Axial laminar flow in a horizontal annular pipe is influenced by the presence of buoyancy-induced secondary flows that are caused by the heat flow from the inner cylinder. A numerical study is presented for the fully developed region of the buoyancy-affected flow. The distributions of the axial and cross-sectional velocities are calculated along with the temperature variation in the cross section. Results are presented for a range of values of the Rayleigh number, the Prandtl number, and the radius ratio of the annulus. The Nusselt number increases significantly with the Rayleigh number; yet the corresponding increase in the friction factor is found to be rather small. Distributions of secondary flow and isotherms over the cross section are presented for different values of the parameters. In each half of the annulus on either side of the vertical centerline, the secondary flow displays a single-eddy pattern at low Rayleigh numbers and changes to a double-eddy pattern at high values.

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