hancement ratio ε = hfined tube/hplain tube
10
data of Wanniarachchi et al. (1985) for
R-113
Marto et al. (1988)
e = 1.0 mm
steam condensing on a family of cooled
w = 1.0 mm
copper tubes (coolant temperature Tc,
coolant velocity Uc) with a root diameter
8
of 19.05 mm and having rectangular in-
tegral fins, 1 mm thick and 1 mm high.
The figure also shows the predictions of
theoretical models of Beatty and Katz
6
(1948), Honda and Nozu (1987b), and
hfinned
Honda et al. (1987). The Beatty-Katz mod-
ζ = 0.95
hplain
el overpredicts the experimental data at
Honda et al. (theory)
(1987)
small fin spacings, but underpredicts it
4
at large fin spacings. The Honda-Nozu
model underpredicts the data, and the
Webb et al. (theory)
(1985)
discrepancy gets worse as the fin spac-
ζ = 0.85
ing increases. The predictions of Honda
2
et al. (1987) appear to fit the data best,
Beatty and
Area
Katz (theory)
except at small fin spacings where the
Ratio
(1948)
flooding is very nearly complete. Al-
though not shown here, the model pro-
posed by Adamek and Webb (1990) pre-
0
1
2
3
dicts the steam data within 10 to 15 %
δ (mm)
Figure 24. Effect of fin spacing on the enhance- over the complete range of fin spacings.
ment ratio for R-113 condensing on horizontal in-
The same family of copper tubes as
tegral-fin tubes. Adapted from Marto et al. (1988). used by Wanniarachchi et al. (1985) for
steam were tested by Marto et al. (1988)
for R-113. These results together with the theoretical predictions of Beatty and Katz (1948),
Honda et al. (1987) and Webb et al. (1985) are shown in Figure 24. The Beatty-Katz model
vastly underpredicts the data. The Webb et al. (1985) theory provides a much better
prediction at ζ = 0.85 than at ζ = 0.95. However, the closest agreement with the data is
achieved by using the Honda et al. (1987) model.
From the brief discussion here and more extensive coverage elsewhere, e.g., Marto
(1988), it appears that virtually all available experimental data pertain to steam and
currently popular refrigerants such as R-11, R-12, R-22 and R-113. The data for R-152a,
which is a promising alternative to CFCs, are just beginning to appear. Cheng and Tao
(1994) are perhaps the first to report experimental work on condensation of R-152a on
plain and finned tubes. They conclude that i) the simple Nusselt theory predicts within 15
% the data for condensation on a single smooth tube, ii) the performance of a single
smooth tube with R-152a condensing on its outside is 2025 % better than that obtained
with R-12, and iii) the integral fin tube provides enhancement of 4 to 10 times that of a
plain tube.
Effect of interfacial shear
It is well known that the interfacial shear stress of the liquid/vapor interface increases
the heat transfer coefficient if the vapor and liquid (condensate) flow in the same direc-
tion, and decreases it if the two flow in opposite directions. For condensation on plain
(smooth) tubes, the effect of vapor velocity has been studied extensively, see for example,
Memory and Rose (1986), Honda et al. (1986) and Fujii (1991). However, the correspond-
38
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