Manipulating the heat transfer of pool boiling by tuning the bubble dynamics with mixed wettability surfaces

Boiling heat transfer is widely involved in various energy conversion and utilization systems owing to its high heat transfer ecoefficiency even at low temperature differences. It is still a great challenge to control the boiling behaviors due to their multiple influencing factors and their randomness of bubble behaviors. The wettability greatly affects the boiling heat transfer. Hydrophobic surfaces can promote bubble nucleation and increase the heat transfer coefficient of nucleate boiling, while hydrophilic surfaces can promote bubble detachment, inhibiting film boiling, and hence increasing the critical heat flux. In order to meet such mixed requirements of wettability at different boiling stages, we propose a scalable and affordable method to fabricate the hydrophilic / hydrophobic mixed wettability surface. Compared with pure hydrophilic or superhydrophobic surfaces, the mixed surfaces exhibit higher heat transfer coefficient than the hydrophilic surface at low superheat degrees but larger critical heat flux than the hydrophobic surface at high superheat degrees. Therefore, the mixed wettability surfaces are more adaptable for a wider range of inputted heat flux. Such boiling heat transfer characteristics is the resultant of the unique bubble dynamics. The bubbles are likely to pinned on the hydrophobic spots but more easily detach from hydrophilic area. Tuning the hydrophilic / hydrophobic area ratio can manipulate the bubble dynamics. A numerical model is used to mimic the boiling on the hydrophilic / hydrophobic mixed surface, which agrees with the experimental results. And then the optimal area ratio is suggested by a boiling phase diagram based on a large number of simulations with various area ratios. Besides proposing a scalable and affordable way to fabricate the hydrophilic / hydrophobic mixed wettability surface, we also provide details of bubble dynamics on the pure or mixed wettability surfaces. Such details facilitate to understand the enhancement mechanisms of boiling heat transfer on mixed surface, which is still an open issue. The effects of the area ratios on the heat transfer coefficient are also discussed, which would provide a guideline to fabricate the mixed boiling enhancement surfaces. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Wettability plays a crucial role in boiling heat transfer, with hydrophilic surfaces promoting bubble detachment and hydrophobic surfaces promoting bubble nucleation. Mixed wettability surfaces offer greater adaptability to a wider range of heat flux inputs.