UCLA

Nucleate boiling is ubiquitous from daily lives to engineering applications, and the increasing demand for efficient heat transfer in the industry requires further understanding of it, especially on small scales where the knowledge on conventional scales may no longer apply. This study investigated the effects of heater size, contact angle, surrounding vessel size, and surface structure on nucleate boiling heat transfer occurring on horizontal flat surfaces where the heater size is comparable to the two-dimensional ''most dangerous'' Taylor wavelength, aiming to comprehend the parametric effects on nucleate boiling heat flux and critical heat flux (CHF) and revisit the hydrodynamic theory near critical condition.

Saturated water at one-atmosphere pressure was boiled on horizontal flat copper discs of diameters 1.0, 1.5, and 2.0 cm, respectively. The contact angle was varied from about 10 to 80 deg by controlling thermal oxidation of the discs, while the surrounding vessel size was changed by placing glass tubes of different inner diameters around the discs. The surface structure in the form of microgrooves was fabricated by sanding the disc top surface. Boiling heat transfer data were obtained up to CHF. Boiling curves and CHFs measured under different experimental configurations were compared in terms of each parameter. Rohsenow's correlation was employed to assess the parametric effects on nucleate boiling heat flux quantitatively. Vapor removal patterns were photographed in nucleate boiling regime and near CHF. Vapor jet diameter and the dominant wavelength at water-steam interface were measured from the photographs for the well wetted discs and used to predict the corresponding CHF based on the hydrodynamic theory.

For well wetted surfaces, the boiling curve was insensitive to the heater size, but the CHF increased when the heater size was reduced from 2.0 to 1.0 cm. Improving the wettability delayed the onset of nucleate boiling and shifted the boiling curve to the right while enhancing the CHF substantially. Enlarging the liquid-holding vessel hardly affected the boiling curve at low heat fluxes but improved the CHF slightly. The structured surface featured higher nucleate boiling heat flux resulting from more active nucleation sites but showed no advantage in CHF over the plain surface of similar size and contact angle. In this study, the highest measured CHFs for plain and structured surfaces are close and both about 2.1 times Zuber's CHF prediction for infinite horizontal flat plates. They were obtained on well wetted 1.0-cm-diameter discs surrounded by large vessels.