hancement ratio of 1.39 for a bronze tube is 42 % less than the enhancement ratio of 2.4 for
a copper tube. For R-113, the corresponding figure is less than 5 %. The enhancement ratio
increases as the fin height increases except for steam condensing on bronze tubes when
the trend is opposite. It is also worth noting that the enhancement ratios for R-113 are
much higher than those for steam.
INTERNALLY FINNED TUBES
When saturated vapor flows into a cooled tube, vapor condenses on the tube wall
forming a condensate film on the tube wall. A variety of flow patterns can arise depend-
ing on the mass flux (velocity). For example, when the condensate layer is symmetric and
the liquid/vapor interface is sharply defined, the flow is classified as annular flow. Other
flow possibilities include bubbly flow, dispersed flow, wavy flow, stratified flow, plug
flow, and slug flow. Even for a smooth (plain) tube, the flow is quite complex and difficult
to analyze. The presence of internal fins complicates the situation further, and makes it
extremely difficult, if not impossible, to develop an accurate physics based model describ-
ing the heat and fluid flow processes. Consequently, the discussion in this section, includ-
ing the pressure drop and heat transfer correlations, will be largely based on experimental
studies.
In 1974, two papers, one by Reisbig (1974) and the other by Vrable et al. (1974) reported
on the condensation of R-12 in internally finned tubes. Reisbig found the condensation
heat transfer coefficient to be 2040 % greater than the smooth-tube value. However, he
did not propose any correlation. Vrable et al. (1974), on the other hand, conducted 26
experiments with two different internally finned tubes, varying the inlet reduced pressure
(p/pcr) from 0.18 to 0.46 and the mass flux from 86.7 to 853 kg/m2 s. The more effective
tube was found to give a maximum enhancement of 300 %. The following correlation
represented their data within 30 %:
-0.65
0.8
p
k 2d G
h = 0.01 l h e
Prl .33
0
(101)
dh l
pcr
where
ρ 0.5
l x + (1 - x) G
Gl =
(102)
ρv
G
=
mass flux (velocity)
dh
=
tube hydraulic diameter
x
=
vapor quality
p
=
pressure
pcr
=
critical pressure.
Royal and Bergles (1978) condensed low pressure steam in four internally finned
copper tubes, three of which had spiral fins. The geometric data for these fins are given in
Table 4. Figures 25 and 26 represent the heat transfer and pressure drop data, respectively.
The heat transfer data for finned tubes indicate a significant improvement over the smooth
(plain tube). The highest enhancement ratio of 2.3 is achieved with a 15.9-mm outside-
diameter tube containing 16 spiraled fins of height 1.45 mm and a helix angle of 3.22
degrees (tube 5). It is also interesting to note that the area ratio (A/Ap) for tubes 3 and 4 is
41
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