A study of nucleate boiling heat transfer on hydrophilic, hydrophobic and heterogeneous wetting surfaces

The effect of characteristic of heating surface on nucleate boiling heat transfer is well reported via many previous results. However, until recently, the study of surface influences on nucleate boiling was complicated by coupled surface factors; wettability and surface roughness. This study focuses on nucleate pool boiling under different wetting conditions, in the absence of microscale roughness, which is coupled with wetting phenomena. Heterogeneous boiling occurs on hydrophilic (54°) and hydrophobic (123°) surfaces, even without microstructures that can trap water vapor. In the very low heat flux regime, hydrophobic surfaces offer better nucleate boiling heat transfer than hydrophilic surfaces. Moreover, a heterogeneous wettability surface composed of hydrophobic dots on a hydrophilic surface provides better nucleate boiling heat transfer than a homogeneous wettability surface (hydrophilic or hydrophobic). Analysis of bubble generation and departure was carried out via a high-speed visualization technique in order to understand these experimental results. Based on the bubble analysis, it was inferred that changes in wettability could lead to totally different boiling performance when microstructures are absent. Also, the number of hydrophobic dots and the pitch distance between dots were key parameters for explaining boiling performance under heterogeneous wetting conditions.