Modeling of heat and mass transfer for dropwise condensation of moist air and the experimental validation
Modeling of heat and mass transfer for dropwise condensation of moist air and the experimental validation
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DOI：
10.1016/j.ijheatmasstransfer.2017.12.059
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发表时间：
2018-05
影响因子：
5.2
通讯作者：
Shaofei Zheng;F. Eimann;C. Philipp;T. Fieback;U. Gross
中科院分区：
文献类型：
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作者：
Shaofei Zheng;F. Eimann;C. Philipp;T. Fieback;U. Gross
建立了描述湿空气在冷基体上滴状冷凝过程中液滴生长的单液滴模型。冷凝过程由液滴表面分为两个部分。第一步，即从周围环境到液滴表面的传质过程，由动力学理论和连续流体动力学定律建模，该定律使用两区域概念（Knudsen层和连续区域）在任何液滴尺寸和任何不凝气体（NCG）浓度下制定。第二步，即通过液滴的热传递，由傅立叶热传导定律控制。这三个区域（连续体区域、努森层和液滴内部区域）通过匹配质量流率和能量流率而合并。由此，可以根据不同的条件来评估液滴的生长速率、液滴的成核尺寸、液滴表面和努森层界面处的温度。为此，开发了一种数值算法，以充分详细地反映液滴动力学，包括成核，生长/聚结，滑落/脱落，再成核。将单液滴模型中的生长速率和最小半径引入到生长算法中，对整个冷凝过程进行了模拟。此外，还进行了不同相对湿度下湿空气滴状冷凝实验，对模拟结果进行了验证。结果表明，本文所建立的湿空气滴状冷凝液滴生长模型是可靠的。本文的模型和实验还表明，空气中的水蒸气从自由流向液滴表面的扩散阻力对湿空气滴状冷凝的传热性能有重要影响。
A single droplet model is developed to describe the droplet growth during dropwise condensation of moist air on a cold substrate. The condensation process is divided by the droplet surface into two parts. The first step, i.e. the processes of mass transfer from the surroundings to the droplet surface, is modeled by the Kinetic theory and the laws of continuum fluid dynamics formulated using the two-region concept (Knudsen layer and continuum region) at any droplet size and at any concentration of non-condensable gas (NCG). The second step, i.e. the heat transfer across the droplet, is governed by Fourier’s law of heat conduction. These three regions (the continuum region, the Knudsen layer and the region inside the droplet) are incorporated by the matching both the mass flow rates and the energy flow rates. From these, the droplet growth rate, the nucleation size of droplet, the temperature at the droplet surface and Knudsen layer interface can be evaluated depending on different conditions. For this, a numerical algorithm is developed to reflect the droplet dynamics sufficiently detailed, including nucleation, growth/coalescence, slide-off/fall-off, re-nucleation. This is applied to the simulation of the entire condensation process by putting the growth rate and minimum radius from single droplet model into the growth algorithm. Additionally, dropwise condensation experiments of moist air in different relative humidity (RH) are carried out for validation of the simulation results. Good agreement is obtained which demonstrates that the present droplet growth model for dropwise condensation of moist air is credible. The current model and experiments also indicate that the diffusion resistance of water vapor in air from the free stream toward the droplet surface has a significant influence for the heat transfer performance of dropwise condensation of moist air.