| Vapor condensation process includes mass transfer in gas phase and heat transfer inphase change. Pure vapor condensation is controlled by the conduction of condensate film,while condensation heat transfer in the presence of non-condensable gas is usually dominatedby the mass diffusion of the gas side. Both the interfacial effect of solid-liquid and gas-liquidcan influence the heat transfer characteristic. The solid-liquid interfacial effect is to alter themode of condensation, from common filmwise to dropwise, while the gas-liquid interfacialeffect is to enhance conduction and mass transfer in gas phase diffusion process, by motion ofthe condensate.The most promising way to achieve dropwise condensation is modifying heat transfersurface by utilizing organic coatings with low surface free energy. Several factors in heattransfer of vapor mixture condensation are analyzed. Considering to these effects, thefunctional coating surfaces are prepared for vapor mixture condensation. The mechanism ofheat transfer enhancement for vapor mixture dropwise condensation is systematicallyinvestigated, by establishing mathematical model, experimental test, image acquirement, andso on. A new idea is proposed to enhance heat and mass transfer by using the interfacial effectcome from the motion of condensate droplets.In functional surface preparation, the thickness and stability of the organic coatings areconstrained for its thermal resistance, well thermal conductivity particles are added to modifythe organic fluorocarbon coatings with low surface energy, to enhance the heat conduction ofthe coating obviously. However the surface energy increases with increasing content of thefillings, and the adhesion also decreases with the larger particle size. When the particle size ofthe fillings is reduced to a nanometer magnitude, the functional surface indicates a very stablecharacter in cold and hot alternate circumstance. In order to improve the heat conduction ofthe coating with low surface energy, the paramagnetic iron particles are applied to thecoatings by special coating technology. The gradient dispersing particles in the coating areobtained by the magnetic field. The results indicate that the surface energy of the modifiedcoating varies slighter than that of the common coatings, and meanwhile the apparentappearance and the thermal conductivity are improved. The particle kinds, content of thefillings and the heat treatment technology of the coatings can significantly influence thethermal conductivity, surface free energy and the stability of the modified coating. When the mass concentration of the fillings is 17%, the conductivity of the coating modified by copperpowder can increase by 89.6%. The enhancing degree of the conduction performances for theother modified coatings is as follows: Cu(89.6%)＞CuO(84.8%)＞Cr2O3(83.5%)＞BN(67.7%)＞MFe(56.4%)＞SiC(52.4%)＞Fe(34.8%). Nevertheless, condensing take place on themodified surface, the intrinsical character of dropwise condensation decrease along with thefilling content increasing. Obviously, the effect on the surface energy of the coating must betaken into account, in the course of improving the thermal conductivity of the coatings. Themodified coating shows an outstanding anti-corrosive and anti-fouling performance due to itsrelatively low surface energy and excellent anti-corrosive performance of the fluorocarbonresin.Perfect dropwise condensation (DWC) has been presented on the modified surface withlow surface energy. The experimental results have shown that, heat transfer coefficient ofpure vapor dropwise condensation is higher than that of film condensation (FWC) by 50%.However, when the content of non-condensable gases is 0.5%～5%, the heat transfercoefficient of dropwise condensation may be higher than that of film condensation by morethan 80%. As the content of non-condensable gases (NCG) increases, the thermal resistancein the gas phase increases non-linearly and the thermal resistance of the coating accounts for asmaller fraction of the total thermal resistance in the condensation process of the vapormixture. It can improve the stability, prolong the service life and enhance the anti-corrosiveperformance of the surface with a promise thickness and resistance of the coatings bycontrolling modification technology. In the condensation process of vapor mixture, the heattransfer properties of dropwise condensation surpass those of traditional film condensation.However, the enhancement mechanism of the heat transfer is much different from that of thepure vapor condensation. Through observing and analyzing the images of droplet growth andmovement, the physical and chemical characteristics of condensation surface have a greatimpact on the pattern and movement of the condensate for dropwise condensation. Somespecial phenomena may exist, for example, the local droplet departing diameter larger thanthe critical one, the fixed growing point and departing places on the modified surface. It canbe seen from the movement and growth of the droplets that the disturbance of the dropletdeparting movement to the diffusion boundary layer in gas phase is the main factor in heattransfer enhancement. Experiments of three condensation modes, complete dropwisecondensation, the dropwise and filmwise coexisting condensation and filmwise condensationrespectively, are conducted to find out the mechanism. The experimental results reveal thatthe boundary layer of the gas side mixed up by the departing droplets and the periodicvibration is the main factors for dropwise condensation heat transfer enhancement of thevapor mixture. Based on the field synergetic principle of convection mass transfer, further analysis iscarried out to study the disturbance of the droplets with different patterns and motions underdifferent operating modes and to simulate the flow field in the gas phase both in the stablestate and the dynamic state by the Commercial Software Fluent. The software analogueresults are well agreed with the practical flow field detected by the PIV technology.According to the force analysis, the mathematic model of the departure droplet patterns inflow field is established to show the effect of the vapor velocity on the departing droplet. Theeffect of the vapor velocity direction and magnitude on the diameter and height of thedeparting droplet is investigated to obtain the state of the largest droplet on the condensingsurface. Under the circumstances of the vapor flowing parallel with droplet departingdirection, the diameter of the departing droplet on the vertical surface decreases linearly as theflow rate of vapor increases. The more the condensation surface deviates from the verticaldirection, the more the flow rate of the vapor influences. The counter flow of the vapor with asmall velocity influences slightly. However, when the velocity reaches more than 5m/s, itseffect on the departing diameter gets greater obviously. When the velocity of the vapormixture is relatively small, it is found from the flow field simulated results that the interfacecurvatures influence the flow field slightly. However, when the droplet size is larger than thethickness of the boundary layer, the droplet movement influences more greatly on the flowfield in the boundary layer along with the vapor velocity increasing. The velocity profile inthe diffusion boundary layer varied evidently with the dynamic change of the droplet, due tothe droplet movement take place in the diffusion boundary layer. Eddy flow occurs near thecondensing surface in the gas phase and there is a perpendicular velocity to the condensingsurface during the droplets departing process. The disturbance of the droplets to the gas phasehas increased the concentration gradient of the vapor in the boundary layer. And theperpendicular velocity cooperates well with the mass diffusion direction. But theenhancement degree on the heat transfer in the gas phase of the interracial effect caused bythe droplet movement is not only related to the shape and the dynamics of the droplets, butalso correlated with the velocity direction and magnitude of the vapor. It is resulted from boththe shape and dynamics of the droplets and the relative velocity.Stable dropwise condensation can be achieved by applying the modified coatings, but thethermal resistance of the coating affects the heat transfer coefficient greatly. The dropwisecondensation heat transfer model is established, including the contact angle and the largercoating thermal resistance. Considering the effect of the droplet curvature, the conductionshape factor is introduced to modify the heat transfer equation of the droplet and coating layerin this paper. The predicted results of the model agree very well with the data from the presentexperiments and in literature. The resistance of the coatings and condensate droplets in dropwise condensation for vapor mixture are quantitatively analyzed, according to the linearrelationship of the subcooling and heat flux. The heat transfer coefficient in the gas phase isobtained for both the dropwise and filmwise mode by subtracting the resistance of the dropletand coating from total resistance measured by experiments. The comparative results indicatethat, when the content of noncondensable gases is 1%, 3%, and 5%, the heat transfercoefficient of gas phase for dropwise condensation is 2.4, 2.2 and 1.8 times that of filmwisecondensation respectively, due to the gas-liquid interfacial effect.Three kinds of heat transfer enhancement surfaces are designed specially in this paper, byutilizing the interaction of the gravitational movement of the condensate and the boundarylayer of gas side and making the transversal velocity component cooperate with the massdiffusion process. A slight disturbance to the vital dominant region can realize the greatenhancement of mass diffusion, as the gas-liquid interfacial effect acts directly on the masstransfer boundary layer. Thus the gas-fluid interfacial effect caused by different surfaceconfiguration obviously enhances the condensation heat transfer coefficient of the steam-airmixture, under the gravitational field.
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