Enhancing thermal stability and uniformity in boiling heat transfer using micro-nano hybrid surfaces (MNHS)

Abstract

In two-phase heat transfer, promising issues include not only improving the boiling performance but also the surface temperature uniformity and stability, which indicate that how long the system maintains thermal stability without a failure on the surface. In this study, the merits of micro-nano hybrid surfaces (MNHS) are discussed for enhancing the thermal uniformity/stability and boiling heat transfer performance. Spatial/temporal heat transfer characteristics are evaluated on MNHS using a local temperature-measuring sensor of resistance temperature detector (RTD). We demonstrate that MNHS can enhance not only boiling performance but also thermal uniformity/stability by delaying bubble coalescence with an appropriate design of the location of the nucleation sites and the nucleated bubble size. The confining effects of nucleated bubbles on nanowire (NW) structures and of uniform bubble nucleation on uniformly distributed micro-cavity (MC) structures lead to the reliable enhancement of thermal uniformity/stability as well as critical heat flux (CHF) in pool boiling environments. These combined effects of the NW and MC structures could delay the bubble coalescence phenomenon by catalyzing bubble nucleation dispersedly and quickly at small bubble sizes. When the normalized pitch of the nucleation sites is 1, namely the pitch of the nucleation sites and the bubble departure size have the same dimension, CHF is significantly enhanced, by more than 170%, on an MNHS versus a plain surface by delaying bubble coalescence and maximizing bubble density. Boiling heat transfer using an MNHS represents a breakthrough reducing the spatial and temporal temperature variation at CHF to less than 1/3 and 1/4, respectively, compared with a plain surface.