Model-based assessment of boiling heat transfer enhanced by coatings

Abstract

The present study aims to investigate boiling heat transfer enhancement on coated surfaces, based on mechanistic models. An electrochemical deposition method was used to fabricate coatings on copper surfaces which enhance critical heat flux and heat transfer coefficient of deionized water by 35.5% and 40.1%, respectively, compared with a smooth surface. Bubble dynamics indicates that regardless of surfaces, scaling laws of Db*∝t*0.5 and Db*∝t*0.2 are followed in the inertia-controlled growth stage and the heat-diffusion-controlled growth stage, respectively, concerning normalized bubble diameter (Db*) and normalized bubble time (t*). The coating decreases the bubble departure diameter to one-third to half of that on the smooth surface and increases the departure frequency to three times that on the smooth surface. In addition, the coated surface provides more active nucleation sites which are 1, 2 order magnitude higher than the smooth surface. With these insights, a mechanistic heat transfer model was established by quantifying natural convection, transient heat conduction, and microlayer evaporation, which matches well with the measured pool boiling curve. In the end, critical heat flux was explored experimentally and theoretically. Inspired by the coalesced bubble behavior at high heat flux and the Kandlikar force model, a new force-balance model was proposed by incorporating a surface-dependent surface tension force and adding a new capillary wicking force. The present model presents a better prediction of critical heat flux, verified by the current measurement and the literature.