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The Influence of System Pressure on Bubble Coalescence in Nucleate Boiling

机译：系统压力对成核沸腾气泡合并的影响

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摘要

Boiling is one of the most effective heat transfer mechanisms. In spite of a long time of research, the physical fundamentals are still not sufficiently understood. Pursuing the objective to predict heat transfer based on physical and geometrical properties, experimental and numerical investigations are conducted at the institute of the authors. The focus of the presented research is the coalescence of two single bubbles under varying pressure conditions. In the experiment a thin stainless-steel foil is used as a Joule heater. The experiments were performed in a pressure range of 300-1000 mbar using FC72 as working fluid. Two types of heaters with a distance between two artificial nucleation sites of 300 μm (type 3) and 500 μm (type 5) were used. The experimental results indicate a strong dependence of the occurrence of bubble coalescence on pressure. For the type 5 heater, a Gaussian distribution for the coalescence frequency when plotted over pressure is observed. Experimental results with the type 3 heater show a similar distribution of the frequency with a shifted maximum. Further, it is shown that during bubble coalescence a small droplet can remain inside the bubble and enhance the heat transfer, which is attributed to an additional thin film region. The formation of this remaining droplet is sensitive to system pressure. Numerical investigations of bubble coalescence were conducted with the computational fluid dynamics (CFD) software OpenFOAM. In OpenFOAM, dynamic mesh handling allows high spatial resolution at the phase boundary, which is captured with the volume-of fluid method. Evaporation and a subgrid microscale model were implemented in the flow solver to account for evaporation at the phase boundary and the three-phase contact line. The results show a strong dependence of bubble dynamics and coalescence on contact angle and bubble growth rate. Although it was possible to observe the creation of the residual droplet, more effort needs to be put into finding appropriate initial conditions.
机译：沸腾是最有效的传热机制之一。尽管进行了长时间的研究，但仍未充分了解物理基础。为了基于物理和几何特性预测热传递，在作者协会进行了实验和数值研究。本研究的重点是在变化的压力条件下两个单个气泡的合并。在实验中，将薄不锈钢箔片用作焦耳加热器。使用FC72作为工作流体，在300-1000 mbar的压力范围内进行了实验。使用两种类型的加热器，两个人工成核位置之间的距离为300μm（类型3）和500μm（类型5）。实验结果表明，气泡聚结的发生对压力的强烈依赖性。对于5型加热器，在压力上绘制时会观察到聚结频率的高斯分布。 3型加热器的实验结果表明，频率的分布相似，最大值发生了偏移。此外，显示出在气泡聚结期间，小液滴可保留在气泡内部并增强传热，这归因于额外的薄膜区域。残留液滴的形成对系统压力敏感。使用计算流体动力学（CFD）软件OpenFOAM对气泡聚结进行了数值研究。在OpenFOAM中，动态网格处理允许在相边界处实现高空间分辨率，这可以通过体积法来捕获。在流动求解器中实现了蒸发和亚网格微尺度模型，以解决相边界和三相接触线处的蒸发。结果表明，气泡动力学和聚结强烈依赖于接触角和气泡生长速率。尽管可以观察到残留液滴的产生，但是需要花费更多的精力来寻找合适的初始条件。

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  • 来源
    《Heat Transfer Engineering》 |2014年第8期|420-429|共10页
  • 作者

    AXEL SIELAFF; JOCHEN DIETL; STEFAN HERBERT; PETER STEPHAN;

  • 作者单位

    Institute of Technical Thermodynamics, Technische Universitaet Darmstadt, Petersenstrasse 17, 64287 Darmstadt, Germany;

    Institute of Technical Thermodynamics, Technische Universitaet Darmstadt, Germany;

    Institute of Technical Thermodynamics, Technische Universitaet Darmstadt, Germany;

    Institute of Technical Thermodynamics, Technische Universitaet Darmstadt, Germany,Center of Smart Interfaces, Technische Universitaet Darmstadt, Germany;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);
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