Prediction of dynamic contact angles and bubble departure diameters in pool boiling using equilibrium thermodynamics
Prediction of dynamic contact angles and bubble departure diameters in pool boiling using equilibrium thermodynamics
复制标题
使用平衡热力学预测池沸腾中的动态接触角和气泡离开直径
DOI：
10.1016/j.ijheatmasstransfer.2017.07.013
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发表时间：
2017
影响因子：
5.2
通讯作者：
Ardron K
中科院分区：
文献类型：
--
作者：
Ardron K
根据平衡态热力学原理，提出了一个预测池沸腾中动态接触角和气泡脱离直径的模型。所示的方法给出了很好的协议与实验测量池沸腾在很宽的压力范围内，液体过热和工作流体。该模型表明，对于与高Jakob数（> 100）沸腾相关的快速生长的气泡，气泡形状由流体阻力而不是表面张力主导，导致气泡在生长期间具有主要的半球形形状。对于与较低Jakob数（< 50）相关的较慢生长的气泡，表面张力似乎主导流体阻力，导致气泡采用具有保持接近热力学接触角的接触角的球冠形状。这一观察结果与实验是一致的。该模型表明，在高压沸腾的热力学接触角的系统有一个控制的影响，气泡的离开尺寸。在高压沸腾的水的分离直径的分析支持的观点，热力学接触角是强烈依赖于温度，它是这种温度依赖性，是负责在高压沸腾中观察到的小气泡分离尺寸。以前的气泡脱离模型已经无法解释气泡脱离尺寸的压力依赖性。提出了水的热力学接触角与温度的关系式。目前的气泡分离模型需要一种计算气泡生长速率的方法，因为后者决定了作用在附着气泡上的流体阻力。为此，开发了一种新的预测气泡生长的方法，根据Scriven的解决方案传导控制气泡生长，与校正，考虑到不均匀的液体蒸发，微层蒸发，生长速度对气泡形状的影响，和冷却效果的微层加热表面上。预测方法给出了令人满意的协议与气泡的生长数据为广泛的池沸腾实验，也与微观尺度的CFD模拟的结果，个别气泡的增长。
A model is proposed for predicting dynamic contact angles and bubble departure diameters in pool boiling based on principles of equilibrium thermodynamics. The method is shown to give good agreement with experimental measurements for pool boiling over a wide range of pressures, liquid superheats and working fluids. The model suggests that for fast-growing bubbles associated with boiling at high Jakob Numbers (> 100) the bubble shape is dominated by fluid drag forces rather than surface tension forces, resulting in the bubbles having a predominantly hemispherical shape during growth. For slower-growing bubbles associated with lower Jakob numbers (< 50) surface tension forces appear to dominate over fluid drag forces, resulting in the bubbles adopting a spherical cap shape with a contact angle that remains close to the thermodynamic contact angle. This observation is in accordance with experiment. The model implies that in high pressure boiling the thermodynamic contact angle for the system has a controlling influence on bubble departure sizes. Analysis of departure diameters in high pressure boiling of water supports the view that the thermodynamic contact angle is strongly dependent on temperature, and that it is this temperature dependence that is responsible for the small bubble departure sizes observed in high pressure boiling. Previous models of bubble departure have been unable to explain the pressure dependence of bubble departure size. A correlation of the thermodynamic contact angle against temperature for water is proposed. The current model of bubble departure requires a means of calculating the bubble growth rate, as the latter determines the fluid drag forces acting on an attached bubble. A new predictive method for bubble growth is developed for this purpose, based on Scriven’s solution for conduction controlled bubble growth, with corrections to take account of non-uniform liquid superheat, microlayer evaporation, the effect of growth rate on the bubble shape, and the cooling effect of the microlayer on the heated surface. The predictive method gives satisfactory agreement with bubble growth data for a wide range of pool boiling experiments and also with results of microscale CFD simulations of the growth of individual bubbles.
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