Enhancement of Liguid-Solid Contact in Miniaturization Phenomena

本文研究了两种沸腾现象：微小气泡发射沸腾和液滴微型化沸腾中液固接触的增强。在前者中，在受热表面上产生的气相不是覆盖表面，而是以许多微小的蒸汽泡的形式出现，喷射到过冷的液体中，在过冷池中沸腾。然而，在后一种情况下，当液体体积撞击加热表面时，会产生许多微小的液滴分散到大气中，这被称为液滴沸腾。液固接触的增强是通过微小气泡的喷射和微小液滴的分散引起的扰动来实现的，这些扰动分别由空隙探针和压电电位方法测量。比较了气泡喷射和液滴扩散的周期。在微型化沸腾情况下，利用埋设在受热面下的热电偶监测受热面温度随时间的变化，评估微型化过程的平均表面温度和热流密度，并与微小气泡发射沸腾过程的稳态池沸腾曲线进行比较。针对液固接触沸腾，将低熔点的熔体放入盛满水的池中，在稳态下进行液液直接接触沸腾。在过热度20 ~ 200K范围内得到了沸腾曲线。结果表明，尽管熔体与水之间存在剧烈的相互作用，但热流密度低于1 MW/m^2。结果表明，三种微型化沸腾现象均发生在相同的阈值温度下，在180 ~ 210℃范围内，微型化发生的机理相同。

This research concerns with augmentation of liquid-solid contact in two boiling phenomena : minute bubble emission boiling and miniaturization boiling of droplets. In the former, vapor phase generated on a heating surface, instead of covering the surface, appear in many minute-size vapor bubbles ejecting into subcooled liquid, in a subcooled pool boiling.In the latter, however, a liquid volume, upon its impingement upon a heating surface, produces numerous minute-size droplets dispersing into the atmosphere, referred to as liquid droplet boiling. Enhancements in liquid-solid Contact are achieved through disturbances due to the ejection of minute bubbles and dispersion of minute droplets which were measured by means of the void probe and piezoelectric potential methods, respectively. The periods of bubble ejection and droplet dispersion were compared. In the miniaturization boiling case, the timewise variation of heating surface temperature was monitored using thermocouples that were embedded underneath the heating surface, from which the time-wise average surface temperature and heat flux during miniaturization process were evaluated and compared with steady-state pool boiling curve of the minute bubble emission boiling process.In connection with the liquid-solid contact boiling, the liquid-liquid, direct-contact boiling was performed at steady state by placeing a molten ally of low melting point into a pool filled with water. The boiling curve was obtained in the range of the superheat from 20 to 200K.It was disclosed that the heat flux was under 1 MW/m^2, although a violent interaction was observed between the melt and the water.It is concluded from a comparison based on these measured results that the three miniaturized boiling phenomena occur at the same threshold temperature, in the range of about 180 to 210ﾟC with miniaturization taking place under the same mechanism.