Natural convection heat transfer in concentric and eccentric annuli made of two isothermal horizontal circular cylinders is numerically investigated for Rayleigh numbers less than 5.0 × 104 which is based on the difference of radii. Bipolar coordinates are used for eccentric annuli, and it is found that for very small eccentricity the overall thermal behavior of the annuli exhibits that of the exactly concentric cylinders. The maximum deviation of the local heat-transfer coefficient of the cylinder walls remains, for example, for ε = 0.01, Ri/R0 = 0.3846 and RaL = 1.0 × 104, within a meager 5 percent. The parametric effect on the heat-transfer characteristics is discussed with respect to the diameter ratio for concentric cylinders, and eccentricity and azimuthal angular location of the inner cylinder for eccentric annuli. Output is displayed in terms of streamlines, isothermal contours, radial temperature distribution and equivalent thermal conductivities. Convection patterns are explained in detail.

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