Coupled effects of surface wettability and wall subcooling on vapor condensation: A phase-change lattice Boltzmann study

A comprehensive understanding of condensation mechanisms is essential to improve energy conversion efficiency and guide the design of high-performance materials. Among the various factors affecting vapor condensation, surface wettability and wall temperature are particularly influential. In this work, a phase-change lattice Boltzmann method is utilized to analyze the vapor condensation process. The evolution of a single condensate droplet on surfaces with different wettability characteristics was analyzed, focusing on the effects of wall subcooling on droplet nucleation, detachment, and migration behaviors on hydrophilic and hydrophobic interfaces. Furthermore, large-scale condensation processes involving droplet coalescence were investigated to elucidate the coupled mechanisms between droplet dynamics and interfacial heat transfer. The results reveal that hydrophobic surfaces can sustain a large population of microdroplets with low surface coverage, promoting efficient heat transfer. This effect causes that as the subcooling degree of the wall surface increases, the contact angle corresponding to the maximum heat transfer performance under the current operating conditions shifts in the direction of a larger value. At higher degrees of wall subcooling, surfaces with larger contact angles demonstrate superior heat transfer performance. Specifically, when the wall subcooling is 0.088, 0.132, and 0.176, increasing the contact angle from 51.8° to 144.0° enhances the average wall heat flux by 28.82%, 45.18%, and 55.70%, respectively. Furthermore, for hydrophilic (51.8°) and hydrophobic (144.0°) surfaces, increasing the wall subcooling from 0.088 to 0.220 results in average wall heat flux gains of 121.69% and 289.18%, respectively. On hydrophilic surfaces, droplets nucleate rapidly and exhibit a longer detachment period and larger departure diameter, whereas on hydrophobic surfaces, nucleation is delayed, and droplets detach more readily due to lower adhesion. Furthermore, gradient wettability surfaces are proposed and the coupling mechanism between droplet self-transport and condensation heat transfer is examined. The self-propelled motion of droplets accelerates vapor condensation, leading to a more pronounced improvement in thermal performance as the wettability gradient increases.