Heat transfer enhancement caused by the controlled production of sliding vapor bubbles in laminar subcooled flow in a narrow channel

The capacity of sliding vapor bubbles to increase the heat transfer rate in highly subcooled diabatic flow in a horizontal minichannel was investigated experimentally. The apparatus consisted of a minichannel (1.4 x 23.2 x 357 mm) with a heated test surface forming one wide wall of the channel. The electronics cooling fluid, Novec 649, was the working fluid. The vapor bubbles were produced on an electrically heated Ni-Cr wire positioned across the channel width at one streamwise location. Bubble diameters ranging from 0.2 to 7.5 mm were produced for flows with single-phase Reynolds numbers between 127 and 875. Full-field thermal imaging of the test surface was accomplished with liquid crystal thermography. Color video recordings of thermochromic liquid crystals coated on the test surface and high-speed gray-scale videos of the bubble motions within the channel were superimposed so that the temperature field on the test surface could be correlated to observed bubble dynamics.;Increases in the heat transfer coefficient of up to 64% were observed and compared to single-phase liquid flow under the same conditions. These enhancements were sustained over large streamwise distances so that a streamwise plateau in local Nusselt number is produced. An empirical correlation was proposed that predicts these enhancements based on the number and size of the bubbles, the channel size, and the fluid properties.;Vapor bubbles produced on the heated wire were observed to persist much farther downstream than is anticipated by standard theory. These bubble lifetimes were attributed to a combination of internal superheating before the bubble is released, heating from the test surface, and the evolution of a heated layer of liquid around the bubble. To investigate this hypothesis, a numerical model was developed which predicts the temperature distribution around a single superheated vapor bubble sliding in subcooled flow between heated parallel plates. These results were used to estimate the time required for a superheated bubble to cool to saturation temperature and start condensing. The model results agree with the detailed observations of the evolution of a single large vapor bubble.