It is observed that the outer surface of the liquid

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Unformatted text preview: or condensation on the outer surfaces of vertical tubes or plates increases in the flow direction due to the increase of the liquid film thickness . The flow of liquid film exhibits different regimes, depending on the value of the Reynolds number. It is observed that the outer surface of the liquid film remains smooth and wave-free for about Re 30, as shown in Figure 10–23, and thus the flow is clearly laminar. Ripples or waves appear on the free surface of the condensate flow as the Reynolds number increases, and the condensate flow becomes fully turbulent at about Re 1800. The condensate flow is called wavy-laminar in the range of 450 Re 1800 and turbulent for Re 1800. However, some disagreement exists about the value of Re at which the flow becomes wavy-laminar or turbulent. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 535 535 CHAPTER 10 δ–y Heat Transfer Correlations for Film Condensation Below we discuss relations for the average heat transfer coefficient h for the case of laminar film condensation for various geometries. dx Shear force du µl — (bdx) dy 1 Vertical Plates Consider a vertical plate of height L and width b maintained at a constant temperature Ts that is exposed to vapor at the saturation temperature Tsat. The downward direction is taken as the positive x-direction with the origin placed at the top of the plate where condensation initiates, as shown in Figure 10–24. The surface temperature is below the saturation temperature (Ts Tsat) and thus the vapor condenses on the surface. The liquid film flows downward under the influence of gravity. The film thickness and thus the mass flow rate of the condensate increases with x as a result of continued condensation on the existing film. Then heat transfer from the vapor to the plate must occur through the film, which offers resistance to heat transfer. Obviously the thicker the film, the larger its thermal resistance and thus the lower the rate of heat transfer. The analytical relation for the heat transfer coefficient in film condensation on a vertical plate describ...
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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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