EFFECTS OF SURFACE WETTABILITY AND INCLINATION ON BUBBLE BEHAVIORS AND POOL BOILING HEAT TRANSFER

Bubble behaviors have significant influences on pool boiling heat transfer, and has been investigated extensively. However, most of the studies were conducted on horizontal upward-facing surfaces. To design the boiling surfaces with non-horizontal orientations, it is necessary to understand the surface effects on the bubble behaviors on surfaces with varied inclinations and wettabilities. In this study, the polished side of a silicon wafer is used as the surface. Five different wetting surfaces are fabricated by laser ablation and silanization grafting via chemical vapor deposition, which include the hydrophilic, hydrophobic surfaces super-hydrophilic and super-hydrophobic surfaces. The surface inclination is varied between the horizontal up-facing orientation (0°) and the vertical orientation (90°). Air bubble formation from a submerged orifice is used to study bubble behaviors. A thin-film resistive heater fabricated on the unpolished side of a silicon wafer substrate is used as the heating element. Air bubble behaviors are recorded by a high-speed camera, and compared with the vapor bubbles from pool boiling. In addition, the effects of surface contact angle hysteresis (CAH) on bubble dynamics are investigated by testing two hydrophobic surfaces with similar apparent contact angles but different CAH. This study is useful for the research and development of engineered surfaces for the enhancement of boiling heat transfer.