| XU Jin-ping,JIA Yi-cong,JI Xu,FAN Quan-hai,ZHANG Hai-lin.Condensation Heat Transfer Enhancement by Copper-based Superhydrophobic Surface Modification[J],52(7):306-314 |
| Condensation Heat Transfer Enhancement by Copper-based Superhydrophobic Surface Modification |
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| DOI：10.16490/j.cnki.issn.1001-3660.2023.07.028 |
| KeyWord:superhydrophobic coating contact angle filmwise condensation dropwise condensation heat transfer coefficient |
| Author | Institution |
| XU Jin-ping |
School of Energy and Environmental Science, Yunnan Normal University, Kunming , China |
| JIA Yi-cong |
School of Energy and Environmental Science, Yunnan Normal University, Kunming , China |
| JI Xu |
School of Energy and Environmental Science, Yunnan Normal University, Kunming , China |
| FAN Quan-hai |
School of Energy and Environmental Science, Yunnan Normal University, Kunming , China |
| ZHANG Hai-lin |
School of Energy and Environmental Science, Yunnan Normal University, Kunming , China |
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| Abstract: |
| Hydrophobic copper stearate powder was prepared by the reaction of anhydrous copper sulfate with sodium stearate. It was mixed with epoxy resin in the ratio of 5∶1 by mass and coated on the copper sheet uniformly by electrostatic spraying. The powder was cured in an oven at 80 ℃ for 12 h and then removed. The light blue copper stearate powder evenly covered the copper-based surface, and the film was stable and unlikely to peel off. The surface water droplets appeared as full spheres and did not appear to be spreading. SEM images and the microscopic morphology showed scale-shaped and staggered stacking arrangement, forming a micro- and nano-graded structure. Its static contact angle reached 154.3°, and the superhydrophobic film layer was successfully constructed on the copper surface. The same process conditions were used to construct a superhydrophobic film on the coil heat exchanger surface to realize the transformation of its surface filmwise condensation to dropwise condensation and to strengthen the heat and moisture transfer during water vapor condensation. The steam generation system was used to provide constant steam to the coil heat exchanger without interruption, as well as to enable high-precision temperature and flow measurement. When water vapor came into contact with the wall of the copper tube below its saturation temperature, the condensate quickly spread into a liquid film and adhered to the wall. It continuously accumulated until gravity was greater than the viscous force and slowly slides off from the wall. When the surface of the heat exchanger coil was transformed into a superhydrophobic state, the condensate could not wet the wall surface well and formed multiple small droplets and then dripped down rapidly, which could effectively achieve droplet jumping and self-removal. The heat transfer performance of the tube was studied in convective heat transfer experiments. Due to the good balance between droplet nucleation and separation, the heat transfer coefficient of the superhydrophobic tube was increased by 38% compared with that of the copper tube. The superhydrophobic film did not peel off or lose the superhydrophobic performance when the high temperature air flowed through. The variation of temperature and humidity of water vapor inlet and outlet and the variation of water temperature in the condenser inlet and outlet was recorded. Dropwise condensation was difficult to produce and maintain for a long time on conventional metals. The hydrophobic coating was prepared by electrostatic spraying on the copper-based heat exchanger surface to realize the transformation from filmwise condensation to dropwise condensation during the condensation. The prepared hydrophobic material and thermosetting powder with a ratio of 5∶1 showed the best stability performance with a contact angle of 154.3° and the best coating performance. The gas condensation experiments on the heat exchanger surface showed that the superhydrophobic filmwise could effectively achieve droplet jumping and self-removal, and the high temperature airflow did not cause the superhydrophobic filmwise to peel off or lose its superhydrophobic performance, and the heat transfer coefficient was 38% higher than that of conventional coils. |
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