Noncircular ducts heat transfer

The convective heat-transfer coefficient and friction factor for laminar flow in noncircular ducts can be calculated from empirically or analytically determined Nusselt numbers, as given in Table 5. For turbulent flow, the circular duct data with the use of the hydrauhc diameter, defined in equation 10, may be used. 

Table 5. Correlations for Heat-Transfer and Darcy Friction Coefficients for Noncircular Laminar Duct Flow ...

A significant heat-transfer enhancement can be obtained when a noncircular tube is used together with a non-Newtonian fluid. This heat-transfer enhancement is attributed to both the secondary flow at the comer of the noncircular tube (23,24) and to the temperature-dependent non-Newtonian viscosity (25). Using an aqueous solution of polyacrylamide the laminar heat transfer can be increased by about 300% in a rectangular duct over the value of... 

When dealing with noncircular ducts it is common to assume that the equations for the heat transfer rate for a circular pipe can be applied to the noncircular duct provided that the correct equivalent diameter is used for the noncircular duct. Now for a circular pipe ... 

Irvine, T. R. Noncircular Duct Convective Heat Transfer, in W. lbele (ed.), Modern Developments in Heat Transfer, Academic Press, Inc., New York, 1963. 

Flow in Noncircular Ducts The length scale in the Nusselt and Reynolds numbers for noncircular ducts is the hydraulic diameter, D), = 4AJp, where A, is the cross-sectional area for flow and p is the wetted perimeter. Nusselt numbers for fully developed laminar flow in a variety of noncircular ducts are given by Mills (Heat Transfer, 2d ed., Prentice-Hall, 1999, p. 307). For turbulent flows, correlations for round tubes can be used with D replaced by l. ... 

For flows through noncircular cross sections and ducts, the heat transfer correlations developed for pipes can be used based on a hydraulic diameter,... 

For a circular duct, the hydraulic diameter is equal to its physical diameter. For a noncircular duct, it is convenient to use the hydraulic diameter to substitute for the characteristic physical dimension. However, for ducts with very sharp corners (e.g., triangular and cusped ducts), the use of the hydraulic diameter results in unacceptably large errors in the turbulent flow friction and heat transfer coefficients determined from the circular duct correlation. Other dimensions have been proposed as substitutes for hydraulic diameter. These equivalent diameters, provided for specific ducts only, will be presented elsewhere in this chapter. 

J. P. Hartnet, and T. F. Irvine Jr., Nusselt Values for Estimating Liquid Metal Heat Transfer in Noncircular Ducts, AIChE J., (3) 313-317,1957. 

P. C. Bandopadhayay, and C. M. Ambrose, A Generalized Length Dimension for Noncircular Ducts, Lett. Heat Mass Transfer, (7) 323-328,1980. 

J. P. Zarling, Application of Schwarz-Neumann Technique to Fully Developed Laminar Heat Transfer in Noncircular Ducts, J. Heat Transfer, (99) 332-335,1977. 

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