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

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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