Numerical Investigation of Flow Structure and Heat Transfer Produced by a Single Highly Confined Bubble in a Pressure-Driven Channel Flow

Abstract

Confined bubbly flows in millimeter-scale channels produce significant heat transfer enhancement when compared to single-phase flows. This enhancement has been demonstrated in experimental studies, and some of these studies conclude that the enhancement persists even in the absence of active nucleation sites and bubble growth. This observation leads to the hypothesis that the enhancement is driven by a convective phenomenon in the liquid phase around the bubble instead of sourcing from microlayer evaporation or active nucleation. Presented here is a numerical investigation of flow structure and heat transfer due to a single bubble moving through a millimeter-scale channel in the absence of phase change. The simulation includes thermal boundary conditions designed to match those of a recent experiment. The channel is horizontal with a uniform-heat-generation upper boundary condition and an adiabatic lower boundary condition. The Lagrangian framework allows the simulation of a channel of arbitrary length using this smaller computational domain. The fluid phases are modeled using the Volume-of-Fluid method with full geometric reconstruction of the liquid/gas interface. The liquid around the bubble moves as a low-Reynolds-number unsteady laminar flow. In a square region from the trailing edge of the contact line to one nominal bubble diameter behind the bubble, the area-averaged Nusselt number is, at its greatest, 4.7 times the value produced by a single-phase flow. Bubble shape and speed compare well to observations from the recent experiment. The heat transfer enhancement can be attributed to flow structures created by bubble motion. Multiple regions have been observed and are differentiated by their respective vortex characteristics. The primary region exists directly behind the bubble and exhibits the highest enhancement in heat transfer. It contains channel-spanning vortices that move cold fluid along the centerline and edge of the vortices from near the far wall of the channel to the heated wall. The cold fluid delivered by this motion tends to thin the thermal gradient region near the wall and directly behind the bubble and results in the highest local heat transfer coefficients. This vortex drives a bulk exchange of fluid across the channel and elongates the area of heat transfer enhancement to several bubble diameters. The secondary region is a set of vortices that exist to the side and slightly behind the bubble. These vortices rotate at a shallow angle to the primary flow direction and are weaker than those in the other regions.