The Influence of System Pressure on Bubble Coalescence in Nucleate Boiling
The Influence of System Pressure on Bubble Coalescence in Nucleate Boiling
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DOI：
10.1080/01457632.2013.830917
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发表时间：
2014-03
影响因子：
2.3
通讯作者：
A. Sielaff;J. Dietl;S. Herbert;P. Stephan
中科院分区：
文献类型：
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作者：
A. Sielaff;J. Dietl;S. Herbert;P. Stephan
沸腾是最有效的传热机制之一。尽管进行了长时间的研究，但人们仍然没有充分了解物理基础。为了达到根据物理和几何性质预测换热的目的，作者在研究所进行了实验和数值研究。本文研究的重点是两个单一气泡在不同压力条件下的聚结。在实验中，使用了一种薄的不锈钢箔作为焦耳加热器。实验在300-1000mbar的压力范围内进行，以FC72为工质。使用两种类型的加热器，两个人工成核点之间的距离为300μm(类型3)和500μm(类型5)。实验结果表明，压力对气泡聚并的发生有很强的依赖性。对于5型加热器，在压力下绘制聚结频率图时，聚结频率呈高斯分布。在3型加热器上的实验结果表明，频率分布相似，但最大值发生了漂移。此外，研究还表明，在气泡合并过程中，小液滴可以留在气泡内部并增强传热，这归因于一个额外的薄膜区域。这一剩余液滴的形成对系统压力很敏感。利用计算流体力学(CFD)软件OpenFOAM对气泡聚并过程进行了数值研究。在OpenFOAM中，动态网格处理允许高空间分辨率的相边界，这是由流体体积方法捕获的。在流动解算器中采用蒸发和亚网格微尺度模型来考虑相边界和三相接触线上的蒸发。结果表明，接触角和气泡生长速率对气泡的动力学和聚结有很强的依赖性。虽然可以观察到残留液滴的产生，但需要付出更多努力来寻找适当的初始条件。
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.