Mixed convection of air with Prandtl number of 0.7 in eccentric horizontal annuli with isothermal wall conditions was numerically investigated. The inner cylinder is at a higher temperature and the inner and outer cylinders rotate either clockwise or counter-clockwise at a constant angular velocity. The effects of parameters such as the eccentricity of the axes, the Rayleigh number and rotation direction were studied. The two-dimensional Navier-Stokes and energy equations were numerically solved in an orthogonal grid by employing a finite-volume method based on the SIMPLE algorithm. This orthogonality was achieved by employing a cylindrical bipolar coordinate system. Use of this coordinate system has the privileges of orthogonal coordinate and simplicity of computing different fluxes involved. Mean and local heat transfer results were obtained. The physics of the flow and the heat transfer is revealed by the streamlines and the isotherms obtained from the numerical solutions. Flow and heat transfer are influenced by the orientation and eccentricity of the inner cylinder. The present numerical results are in good agreement with the results of others for pure natural convection.

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