Effects Of Micro/nanostructures On Dropwise Condensation Heat Transfer

Condensation is a phase change commonly found in nature and industrial production and life.Dropwise condensation has an excellent heat transfer effect,which is one order of magnitude higher than membrane condensation.Therefore,the study of the droplet condensation heat transfer process not only has a high scientific value,but also has broad application prospects.Many researchers have also been exploring how to promote drop condensation by modifying the condensed surface.However,the research on the micro-nano scale has become a hot spot in recent years.In this paper,technology of copper wire sintering was used to get the micron-scale heterogeneous surface on the copper surface.On the hydrophilic-superhydrophobic hybrid surface,the width of the hydrophilic region is about 35 μm and the superhydrophobic region is 65 μm.This non-silicon micromachining method has not been reported in the previous literature.On the basis of this technology,a hydrophobic-superhydrophobic hybrid surface was prepared at the same time.In this paper,heat transfer experiments were conducted on the two hybrid surfaces and nano-grass surfaces,and the polymer surface was used as a control experiment.The experiment was carried out in a pure steam environment.The experimental steam temperature in the experimental section was about 60 °C and the flow rate was 3 m/s.Cold side water temperature ranged from 0.5 °C to 40 °C,adjusting the supercooling of the experimental surfaces.The experimental results show that the condensation heat transfer coefficients of the four surfaces:hydrophilic-superhydrophobic surface<nano-grasses surface<hydrophobic-superhydrophobic surface<polymer surface.In addition,there were three surfaces except the polymer surface exhibiting the "failure"phenomenon.Due to the existence of nanostructures on the three kinds of failure surfaces,a detailed study was conducted on the surface of nano-grass surface.The heat transfer coefficient of fresh nano-grass surface reduced with the increase of superficial undercooling,from 90.46 kW/m2K to 63.7 kW/m2K,which was 3.17 to 4.5 times higher than that of membrane condensation.It was found through a high-speed camera that the droplet fell off in turn by jumping,rolling,sliding with the increase of undercooling,.With the long term experiments,the surface of nano-grass gradually failed,the final heat transfer coefficient was reduced to 35 kW/m2K,condensate surface also appeared"flooding".That was caused by the fracture of nanostructure which was confirmed by scanning electron microscopy.The experimental results show that the heat transfer coefficient of the polymer surface is between 110 kW/m K and 80 kW/m2K,higher than that of the fresh nano-grass surface.Through the model of nano-grass surface heat transfer thermal resistance,we found that the the steam thermal resistance in the nano-grass was too enormous.Besides,the heat transfer characteristics of fresh nano-grass and the droplet morphology were explained by establishing the model of droplet infiltrating the nano-grass surface and the model of droplet shedding.