Enhancement of Heat Transfer Behind Sliding Bubbles

Abstract

The time-dependent temperature distribution on an inclined, thin-foil uniform-heat-generation heater was used to infer the heat transfer enhancement caused by the passage of an FC-87 bubble sliding beneath the lower surface of the heater. A two-camera system was used: One camera recorded color images of a liquid crystal layer applied to the upper (dry) side of the heater while a second camera simultaneously recorded the position, size, and shape of the bubble from below. The temperature response of the heater could then be correlated directly to the bubble characteristics at any given time during its passage. The data along the line bisecting the bubble wake from the nine bubbles comprising 54 bubble images were analyzed. The heat transfer in the wake behind the sliding cap-shaped bubbles is very effective compared with the natural convection that occurs before the passage of the bubble. The maximum values of heat transfer coefficient in the range of 2500 W/m2 K were produced in very sharply peaked curves. The point of maximum cooling measured as a fraction of the local driving temperature difference before the bubble passage was identified and correlated with some success to the streamwise length of the bubble. The location of the maximum heat transfer coefficient was reasonably correlated with bubble width. The level of the maximum heat transfer coefficient when cast as a Nusselt number based on bubble width grew to a saturation value as the bubble moved across the plate. A constant value of Nusselt number requires that the heat transfer coefficient falls as the bubble grows past some critical bubble size. This behavior was observed for the larger cap-shaped bubbles.