Abstract
Slug flow is a multiphase flow pattern characterized by the occurrence of long gas bubbles (Taylor bubbles) separated by liquid slugs. This multiphase flow regime is present in many and diversified natural and industrial processes, at macro and microscales, such as in eruption of volcanic magmas, oil recovery from pre-salt regions, micro heat exchangers, and small-sized refrigerating systems. Previous studies in the literature have been mostly focused on tubular gas bubbles flowing in Newtonian liquids. In this work, results from several numerical simulations of tubular gas bubbles flowing in a shear thinning liquid in microchannels are reported. To simulate the shear thinning behavior, carboxymethylcellulose (CMC) solutions with different concentrations were considered. The results are compared with data from bubbles flowing in Newtonian liquids in identical geometric and dynamic conditions. The numerical work was carried out in computational fluid dynamics (CFD) package Ansys Fluent (release 16.2.0) employing the volume of fluid (VOF) methodology to track the volume fraction of each phase and the continuum surface force (CSF) model to insert the surface tension effects. The flow patterns, the viscosity distribution in the liquid, the liquid film thickness between the bubble and the wall, and the bubbles shape are analyzed for a wide range of shear rates. In general, the flow patterns are similar to those in Newtonian liquids, but in the film, where a high viscosity region is observed, the thickness is smaller. Bubble velocities are smaller for the non-Newtonian cases.
Highlights
Gas–liquid slug flow, known as Taylor bubble flow, is a multiphase flow pattern characterized by the presence of long bullet-shaped bubbles, occupying almost all of the cross-section of the tube, separated by stagnant liquid or by liquid flowing co-currently or counter-currently [1,2,3,4,5]
The volume of fluid (VOF) methodology [40] included on this package was used as the gas–liquid interface tracking technique coupled with the geometric reconstruction scheme [41] that is based on a piecewise-linear approach to define the interface between phases
StudyThe wasstudy number, and by the appearance of viscoelastic for CMC concentrations above was conducted for a 0.5% CMC aqueous solution, for which rheological data is available and the shear thinning behavior can be observed with negligible viscoelastic effects
Summary
Gas–liquid slug flow, known as Taylor bubble flow, is a multiphase flow pattern characterized by the presence of long bullet-shaped bubbles, occupying almost all of the cross-section of the tube, separated by stagnant liquid or by liquid flowing co-currently or counter-currently [1,2,3,4,5]. In the most common case, the Taylor bubble outruns the liquid phase leading to the development of a liquid film between the lateral surface of the bubble and the channel wall [1,2,3,6,7] This flow pattern can occur in channels of different shapes (e.g., square, circular, or triangular cross section), dimensions (micro or macroscale) [1] and orientations (horizontal, vertical, and inclined channels) [3]. Due to the spatial and temporal variation of shear stresses in slug flow, and the corresponding direct effect on the viscosity fields, this characterization entails an extra degree of complexity This challenge has been already addressed numerically for macroscale systems [20], but a systematic study for microscale systems is still lacking
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