves based on high-fidelity vibrational relaxation and dissociation models

This work evaluates high-fidelity vibrational-translational (VT) energy relaxation and dissociation models for pure O2 normal shockwave simulations with the direct simulation Monte Carlo (DSMC) method. The O2–O collisions are described using ab initio state-specific relaxation and dissociation models. The Macheret-Fridman (MF) dissociation model is adapted to the DSMC framework by modifying the standard implementation of the total collision energy (TCE) model. The O2–O2 dissociation is modeled with this TCE+MF approach, which is calibrated with O2–O ab initio data and experimental equilibrium dissociation rates. The O2–O2 vibrational relaxation is modeled via the Larsen-Borgnakke model, calibrated to experimental VT rates. All the present results are compared to experimental data and previous calculations available in the literature. It is found that, in general, the ab initio dissociation model is better than the TCE model at matching the shock experiments. Therefore, when available, efficient ab initio ...

Maximum entropy modeling of vibrational-translational energy exchange in O2+O collisions

This work proposes a vibrational-translational energy exchange model based on quasiclassical trajectory (QCT) calculations of O2 + O and Maximum Entropy (ME) considerations. QCT calculations show that collisions favor small vibrational transitions. This favoring drops off first exponentially and then linearly as ∆v increases. Such trends are captured by the new ME-QCT-VT model. The model is verified by comparing its stateto-state cross sections and rates to those calculated directly by QCT. The state-to-state rates match within 30% using 11 fitted parameters for the O2 +O system that has approximately 3,000 ro-vibrational states. In addition, the implementation of the model in direct simulation Monte Carlo (DSMC) method is discussed. Adiabatic DSMC calculations show that the model satisfies detailed balance. The vibrational temperatures and distribution functions predicted by DSMC calculations with the ME-QCT-VT model match masterequation calculations with the complete set of state-to-state rates. The proposed model makes it feasible to upscale ab-initio simulation to DSMC and CFD calculations of a full flowfield.

Consistent post-reaction vibrational energy redistribution in DSMC simulations using TCE model

The direct simulation Monte Carlo (DSMC) method has been widely applied to study shockwaves, hypersonic reentry flows, and other nonequilibrium flow phenomena. Although there is currently active research on high-fidelity models based on ab initio data, the total collision energy (TCE) and Larsen-Borgnakke (LB) models remain the most often used chemistry and relaxation models in DSMC simulations, respectively. The conventional implementation of the discrete LB model, however, may not satisfy detailed balance when recombination and exchange reactions play an important role in the flow energy balance. This issue can become even more critical in reacting mixtures involving polyatomic molecules, such as in combustion. In this work, this important shortcoming is addressed and an empirical approach to consistently specify the post-reaction vibrational states close to thermochemical equilibrium conditions is proposed within the TCE framework. Following Bird’s quantum-kinetic (QK) methodology for populating post-r...

Comparison between phenomenological and ab-initio reaction and relaxation models in DSMC

New state-specific vibrational-translational energy exchange and dissociation models, based on ab-initio data, are implemented in direct simulation Monte Carlo (DSMC) method and compared to the established Larsen-Borgnakke (LB) and total collision energy (TCE) phenomenological models. For consistency, both the LB and TCE models are calibrated with QCT-calculated O2+O data. The model comparison test cases include 0-D thermochemical relaxation under adiabatic conditions and 1-D normal shockwave calculations. The results show that both the ME-QCT-VT and LB models can reproduce vibrational relaxation accurately but the TCE model is unable to reproduce nonequilibrium rates even when it is calibrated to accurate equilibrium rates. The new reaction model does capture QCT-calculated nonequilibrium rates. For all investigated cases, we discuss the prediction differences based on the new model features.

DSMC simulation of microstructure actuation by Knudsen thermal forces including binary mixtures

Complex and non-intuitive flow structures controlled only by thermal gradients can be observed in rarefied gas flows. A force of thermophoretic nature, often referred to as Knudsen or radiometric force, can become dominant in microflow applications. A Microelectromechanical In-plane Knudsen Radiometric Actuator (MIKRA) that exploits these forces has been developed and tested at Purdue. Previous efforts used DSMC to understand the MIKRA flow structure and validate numerical modeling for simple gases. This work investigated more realistic boundary conditions as well as species separation around the MIKRA beams for Xe-He and N2-H2O mixtures. The main goal of this work was to run DSMC simulations of the MIKRA sensor to not only understand how it can be accurately modeled, but to look at future applications for gas mixture sensing as well.

Microscale In-Plane Knudsen Radiometric Actuator: Design, Characterization, and Performance Modeling

Thermally driven gas flow is exploited in a microscale device for the purpose of gas pressure dependent actuation in rarefied environments. The device relies on the in-plane motion of a shuttle mass in response to Knudsen thermal forces. The shuttle response is measured capacitively and the magnitude is used to estimate ambient pressure. Combined with heating element resistance, these measurements, in principle, enable the simultaneous determination of gas composition if the constituents are known. Numerical simulations using direct simulation Monte Carlo are carried out to elucidate the mechanisms of force production. Results reveal highly vortical flow structures forming high pressure regions at the shuttle surface. The experimental and numerical results are expressed in terms of a non-dimensional Knudsen force coefficient, allowing force magnitude to be estimated for dynamically similar geometries. [2016-0228]