Falling-droplet-enhanced filmwise condensation in the presence of non-condensable gas
Falling-droplet-enhanced filmwise condensation in the presence of non-condensable gas
复制标题
DOI：
10.1016/j.ijheatmasstransfer.2019.05.110
复制
发表时间：
2019-09
影响因子：
5.2
通讯作者：
Rongfu Wen;Xing-Dong Zhou;Benli Peng;Z. Lan;Ronggui Yang;Xuehu Ma
中科院分区：
文献类型：
--
作者：
Rongfu Wen;Xing-Dong Zhou;Benli Peng;Z. Lan;Ronggui Yang;Xuehu Ma
在不凝结气体(NCG)存在的情况下强化凝结换热对于广泛的能源密集型工业应用具有重要意义。NCG存在下的蒸汽冷凝换热性能主要受冷凝面附近扩散层中蒸汽分子的初始成核和传质控制。大多数基于加速冷凝液去除的方法不能有效地改善凝结成核和蒸汽传输，这使得在NCG存在的情况下强化蒸汽凝结具有挑战性。在这里，我们提出了一种亲水性铜表面与间隔氟碳涂层疏水凸起，以实现下降液滴增强膜状凝聚在NCG的存在。由于亲水表面成核势垒的降低，水蒸气可以迅速成核并在表面形成一层薄薄的液膜。这种冷凝膜可以周期性地从间隔疏水凸起中移除，以防止液膜沿垂直表面的厚度增长，这归因于凝析液体在疏水凸起上的表面粘附性降低。更重要的是，从疏水凸起中排出的凝析液会以液滴的形式脱落，对NCG扩散边界层产生强烈的扰动。数值计算和可视化实验定量地表明，液滴下落可以显著改善水蒸气从主体水蒸气到凝结表面的传输，从而促进液滴的生长。实验证明，在NCG存在的情况下，强化膜状冷凝的高性能传热优于传统的膜状冷凝和滴状冷凝，同时通过使用耐用的氟碳涂层凸块避免了超薄疏水涂层的耐久性问题。
Enhancing condensation heat transfer in the presence of non-condensable gas (NCG) is of fundamental importance for a wide range of energy-intensive industrial applications. Heat transfer performance of vapor condensation in the presence of NCG is dominated by the initial nucleation and mass transfer of vapor molecules in the diffusion layer near the condensing surface. Most of the approaches based on accelerating condensate liquid removal cannot effectively improve the nucleation and vapor transport, which makes it challenging to enhance vapor condensation in the presence of NCG. Here, we present a hydrophilic copper surface with interval fluorocarbon-coated hydrophobic bumps to enable falling-droplet-enhanced filmwise condensation in the presence of NCG. Benefiting from the reduced nucleation energy barrier on the hydrophilic surface, water vapor can rapidly nucleate and form a thin liquid film on the surface. Such condensate film can be periodically removed from the interval hydrophobic bumps to prevent the thickness growth of liquid film along the vertical surface, which is attributed to the surface adhesion reduction of condensate liquid on the hydrophobic bumps. More importantly, the removed condensate liquid departing from the hydrophobic bumps can fall off in the form of droplets to strongly disturb the NCG diffusion boundary layer. Numerical calculations and visualization experiments quantitatively reveal that the falling droplets can significantly improve water vapor transport from the bulk vapor to condensing surface for droplet growth. High-performance heat transfer of the enhanced filmwise condensation in the presence of NCG is experimentally demonstrated to be better than both the conventional filmwise and dropwise condensation while avoiding the durability issues of ultra-thin hydrophobic coatings by utilizing durable fluorocarbon-coated bumps.