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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

## Abstract

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 $25–50mm$ 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 $(G∕Gmf<5)$ and with small diameter wires $(1–6mm)$. 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 $dp=145–330μm$ (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 $(±5–10%)$ to the maximum fluidizing rate tested, typically $8–12×Gmf$. A correlation for maximum Nusselt number is developed.

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## Figures

Figure 1

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

Figure 2

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

Figure 3

Terminal assembly

Figure 4

Sample temperature distribution based on Eq. 4

Figure 5

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

Figure 6

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

Figure 7

Mean Nusselt number versus dt∕dp for different particle sizes

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

## Errata

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