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

在高热容受热面上的过冷池沸腾中，形成大的聚结气泡，并反复分离，并在底部停留在薄薄的富液层(称为液体宏观层)上。当富含液体的层在大的聚结气泡脱离之前干燥时，临界热流密度就出现了。另一方面，在热容较低的情况下，即使在接近临界热流密度的高热流密度下，也会在很短的时间内形成大量细小气泡并崩塌，这些细小气泡的合并触发了临界热流密度。因此，弄清过冷池沸腾临界热流密度的测量方法，包括电导探针法和电容探针法，对于解决过冷池沸腾临界热流密度的机理具有重要意义。在电导探针法中，使用针尖直径约5μm的探头高精度测量汽液两相结构。测量员…在0K到30K的过冷度范围内，水在朝上的表面沸腾时的沸腾温度更高。结果发现，与大的聚结气泡相对应的探测信号在距离加热面几乎相同的高度消失。该高度被认为是液体大分子层。测量到的宏观层比先前提出的饱和沸腾关联式外推到更高的热流密度时要厚得多，这很可能是CHF随着过冷度的增加而增加的原因。电容探头具有足够高的灵敏度，可以检测从发电到崩溃的气泡周期，该周期在0.1毫秒内终止。或者更少。在过冷度为0K~70K的范围内，测量了水在上升表面沸腾时一次气泡的生长时间和等待时间。当过冷度超过10K时，生长时间随过冷度的增加而迅速缩短。等待时间随过冷时间的增加呈现出先增大后减小的趋势，这与经典形核理论的预测不一致。较少

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