HEAT-TRANSFER TO BUBBLES UNDER A HORIZONTAL TUBE

The mechanisms of boiling heat transfer on the outside of a horizontal tube are multifarious and include contributions from site nucleation, micro-layer evaporation and liquid disturbance. Previous work has sho wn the strong influence of sliding bubbles which disturb the liquid an d can leave a thin evaporating layer under the bubble. Results of expe riments designed to determine the local temperature profiles around in dividual bubbles in water as they slide around the periphery of a thin downward-facing curved surface are presented. High-speed video record ing of thermochromic paint on the surface followed by hue analysis and processing allowed detailed examination of a small section of the cur ved surface. The strong influence of liquid disturbance around the bub ble on the local heat transfer is demonstrated. Under the low heat flu x conditions of the experiments, the liquid layer under the bubble was found to dry out yielding a high-temperature spot which was quenched by the surrounding liquid when the bubble moved away. (C) 1998 Elsevie r Science Ltd.