Enhancement of boiling heat transfer under sub-atmospheric pressures using biphilic surfaces

Surface wettability of a heating surface is one of the most important factors affecting boiling performance. While a biphilic surface (with juxtaposed hydrophilic and hydrophobic regions) is known as a promising technique to enhance water pool boiling at the atmospheric pressure, there is no research regarding its potential for sub-atmospheric applications. In the present study, we have investigated the characteristics of pool nucleate boiling on biphilic surfaces at sub-atmospheric pressures. Biphilic surfaces were made by applying Ni-TFEO (tetrafluoroethylene oligomer) electroplating (with a contact angle of about 140°) on a copper surface. The heat transfer performance of various biphilic surfaces (with different hydrophobic spot diameters and pitches) were measured in the pressure range from atmospheric to 6.9 kPa. At a pressure of 14.0 kPa, the wall superheat at the onset of nucleate boiling was reduced by 12 K on a biphilic surface compared with a mirror-finished copper surface. The experiment with three different biphilic patterns revealed that smaller pitch and diameter of the hydrophobic spots were favorable to heat transfer at 14.0 kPa. The enhancement of HTC over Kutateladze's correlation reached 270%. A sharp transition from continuous to intermittent boiling, resulting in large deterioration of HTC, was observed on a biphilic surface at a much lower pressure than that on a copper surface. Boiling performance was less affected by the pressure level above the transition pressure.