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.
