Several industrial activities require the introduction of mechanical agitation and mixing process in vessels with heated walls. This operation is performed in order to accelerate a certain physical or chemical desired homogeneity. Most of the numerical studies presented for the mixing and mechanical agitation process were based on the enhancement of the hydro-dynamic performance and energy consumption, while comparatively less research is carried out on thermal phenomena in the literature. Based on this deficiency, the adoption of computational fluid dynamics (CFD) to study mixed convection in a mechanically stirred tank is the subject of the present paper. The numerical investigation focuses on a 3D unsteady laminar flow in a cylindrical tank equipped with an anchor and filled with an alumina-water nanofluid. The wall of this tank is exposed to a constant hot temperature. However, the anchor, the bottom of the tank and the free surface are assumed adiabatic and the nanofluid has an initial cold temperature. The rotary flow is governed by the equations that describe mixed convection, introducing the buoyancy force in the vertical direction into momentum equations. The Richardson number and the volume fraction of the alumina nanoparticles have been considered as control parameters in order to demonstrate their effect on the thermo-hydrodynamic behavior during the temporal evolution of the thermal state. The numerical results show that the predominant nature of the flow generated by the anchor is no longer tangential in the stirred tank by increasing the Richardson number in the unsteady state. In addition, a relative increase in the average Nusselt number is directly attributed to the increase in buoyancy effects on the one hand, and to the volume fraction of alumina nanoparticles on the other hand.