RÉSUMÉ

Mixed convective heat transfer around a heated or cooled sphere is analyzed for a wide range of Reynolds and Richardson numbers. The complexity of numerical algorithms coupling the momentum and heat transfer equations limits the number of such direct simulation. The coupled flow and thermal field are solved through an upwind based finite volume method. The numerical accuracy of our code was validated by comparing with several published results. The role of the temperature-induced baroclinic vorticity on the wake is analyzed. The increase or decrease of surface temperature delays or enhances, respectively, the critical Reynolds number for flow separation. The buoyancy-induced jet due to the heated sphere delays the flow separation and enhances the drag coefficient as well as the rate of heat transfer. For a cooled sphere, the opposing effect on the inertial forces lowers the critical Re for flow separation, the wake size increases and the thermal boundary layer becomes thicker compared to the Ri ≥ 0 case. The drag coefficient and rate of heat transfer are evaluated as functions of Ri (−0.5 ≤ Ri ≤ 1.5) and Re (Re ≤ 200), respectively. The correlation formula for the Nusselt number valid for forced convection is extended for the mixed convection regime by introducing the concept of effective temperature. Our computed results for heat transfer are found to be close to the estimated values obtained by the modified correlation formula for a moderate range of Reynolds numbers.