Depression of Incipient Boiling Temperature by Impingement of Boiling Bubbles Rapidly Generated on a Small Heater Close to the Wall

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The inception of boiling in a highly-wetting liquid on a foil heater surface subjected to the impingement of boiling bubbles generated on a fine wire heated pulsewise were investigated in an attempt to reduce the temperature overshoot before boiling incipience. The induction of boiling in ethyl alcohol and FC-72 by the impingement of boiling bubbles on a foil surface heated stepwise was examined. The extent to which the superheat of the foil surface can be reduced by the induction of boiling was also examined. The following conclusions were obtained.(1) Boiling is induced on both vertical and horizontal foil heat transfer surfaces at a location directly below the wire whenever boiling bubbles from the wire impinge on the heated foil surface before boiling begins spontaneously at a high wall superheat, regardless of the magnitude of the wall superheat at the time of impingement, even under negative superheat.(2) Boiling induced locally propagates rapidly along the heat transfer surface of the foil, and then nucleate boiling develops on the entire surface. The wall temperature then decreases to that of steady-state nucleate boiling. The extension rate of boiling area due to the propagation increases significantly with increasing wall superheat above approximately 60 K (ethyl alcohol).(3) Boiling can also be induced when the wire is heated pulsewise several seconds before the onset of heating of the heat transfer surface.(4) The maximum superheat of the heat transfer surface during stepwise heating is significantly lowered by the induction of boiling. However, there seems to be a lower limit for this maximum superheat when boiling is induced. The value of this limit may depend on the stepwise heat generation rate on the heat transfer surface.(5) Various plausible mechanisms are proposed for the induction of boiling.