Effects of Heater Surface Characteristics on Bubble Sliding Induced Heat Transfers during Upward Subcooled Nucleate Boiling

In the present study the effects of surface wettability and roughness on wall heat transfers during bubble sliding are investigated via isolated bubbles on a vertical heater. Bubbles were generated from artificial cavities on stainless steel heaters. The test liquid was de-gassed, de-ionized water and a borosilicate glass vessel was filled with the test liquid. By high resolution imaging technique, the bubble dynamic parameters such as temporal evolution of bubble size, bubble base diameter and bubble sliding over heater for different surface characteristics were measured. The experimental values of different bubble dynamic parameters were used in the numerical models to calculate the associated heat transfers induced by bubble sliding.

We found that with the decrease of surface wettability for similar roughness, bubble sliding distances and the bubble induced areas increase. It enhances the transient heat conduction. For rough surfaces, sliding velocities were found higher than that of smooth surfaces. It is to be noted that the wettability of these surfaces is decreased while the roughness is increased. The transient conduction heat transfers are found generally higher in this study when the surface becomes rougher. An optimal roughness of Rq=90 nm showed more than 1.4 times augmented heat transfer over the smooth surfaces due to the interaction of surface roughness with the microlayer. These kind of surfaces even transfers higher heat flux than the roughest surface, studied in this work. The contribution of microlayer evaporation beneath sliding bubbles was not found significantly influential to the total heat flux. Future work will focus on the detail understandings of surface characteristics effects on the recovery of thermal boundary layer and the microlayer behavior during bubble sliding.