Buoyancy-driven heat transfer analysis in two-square duct annuli filled with a nanofluid

The natural convection fluid flow and heat transfer in the annuli of two differentially-heated square ducts filled with the TiO2-water nanofluid are investigated numerically. The outer duct is maintained at a constant temperature Tc while the inner duct is kept at a differentially higher constant temperature Th. The governing equations written in terms of the primitive variables are solved using the finite volume method and the SIMPLER algorithm. Through a parametric study conducted, the effects of the Rayleigh number, the aspect ratio of the annulus, and the volume fraction of the nanoparticles on the fluid flow and heat transfer are investigated. To verify the numerical procedure, two different natural convection simulations are conducted using the proposed code, and the results are found to be in good agreement with the existing results already available in the literature. The numerical outcome of the present study shows that, by increasing the width of the gap between the ducts and also the Rayleigh number, multiple eddies are developed in the gap between the top walls of the square ducts. The eddies formed demonstrate the characteristics of the Rayleigh-Bénard convective type. Moreover, it is observed from the results that, the average Nusselt number increases by increasing the volume fraction of the nanoparticles.