Effects of flow fluctuations and buoyancy on heat transfer from a sphere in non-Newtonian nano fluids: Potential for intensification

Abstract

This work numerically explores the combined effects of buoyancy-induced flow and pulsating velocity for nanofluids (power-law type fluids) over a sphere to ascertain the potential of this approach for process intensification. The pertinent field equations are solved numerically for a range of dimensionless variables as, Richardson number (0 ≤ Ri ≤ 5), amplitude of fluctuations (0 ≤ A ≤ 0.8), frequency of pulsation (π/4 ≤ ω* ≤ π), Reynolds number (5 ≤ Re ≤ 100), power-law index (0.3 ≤ n ≤ 1.5) and Prandtl number (1 ≤ Pr ≤ 100). The aiding-buoyancy flow suppresses the transient effects of the flow pulsations. The recirculation length (wake) is considerably shortened as the buoyancy-driven flow strengthens. Even a weak buoyancy-induced flow (for instance, Ri = 1) is sufficient to mar the influence of pulsatile velocity. The Richardson and Prandtl numbers do not influence the phase difference in the growth of the momentum boundary layer. Overall, the improvement in the Nusselt number is significantly lowered by the aiding-buoyancy mixed convection than that in the forced convection regime. While this may seem like a detrimental effect on the overall heat transfer, this offers a potential tool for modulating the rate of heat transfer for the processing of temperature-sensitive materials wherein the main consideration is to limit the maximum temperature in the system. The present results can be used to devise an operating envelope of conditions depending upon the envisaged application. Finally, the mean Nusselt number is correlated by using the modified velocity and viscosity scales accounting for the contribution of the mean flow and the buoyancy-induced flow.