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RESEARCH PAPERS: Heat Transfer Enhancement

Thermohydraulics of Laminar Flow Through Rectangular and Square Ducts With Transverse Ribs and Twisted Tapes

[+] Author and Article Information
Debashis Pramanik

Mechanical Engineering Department, Bengal Engineering and Science University, Shibpur Howrah 711 103, West Bengal, Indiadebashis_pramanik@yahoo.co.in

Sujoy K. Saha1

Mechanical Engineering Department, Bengal Engineering and Science University, Shibpur Howrah 711 103, West Bengal, Indiasujoy_k_saha@hotmail.com

1

Corresponding author.

J. Heat Transfer 128(10), 1070-1080 (Apr 29, 2006) (11 pages) doi:10.1115/1.2345432 History: Received November 03, 2005; Revised April 29, 2006

The heat transfer and the pressure drop characteristics of laminar flow of viscous oil through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and fitted with twisted tapes have been studied experimentally. The tapes have been full length, short length, and regularly spaced types. The transverse ribs in combination with full-length twisted tapes have been found to perform better than either ribs or twisted tapes acting alone. The heat transfer and the pressure drop measurements have been taken in separate test sections. Heat transfer tests were carried out in electrically heated stainless steel ducts incorporating uniform wall heat flux boundary conditions. Pressure drop tests were carried out in acrylic ducts. The flow was periodically fully developed in the regularly spaced twisted-tape elements case and decaying swirl flow in the short-length twisted tapes case. The flow characteristics are governed by twist ratio, space ratio, and length of twisted tape, Reynolds number, Prandtl number, rod-to-tube diameter ratio, duct aspect ratio, rib height, and rib spacing. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. The performance of the geometry under investigation has been evaluated. It has been found that on the basis of both constant pumping power and constant heat duty, the regularly spaced twisted-tape elements in specific cases perform marginally better than their full-length counterparts. However, the short-length twisted-tape performance is worse than the full-length twisted tapes. Therefore, full-length twisted tapes and regularly spaced twisted-tape elements in combination with transverse ribs are recommended for laminar flows. However, the short-length twisted tapes are not recommended.

Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 2

Schematic diagram of experimental setup

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Figure 3

Three junction thermopile

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Figure 4

Axial variation of duct outside wall temperature, AR=1

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Figure 5

Axial pressure drop results for the cases of y=2.5 and s=2.5

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Figure 14

(fRe)Sw versus Sw—1=0.7, e∕Dh=0.0441, P∕e=20

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Figure 1

(a) Full-length twisted-tape insert inside a duct, (b) regularly spaced twisted-tape elements, and (c) transverse rib turbulators

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Figure 6

Duct cross sections showing rib and twisted-tape inside ducts

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Figure 7

Peripheral wall temperature variation—Regularly spaced twisted-tape elements, y=2.5, s=2.5, TZ-tape zone, RZ-rod zone, AR=0.333

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Figure 8

Effect of Prandtl number on Nusselt number, (a) short-length twisted tape and (b) regularly spaced twisted-tape elements

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Figure 9

Comparison of present friction factor data with other data (short-length twisted tape)

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Figure 10

Comparison of present friction factor data with other data (regularly spaced twisted-tape elements)

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Figure 11

Comparison of present Nusselt number data with other data (short-length twisted tape)

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Figure 12

Comparison of present Nusselt number data with other data (regularly spaced twisted-tape elements)

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Figure 13

(fRe)Sw versus Sw—AR=1, e∕Dh=0.0441, P∕e=10 (short-length twisted tape)

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Figure 18

Num((μb∕μw)−0.14)∕5.172 versus Sw.Pr0.565-AR=1, e∕Dh=0.0441, P∕e=10

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Figure 19

Num((μb∕μw)−0.14)∕5.172 versus Sw.Pr0.565-1=0.7, e∕Dh=0.0441, P∕e=20

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Figure 20

Num((μb∕μw)−0.14)∕5.172 versus Sw.Pr0.565-1=0.9, AR=0.333, P∕e=10

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Figure 21

Num((μb∕μw)−0.14)∕5.172 versus Sw.Pr0.565-1=0.9, AR=0.5, e∕Dh=0.0441

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Figure 15

(fRe)Sw versus Sw—1=0.9, AR=0.333, P∕e=10

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Figure 16

(fRe)Sw versus Sw—1=0.9, AR=0.5, e∕Dh=0.0441

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Figure 17

(fRe)Sw versus Sw—AR=1, e∕Dh=0.0441, P∕e=10 (regularly spaced twisted-tape elements)

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