Sliding bubble heat transfer on a tube under heating and cooling conditions

It has been shown previously that much of the heat transfer which occurs when boiling on the outside of tube bundles is due to bubbles which slide around the tubes rather than nucleation at the surface. It is thought that these bubbles create a thin layer of liquid underneath them which evaporates causing high heat transfer rates. In this work an experiment is described which allows either independent heating or cooling of a single tube within a bundle under normal boiling conditions. The results confirm the important influence of sliding bubbles and indicate that the evaporation heat flow rate is typically twice that for condensation under the same conditions. Theoretical considerations show that the experimental heat transfer rates encountered can be explained by thermofluid analysis of the liquid layer.