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Research Papers: Natural and Mixed Convection

Effects of Various Physical and Numerical Parameters on Heat Transfer in Vertical Passages at Relatively Low Heat Loading

[+] Author and Article Information
Amir Keshmiri

School of Mechanical, Aerospace and Civil Engineering (MACE),  The University of Manchester, Manchester, M13 9PL, UK; Advanced Design Group AECOM, 1 New York St., Manchester, M1 4HD, UKa.keshmiri@manchester.ac.uk

J. Heat Transfer 133(9), 092502 (Aug 01, 2011) (8 pages) doi:10.1115/1.4003925 History: Received September 24, 2010; Revised March 19, 2011; Accepted April 04, 2011; Published August 01, 2011; Online August 01, 2011

The present work is concerned with the modeling of buoyancy-modified mixed convection flows, such flows being representative of low-flow-rate flows in the cores of Gas-cooled Reactors. Three different eddy viscosity models (EVMs) are examined using the in-house code, “CONVERT. ” All fluid properties are assumed to be constant, and buoyancy is accounted for within the Boussinesq approximation. Comparison is made against experimental measurements and the direct numerical simulations (DNS). The effects of three physical parameters including the heat loading, Reynolds number, and pipe length on heat transfer have been examined. It is found that by increasing the heat loading, three thermal-hydraulic regimes of “early onset of mixed convection,” “laminarization,” and “recovery” were present. At different Reynolds numbers, the three thermal-hydraulic regimes are also evident. The k-ε model of Launder and Sharma was found to be in the closest agreement with consistently normalized DNS results for the ratio of mixed-to-forced convection Nusselt number (Nu/Nu0 ). It was also shown that for the “laminarization” case, the pipe length should be at least “500× diameter” in order to reach a fully developed solution. In addition, the effects of two numerical parameters namely buoyancy production and Yap length-scale correction terms have also been investigated and their effects were found to be negligible on heat transfer and friction coefficient in ascending flows.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Heat transfer impairment and enhancement in ascending and descending mixed convection flows (Re = 5,300; Pr = 0.71)

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Figure 2

Normalized Nusselt number impairment and enhancement against the buoyancy parameter for different Reynolds numbers

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Figure 3

Mean flow and turbulence profiles for case (C) at different streamwise locations obtained using the Launder–Sharma model in CONVERT

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Figure 4

Effects of including the buoyancy production term on the heat transfer impairment/enhancement

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Figure 5

Effects of the heat flux models on the buoyancy production term [m2 /s3 ] using (a) GGDH and (b) SGDH

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