Boiling Heat Transfer Characteristics with Highly Wettable Heated Surface under Forced Convection Conditions

Abstract

Under forced convection and atmospheric pressure conditions, heat transfer characteristics were studied using an annulus channel of a heater rod with highly-wettable surface. Improvement of phase change heat transfer under boiling and condensation process requires that either liquid heated or vapor cooled can contact a heat transfer surface effectively, but either a vapor layer or a liquid film can be formed repeatedly on the surface during boiling and condensation processes. As oxide semiconductor materials are known to be highly wettable, TiO 2 was coated on a heater rod surface. The Leidenfrost temperature for the TiO 2 coated surface was higher than that for the uncoated surface. Under mass flux conditions from 600 to 1 700 kg/m 2 s, the heater rod surface temperature at variable power conditions was measured. A higher heat flux at the point where the forced convection vaporization started and a larger critical heat flux were observed for the TiO 2 coated surface than that for the uncoated surface.Key Words: Forced Convection, Critical Heat Flux, Wettable Heater Surface, Oxide Semiconductor Coating ObjectivesPower plants, especially nuclear power plants of a light water reactor, occupy the major portion of electric power generation in Japan. Heat transfer enhancements of reactor cores and steam generators are always requested, though much effort has been made so far. For example, in boiling water nuclear reactors, improvements in critical power of thermal limits within reactor fuel assemblies have proceeded from hydrodynamic point of view under annular-dispersed two-phase flow conditions(1), (2) . In recent years, two major photo-effects are found of photocatalytic reaction and super-hydrophilic effects. The practical application of the photocatalytic reaction to corrosion reduction is under development (3) . Improvement of phase change heat transfer under boiling and condensation processes requires that either liquid heated or vapor cooled may come in contact with a heat transfer surface more effectively, while either a vapor layer or a liquid film can be * Received 30th July, 2003 (No. 03-4181) * * Toshiba Corporation, 8 Shin-sugita-cho, Isogo-ku, Yokohama, Kanagawa 235-0082, Japan. E-mail: mitutake@hflwing.tokai.jaeri.go.jp * * * Toshiba Corporation, 4-1 Ukishima-cho, Kawasaki-ku, Kawasaki, Kanagawa 210-0862, Japan formed repeatedly on the surface during boiling and condensation processes. In the phase change processes, it is important from the heat transfer point of view, that liquid film should cover smoothly and quickly over no-wetted region where vapor is generated on a heated surface. There are several researches published relating to hydrophilic effects on heat transfer characteristics (4), (5) . They are liquid film downward flow evaporation heat exchanger, pool boiling critical heat flux, evaporation of liquid droplets on oxide metal surfaces. Recently, the experimental study was reported on the radiation induced surface activation effect (6), (7) which showed enhancement in wettability for wat...

Jet ejection from droplets near the Leidenfrost temperature

“…Plate 2 had a much larger surface roughness than plate 1. Figure 3 shows the measured droplet lifetimes on plate 1 for droplets dropped from different heights (Mitsutake et al 2004) have been used in previous studies to prevent the droplet from escaping and to enable the evaporation time to be easily determined. However, a flat surface was selected in our experiments to enable the instantaneous droplet behavior to be imaged at impact.…”