Nanostructure-Supported Evaporation Underneath a Growing Bubble.

Abstract

High porosity nanostructured coatings have been extensively studied for their use in enhancing liquid-to-vapor phase change due to their ability to wick liquids laterally across surfaces during boiling. Although the effect of these coatings on the maximum heat transfer rate achievable (the critical heat flux) is now well understood, the impact on boiling efficiency (the heat transfer coefficient) is less clear. In this work, a novel experimental apparatus is used to take heat transfer measurements beneath growing and departing bubbles on nanostructured surfaces. By independently tuning the surface heat flux and bubble departure time, IR thermography is used to directly visualize and characterize surface superheat, heat flux, and heat transfer coefficient during the highly transient bubble ebullition cycle. It is shown that although flat surfaces exhibit large spatial and temporal variations in surface temperature and heat flux, the nanostructured coatings produce a uniform temperature profile with enhanced heat transfer due to evaporation from the nanostructure-supported liquid films beneath the bubble. This work demonstrates the relative importance of advancing and receding contact lines, as well as the quenching process, on the overall thermal performance of structured and nonstructured surfaces. It is seen that the combined effects produce a net increase in heat transfer coefficient of over 30%, averaged over the entire ebullition cycle and throughout the entire area of influence. Additionally, the impact of viscous resistance and the importance of the nanostructure dry-out has been studied by tuning the ebullition cycle time to create dry spots. This work shows for the first time the role of nanostructured coatings and thin-film evaporation during nucleate boiling, and it provides a framework to understand the complicated nature of nanostructured boiling across all portions of the boiling curve from nucleation to critical heat flux.