Condensation heat transfer of steam on vertical dropwise and filmwise coexisting surfaces with a thick organic film promoting dropwise mode

This article describes an experimental investigation of the condensation heat transfer of steam on dropwise and filmwise coexisting (DFC) surfaces, on which dropwise and filmwise exist simultaneously at adjacent positions. A fluor-containing organic film with thickness of more than 1 mum was coated on the dropwise regions of the external surface of a brass tube to promote dropwise condensation. The surfaces were divided horizontally into many regions according to the designed dropwise and filmwise condensation area ratio. The area ratio of dropwise regions and filmwise regions in the present study was fixed at 50%:50% for all the six surfaces, while the numbers of dropwise and filmwise regions ranged from 2 to 16. Experiments were conducted at atmospheric pressure and the treated tube was oriented vertically in the condensing chamber. It was found that the heat transfer performance for DFC surfaces increases with increasing number of dropwise and filmwise regions, and an enhancement ratio of 1.27 to 1.96 is realized compared with the results for bare surface. Visual observation revealed that the appearance of condensation near the boundary region between the dropwise and filmwise regions was dependent on the relative positions of the two condensation regions. The condensate flowed smoothly across the boundary for dropwise condensation in the upper region. With filmwise condensation in the upper region, a condensate ring was formed at the interface and was retained at the interface for a short period of time before collapsing and then continued to flow downward through the dropwise condensation region. The condensate ring made a remarkable contribution to the condensation heat transfer enhancement for DFC surfaces. Finally, the results also showed that the heat transfer enhancement for dropwise and filmwise coexisting surfaces depends not only on the area ratio on DFC surfaces, but also on the surface subcooling degree. An optimal coordinating condition between these two factors might realize a maximum heat transfer enhancement ratio.