Mixed convection heat transfer from a cylinder in power-law fluids: effect of aiding buoyancy

Abstract

Mixed convection heat transfer from an isothermally heated horizontal cylinder immersed in incompressible power-law fluids is considered here in the steady flow regime when both the imposed flow and the buoyancy induced motion are in the same direction, i.e., the so-called buoyancy aiding configuration. Within the framework of the Boussinesq approximation, the suitable forms of the momentum and thermal energy equations for the power-law fluid model have been solved numerically using the finite volume based FLUENT (version 6.3) solver for the following ranges of conditions: the buoyancy parameter (Richardson number, 0 ≤ Ri ≤ 2), power-law index (0.2 ≤ n ≤ 1.8), Reynolds number (1 ≤ Re ≤ 40), and Prandtl number (1 ≤ Pr ≤ 100). In particular, the effects of these dimensionless parameters on the detailed local and global kinematics of flow and heat transfer characteristics such as streamline, vorticity, and pressure profiles, individual and total drag coefficients, and local and average Nusselt numbers have been presented. The wake size shows trends which are qualitatively similar to that seen in the pure forced convection (Ri = 0) regime, though it decreases with increasing Richardson number (Ri) and/or Prandtl number (Pr). The pressure coefficient decreases with the increasing values of Reynolds number (Re) and Prandtl number (Pr) and with decreasing Richardson number (Ri). It is, however, seen to be relatively insensitive to the value of power-law index (n). Both drag coefficients and average Nusselt number are augmented with the increasing buoyancy effects, Reynolds (Re) and, Prandtl (Pr) numbers. An increase in the shear-thinning tendency of the fluid enhances the drag and heat transfer, whereas both of these are generally reduced in shear-thickening fluids. Similarly, the fluid behavior also modulates the role of Richardson number (Ri); namely, the pressure forces dominate over the viscous forces in shear-thinning fluids and vice versa in shear-thickening fluids. Therefore, the buoyancy effects are found to be stronger in shear-thinning fluids and/or at low Reynolds numbers than that in shear-thickening and/or at high Reynolds numbers.