Bubble-wake interactions of a sliding bubble pair and the mechanisms of heat transfer

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, namely time- resolved particle image velocimetry (PIV), high speed video and high speed infrared thermography. These allow for the fluid motion associated with bubble-wake interactions to be studied, along with the resultant changes in the trailing bubble interface and convective heat transfer. This work has revealed that for an intermediate bubble size, in-line bubble pairs adopt a configuration in which their paths are 180 degrees 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 additional complexity to these bubble-wake interactions, since the trailing bubble can momentarily decrease convective heat transfer 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 introducing multiple bubbles to the surface remains an effective mechanism of enhancing convective heat transfer.