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Unformatted text preview: e condensate at the wall is zero because of the “noslip” 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 (108) 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, crosssectional 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 crosssectional 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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 Spring '10
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