Bubble-wake interactions of a sliding bubble pair and the mechanisms of heat transfer R. O’Reilly Meehan a,⇑ , B. Donnelly a , K. Nolan a , D.B. Murray b a Thermal Management Research Group, Efficient Energy Transfer (gET) Department, Nokia Bell Labs, Blanchardstown Business & Technology Park, Snugborough Rd, Dublin 15, Ireland b Dept. of Mechanical & Manufacturing Engineering, Trinity College Dublin, Dublin 2, Ireland article info Article history: Received 22 August 2016 Received in revised form 21 December 2016 Accepted 5 January 2017 abstract An experimental investigation is reported for the bubble-wake interactions that occur between an in-line air bubble pair sliding under an inclined surface in quiescent water. Three experimental techniques are utilised to study this flow: time-resolved particle image velocimetry (PIV), a new edge-based bubble tracking algorithm incorporating high speed video and high speed infrared thermography. These tech- niques allow for a novel characterisation of sliding bubble-wake interactions in terms of their associated fluid motion, the fluid-induced changes in the trailing bubble interface and the resulting surface convec- tive heat transfer. As these interactions are ubiquitous to multiphase flows, such knowledge is pertinent to many industrial applications, including the optimisation of two-phase cooling systems. This work has revealed that for an intermediate bubble size, in-line bubble pairs adopt a configuration in which their paths are 180° out of phase. Upon entering the fluid shed from the near wake of the leading bubble at each local extremum, the trailing bubble is accelerated both in the direction of buoyancy and in the spanwise direction corresponding to that of the shed fluid structure. This causes significant, high-frequency changes in the interface of the trailing bubble, which recoils and rebounds during this interaction. Surface heating adds further complexity to the bubble-wake interaction process due to the disruption of the thermal boundary layer at the surface. It is found that the trailing bubble can momentarily decrease local convective heat transfer levels by displacing the cool fluid introduced to the surface by the leading bubble. However, the amplified fluid mixing and local heat transfer enhancement of 7–8 times natural convection levels observed at the trailing bubbles mean that the net effect of the trailing bubble is to enhance convective heat transfer. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The dynamics of bubbles moving through a fluid are rich, com- plex and multi-layered. Considering that they also have applica- tions in diverse fields such as chemical engineering, water treatment and thermal management, it is no surprise that there has been considerable recent research into these flows. Despite this, the current understanding of two phase flows remains incom- plete, particularly in terms of the coupling between the bubble dynamics, fluid mechanics and convective heat transfer enhance- ment [1]. To date, both vapour and gas bubbles have been found to enhance the convective heat transfer rate between the fluid and an adjacent heated surface [2]. A significant portion of this enhancement results from the turbulent disruption of the sur- rounding fluid induced by the bubble wake structure. Indeed, much of the behaviour of these flows stems from the bubble wake [1,3,4]. Extensive work exists on freely rising bubbles describing the bubble shape [5,6], path [7,8], dynamics [9], wake structures [10–12] and bubble–bubble interactions [13,14]. Bubbles in more constricted geometries, however, have received comparatively less attention. One configuration of interest is the interaction between a gas bubble and a heated inclined surface, which is pertinent to applications such as two-phase shell and tube heat exchangers. At intermediate surface inclination angles, gas bubbles have been observed to slide under the surface, resulting in significant convec- tive heat transfer enhancement [2]. Much of the literature on the heat transfer associated with these sliding bubbles does not address the underlying fluid motion that causes this enhancement. Furthermore, the majority of studies on sliding bubbles to date have been limited to a single bubble, whereas the aforementioned industrial applications will involve the interactions between multiple bubbles. The current study will seek to address these issues by exploring the dynamics of bubble-wake interactions for http://dx.doi.org/10.1016/j.ijheatmasstransfer.2017.01.017 0017-9310/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: rudi.oreilly_meehan@nokia.com (R. O’Reilly Meehan). International Journal of Heat and Mass Transfer 108 (2017) 1347–1356 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt