As was the case in forced convection involving a

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Unformatted text preview: e condensate at the wall is zero because of the “no-slip” condition and reaches a maximum at the liquid–vapor interface. The temperature of the condensate is Tsat at the interface and decreases gradually to Ts at the wall. As was the case in forced convection involving a single phase, heat transfer in condensation also depends on whether the condensate flow is laminar or turbulent. Again the criterion for the flow regime is provided by the Reynolds number, which is defined as Re Dh l l l 4 Ac p l 4 l l · 4m pl l l l (10-8) where Dh p Ac l l · m 4Ac /p 4 hydraulic diameter of the condensate flow, m wetted perimeter of the condensate, m p wetted perimeter film thickness, m2, cross-sectional area of the condensate flow at the lowest part of the flow density of the liquid, kg/m3 viscosity of the liquid, kg/m · s average velocity of the condensate at the lowest part of the flow, m/s mass flow rate of the condensate at the lowest part, kg/s l l Ac The evaluation of the hydraulic diameter Dh for some common geometries is illustrated in Figure 10–22. Note that the hydraulic diameter is again defined such that it reduces to the ordinary diameter for flow in a circular tube, as was done in Chapter 8 for internal flow, and it is equivalent to 4 times the thickness of the condensate film at the location where the hydraulic diameter is evaluated. That is, Dh 4 . The latent heat of vaporization hfg is the heat released as a unit mass of vapor condenses, and it normally represents the heat transfer per unit mass of condensate formed during condensation. However, the condensate in an actual L D L D δ δ p=L Ac = L δ 4A — Dh = — c = 4δ p (a) Vertical plate p = πD Ac = π Dδ 4A — Dh = — c = 4δ p (b) Vertical cylinder δ p = 2L Ac = 2Lδ 4A — Dh = — c = 4δ p (c) Horizontal cylinder FIGURE 10–22 The wetted perimeter p, the condensate cross-sectional area Ac, and the hydraulic diameter Dh for some common geometries. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 534 534 HEAT TRANSFER condensation process is cooled further to some average temperature between Tsat and Ts, releasing more heat in the process. Ther...
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