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Research Papers: Natural and Mixed Convection

An Experimental Study of Mixed Convection in Vertical, Open-Ended, Concentric and Eccentric Annular Channels

[+] Author and Article Information
S. Tavoularis

e-mail: stavros.tavoularis@uottawa.ca
Department of Mechanical Engineering,
University of Ottawa,
161 Louis Pasteur,
Ottawa, Ontario K1N 6N5, Canada

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 5, 2012; final manuscript received February 18, 2013; published online June 17, 2013. Assoc. Editor: Ali Ebadian.

J. Heat Transfer 135(7), 072502 (Jun 17, 2013) (9 pages) Paper No: HT-12-1418; doi: 10.1115/1.4023748 History: Received August 05, 2012; Revised February 18, 2013

The effect of eccentricity on heat transfer in upward flow in a vertical, open-ended, annular channel with a diameter ratio of 0.61, an aspect ratio of 18:1, and both internal surfaces heated uniformly has been investigated experimentally. Results have been reported for eccentricities ranging from the concentric case to the near-contact case and three inlet bulk Reynolds numbers, equal approximately to 1500, 2800, and 5700. This work complements our recently reported experimental results on natural convection in the same facility. The present results are deemed to be largely in the mixed convection regime with some overlap with the forced convection regime and likely to include cases with laminar, transitional, and turbulent flows in at least a part of the test section. Small eccentricity had an essentially negligible effect on the overall heat transfer rate, but high eccentricity reduced the average heat transfer rate by up to 60%. High eccentricity also resulted in wall temperatures in the narrow gap region that were much higher than those in the open channel. The concentric-case Nusselt number was higher than the Dittus–Boelter prediction, whereas the highly eccentric-case Nusselt number was significantly lower than that.

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Figures

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Fig. 1

Schematic diagram of the experimental apparatus

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Fig. 2

Sketch of the annular duct cross section showing positions of thermocouples and foil gaps

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Fig. 3

Representative wall temperature measurements along the annulus for different eccentricities; ○, ◇, and □ denote the readings of thermocouples S0i, S90i, and S180i, respectively, whereas +, ×, and are for the corresponding thermocouples on the outer cylinder; Re = 2800

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Fig. 4

Azimuthal temperature variation for the inner (a) and outer (b) cylinders at z/H = 0.5; e = 0 (), 0.1 (□), 0.3 (×), 0.5 (+), 0.7 (◇), 0.8 (○) and 0.9 (♦); Re = 2800

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Fig. 5

Azimuthally averaged temperature variation along the annulus for various eccentricities and Reynolds numbers (symbols are the same as in Fig. 4)

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Fig. 6

Circumferentially averaged wall temperature rise at z/H = 0.5; □: Re = 1500, ▿: Re = 2800, ◇: Re = 5700; △: natural convection measurements by CT at Re ≈ 900. Smooth lines approaching constant asymptotes at e = 0 and 1 have been fitted to all data sets.

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Fig. 7

Ratio of Richardson number to the Richardson number for natural convection at the same eccentricity for concentric (○) and highly eccentric (△; e = 0.9) annular channels at midheight

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Fig. 8

Application of the Jackson and Hall criterion (dashed line) to test the influence of buoyancy forces on the heat transfer coefficient in concentric (●) and highly eccentric (▴; e = 0.9) annular channels at midheight; open symbols represent corresponding natural convection measurements by CT

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Fig. 9

Azimuthal variation of the local Nusselt number for the inner (a) and outer (b) cylinders at z/H = 0.5; e = 0 (), 0.1 (□), 0.3 (×), 0.5 (+), 0.7 (◇), 0.8 (○) and 0.9 (♦); Re = 2800

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Fig. 10

Azimuthally averaged Nusselt number versus eccentricity at z/H = 0.5, □; Re = 1500, ▿; Re = 2800, ◇; Re = 5700, △: natural convection measurements by CT at Re ≈ 900. Smooth lines approaching constant asymptotes at e = 0 and 1 have been fitted to all data sets; uncertainty bars have been drawn for the concentric case results, as representative of all results.

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Fig. 11

Nusselt number versus Reynolds number at z/H = 0.5; closed symbols correspond to present results, whereas open symbols correspond to natural convection; circles denote concentric cases, whereas triangles denote highly eccentric (e = 0.9) cases; solid lines: exponential curves fitted to the present data; dashed line: laminar flow in pipes; dotted line: El-Genk and Rao correlation; dash and dot line: Dittus–Boelter correlation; uncertainty bars have been drawn for the concentric case results.

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