Mechanism and Optimization of Enhancement of Filmwise Condensation Heat Transfer by Coexistence With Dropwise Condensation Sections

本研究的目的是通过在膜状表面之间分配有机涂层促进的最佳宽度的滴状冷凝表面来获得较高的膜状冷凝热流，并获得较高的冷凝系统的平均总换热系数。众所周知，在相同的表面过冷度下，滴状冷凝的换热速率是膜状冷凝的几到几十倍。但滴状冷凝很难长时间保持，因此在实际应用中，几乎所有的冷凝设备都是在假设存在膜状冷凝的情况下设计的。滴状冷凝的促进剂有一个有限的寿命，在这段时间接近尾声时，表面的拒水性恶化为滴状和膜状的混合冷凝。因此，需要寻找一种永久的滴状缩合促进剂来实现高性能的冷凝。在这方面，具有低表面能的有机材料c…更适用于冷凝面。然而，在聚合物涂层的情况下，应该克服两个相互矛盾的难题。第一个问题是涂层必须与金属基材有很长一段时间的Fn-m附着力。通常，这需要更大的厚度。第二个困难是这些有机材料的导热系数很低。这就要求涂层厚度小于5m。在本研究中，通过使聚四氟乙烯促进的足够宽度的滴状冷凝区在膜状区之间共存，积极地研究了获得更高膜状冷凝热流的方法。对于水平方向，检查了几种不同的间距，在胶片方向的上方布置了一个滴状部分，以清楚地说明液滴落到胶片方向上的效果。膜状部分的热流密度随着滴状部分高度的增加而增加，当滴状部分高度达到2 mm时，热流密度减小。通过改变夹在恒定宽度的滴状段之间的膜状段的宽度，也测试了液滴强化换热的膜状部分的程度。膜状部分的热流密度在宽度为5~3 mm时突然增大。结果表明，膜状截面强化冷凝换热存在一个最佳的截面宽度。较少

This research aimed at obtaining higher filmwise condensation heat flux by distributing dropwise condensation surfaces of optimal width promoted by an organic coating among filmwise surfaces, and to get higher mean overall heat transfer coefficients in condensing systems. It is well known that heat transfer rate of dropwise condensation are several to several ten times those for filmwise condensation at the same surface subcooling. But it is difficult to maintain dropwise condensation for long periods of time, so in practice almost all condensing equipment is designed on the assumption that filmwise condensation will exist. Promoters of dropwise condensation have a finite lifetime, and the water repellency of the surface deteriorates to mixed dropwise and filmwise condensation near the end of that period. Therefore there is a need to discover a permanent promoter for dropwise condensation to realize high-performance condensers.In this regard, organic materials with low surface energy c … More an be applied to the condensing surface. In the case of polymer coating, however, two contradictory difficult problems should be overcome. The first problem is that the coating must have fn-m adhesion with the metal substrate for a long period of time. Generally, this requires a larger thickness. The second difficulty is the low thermal conductivity of these organic materials. That requires the thickness of the coating to be less than 5 m.In this research methods were examined of obtaining higher filmwise condensation heat flux positively by making dropwise condensation areas of adequate width promoted by Teflon coexist among filmwise areas. Several different spacing were examined for the horizontal orientation, arranging a dropwise section above a filmwise one, to make clear the effect of the drops falling down to the filmwise section. The heat flux of the filmwise part increased with increasing the height of the dropwise part up to 2 mm, but then decreased above that. The extent of the filmwise part that was augmented in heat transfer by drops was also tested by changing the width of the filmwise section sandwiched between dropwise sections of constant width. The heat flux of the filmwise part increased abruptly at the width between 5 and 3 mm. Consequently, it was shown that there exists an optimum width for each section for enhancing condensation heat transfer in the filmwise sections. Less