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Research Papers: Natural and Mixed Convection

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

[+] Author and Article Information
G. H. Choueiri

Department of Mechanical Engineering,  University of Ottawa, 161 Louis Pasteur, Ottawa, ON K1N 6N5, Canadastavros.tavoularis@uottawa.ca

S. Tavoularis1

Department of Mechanical Engineering,  University of Ottawa, 161 Louis Pasteur, Ottawa, ON K1N 6N5, Canadastavros.tavoularis@uottawa.ca

1

Corresponding author.

J. Heat Transfer 133(12), 122503 (Oct 05, 2011) (9 pages) doi:10.1115/1.4004428 History: Received April 30, 2010; Revised June 13, 2011; Published October 05, 2011; Online October 05, 2011

The effects of eccentricity on the natural convection heat transfer from a vertical open-ended cylindrical annulus with diameter ratio of 1.63 and aspect ratio of 18:1 have been investigated experimentally. Within the range of present conditions, and with the possible exclusion of the highest eccentricities, it was found that the flow was thermally fully developed in a considerable section of the apparatus, as indicated by the linear variation of wall temperature with height. This made it possible to estimate the mass flow rate from the wall temperature gradient in the mid-section of the annulus, and use it to calculate the bulk Reynolds number, which was found to be weakly sensitive to eccentricity for a constant wall heat flux and to increase with increased wall heat flux. With the exception of the very low eccentricity range in which it was insensitive to eccentricity, the overall heat transfer rate diminished monotonically with increasing eccentricity. Plots of the local azimuthal variation of the Nusselt number showed that, at low eccentricities, the heat transfer rate increased near the wider gap but decreased near the narrower gap. The average Nusselt number was found to decrease measurably with increasing eccentricity and to increase slightly with increasing heat flux within the examined range. In contrast, the Grashof number was found to be much more sensitive to changes in heat flux and only had a weak dependence on eccentricity.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic diagram of the experimental apparatus

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Figure 2

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

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Figure 3

Variations of the room temperature and the difference between the average annulus wall temperature and the room temperature during a 50 h time interval

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Figure 4

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; longer dashed lines correspond to q″* = 0.53 and shorter ones to q″* = 1.39

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Figure 5

Azimuthal temperature variation for the inner (a) and outer (b) cylinders at z/H = 0.5.

, □, ×, +, ⋄, ○, and ◊ represent eccentricities of 0, 0.1, 0.3, 0.5, 0.7, 0.8, and 0.9, respectively

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Figure 6

Azimuthally averaged temperature variation along the annulus for various eccentricities.

, □, ×, +, ⋄, ○, and ◊ represent eccentricities of 0, 0.1, 0.3, 0.5, 0.7, 0.8, and 0.9, respectively; longer dashed lines correspond to q″* = 0.53 and shorter ones to q″* = 1.39.

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Figure 7

Variation of normalized average wall temperature gradient with height; the vertical axis scale corresponds to the concentric case (e = 0), but each curve for the increasingly eccentric cases has been shifted upwards by three units compared to the previous one (symbols same as in Fig. 6)

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Figure 8

Variation of average wall temperature gradient with eccentricity at z/H = 0.5; q″* = 0.53 ( ○ ) and 1.39 (•)

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Figure 9

Variation of inlet Reynolds number with eccentricity for q″* = 0.53 ( ○ ) and 1.39 (•)

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Figure 10

Variations of average wall temperature rise ( ▪ ) and bulk fluid temperature rise ( ▴ ) with eccentricity at z/H = 0.5; q″* = 0.53 (a) and 1.39 (b)

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Figure 11

Nusselt number versus eccentricity for q″* = 0.53 ( ○ ) and 1.39 (•)

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Figure 12

Variation of the local Nusselt number with azimuthal position for different eccentricities for q″* = 0.53 (a) and 1.39 (b); symbols as in Fig. 6

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Figure 13

Modified Grashof number versus eccentricity for q″* = 0.53 ( ○ ) and 1.39 (•)

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