Flow structures and heat transfer enhancement in the wakes of sliding bubbles

Abstract

The motion of a bubble through a fluid has attracted considerable scientific attention for many years due to the complex, interesting fluid dynamics in their wakes, coupled to a rich and varied interface motion. Additionally, vapour and gas bubbles have been found to significantly increase convective heat transfer rates between a heated surface and the surrounding fluid. Bubbles play a key role within two phase heat exchangers, in addition to applications in chemical engineering, water treatment and medicine. Of particular interest are two-phase cooling systems, which can achieve heat transfer coefficients considerably larger than their single-phase counterparts. However, a widespread implementation of these systems has yet to occur. This is due to their considerable size and to reliability issues resulting from the complexity of the flow, since the mechanisms involved in this bubble motion are dynamic and are often poorly understood. Although numerous studies exist for bubbles rising in an unbounded medium, that of bubbles rising in constricted geometries has received less attention. The particular case of a gas bubble sliding underneath an inclined surface in a quiescent medium is of key importance in the above applications. In particular, the wake of a sliding bubble and how it influences bubble interactions has received little to no attention in the literature. This study experimentally investigates air bubbles sliding under an inclined surface in quiescent water in terms of the bubble mechanics, fluid motion and resulting heat transfer. Time-resolved particle image velocimetry (PIV) is utilised in three measurement planes to study the flow features in the wakes of sliding bubbles for a range of bubble diameters and surface inclination angles. High speed imaging and advanced analytical techniques are used to capture the dynamics of the bubble and the motion of its interface. High speed, high resolution infrared thermography is used to measure the two dimensional transient convective surface heat transfer. An experimental setup has been designed and built to facilitate these measurements, while purpose specific code has been developed to analyse the experimental data in detail. These measurements are performed both for single bubbles and an in-line bubble pair. Analysis of the measured velocity and vorticity fields reveals a wake structure consisting of a near wake that moves in close proximity to the bubble, shedding vorticity at the extrema of the bubble path. Downstream of the bubble in the far wake, these structures evolve into asymmetrical, oppositely-oriented hairpin vortices that are generated in the near wake. These hairpin vortices bear similarities to those observed behind freely rising bubbles and near-wall bluff bodies and are found to cause significant motion of the bulk fluid. This fluid motion is key to the convective heat transfer enhancement associated with a sliding bubble. Sliding bubbles were also found to provide local heat transfer enhancement of up to 6 times natural convection levels, which evolves dynamically as the bubble traverses the surface, spreading over a large area and affecting heat transfer rates. The current work links the bubble wake to the centroidal and interfacial dynamics of the bubble, in addition to quantifying the bubblewake interactions in terms of fluid flow, bubble dynamics and surface heat transfer. This in-depth description of complex flow phenomena will be key in the future optimisation of multiphase convective heat transfer.