Thermo-Hydrodynamics of Heat Transfer Enhancement During a Gas-Liquid Taylor Bubble Flow in a Mini/Microchannel

Abstract

Taylor bubble flow in microchannel systems play an important role in many industrial applications such as two-phase flow micro heat exchangers, pulsating heat pipes, monolithic reactors, digital microfluidics, microscale mass transfer process, fuel cells, etc. Taylor bubble is formed in capillary tubes when gas-liquid or liquid-liquid flows with particular range of flow rate ratios. In this work, a 3D numerical study has been carried out using the volume-of-fluid (VOF) method on commercially available Ansys Fluent® for the formation of (i) isolated Taylor bubble and (ii) a train of Taylor bubbles, in a square channel of side 1.0 mm. At inlet of the capillary tube, gas and liquid flows in co-current flow arrangement neglecting the nozzle thickness. Constant thermo-physical properties are considered for solid and fluid. The flow is hydrodynamically developing at the inlet of the channel. Taylor bubble formed gets stabilized at the entry section of the fluid channel. The Taylor bubble then passes through a square channel (01 mm2 ) carved on a solid substrate of size 3×2×30 mm3 . The flow is hydrodynamically fully developed and thermally developing inside this channel. Constant heat flux is applied on the bottom wall of the substrate (3 × 30 mm2 ), while all other surfaces exposed to the ambient are insulated. To avoid the end effect, the fluid again passes through a capillary tube after it travels the full length of the substrate. Thus, three-dimensional Navier-Stokes and energy equations are solved simultaneously. No slip boundary condition is imposed on the inner walls of the channel. Sufficiently fine mesh considered near the boundary to capture liquid film surrounding a Taylor bubble interface near the wall. The liquid film is maintained without its breakup throughout the length of the channel. The numerical simulations are carried out for the substrate wall to fluid thermal conductivity ratio (ksf ∼ 10−646), ratio of substrate thickness to channel depth (δsf ∼ 1−3) and capillary number (Ca ∼ 0.005 − 0.007). The objective of this study is to explore heat transfer enhancement due to injection of Taylor bubbles in steady flow, which causes disturbance in the flow field at its head and tail, resulting in mixing in the fluid that shows reduction in wall temperature compared to single phase liquid flow in the channel. The axial variation of bulk fluid temperature shows a footprint of a Taylor bubble at its back end. The effect of bubble length and the frequency of bubble occurrence is also studied.