4. Conclusions

Extensive experimental work has been performed on integral-fin tubes and has shown that geometry is not the only point of interest for the enhancement of heat transfer. Researchers have reported the optimum fin dimensions for a range of condensing fluids [24–26, 28, 30]. The work of Honda et al. [12] successfully predicts the condensate retention on integral-fin tubes. Reliable heat-transfer models (e.g. [1, 55]) accounting for the combined effects of surface tension and gravity on heat transfer have been developed and are readily available for design engineers.

A reasonable amount of experimental work is reported on condensation heat transfer on enhanced pin-fin tubes. Work of previous researchers has shown the superior performance of such tubes over equivalent integral-fin tubes. The extent of condensate retention and formation of many sharp surfaces enhancing surface-tension effects on pin-fin tubes are identified to be the important parameters contributing towards the heat-transfer enhancement. The model presented by Ali and Briggs [2] is available to predict heat transfer on the pin-fin tubes reasonably by accounting the effect of both gravity and surface-tension condensate drainage.

#### Nomenclature



4. Conclusions

116 Heat Exchangers– Advanced Features and Applications

engineers.

Nomenclature

A constant in Eq. (27)

Af surface area of fin flank

Ar surface area of inter-fin spacing

Extensive experimental work has been performed on integral-fin tubes and has shown that geometry is not the only point of interest for the enhancement of heat transfer. Researchers have reported the optimum fin dimensions for a range of condensing fluids [24–26, 28, 30]. The work of Honda et al. [12] successfully predicts the condensate retention on integral-fin tubes. Reliable heat-transfer models (e.g. [1, 55]) accounting for the combined effects of surface tension and gravity on heat transfer have been developed and are readily available for design

Figure 7. Schematic representation of pin-fin tube identifying five regions for heat transfer (after Ali and Briggs [2]).

A reasonable amount of experimental work is reported on condensation heat transfer on enhanced pin-fin tubes. Work of previous researchers has shown the superior performance of such tubes over equivalent integral-fin tubes. The extent of condensate retention and formation of many sharp surfaces enhancing surface-tension effects on pin-fin tubes are identified to be the important parameters contributing towards the heat-transfer enhancement. The model presented by Ali and Briggs [2] is available to predict heat transfer on the pin-fin tubes reasonably by accounting the effect of both gravity and surface-tension condensate drainage.

Ad outside surface area of a smooth tube with outside diameter, d



m\_ mass flow rate of condensate

118 Heat Exchangers– Advanced Features and Applications

Nud average Nusselt number by Honda and Nozu [55] model, defined by Eq. (26)

Nudf Nusselt number for flooded region, Honda and Nozu [55]

NuP vapour-side Nusselt number for a vertical plate

n total number of pins per circumference

q heat flux on outside of the test tube

qplain heat flux through plain or smooth tube

Re condensate Reynolds number

qtip,flood heat flux to fin tip in flooded part of the tube

r radius of curvature of the vapour-liquid interface

t fin-tip thickness or longitudinal pin-tip thickness

rb radius of curvature of the vapour-liquid interface at fin bottom

S distance along the vapour-liquid interface measured from the fin tip

T~ wf dimensionless average wall temperatures at fin root in flooded region

T~ wu dimensionless average wall temperatures at fin root in unflooded region

rt radius of curvature of the vapour-liquid interface at fin tip

s fin spacing at fin root or longitudinal pin spacing at pin root

Q total heat-transfer rate through the test tube

qflank heat flux to fin flank in unflooded part of the tube

qint heat flux to inter-fin spacing in unflooded part of the tube

p fin pitch

qtip heat flux to fin tip

Ro fin or pin-tip radius Rr fin or pin-root radius

Sm total fin arc length

sc circumferential pin spacing

NuT vapour-side Nusselt number for a horizontal tube

Nudu Nusselt number for unflooded region, Honda and Nozu [55]


#### Greek Letters


σ surface tension

