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Rarefied flow separation in microchannel with bends

Published online by Cambridge University Press:  28 August 2020

Minh Tuan Ho Open the ORCID record for Minh Tuan Ho [Opens in a new window] ,
Jun Li Open the ORCID record for Jun Li [Opens in a new window] ,
Wei Su ,
Lei Wu Open the ORCID record for Lei Wu [Opens in a new window] ,
Matthew K. Borg ,
Zhihui Li  and
Yonghao Zhang Open the ORCID record for Yonghao Zhang [Opens in a new window]
Minh Tuan Ho
Affiliation:
James Weir Fluids Laboratory, Department of Mechanical and Aerospace Engineering, University of Strathclyde, GlasgowG1 1XJ, UK
Jun Li*
Affiliation:
Center for Integrative Petroleum Research, College of Petroleum Engineering and Geosciences, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
Wei Su
Affiliation:
James Weir Fluids Laboratory, Department of Mechanical and Aerospace Engineering, University of Strathclyde, GlasgowG1 1XJ, UK
Lei Wu
Affiliation:
Department of Mechanics and Aerospace Engineering, Southern University of Science and Technology, Shenzhen518055, PR China
Matthew K. Borg
Affiliation:
School of Engineering, University of Edinburgh, EdinburghEH9 3FB, UK
Zhihui Li
Affiliation:
Hypervelocity Aerodynamics Institute, China Aerodynamics Research and Development Center, Mianyang621000, PR China
Yonghao Zhang*
Affiliation:
James Weir Fluids Laboratory, Department of Mechanical and Aerospace Engineering, University of Strathclyde, GlasgowG1 1XJ, UK
*
†Email addresses for correspondence: junli@kfupm.edu.sa, yonghao.zhang@strath.ac.uk
†Email addresses for correspondence: junli@kfupm.edu.sa, yonghao.zhang@strath.ac.uk
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Abstract

Based on an accurate numerical solution of the kinetic equation using well-resolved spatial and velocity grids, the separation of rarefied gas flow in a microchannel with double rectangular bends is investigated over a wide range of Knudsen and Reynolds numbers. Rarefaction effects are found to play different roles in flow separation (vortex formation) at the concave and convex corners. Flow separations near the concave and convex corners are only observed for a Knudsen number up to $0.04$ and $0.01$, respectively. With further increase of the Knudsen number, flow separation disappears. Due to the velocity slip at the solid walls, the concave (convex) vortex is suppressed (enhanced), which leads to the late (early) onset of separation of rarefied gas flows with respect to the Reynolds number. The critical Reynolds numbers for the emergence of concave and convex vortices are found to be as low as $0.32\times 10^{-3}$ and $30.8$, respectively. The slip velocity near the concave (convex) corner is found to increase (decrease) when the Knudsen number increases. An adverse pressure gradient appears near the concave corner for all the examined Knudsen numbers, while for the convex corner it only occurs when the Knudsen number is less than $0.1$. Due to the secondary flow and adverse pressure gradient near the rectangular bends, the mass flow rate ratio between the bent and straight channels of the same length is a non-monotonic function of the Knudsen number. Our results clarify the diversified and often contradictory observations reported in the literature about flow rate enhancement and vortex formation in bent microchannels.

JFM classification

Micro-/Nano-fluid dynamics: Micro-/Nano-fluid dynamics Rarefied Gas Flow: Rarefied Gas Flow
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 901 , 25 October 2020 , A26
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Copyright
© The Author(s), 2020. Published by Cambridge University Press

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