The Effect Research Of Water Vapor Dropwise Condensation Heat Transfer

Compared with other heat transfer means, dropwise condensation has a very high heat transfer coefficient,which can greatly improve the efficiency of heat transfer, reduce the heat transfer area and costs for heat exchanger. So the dropwise condensation of water vapor will be promising in the industrial applications.First, this paper summarized the surface modification methods achieving dropwise condensation of water vapor, and found that the formation of gradient surface energy would be the future development direction of surface modification technology which could be obtained by molecular self-assembly and ion beam dynamic mixing technology to make the appropriate follow-up treatment. Second, the factors that impact on heat transfer of dropwise condensation were studied by numerical simulation method, such as: the condensation heat transfer coefficient and the thermal stress distribution on different substrate materials, the heat transfer inside the droplet considering the thermocapillary convection on gas-liquid interface, the effect of droplet shedding diameter and rate due to droplet contact angle of the substrate surface. Some conclusions were drawn:(1) Condensing heat transfer coefficients didn’t change with the thermal conductivity of the substrate. Thermal stress caused by thermal expansion inconsistencies was localed at the substrate covered by large liquid and interface of surface modification materials and metal, the maximal value was 17.13 MPa .(2) The symmetry vortex was found in the droplet caused by thermocapillary convection, which could effectively promote the heat transfer inside the droplet, especially at the edge of the droplet. The temperature contour in the droplet turned from the“level-line”to“arc-line”due to the thermocapillary convection. The temperature difference of the up-down surface varied from 16℃to 3.5℃. The substrate heat transfer coefficient of a 2mm diameter droplet increased to 47.1% by thermocapillary convection.(3) The hysteresis contact angle increased with the apparent equilibrium contact angle and surface roughness. The hysteresis contact angle almost kept a constant value at the surface roughness reached 0.01. The falling diameter of the droplet increased with the hysteresis contact angle. The larger the balance contact angle between droplet and substrate was, the faster droplets fell.