The heat of vaporization hfg released as the vapor

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Unformatted text preview: iameters (Fig. 10–20). In film condensation, the surface is blanketed by a liquid film of increasing thickness, and this “liquid wall” between solid surface and the vapor serves as a resistance to heat transfer. The heat of vaporization hfg released as the vapor condenses must pass through this resistance before it can reach the solid surface and be transferred to the medium on the other side. In dropwise condensation, however, the droplets slide down when they reach a certain size, clearing the surface and exposing it to vapor. There is no liquid film in this case to resist heat transfer. As a result, heat transfer rates that are more than 10 times larger than those associated with film condensation can be achieved with dropwise condensation. Therefore, dropwise condensation is the preferred mode of condensation in heat transfer applications, and people have long tried to achieve sustained dropwise condensation by using various vapor additives and surface coatings. These attempts have not been very successful, however, since the dropwise condensation achieved did not last long and converted to film condensation after some time. Therefore, it is common practice to be conservative and assume film condensation in the design of heat transfer equipment. 10–5 I FILM CONDENSATION We now consider film condensation on a vertical plate, as shown in Figure 10–21. The liquid film starts forming at the top of the plate and flows downward under the influence of gravity. The thickness of the film increases in the flow direction x because of continued condensation at the liquid–vapor interface. Heat in the amount hfg (the latent heat of vaporization) is released during condensation and is transferred through the film to the plate surface at temperature Ts. Note that Ts must be below the saturation temperature Tsat of the vapor for condensation to occur. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 533 533 CHAPTER 10 Typical velocity and temperature profiles of the condensate are also given in Figure 10–21. Note that the velocity of th...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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