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

过冷池沸腾高热通量受热面附近的液-汽结构

• 批准号: 13650204
• 负责人: SAKASHITA Hiroto
• 金额: $ 1.28万
• 依托单位: HOKKAIDO UNIVERSITY
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 2001
• 资助国家: 日本
• 起止时间: 2001 至 2002
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-13650204/
• 关键词: Subcooled boiling; Conductance probe; Capacitance probe; Critical heat flux; Liquid macrolayer; Primary bubble; Growing time; Waiting time; マクロ液膜厚さ; 静電容量法; 触針法; 気泡寿命; プール沸騰; 気液挙動; 測定

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

在高热容量受热面的过冷池沸腾过程中，形成大的聚结气泡并反复分离，在气泡底部留下一层薄薄的富液层（称为液体巨层）。当富液层在大的凝聚气泡脱离之前干燥时，就会出现临界热流密度。另一方面，由于热容量较低，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临界热流密度。因此，弄清液体巨层的结构和短寿命细气泡的行为对解决过冷池沸腾临界热流的机理具有重要意义。本研究开发了两种测量方法；一种电导探头法和电容探头法。在电导探针法中，采用尖端直径约5μm的探针测量液汽结构，测量精度较高。在过冷0 ~ 30K范围内，对水在朝上表面沸腾进行了更多的测试。结果表明，大气泡所对应的探测信号在离受热面几乎相同的高度处消失。这个高度被认为是液体巨层。测量到的巨层比先前提出的饱和池沸腾与更高热通量的相关性的外推要厚得多，这很可能是CHF随着过冷度的增加而增加的原因。提出了一种电容探针法，用于高精度测量过冷沸腾过程中细小气泡的行为。电容探头具有很高的灵敏度，足以检测气泡周期从产生到崩溃，在0.1msec内终止。或更少。在过冷0 ~ 70K范围内，测量了水在上表面沸腾时的初级气泡的生长时间和等待时间。在10K以上，随着过冷度的增加，生长时间迅速缩短。等待时间随亚冷时间的增加呈现先增大后减小的趋势，这与经典成核理论的预测不一致。少