Cen58933ch10qxd 942002 1238 pm page 532 532 heat

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Unformatted text preview: d core Quality Transition flow Annular flow Slug flow Bubbly flow x=0 Forced convection Coefficient of heat transfer FIGURE 10–19 Different flow regimes encountered in flow boiling in a tube under forced convection. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 532 532 HEAT TRANSFER in the two-phase flow (bubbly flow, slug-flow, annular-flow, and mist-flow) cases, but they are beyond the scope of this introductory text. A crude estimate for heat flux in flow boiling can be obtained by simply adding the forced convection and pool boiling heat fluxes. 10–4 80°C 80°C Droplets Liquid film (a) Film condensation (b) Dropwise condensation FIGURE 10–20 When a vapor is exposed to a surface at a temperature below Tsat, condensation in the form of a liquid film or individual droplets occurs on the surface. Cold 0 plate y x g · m(x) Vapor, Liquid-vapor interface T(y) Ts Tsat Temperature profile Tv, Velocity (y) profile Liquid, l FIGURE 10–21 Film condensation on a vertical plate. I CONDENSATION HEAT TRANSFER Condensation occurs when the temperature of a vapor is reduced below its saturation temperature Tsat. This is usually done by bringing the vapor into contact with a solid surface whose temperature Ts is below the saturation temperature Tsat of the vapor. But condensation can also occur on the free surface of a liquid or even in a gas when the temperature of the liquid or the gas to which the vapor is exposed is below Tsat. In the latter case, the liquid droplets suspended in the gas form a fog. In this chapter, we will consider condensation on solid surfaces only. Two distinct forms of condensation are observed: film condensation and dropwise condensation. In film condensation, the condensate wets the surface and forms a liquid film on the surface that slides down under the influence of gravity. The thickness of the liquid film increases in the flow direction as more vapor condenses on the film. This is how condensation normally occurs in practice. In dropwise condensation, the condensed vapor forms droplets on the surface instead of a continuous film, and the surface is covered by countless droplets of varying d...
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