Flow in a three-dimensional narrow-gap horizontal annulus with nanofluid

The three-dimensional (3D) natural convection utilizing a nanofluid in a narrow-gap horizontal annulus is investigated. The influence of Rayleigh number (Ra) and nanoparticle volume fraction (φ) on flow dynamics characteristics and heat transfer is investigated. Ra varies between 8000 and 10 000, whereas φ is in the range of 0–0.09. The findings demonstrate that an increase in the Rayleigh number correlates with a rise in the maximum stream function value, resulting in more isothermal deformation. Moreover, increased nanoparticle volume fractions lead to diminished flow and isothermal deformation. The flow roll size is affected by Ra and φ. At a nanoparticle volume fraction of φ = 0, the flow exhibits a crescent-shaped primary roll alongside an opposing tadpole-shaped roll; when φ grows to 0.04, the small opposing roll decreases into a strip structure, and it vanishes completely at φ = 0.08. Instability onset occurs at the end walls. The flow progressively converges toward the center over time, amplifying roll intensity and dimensions, resulting in considerable isotherm distortion. Three-dimensional circulation patterns are established, encompassing transverse and primary rolls alongside the axial movement of the primary roll. As φ increases, the average Nusselt number (Nu¯i) first decreases and subsequently rises for φ greater than 0.06. Comprehensive simulations produce generalized correlations for Nu¯i related to Ra and φ, exhibiting a divergence of under 3.5% from the correlation equation.