A numerical study based on the analysis of laminar natural convection in a concentric cylindrical annulus heat exchanger is investigated. The operating fluid is confined between two horizontal concentric cylinders which are maintained at different uniform temperatures. The governing equations the flow (of continuity, momentum and energy) are numerically solved via finite volume method (FVM). The investigation is performed for Rayleigh number and volume fraction of nanoparticles in the range of 10 3-10 5 and 0-12%, respectively. The effective thermal conductivity and viscosity of the nanofluids mixture are calculated via Maxwell-Garnett model (MG-model) and Brinkman model, respectively. The results are presented in terms of isotherms, fluid flow patterns and Nusselt number distribution function of Rayleigh number and the volume fraction of silver nanoparticles. The results are also discussed in detail. Results are discussed in detail. It is found that a very good agreement exists between the present results and those from the literature. It is found that fluid flow intensity and heat transfer rate increase with the increase of the nanoparticles volume fraction and Rayleigh number, Also, the thermal effectiveness depends on the Rayleigh number and the volume fraction of the nanoparticles.

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