Enhanced boiling heat transfer of water on a liquid-infused surface

Abstract

The aim of this study was to experimentally demonstrate a counter-intuitive phenomenon that a surface covered with a liquid has the potential for enhancing heat transfer for the boiling of water over it. To this end, a highly-wetting surface with a zero contact angle for multiple liquids, i.e., an Ultra-Omniphilic Surface (UOS) was prepared on aluminum (Al 6061 alloy) using a simple and easy-to-implement bulk micro-manufacturing approach and a non-boiling liquid (NBL) was infused over this surface to occupy its sub-surface micro/nano-cavities. The resulting liquid-infused UOS is called a Binary Surface (BiS) for it has two distinct superficial phases — solid phase as islands and liquid phase as NBL puddles. Saturated nucleate pool boiling experiments were conducted on the BiS and the critical heat flux (CHF) and the boiling heat transfer coefficient (HTC) were measured. The results were compared with the UOS and a plain/polished surface (PS) prepared from the same aluminum alloy sample. In addition, high-speed visualization was employed for capturing the bubble dynamics at different heat fluxes and parameters such as bubble departure diameter (Dd), bubble departure frequency (f), and nucleation site density (NSD) were measured. The results revealed that the nucleate pool boiling performance of water on the BiS surpasses both the PS and the UOS. The HTC on the BiS was 1.33 times and two times larger than the UOS and the PS, respectively. The CHF obtained on the BiS was comparable to that on the UOS and 1.47 times larger than that on the PS even though a considerable portion of the BiS surface area was covered with the NBL and unavailable for boiling. Remarkably, an inspection of the high-speed videos has suggested the presence of the same NBL as the reason for the better boiling heat transfer performance of the BiS. The NBL that was spread over the BiS as puddles was found to (1) prevent the growth of large vapor bubbles and (2) extend the isolated bubble regime by delaying the lateral coalescence of adjacent bubbles. A comparison of the results between UOS and BiS suggests that – as far as boiling enhancement is concerned – mechanisms for tackling vapor bubbles could be superior to those that involve improving surface wettability.