Numerical Analysis of Boiling Bubble Growth under Electric Fields

通过数值分析揭示了电-水动力效应强化换热的机理。仿真代码的开发分为以下三个阶段：(A)无电场、无相变情况，(B)有电场、无相变情况，(C)有电场、有相变情况。首先，为了验证(B)阶段的程序，模拟了均匀电场下R113气泡的伸长率，并与Garton等人的实验结果进行了对比，验证了结果。在直径为0.4mm的下圆盘电极上放置一个初始直径为0.2mm，接触角为15 / kg的R113气泡。上圆盘电极直径1mm，两电极之间的距离1mm。施加到电极上的电压分别为5、10和20kV。在每个施加电压下，气泡都会变形并与下电极分离。粗略估计，在5kV的情况下，静电力是重力的100倍。这一结果定性地解释了为什么在非均匀场下会产生细小的沸腾气泡。为了验证最后阶段(C)的程序，模拟了氦气泡在加热板上的行为。试验计算条件为初始气泡直径20mum，表面超热0.1K，大气压。所得气泡偏离直径为55 μ m，与已有的实验结果定性一致。当施加非均匀电场时，氦气泡的气泡行为与R113气泡的结果几乎相同。

Numerical analysis has been performed to disclose the mechanisim of heat transfer enhancement by EHD (Electro-Hydro-Dynamic) effects. The simulation code has been developed in the following three stages ; (A) the case without electric field and phase change, (B) the case with electric field and without phase change, and (C) the case with electric field and phase change. First, to check the program in stage (B), elongation of R113 bubble under uniform electric fields was simulated and confirmed by comparing the results with the experiment reported by Garton et al.The second, bubble diparture under non-uniform electric fields were simulated. An R113 bubble initially at a diameter of 0.2mm with a contact angle of 15ﾟ are placed on the lower disc electrode of 0.4mm in diameter. Upper disc electrode has 1mm in diameter and the distance between two electrodes is 1mm. The voltage applied to the electrodes are 5,10 and 20kV.The bubble was deformed and separated from the lower electrode in each applied voltage. It was found by the rough estimation in case of 5kV that the electrostatic force is 100 times as much as the gravity force. This results gives us a qualitative explanation why fine boiling bubbles are created under non-uniform fields.To check the program in final stage (C), behavior of helium bubble on a heated plate was simulated. Calculation conditions tested are iniial bubble diameter of 20mum, surface super heat of 0.1K and atmospheric pressure. The bubble departure diameter obtained is 55mum which qualitatively agrees with a reported experimantal result. When we applied non-uniform electric field on the helium bubble, the bubble behavior was almost the same as the results in R113 bubble.