Bhagat, A and Gijare, H and Dongari, Nishanth
(2019)
Modeling of Knudsen Layer Effects in the Micro-Scale Backward-Facing Step in the Slip Flow Regime.
Micromachines, 10 (2).
pp. 1-15.
ISSN 2072-666X
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.
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