Flow Condensation In Mini-channel With Surface Of Different Wettability

The condensation process mostly happens on the surface.The need for surface wettability is different under different conditions.In the incipient stage of condensation,the hydrophilic surface is more likely to become the nucleation site.Therefore,the droplet nucleation rate and density are higher.However,as the condensation proceeds,the liquid on hydrophilic surface is hard to remove,so that the heat transfer efficient deteriorates.This contradiction has prompted this study into mixed wettability surfaces.This kind of surface effectively combines the advantages of both wettabilities,more importantly,creates a surface energy gradient on the surface that provides the self-driven force,making spontaneous movement of the droplet from the hydrophobic region to the hydrophilic region,which greatly accelerates the rate of dropwise condensation refreshment.So the combined effect of surface with mixed wettabilities is more advantageous than surface with uniform wettability.In this experiment,screen diffusion welding and polytetrafluoroethylene solution dropping method were used to prepare the hybrid surface.The screen was used as a mask.When the screen reached the softening temperature,the copper surface sintered together with the wire mesh node under the pressure of the weight.Then the sintered sample is dripped in a polytetrafluoroethylene solution and subjected to a high temperature hardening film.The copper surface sintered with the screen is not affected by the hydrophobic coating.The array of hydrophilic dots is formed after the wire is removed.the surface around the hydrophilic dots is covered by the coating to become hydrophobic.The polymer coating has good durability in high temperature and pressure,and can continuously cause dropwise condensation.The modified surface of this experiment is mainly applied to microchannels,and the microchannel heat exchanger is an effective method to solve the high heat dissipation of electronic devices.When the droplets are driven by steam in a long channel,they slide over the surface which is easily to be damage.Thus,the durable hydrophobic film is more advantageous than other structure.The mini-channel with a hydraulic diameter of 1.5 mm was produced,hoping to study the heat transfer enhancement of mini-channel with surface modification.The bottom surfaces(heat exchange surface)of three mini-channels were modified to be completely hydrophilic,completely hydrophobic and mixed,respectively.The other surfaces of a channel were adiabatic.The mass flow rate of and vapor quality was controlled at 10-60 kg/m2s and 0.3-1,respectively.The experimental results show that the heat transfer coefficient of the hydrophilic-hydrophobic mixed surface is 5.54 times that of the completely hydrophilic surface and 1.89 times that of the completely hydrophobic surface.The two-phase flow pressure drop of mixed surface is similar to the hydrophobic surface,but slightly higher than the hydrophilic surface.Through the visualization of high-speed photography,a series of dynamics of gas-liquid two-phase flow on the surface can be found,so as to obtain a mechanism for heat transfer enhancement of the mixed surface:1)hydrophobic surface and hybrid surface on which dropwise condensation occurs showed better heat transfer characteristic than hydrophilic surface.2)the hydrophilic dots provide sites for preferential nucleation of droplets.The condensation rate of the entire surface is increased.3)With surface energy gradient,the hydrophilic dots act as the center to attract the surrounding droplets,so that the droplets on the PTFE membrane converge toward the hydrophilic dots,accelerating the renewal rate of the droplet condensation on the hydrophobic region.4)Droplets attracted by the hydrophilic dots get deformation,and the contact angle is reduced to around 90 degrees,which reduces the thermal resistance.5)The frequency of droplets being flushed by steam on the pro-hydrophobic mixing surface is increased.This study provides a new way for the design of micro/mini-channel condensers in the future,such as printed circuit heat exchangers,micro heat pipes and so on.