Enhancement of Liguid-Solid Contact in Miniaturization Phenomena

本研究主要针对两种沸腾现象：微小气泡喷射沸腾与液滴微小化沸腾，探讨强化液固接触之方法。在前者中，在加热表面上产生的蒸汽相，而不是覆盖表面，出现在许多微小的气泡喷射到过冷液体中，在过冷池沸腾中。然而，在后者中，液体体积，当其撞击在加热表面上时，产生许多微小的液滴分散到大气中，称为液滴沸腾。液-固接触的增强是通过由于微小气泡的喷射和微小液滴的分散而引起的扰动来实现的，所述微小气泡的喷射和微小液滴的分散分别通过空隙探针和压电电势方法来测量。比较了气泡喷射和液滴分散的周期。在小型化沸腾情况下，利用埋置在加热表面下的热电偶监测加热表面温度随时间的变化，计算了小型化过程中的平均表面温度和热流密度，并与微小气泡喷射沸腾过程的稳态池沸腾曲线进行了比较。通过将低熔点的熔融合金放入充满水的池中，在稳态下进行直接接触沸腾。在20 ~ 200 K的温度范围内得到了沸腾曲线，发现虽然熔体与水之间存在强烈的相互作用，但热流密度小于1 MW/m^2，并通过比较得出结论：这三种小型沸腾现象发生在相同的临界温度下，在约180至210摄氏度的范围内，在相同的机制下发生小型化。

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.