Rheological and porous effects on mixed convection: Power-law fluid flow in a finned annular configuration

Abstract

This study numerically investigates mixed-convection heat transfer of non-Newtonian power-law fluids in the annular space between a rotating outer cylinder and a stationary finned inner cylinder filled with a porous medium. The outer cylinder rotates at constant angular velocity and is maintained at a temperature lower than the finned inner cylinder, producing combined forced and buoyancy-driven flow. The governing equations were solved using the finite volume method implemented in Ansys CFX, with the fluid’s rheology modelled by the Ostwald’s law model. Parametric ranges include: Reynolds numbers from 5 to 35, Richardson numbers from 0 to 3, Darcy numbers from 10^‒5 to 10^‒2, and power-law indices of 0.6, 1.0 and 1.6. The results show a transitional behaviour at a Richardson number of approximately 2.6 (for a Darcy number of 10^‒2), beyond which the average Nusselt number becomes almost independent of the power-law index. Increasing rotational forcing enhances circulation and heat transfer, and under strong buoyancy the shear-thinning fluid (power-law index 0.6) yields up to 15–20% higher average heat transfer compared with the Newtonian case. The novelty of this work lies in the joint investigation of internal fins, outer-wall rotation and porous resistance in non-Newtonian media, which produces distinct flow regimes and offers practical guidance for porous heat exchanger and rotating-machine cooling designs.