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RESEARCH PAPERS: Forced Convection

Heat Transfer to Small Horizontal Cylinders Immersed in a Fluidized Bed

[+] Author and Article Information
J. Friedman1

Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario, M5B 2K3, Canadajfriedma@ryerson.ca

P. Koundakjian

Department of Mechanical Engineering, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, N2L 3G1, Canada

D. Naylor, D. Rosero

Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario, M5B 2K3, Canada

1

Corresponding author.

J. Heat Transfer 128(10), 984-989 (Mar 22, 2006) (6 pages) doi:10.1115/1.2345425 History: Received May 31, 2005; Revised March 22, 2006

Heat transfer to horizontal cylinders immersed in fluidized beds has been extensively studied, but mainly in the context of heat transfer to boiler tubes in coal-fired beds. As a result, most correlations in the literature have been derived for cylinders of 2550mm diameter in vigorously fluidizing beds. In recent years, fluidized bed heat treating furnaces fired by natural gas have become increasingly popular, particularly in the steel wire manufacturing industry. These fluidized beds typically operate at relatively low fluidizing rates (GGmf<5) and with small diameter wires (16mm). Nusselt number correlations developed based on boiler tube studies do not extrapolate down to these small size ranges and low fluidizing rates. In order to obtain reliable Nusselt number data for these size ranges, an experimental investigation has been undertaken using two heat treating fluidized beds; one a pilot-scale industrial unit and the other a lab-scale (300mm diameter) unit. Heat transfer measurements were obtained using resistively heated cylindrical samples ranging from 1.3 to 9.5mm in diameter at fluidizing rates ranging from approximately 0.5×Gmf (packed bed condition) to over 10×Gmf using aluminum oxide sand particles ranging from dp=145330μm (50–90 grit). It has been found that for all cylinder sizes tested, the Nusselt number reaches a maximum near 2×Gmf, then remains relatively steady (±510%) to the maximum fluidizing rate tested, typically 812×Gmf. A correlation for maximum Nusselt number is developed.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic cross-section of lab-scale test bed and sample

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Figure 2

Bed pressure at the distributor as a function of flow rate for a 70 grit (200μm) bed, 240mm deep

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Figure 3

Terminal assembly

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Figure 4

Sample temperature distribution based on Eq. 4

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Figure 5

Standard correlations and data comparison for a 3.18mm sample in a bed of 200μm particles

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Figure 6

Nu versus G∕Gmf for samples in a 70 grit (200μm) bed

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Figure 7

Mean Nusselt number versus dt∕dp for different particle sizes

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Figure 8

Equation 10 plotted with all data, as well as data from Grewal and Saxena (see Ref. 1): (a) Silica sand dp=451μm, dt=12.7mm, (b) silicon carbide dp=178μm, dt=12.7mm, (c) silicon carbide dp=362μm, dt=12.7mm, (d) silica sand dp=504μm, dt=12.7mm, (e) silica sand dp=167μm, dt=12.7mm, and (f) alumina sand dp=259μm, dt=12.7mm

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