Early Evaporation of Microlayer for Boiling Heat Transfer Enhancement.

Abstract

For over five decades, an enhancement in pool boiling heat transfer has been achieved by altering the surface wetting, wickability, roughness, nucleation site density, and providing separate liquid/vapor pathways. In this work, a new enhancement mechanism based on the early evaporation of the microlayer is discovered and validated. The microlayer is a thin liquid film present at the base of a vapor bubble. The presence of microridges on the silicon dioxide surface partitions the microlayer and disconnects it from the bulk liquid, causing it to evaporate sooner, thus leading to increase in the bubble growth rate, heat transfer, departure frequency, and critical heat flux (CHF). Compared to a plain surface, an ∼120% enhancement in CHF is obtained with only an ∼18% increase in surface area. A CHF enhancement map is developed on the basis of the ridge height and spacing, resulting in three regions of full, partial, and no enhancement. The new mechanism is validated by comparing the growth rate of a laser-created vapor bubble on a ridge-structured surface and a plain surface, and the corresponding prediction of the CHF enhancement is found to be in good agreement with the experimental boiling data. This discovery opens up a new field of CHF enhancement and can potentially be coupled with existing techniques to further push the limits of boiling heat transfer.