Numerical study of mixed convection of nanofluid in a concentric annulus with rotating inner cylinder

In this work, the steady and laminar mixed convection of nanofluid in horizontal concentric annulus with rotating inner cylinder is investigated numerically. The inner and outer cylinders are kept at constant temperature Ti and To respectively, where Ti > To. The annular space is filled with Alumina-water nanofluid. The governing equations with the corresponded boundary conditions in the polar coordinate are discretized using the finite volume method where pressure-velocity coupling is done by the SIMPLER algorithm. Numerical results have been obtained for Rayleigh number ranging from 102 to 105, Reynolds number from 1 to 300 and nanoparticles volume fraction from 0.01 to 0.06. The effects of the Reynolds and Rayleigh numbers, average diameter of nanoparticles and the volume fraction of the nanoparticles on the fluid flow and heat transfer inside the annuli are investigated. According to the results, the average Nusselt number decreases with increasing the Reynolds number. However, the average Nusselt number increases by increasing the Rayleigh number. Moreover, the maximum average Nusselt number occurs for an optimal nanoparticle volume fraction except situations that heat conduction predominates over the heat convection. In these conditions the average Nusselt number is close to unity.