Lattice Boltzmann simulations for transition from dropwise to filmwise condensation on hydrophobic surfaces with hydrophilic spots

Transition from dropwise to filmwise condensation of a dry saturated vapor on downward-facing smooth subcooled horizontal hydrophobic walls with hydrophilic spots is simulated numerically using the newly developed phase-change lattice Boltzmann method. Dynamic behaviors including growth, coalescence and departure of condensate droplets from cooler surface are investigated. Effects of wall subcooling and wettability of the cooler surface on droplet departure diameter, average cycle time and nucleation time are presented. At small wall subcoolings, droplets are formed on hydrophilic spots of the hydrophobic surface. With increasing wall subcoolings, coalescence of droplets and their subsequent departure are observed. Further increase of the wall subcooling leads to transition from dropwise condensation to filmwise condensation mode, and droplets fall down from the subcooled surface at locations according to Taylor's unstable wavelength. The dropwise condensation curve in terms of average heat flux versus wall subcooling from droplet nucleation to peak condensation heat flux and its subsequent transition to filmwise condensation is obtained numerically for the first time. It is found that the location where droplet or liquid film in contact with the downward-facing surface has a high local heat flux, especially around the triple-phase contact line. The degree of wall subcooling corresponding to peak condensation heat flux increases with the increasing pitch distance between hydrophilic spots. The cooler's hydrophobic surface with a smaller contact angle has a lower peak condensation heat flux which occurs at a smaller degree of wall subcooling. During transition from dropwise to filmwise condensation, there is a graduate reduction in wall heat flux under constant wall temperature conditions, and a sudden drop in the average wall temperature of the subcooled surface under constant wall heat flux conditions.