Nucleate pool boiling heat transfer augmentation on hydrophobic self-assembly mono-layered alumina nano-porous surfaces

摘要

In this study, we have observed substantial pool boiling heat transfer augmentation by creating one-dimensionally grown Alumina Nano Porous Surface (1-D ANPS). Pre-organized numerous nano pores have structural uniqueness such as enlarged surface area, increase in active nucleation sites, and liquid thin film evaporation at vapor-liquid menisci of the pore mouth. At the same time, the surface wetting of a cylindrical heater is modified with a hydrophobic Self-Assembled Monolayer (SAM) coating. Enhanced water contact angle of the 1 -D ANPS yields substantial active nucleation site increase by preventing liquid impregnation into nano pores, which play a role of trapping non-condensible gas. The combination of structural uniqueness of the 1-D ANPS and surface wettability modification achieves significant heat transfer coefficient (HTC) enhancement in the nucleate pool boiling regime. Also, HTC improvement is closely related to bubble interactions on the cylindrical heating surface while they are moving along the periphery. For a hydrophobic surface, bubble sweeps along the heater periphery and efficiently removes adjacent premature bubbles. It leads to reduced waiting time. Enlarged bubbles carry more heat from the surface and yield huge internal/external convection. In order examine bubble motions, an analytical model and high speed camera were used.