Heat transfer during condensing droplet coalescence
Heat transfer during condensing droplet coalescence
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DOI：
10.1016/j.ijheatmasstransfer.2018.07.005
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发表时间：
2018-12
影响因子：
5.2
通讯作者：
S. Adhikari;A. Rattner
中科院分区：
文献类型：
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作者：
S. Adhikari;A. Rattner
滴状冷凝产生的热通量比膜状冷凝高一个数量级。聚结是超过小阈值尺寸的冷凝液滴的主要生长模式（例如，在1atm下，水的半径为r bbb20 μm），但之前没有研究定量评估聚结过程中的传热。以前的水滴凝结模型通常将聚结描述为瞬时事件，传热速率逐步降低。然而，准稳定液滴温度分布的凝聚和恢复需要有限的时间，在此期间，液滴直接冷凝换热速率逐渐衰减。此外，在此期间，液滴可能振荡，反复清除周围表面，导致高总热通量。本研究采用流体体积（VOF）模拟来定量评估液滴合并过程中的这两个瞬态传热过程。结果表明，传热衰减的直接机制可以用一个时间常数τ的衰减指数函数来表示。模拟执行确定τ(r 1, Rt)(1μm⩽r 1⩽25μm, 1⩽Rt⩽4)r 1在哪里的半径较小的液滴和Rt是两个合并液滴半径之间的比例。对于大气压下的水，这跨越了大部分热传递发生在表面上的液滴大小范围（~ 80%）。对于所研究的液滴尺寸范围、流体性质和表面条件，提出了τ (r 1, Rt)的简单相关性。这些模拟还用于确定由于液滴凝聚时周围表面的反复清除而导致的传热增强的数量级。研究结果可以改进水滴凝结模型，更准确地预测凝聚过程中的瞬态传热。
Dropwise condensation can yield heat fluxes up to an order of magnitude higher than filmwise condensation. Coalescence is the primary mode of growth for condensing droplets above a small threshold size (eg, radius r> 2 μm for water at 1 atm), but no prior studies have quantitatively assessed heat transfer during coalescence. Previous models of dropwise condensation have generally described coalescence as an instantaneous event, with a step reduction in heat transfer rate. However, coalescence and recovery of a quasi-steady droplet temperature profile requires a finite time, during which the direct droplet condensation heat transfer rate gradually decays. Additionally, during this period, the droplet may oscillate, repeatedly clearing the surrounding surface and resulting in high overall heat fluxes. This study employs Volume-of-Fluid (VOF) simulations to quantitatively assess these two transient heat transfer processes during droplet coalescence. It is shown that the direct mechanism of gradual heat transfer decay can be represented by a decaying exponential function with a time constant τ. Simulations are performed to determine τ (r 1, Rt) for (1 μ m⩽ r 1⩽ 25 μ m; 1⩽ Rt⩽ 4) where r 1 is the radius of the smaller droplet and Rt is the radius ratio between the two merging droplets. For water at atmospheric pressure this spans the range of droplet sizes through which most of the heat transfer occurs on a surface (∼ 80%). A simple correlation is proposed for τ (r 1, Rt) for the studied droplet size range, fluid properties, and surface conditions. These simulations are also employed to determine the order of magnitude of heat transfer enhancement due to repeated clearing of the surrounding surface as droplets coalesce. Findings can inform improved models of dropwise condensation that more accurately predict transient heat transfer during coalescence events.