This paper concerns a theoretical investigation of forced and mixed convection heat transfer in a vertical concentric annulus. An implicit finite difference technique is developed to study the effects of temperature-dependent fluid properties, which are represented by power law relations. The fluid under consideration in this study is air, which is an ideal gas with Pr = 0.72. Computations are made with a radius ratio of 0.25. The inner wall is heated at UHF, and the outer wall is heated at UHF or is insulated. The axial distribution of the wall-to-bulk temperature ratio is found to undergo a maximum or minimum, depending on whether the wall is heated or insulated. Fluid property variations enhance the local Nusselt numbers. It is shown that, depending on the length of the duct, heat transfer can be appreciably affected by fluid property variation. For the UHF cases studied here, free convection effects enhance the local Nusselt numbers at intermediate axial distances.

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