Nanofluid buoyancy-driven heat transfer in three-dimensional horizontal annuli

The effect of Rayleigh number and nanoparticle concentration on the flow dynamics and natural convection in three-dimensional large- and moderate-gap annuli is analyzed in detail in this work. A nanofluid is applied as the working fluid, with typical nanoparticle concentration ranging from zero to nine percent. The results indicate that the flow strength and deformation of isotherms becomes less intensive with an increase in the nanoparticle concentration for both large- and moderate-gap annuli. There exists a core region in the large-gap annulus, where the flow is two-dimensional and the value of the local Nusselt number is almost uniform in the axial direction. The core region exists between two secondary rolls, which are adjacent to the primary end rolls. The secondary flow decreases in size and finally disappears with an increase in the nanoparticle concentration. For the moderate-gap annulus, the space between the two end walls near the top portion is filled by multiple rolls, and an increase in the nanoparticle concentration gradually reduces the number of the rolls. The locations of the descending and ascending rolls relate to the maximum peaks of the Nusselt number on the inner and outer cylinders, respectively. The heat transfer enhancement only occurs at low nanoparticle concentrations (phi < 3%) and higher Rayleigh numbers (Ra > 10(5)) for a large-gap annulus. Otherwise, in general, a deterioration in heat transfer is observed when the nanoparticle concentration increases from 1 to 9% for both large- and moderate-gap annuli. (C) 2020 Elsevier Masson SAS. All rights reserved.