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Unformatted text preview: liquid, W/m · °C
height of the vertical plate, m
surface temperature of the plate, °C
saturation temperature of the condensing fluid, °C At a given temperature,
l and thus l
l except near the critical
point of the substance. Using this approximation and substituting Eqs. 10–14
kl
and
and 10–18 at x
L into Eq. 10–8 by noting that x L
hx L
4
h
(Eqs. 10–19 and 10–21) give
hvert
3xL
Re 4g l ( ) l 3 3 2
l 4g
3 2
l
2
l 3 kl
hx kl
4g
2 3h
3l
vert/4 L 3 (1023) Then the heat transfer coefficient hvert in terms of Re becomes
hvert 1.47kl Re 1/3 g
2
l 1/3 , 0 Re 30 (1024) l The results obtained from the theoretical relations above are in excellent
agreement with the experimental results. It can be shown easily that using
property values in Eqs. 10–22 and 10–24 in the specified units gives the condensation heat transfer coefficient in W/m2 · °C, thus saving one from having cen58933_ch10.qxd 9/4/2002 12:38 PM Page 538 538
HEAT TRANSFER hvert W
m kg kg J
s2 m3 m3 kg m · °C
kg
m · s · °C · m 3 1/4 J
W3
m1
s m6 m3 · °C3 °C
1/4
W4
m8 · °C4
W/m2 · °C FIGURE 10–25
Equation 10–22 gives the
condensation heat transfer coefficient
in W/m2 · °C when the quantities are
expressed in the units specified
in their descriptions. to go through tedious unit manipulations each time (Fig. 10–25). This is also
true for the equations below. All properties of the liquid are to be evaluated at
the film temperature Tf (Tsat Ts)/2. The hfg and are to be evaluated at
the saturation temperature Tsat.
Wavy Laminar Flow on Vertical Plates At Reynolds numbers greater than about 30, it is observed that waves form at
the liquid–vapor interface although the flow in liquid film remains laminar.
The flow in this case is said to be wavy laminar. The waves at the liquid–
vapor interface tend to increase heat transfer. But the waves also complicate
the analysis and make it very difficult to obtain analytical solutions. Therefore, we have to rely on experimental studies. The increase in heat transfer due
to the wave effect is, on average, about 20 percent, but it can exceed 50 percent. The exact amount o...
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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.
 Spring '10
 Ghaz

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