Experimental Research On Dropwise Condensation Of Vapor

The heat transfer of vapor condensation is widely applied in the fields such as spaceflight, power production and chemical industry. The improvement of its heat transfer performance is of significance for energy conservation, raw materials and project expenditure saving. The dropwise condensation is an ideal way to enhance the performance of condensation by surface tension, since its heat transfer coefficients are several times even scores times those for filmwise condensation, thus its applications in industry will engender magnitude benefit.As experimental part of the scheme "Microscopic Mechanism of Dropwise Condensation and its Stability" (NSFC No.50276016), an apparatus for dropwise condensation experiment is designed and installed. By means of spreading hydrophilic material on the inner surface of observe-window-glass, the problem of steam fogging on the glass is solved and the scenes of dropwise condensation on the target surfaces can be observed clearly.In order to reveal the rule that the heat transfer performance changes with contact angles, a series of vapor condensation experiments are carried out on the brass-based walls. Dropwise condensation is realized on the walls coated by 10 kinds of materials such as Ethyl Cyanoacrylate Adhesive, stearic acid, PTFE, etc. The dropwise condensation heat transfer coefficient is from 2.28 times to 6.65 times those for filmwise condensation on the correlated brass wall, the Ethyl Cyanoacrylate Adhesive surface is the best surface for heat transfer among the surfaces studied. On the walls coated by other 5 kinds of materials such as -(CF)_n-, Phthalic Acid, Polyethyleneglycol Succinate and so on, the dropwise condensation phenomena doesn't appear, however, the heat transfer performance is strengthened and the coefficient of condensation is from 1 times to 1.97 times those for film condensation. In the range of little contact angles the rule is found to be reasonable that the heat transfer coefficient increases with the contact angle, the trend that the heattransfer coefficient change is identical to the trend that the decrements of chemical potential change with contact angles. An equation which describes the relationship between contact angles and heat transfer coefficient is derived from the data obtained.Different cases that the condensate of vapor in air aggregates on different surfaces of evaporator and falls from the surface were analyzed with theories of dropwise condensation. The results show that on the hydrophobic surface in heat pump evaporator frosting can be delayed and defrosting can be speeded under certain condition. A new way is brought forth for energy conservation in the heat pump evaporator and is of importance to abate urgent situation in electric power expenditure.