An experimental study on the heat transfer by a single bubble wake rising near a vertical heated wall

Abstract

We experimentally investigate the effect of a single bubble wake on the convective heat transfer (cooling) on a vertical heated wall in a quiescent water (room temperature) tank. The wall temperature is measured with an infrared camera, and gas-liquid velocity fields are also measured simultaneously using a two-phase particle image velocimetry. We consider two conditions of bubble that rises along two-dimensional zigzagging and straight paths, while varying the bubble-wall distance. When it is sufficiently close to the wall for the zigzagging bubble to impact on the wall, the local heat transfer increases up to eight times compared to that by natural convection. As the bubble-wall distance increases, the amount of heat transfer enhancement decreases, while there is a time delay between the instants of maximum Nusselt number and closest bubble-wall distance. This indicates that the bubble-induced liquid flow is the main source of heat transfer, rather than the physical collision to the wall, confirmed by the strongest correlation between the wall-normal liquid velocity and Nusselt number variation. Although the instantaneous maximum rise is smaller, the total augmentation of heat transfer from the wall is maximized at an intermediate distance (about 2.5 times of bubble size), which is determined by the effective time span and area under the effect of bubble wake. When the bubble rises in a straight path, however, the bubble is not effective for heat transfer. Even for the smallest bubble-wall distance, at which the bubble is attracted (and collides) toward the wall, the variation of Nusselt number is quite below 10%. We find that the linearly rising bubble wake exists inside the thermal-boundary layer, while the zigzagging bubble wake significantly disturbs it. Therefore, the main mechanism of the surface cooling is the disruption of a thermal-boundary layer with the entrained bulk liquid by the bubble wake.