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

为了减少沸腾起始前的温度超调，研究了脉冲加热的细金属丝上产生的沸腾气泡冲击箔加热器表面的高润湿性液体的沸腾起始。本文研究了乙醇和FC-72中沸腾气泡撞击分级加热的金属箔表面诱导沸腾的现象。还考察了诱导沸腾可以降低箔表面过热的程度。得出以下结论。(1)在垂直和水平箔传热表面上，在金属丝正下方的位置处，每当来自金属丝的沸腾气泡撞击在加热箔表面上时，都会在高壁厚处自发地开始沸腾，而不管撞击时壁厚的大小，即使在负壁厚下也是如此。(2)局部诱导沸腾沿着传热面迅速传播，然后在整个传热面发展成核态沸腾。然后壁温降低到稳态核态沸腾的温度。由于传播的沸腾面积的扩展速率显着增加，增加壁面温度高于约60 K（乙醇）。(3)当在传热表面开始加热之前几秒钟脉冲式加热金属丝时，也可以引起沸腾。(4)沸腾诱导显著降低了阶梯加热过程中传热表面的最大导热系数。然而，似乎有一个下限，这个最大的沸腾时，诱导。该极限的值可以取决于传热表面上的逐步热生成速率。(5)各种合理的机制提出了诱导沸腾。

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.