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TECHNICAL PAPERS: Natural and Mixed Convection

# Natural Convection Heat Transfer From Horizontal Rectangular Ducts

[+] Author and Article Information
Mohamed E. Ali

Mechanical Engineering Department, King Saud University, P. O. Box 800, Riyadh 11421, Saudi Arabiamali@ksu.edu.sa

J. Heat Transfer 129(9), 1195-1202 (Dec 02, 2006) (8 pages) doi:10.1115/1.2739623 History: Received May 15, 2006; Revised December 02, 2006

## Abstract

Experimental investigations have been reported on steady state natural convection from the outer surface of horizontal ducts in air. Five ducts have been used with aspect ratios ($Γ$=duct height/duct width) of 2, 1, and 0.5. The ducts are heated using internal constant heat flux heating elements. The temperatures along the surface and peripheral directions of the duct wall are measured. Longitudinal (circumference averaged) heat transfer coefficients along the side of each duct are obtained for laminar and transition regimes of natural convection heat transfer. Total overall averaged heat transfer coefficients are also obtained. Longitudinal (circumference averaged) Nusselt numbers are evaluated and correlated using the modified Rayleigh numbers for transition regime using the axial distance as a characteristic length. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers, the aspect ratio, and area ratio for the laminar and transition regimes. The longitudinal or total averaged heat transfer coefficients are observed to decrease in the laminar region and to increase in the transition region. Laminar regimes are obtained only at very small heat fluxes, otherwise, transitions are observed.

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## Figures

Figure 1

Schematic of the experimental system showing the thermocouple locations in the longitudinal (TCW) direction (see text for details)

Figure 2

Circumference averaged dimensionless axial temperature distributions along the duct surface for some selected heat fluxes for duct number 2; (a) associated with transition regime and (b) corresponding to laminar regime

Figure 3

Axial circumference averaged heat transfer coefficient along the duct surface for some selected heat fluxes for duct number 2; (a) associated with transition regime and (b) corresponding to laminar regime (different symbols are assigned for each heat flux only for clarity)

Figure 4

Local circumference averaged Nusselt numbers versus the modified Rayleigh numbers; solid line separating the laminar data (above the line) and the transition data (below the line). The inclined upward arrow shows the transition direction while the downward arrow presents the laminar direction.

Figure 5

Local circumference averaged Nusselt numbers versus the modified Rayleigh numbers for the transition regime. Solid line present the data fit given by Eq. 15.

Figure 6

The overall averaged Nusselt number profiles. Dashed line presents the data fit (Eq. 16) through the critical points marked by circles.

Figure 7

The overall averaged Nusselt numbers for the laminar regime; dashed lines are the fitting data given by Eq. 17 and solid lines are those given by Eq. 18

Figure 8

The overall averaged Nusselt numbers for the transition regime; dashed lines are the fitting data given by Eq. 19 and solid lines are those given by Eq. 20

Figure 9

The overall averaged Nusselt numbers for the transition regime; solid line presents the fitting through the data given by Eq. 21

Figure 10

Comparison of the overall averaged Nusselt numbers for the isoflux surface ducts (symbols) with the analytical (solid line) and the computational (dashed line) having isothermal duct surface

Figure 11

Comparison of the overall averaged Nusselt numbers for the isoflux surface ducts (square symbols) with the experimental (+ symbols and solid line) and the analytical (dashed line) having isothermal duct surface

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