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

研究了铝箔加热器表面高度润湿的液体在脉动加热细丝上产生的沸腾气泡撞击下的沸腾开始，以试图减少沸腾开始前的温度超调。研究了在乙醇和FC-72中，沸腾气泡撞击在逐步加热的铝箔表面上的沸腾诱导。还考察了通过沸腾诱导降低箔表面过热度的程度。得到了以下结论：(1)无论撞击时的壁面过热度大小，即使在负过热度下，当金属丝上的沸腾气泡在高壁面过热度下自发开始沸腾之前，在金属丝正下方的垂直和水平两个受热面上都会发生沸腾。(2)局部诱导的沸腾沿金属箔的换热表面快速传播，然后在整个表面上发展成核沸腾。然后，壁面温度降低到稳定的核态沸腾温度。在约60K以上(乙醇)，随着壁面过热度的增加，沸腾区域的扩展速率显著增加。(3)在受热面开始加热前几秒脉冲加热也能引起沸腾。(4)沸腾的诱导显著降低了分段加热过程中受热面的最大过热度。然而，当沸腾被诱导时，这个最大过热度似乎有一个下限。这一极限值可能取决于换热表面上的逐级产热率。(5)提出了各种可能的沸腾诱导机制。

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.