Parametric Effects of Heater Size, Contact Angle, and Surrounding Vessel Size on Pool Boiling Critical Heat Flux From Horizontal Surfaces

Saturated water at one-atmosphere pressure was boiled on horizontal flat copper disks 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 disks, while the surrounding vessel size was changed by placing glass tubes of different inner diameters around the disks. Nucleate boiling heat transfer data were obtained up to critical heat flux (CHF), where vapor removal pattern was photographed. Vapor jet diameter and the dominant wavelength at water–steam interface were measured from the photographs for the well wetted disks. For well wetted surfaces, the magnitude of CHF increased when the heater size was reduced from 2.0 to 1.0 cm. Improving the wettability enhanced the CHF substantially, whereas the increased size of the liquid-holding vessel had a smaller effect. The highest measured CHF is 233 W/cm² or 2.11 times Zuber's CHF prediction for infinite horizontal flat plates. It was obtained on a 1.0-cm-diameter disk of contact angle about 10 deg surrounded by a large vessel. The CHF for this surface was increased from 203 to 233 W/cm² when the ratio of heater size to surrounding vessel size was decreased from 1 to about 0.