Effect of wettability on nucleate boiling

Abstract

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Heat transfer enhancement at liquid-solid interfaces is often achieved by modifying the surface properties. However, deep efforts to describe the actual role of surface modification only started at the 1980’s and much work has left undone since then. The wettability is a key parameter governing heat, mass and momentum transport at liquid-solid interfaces. However it is usually quantified using macroscopic quantities, which cannot be related with the micro-and-nano-scale phenomena occurring at the interface. In this context, the present paper revises the potential and limitations of using macroscopic apparent contact angles to predict the wetting regimes. Then, these angles are used to relate the wetting regimes with bubble dynamics and heat transfer processes occurring at pool boiling. The results show that the macroscopic angles are useful to establish general trends and differentiate bubble dynamics behaviour occurring for opposite wetting regimes. However, milder wetting changes occurring within each regime, caused, for instance, by surface topography are not well captured by the apparent angle, as the surface topography is not scaled to affect this macroscopic angle, although it can clearly influence the bubble formation and departure mechanisms and, consequently the heat transfer coefficients. In line with this, the concept of the micro-scale contact angle, as introduced by Phan et al. [1] is used here together with a geometrical parameter to include the effect of surface topography, to describe the role of the wettability on bubble dynamics. Based on this analysis, a multi-scale approach is proposed to include the role of wettability on correlations predicting the pool boiling heat transfer coefficients.