Modeling and Preliminary Experiment for Rarefied Gas Flows in Constricted Microchannels

Gaseous slip flows in 3D rectangular microchannels with constrictions have been study numerically, and the experiment using pressure-sensitive-paints (PSP) for polymer microchannel pressure measurements are proposed. Constrictions inside microchannels, either being manufacturing defects or functional design features such as micro-orifices or micro-nozzles, will change the flow pattern because of additional frictional resistance and flow separation. In current research, mass-flowrate reduction due to constrictions has been investigated numerically for air flows in the slip regime, where Knudsen number ranges from 0.003 to 0.07. The results have been compared with both straight microchannels and with 3D analytical solutions. Similar to nozzle cases at macroscale, chocked flows has been observed at the critical pressure ratio of about 1.89. A numerical model including finite inlet and outlet chambers has been used in simulations to evaluate effects of reflection waves. Slip effects have been studied for different accommodation coefficients in presence of constrictions. By implementing multi-species numerical models, thermal induced mass transport has also been studied. Preliminary experiment based on PSP measurement for polymer microchannels has able to generate high spatial resolution pressure data, which are comparable with numerical simulations. Finally, further improvement of experimental setup is discussed.

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Velocity Measurements in Channel Gas Flows in the Slip Regime by means of Molecular Tagging Velocimetry

Micromachines ◽

10.3390/mi11040374 ◽

2020 ◽

Vol 11 (4) ◽

pp. 374 ◽

Cited By ~ 1

Author(s):

Dominique Fratantonio ◽

Marcos Rojas-Cárdenas ◽

Christine Barrot ◽

Lucien Baldas ◽

Stéphane Colin

Keyword(s):

Slip Velocity ◽

Rarefied Gas ◽

Numerical Models ◽

Slip Flow ◽

Diagnostic Methods ◽

Gas Flows ◽

Reconstruction Method ◽

Molecular Tagging Velocimetry ◽

Molecular Tagging ◽

Slip Regime

Direct measurements of the slip velocity in rarefied gas flows produced by local thermodynamic non-equilibrium at the wall represent crucial information for the validation of existing theoretical and numerical models. In this work, molecular tagging velocimetry (MTV) by direct phosphorescence is applied to argon and helium flows at low pressures in a 1-mm deep channel. MTV has provided accurate measurements of the molecular displacement of the gas at average pressures of the order of 1 kPa. To the best of our knowledge, this work reports the very first flow visualizations of a gas in a confined domain and in the slip flow regime, with Knudsen numbers up to 0.014. MTV is cross-validated with mass flowrate measurements by the constant volume technique. The two diagnostic methods are applied simultaneously, and the measurements in terms of average velocity at the test section are in good agreement. Moreover, preliminary results of the slip velocity at the wall are computed from the MTV data by means of a reconstruction method.

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Slip Flow in Rectangular and Annular Ducts

Journal of Basic Engineering ◽

10.1115/1.3650793 ◽

1965 ◽

Vol 87 (4) ◽

pp. 1018-1024 ◽

Cited By ~ 115

Author(s):

W. A. Ebert ◽

E. M. Sparrow

Keyword(s):

Pressure Drop ◽

Rarefied Gas ◽

Slip Flow ◽

Velocity Slip ◽

Density Level ◽

Axial Pressure ◽

Slip Regime ◽

Rarefied Gas Flows ◽

Axial Pressure Gradient ◽

Viscous Shear

An analysis has been performed to determine the velocity and pressure-drop characteristics of moderately rarefied gas flows in rectangular and annular ducts. The density level is such that a velocity slip may occur at the duct walls. In general, it is found that the effect of slip is to flatten the velocity distribution relative to that for a continuum flow; furthermore, the axial pressure gradient is diminished under slip-flow conditions. The conditions characterizing the onset of the slip regime have been determined on the basis of a 2 percent reduction in friction factor relative to the continuum value. For all the geometries studied here, the onset of slip occurred at a Knudsen number of 0.003. The effect of compressibility on the axial pressure drop was also investigated. It was found that compressibility increases the pressure drop primarily through an increase in viscous shear rather than through an increase in momentum flux.

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Establishing a data-based scattering kernel model for gas–solid interaction by molecular dynamics simulation

Journal of Fluid Mechanics ◽

10.1017/jfm.2021.828 ◽

2021 ◽

Vol 928 ◽

Author(s):

Zijing Wang ◽

Chengqian Song ◽

Fenghua Qin ◽

Xisheng Luo

Keyword(s):

Machine Learning ◽

Molecular Dynamics ◽

Md Simulation ◽

Rarefied Gas ◽

Interaction Model ◽

Md Simulations ◽

Dynamics Simulation ◽

Rarefied Gas Flows ◽

Accommodation Coefficients ◽

Scattering Kernel

Scattering kernel models for gas–solid interaction are crucial for rarefied gas flows and microscale flows. However, most existing models depend on certain accommodation coefficients (ACs). We propose here to construct a data-based model using molecular dynamics (MD) simulation and machine learning. The gas–solid interaction is first modelled by 100 000 MD simulations of a single gas molecule reflecting on the wall surface, which is fulfilled by GPU parallel technology. The results showed a correlation of the reflection velocity with the incidence velocity in the same direction, and also revealed correlations that may exist in different directions, which are neglected by the traditional gas–solid interaction model. Inspired by the sophisticated Cercignani–Lampis–Lord (CLL) model, two improved scattering kernels were constructed to better reproduce the probability density of velocity determined from MD simulation. The first one adopts variable ACs which depend on the incidence velocity and the second one combines three CLL-like kernels. All the parameters in the improved kernels are automatically chosen by the machine learning method. Compared with the numerical experiments of a molecular beam, the reconstructed scattering kernels are basically consistent with the MD results.

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Modeling viscous effects on boundary layer of rarefied gas flows inside micronozzles in the slip regime condition

Energy Procedia ◽

10.1016/j.egypro.2018.08.113 ◽

2018 ◽

Vol 148 ◽

pp. 838-845 ◽

Cited By ~ 2

Author(s):

Maria Grazia De Giorgi ◽

Donato Fontanarosa ◽

Antonio Ficarella

Keyword(s):

Boundary Layer ◽

Rarefied Gas ◽

Gas Flows ◽

Viscous Effects ◽

Slip Regime ◽

Rarefied Gas Flows

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Numerical Models of Gas Flow and Heat Transfer in Microscale Channels: Capturing Rarefaction Behaviour Using a Constitutive Scaling Approach

ASME 5th International Conference on Nanochannels, Microchannels, and Minichannels ◽

10.1115/icnmm2007-30097 ◽

2007 ◽

Author(s):

Lynne O’Hare ◽

Jason M. Reese

Keyword(s):

Gas Flow ◽

Rarefied Gas ◽

Numerical Models ◽

Constitutive Relations ◽

Alternative Methods ◽

Navier Stokes ◽

Rarefied Gas Flows ◽

Scaling Models ◽

Flow Features ◽

Microsystem Design

In this paper we discuss the physics of rarefied gas flows at the micro-scale, including discontinuities of momentum and energy at solid boundaries, and the Knudsen layer (a region within one to two mean free paths of any solid surface where non-equilibrium flow features are dominant.) We describe how scaling the constitutive relations of the Navier-Stokes-Fourier equation set can capture key rarefaction behaviour observed in gas microsystems, and how a new implementation of this approach in fully compressible, non-isothermal CFD facilitates the analysis of “real-world” engineering problems. Details of our implementation are given, as are the results of a compressible Couette flow case study, successfully validated against available data sources. We also discuss the relative merits of two published constitutive scaling models, comparing their micro-flow predictions for half-space problems, and contrasting their individual means of application. Some practical implications of using constitutive-relation scaling are explained, and some advantages of the technique compared to alternative methods are outlined. To conclude, we examine some limitations of the method, and outline avenues of research that could potentially broaden the scope of what is a flexible and efficient approach to gas microsystem design using CFD.

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