CFD Analysis of Taylor Bubble in a Co-Flow Microchannel with Newtonian and Non-Newtonian Liquid

TL;DR

This work develops a Volume of Fluid-based CFD model to analyze Taylor bubbles in circular co-flow microchannels, comparing Newtonian fluids with non-Newtonian power-law liquids (CMC solutions). The axisymmetric framework, detailed treatment of surface tension via CSF, and non-Newtonian viscosity η = K dot{γ}^{n−1} enable systematic assessment of bubble length, shape, velocity, and surrounding film thickness as functions of $Ca$, $Ca^*$, and inlet velocity ratios. Key findings show that bubble length generally decreases with higher Capillary numbers and velocity ratios, while bubble velocity and film thickness increase; non-Newtonian rheology reduces bubble length and promotes bullet-shaped noses, with formation frequency rising at higher concentrations. The study also demonstrates that film thickness predictions remain consistent with established Newtonian and non-Newtonian correlations when expressed in terms of $Ca$ or $Ca^*$, providing practical guidance for microfluidic bubble control and mass transfer applications.

Abstract

We present a CFD based model to understand the Taylor bubble behavior in Newtonian and non-Newtonian liquids flowing through a confined co-flow microchannel. Systematic investigation is carried out to explore the influence of surface tension, inlet velocities, and apparent viscosity on the bubble length, shape, velocity, and film thickness around the bubble. Aqueous solutions of carboxymethyl cellulose (CMC) with different concentrations are considered as power-law liquid to address the presence of non-Newtonian continuous phase on Taylor bubble. In all cases, bubble length was found to decrease with increasing Capillary number, inlet gas-liquid velocity ratio, and CMC concentration. However, bubble velocity increased due to increasing liquid film thickness around the bubble. At higher Capillary number and inlet velocity ratio, significant changes in bubble shapes are observed. With increasing CMC concentration, bubble formation frequency and velocity increased, but length decreased.

Figures (14)

• Figure 1: Schematic representation of (a) Taylor bubble formation in a co– flow geometry, (b) surrounding liquid film thickness, radius of nose (R), and bubble velocity ($U_B$) at the nose, (c) liquid film thickness profiles, Zone I: constant film thickness, Zone II: dynamical meniscus, Zone III: nose region.
• Figure 2: (a) 3D schematic representation Taylor bubble formation in a circular microchannel, and (b) computational domain with dimension and imposed boundary condition.
• Figure 3: Comparison of (a) Taylor bubble shape, (b) axial pressure distribution in a unit cell obtained from gupta-2009 and this work for air-water system with $U_G=0.5~m/s$, $U_L=0.5~m/s$, and $\eta_w$= 8.899$\times10^{-4}$ kg/m.s at 0.0085 s, and (c) Taylor bubble length for different injection length ($L_{in}$) at $\eta_w$= 1.003$\times10^{-3}$ kg/m.s, $Q_{G}= 0.47$$\mu L/s$, and $Q_{L}= 2.01$$\mu L/s$ with the results of wangg-2015. The inset shows simulated geometry with varying gas injection position.
• Figure 4: Effect of Capillary number on the bubble length and velocity at fixed operating condition of $\eta _{W} = 8.899\times10^{-4}$ kg/m.s, $U_{L}$ = 0.5 m/s and $U_{G}$ = 0.5 m/s.
• Figure 5: Phase volume fraction contours (color blue: gas, color red: liquid) at 8 ms for $Ca=$(a) 0.0210, (b) 0.0103, (c) 0.0068, (d) 0.0044, and (e) Taylor bubble shape for various surface tension values at $\eta _{W} = 8.899\times10^{-4}$ kg/m.s, $U_{L}$ = 0.5 m/s and $U_{G}$ = 0.5 m/s.