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

滴状冷凝段共存增强膜式冷凝传热机理及优化

基本信息

批准号：
02650144
负责人：
KUMAGAI Satoshi
金额：
$ 1.09万
依托单位：
Tohoku University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1990
资助国家：
日本
起止时间：
1990 至 1991
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-02650144/
关键词：
Condensation heat transfer Filmwise condensation Dropwise condensation Coexistence Heat transfer enhancement Disturbance of liquid film Sweeping effect 熱伝導解析

项目摘要

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

本研究的目的是通过在膜状表面之间分布由有机涂层促进的最佳宽度的滴状冷凝表面来获得更高的膜状冷凝热通量，并且获得更高的冷凝系统的平均总传热系数。在相同的表面过冷度下，滴状冷凝的传热速率是膜状冷凝的几倍到几十倍。但是很难长时间保持滴状冷凝，因此在实践中几乎所有的冷凝设备都是在假设膜状冷凝将存在的基础上设计的。滴状冷凝促进剂具有有限的寿命，并且在该寿命结束时，表面的拒水性劣化为混合的滴状冷凝和膜状冷凝。因此，有必要发现一种用于滴状冷凝的永久促进剂，以实现高性能冷凝器。 ...更多信息可以施加到冷凝表面。然而，在聚合物涂层的情况下，应该克服两个矛盾的困难问题。第一个问题是涂层必须与金属基材长时间保持良好的附着力。通常，这需要更大的厚度。第二个困难是这些有机材料的低导热性。这需要的涂层的厚度小于5米。在这项研究中，方法进行了检查，获得更高的膜状冷凝热通量积极通过使滴状冷凝区域的足够的宽度，促进聚四氟乙烯膜状区域之间共存。对于水平取向，检查了几种不同的间距，将滴状部分布置在膜状部分上方，以清楚液滴下落到膜状部分的效果。膜状部分的热通量随着滴状部分的高度的增加而增加，直到2mm，但随后在高于该高度时减小。通过改变夹在恒定宽度的滴状部分之间的膜状部分的宽度，还测试了通过液滴在热传递中增强的膜状部分的程度。膜状部分的热通量在5和3 mm之间的宽度突然增加。因此，它被证明，存在一个最佳的宽度为每个部分，以提高在膜状部分的冷凝传热。少