Numerical modeling of segmented flows at microscales

Talimi, Vandad (2014) Numerical modeling of segmented flows at microscales. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (17MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Enhancement in heat transfer at micro scales is an active area of study, followed by many researchers. Use of non-boiling two phase flows in the form of segmented flows has become one of the interesting methods for achieving higher cooling (or mass transfer) capacities. Segmented flows (also known as slug flows, plug flows, or Taylor flows) is a series of moving liquid segments which are separated by gas bubbles or another liquid slugs. These flows are known as gas-liquid or liquid-liquid two phase flows, respectively. The effective phenomenon in heat transfer enhancement is internal circulations inside the liquid slugs, which helps to mix the liquid. The main focus of the present thesis is on heat transfer of gas-liquid segmented ows, using numerical simulations based on the concepts of Computational Fluid Dynamics (CFD). As shown in the literature review, large gaps exist in research which need further studies. The target of this research is providing details on hydrodynamics and heat transfer in gas-liquid two phase flows, which can be helpful in understanding the whole process better. Based on the literature reviewed, the numerical modeling can be conducted using single phase or two phase simulations. Both single phase and two phase numerical simulations have been performed in this research. The research has also been conducted using fixed and moving frames of reference. It has been shown that slug flows with shorter slugs provide greater cooling ca- pacities compared with long slugs. Furthermore, it has been shown that liquid film around bubbles reduces the intensity of circulations in liquid slugs and hence reduces the cooling capacity. Therefore, using hydrophobic wall materials has been suggested. The key finding of the present research is how the liquid film around bubbles affects heat transfer, and it has been suggested to include this parameter in the correlations in the future. The comparison of the results with experimental data and/or exact analytical solutions throughout the thesis show good agreements.

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