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Micromachines 2019, 10(2), 118; https://doi.org/10.3390/mi10020118

Modeling of Knudsen Layer Effects in the Micro-Scale Backward-Facing Step in the Slip Flow Regime

Department of Mechanical and Aerospace Engineering, Indian Institute of Technology, Hyderabad, Kandi, Medak 502285, India
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Received: 27 December 2018 / Revised: 25 January 2019 / Accepted: 2 February 2019 / Published: 12 February 2019
(This article belongs to the Special Issue Gas Flows in Microsystems)
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Abstract

The effect of the Knudsen layer in the thermal micro-scale gas flows has been investigated. The effective mean free path model has been implemented in the open source computational fluid dynamics (CFD) code, to extend its applicability up to slip and early transition flow regime. The conventional Navier-Stokes constitutive relations and the first-order non-equilibrium boundary conditions are modified based on the effective mean free path, which depends on the distance from the solid surface. The predictive capability of the standard ‘Maxwell velocity slip—Smoluchwoski temperature jump’ and hybrid boundary conditions ‘Langmuir Maxwell velocity slip—Langmuir Smoluchwoski temperature jump’ in conjunction with the Knudsen layer formulation has been evaluated in the present work. Simulations are carried out over a nano-/micro-scale backward facing step geometry in which flow experiences adverse pressure gradient, separation and re-attachment. Results are validated against the direct simulation Monte Carlo (DSMC) data, and have shown significant improvement over the existing CFD solvers. Non-equilibrium effects on the velocity and temperature of gas on the surface of the backward facing step channel are studied by varying the flow Knudsen number, inlet flow temperature, and wall temperature. Results show that the modified solver with hybrid Langmuir based boundary conditions gives the best predictions when the Knudsen layer is incorporated, and the standard Maxwell-Smoluchowski can accurately capture momentum and the thermal Knudsen layer when the temperature of the wall is higher than the fluid flow. View Full-Text
Keywords: rarefied gas flows; micro-scale flows; Knudsen layer; computational fluid dynamics (CFD); OpenFOAM; Micro-Electro-Mechanical Systems (MEMS); Nano-Electro-Mechanical Systems (NEMS); backward facing step rarefied gas flows; micro-scale flows; Knudsen layer; computational fluid dynamics (CFD); OpenFOAM; Micro-Electro-Mechanical Systems (MEMS); Nano-Electro-Mechanical Systems (NEMS); backward facing step
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Bhagat, A.; Gijare, H.; Dongari, N. Modeling of Knudsen Layer Effects in the Micro-Scale Backward-Facing Step in the Slip Flow Regime. Micromachines 2019, 10, 118.

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