Liquid-Vapor Structure near Heating Surface at High Heat Flux in Subcooled Pool Boiling

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In subcooled pool boiling on heating surfaces with high thermal capacity, large coalesced bubbles are formed and detach repeatedly and rest thin liquid-rich layer (termed liquid macrolayer) on their bottom. Critical heat flux occurs when the liquid-rich layer dries out before detachment of the large coalesced bubbles. On the other hand, with low thermal capacity, numerous fine bubbles are formed and collapse in very short time even at high heat flux near critical heat flux, and coalescence of these fine bubbles triggers the critical heat flux. Therefore, clarifying both the structure of liquid macrolayer and the behavior of short-lived fine bubbles is important to resolve the mechanism of critical heat flux of subcooled pool boiling.The present sutdy developed two measurement methods ; a conductance probe method and a capacitance probe method. In the conductance probe method, the probe with about 5μm tip diameter was used to measure liquid-vapor structure with high accuracy. Measuremen … More ts were made for water boiling on an upward-facing surface in the range of subcooling 0K to 30K. It was found that the probe signals corresponding to the large coalesced bubbles disappear at the almost same height from the heating surface. This height was regarded as the liquid macrolayer. The measured macrolayer was considerably thicker than the extrapolation of the previously proposed correlations for saturated pool boiling to higher heat flux and this is most likely to be the cause that the CHF increases with the increase in subcooling.A capacitance probe method was developed to measure the behavior of fine vapor bubbles in subcooled boiling with high accuracy. The capacitance probe has high sensitivity enough to detect the bubble cycle from generation to collapse that terminates within 0.1msec. Or less. The growing time and the waiting time of primary bubbles were measured for water boiling on upward surfaces in the range of subcooling 0K to 70K. The growing time decreases rapidly with the increase in the subcooling over 10K. The waiting time show the tendency that initially increases and then decreases with increasing subcoolong, which is inconsisitent with the prediction of classical nucleation theories. Less