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Prediction of dynamic contact angles and bubble departure diameters in pool boiling using equilibrium thermodynamics

机译：使用平衡热力学预测水池沸腾中的动态接触角和气泡离开直径

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摘要

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.
机译：提出了一种基于平衡热力学原理预测池沸腾过程中动态接触角和气泡离开直径的模型。结果表明，该方法与各种压力，液体过热和工作流体池沸腾的实验测量结果吻合良好。该模型表明，对于与高雅各布数（> 100）沸腾相关的快速增长的气泡，气泡形状主要受流体阻力而不是表面张力的影响，从而导致气泡在生长过程中主要呈半球形。对于与较低的雅各布数（<50）相关的较慢增长的气泡，表面张力似乎胜过流体的拉力，导致气泡采用球形盖形状，且接触角保持接近热力学接触角。该观察结果与实验一致。该模型表明，在高压沸腾中，系统的热力学接触角对气泡离开尺寸有控制作用。分析高压水沸腾中的离气直径可支持以下观点：热力学接触角与温度密切相关，而正是这种温度依赖性导致在高压沸腾中观察到较小的气泡离析尺寸。以前的气泡离去模型无法解释气泡离去大小与压力的关系。提出了水的热力学接触角与温度的相关性。当前的气泡离开模型需要一种计算气泡增长率的方法，因为后者确定作用在附着气泡上的流体阻力。为此，基于Scriven的传导控制型气泡生长解决方案，开发了一种新的气泡增长预测方法，并进行了修正，以考虑到不均匀的液体过热，微层蒸发，增长率对气泡形状的影响以及微层在加热表面上的冷却效果。该预测方法与广泛的池沸腾实验中的气泡生长数据以及单个气泡生长的微型CFD模拟结果给出了令人满意的一致性。

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  • 来源
    《International Journal of Heat and Mass Transfer》 |2017年第11期|1274-1294|共21页
  • 作者

    Ardron K. H.; Giustini Giovanni; Walker Simon P.;

  • 作者单位

    Nuclear Research Group, Mechanical Engineering Department, Imperial College London, United Kingdom;

    Nuclear Research Group, Mechanical Engineering Department, Imperial College London, United Kingdom;

    Nuclear Research Group, Mechanical Engineering Department, Imperial College London, United Kingdom;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);
  • 原文格式 PDF
  • 正文语种 eng
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