A numerical analysis has been performed on the three-dimensional natural convecrive heat transfer characteristics of a porous medium enclosed by a vertical concentric curved annulus heated from the inner surface and cooled from the outer surface with relation to the thermal insulation layer in the high-temperature ducting system of a high-temperature gas-cooled reactor. Darcy’s law and the Boussinesq approximation are assumed to be applicable. The governing equations are transformed into finite-difference equations, which are numerically solved by a successive over-relaxation procedure for a range of RaDa (a product of the Rayleigh number and the Darcy number) from 100 to 800. Two typical vertical arrangements (case A, in which a 90° bend is attached at the upper part of a vertical straight tube, and case B, in which it is attached at the lower part) were analyzed and compared with each other. The numerical results show that the flow field and the temperature profile have characteristics of those for both horizontal and vertical annuli, and the insulation performance in case B becomes worse than in case A for a whole range of RaDa. Information useful for the thermal insulation design of a high-temperature ducting system was obtained.
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