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Direct numerical simulations of the flow and forced convective heat transfer around a sphere at Reynolds numbers between R e = 500 and R e = 1000 are performed. We investigate the effects of the Prandtl number (P r) on the forced convective heat transfer from a sphere for various fluids having P r = 0. 01 , 0. 1 , 0. 7. At the larger Prandtl number, the convective transport due to the vortex shedding process dominates over the diffusive transport. As the Prandtl number decreases, diffusive effects become important. Moreover, the thermal boundary layer increases with decrements of the Prandtl number, which results in a reduction in the local and mean non-dimensional heat transfer coefficient. It is seen that at R e = 500 and R e = 750 , the vortex shedding process is asymmetric, which results not only in a non-zero lift coefficient, but in an asymmetric temperature field in the wake of the sphere at P r ≥ 0. 1. The dual asymmetry in the flow and the convective heat transfer is smooth out when the Prandtl number reaches P r = 0. 01 as the heat diffusion dominates and asymmetries in the vortex formation zone are no longer relevant in the heat transport. The descend in Prandtl number also produces an attenuation of the temperature fluctuations and thereby, in the turbulent heat transfer. As a direct consequence, two factors emerge at P r = 0. 01 : (1) a lower decay ratio of the temperature in the wake centreline, and (2) a larger wake spread compared to higher Prandtl numbers. [ABSTRACT FROM AUTHOR]

Abstract: The cooling process by natural convection in cylindrical cavities is a phenomenon which takes place in several applications such as solar energy systems. In the present work a storage tank with an internal gas flue is studied experimentally and numerically during its long-term cooling process. The computational domain includes two fluids, i.e. water in the store and air in the chimney, and two external and internal layers of steel separated by polyurethane insulation material. In this paper, the numerical and the experimental analysis of the temperature field inside the tank submitted to an external convection cooling process with a constant convection heat transfer coefficient is presented. The air and the water temperature profiles along the vertical lines are obtained experimentally and numerically, for a cooling period of 90h. The numerical analysis is carried out using a specific CFD code developed for the present work; an axisymmetric domain has been considered. Finally, a detailed description of the phenomena that occur inside the water part of the domain during the cooling process is also provided. [Copyright &y& Elsevier]

• Flow and heat transfer around a sphere under turbulent environment are studied using DNS and LES. • Incoming turbulence largely affects the fluid flow and heat transfer. • Increase of the free-stream turbulence shrinks the vortex formation region and increases drag coefficient. • Turbulence intensity increases the Nusselt number and its fluctuations, especially in the rear zone. • Turbulent heat fluxes in the wake are increased by ve times as turbulent intensity increases up to 10%. Free-stream turbulence is present in many engineering applications and is known to affect both bluff body aerodynamics and heat transfer. In this work, its effects on the heat transfer and the near wake behind a sphere are studied. To do this, direct and large-eddy simulations of the flow at the moderate Reynolds numbers of R e = 1000 and R e = 10 4 , with levels of incoming turbulence up to 10 % and a length scale of O (D) , D being the sphere diameter, are performed. At the Reynolds numbers under consideration, significant changes are observed. Incoming turbulence delays the separation of the laminar boundary layer from the sphere, while at the same time the drag coefficient and the Nusselt number are increased. The incoming level of turbulence also increases the momentum transfer from the surrounding fluid and energises the separated shear-layer. As a consequence, there is a shrinking of the recirculation zone which intensifies the heat transfer from the sphere and thus, the Nusselt number and its fluctuations, especially in the rear zone of the sphere. It is shown that free-stream turbulence increases the turbulent heat flux in the wake, with a larger entrainment of colder fluid from the surroundings, thus producing a faster decay of the temperature at larger levels of turbulence intensity. [ABSTRACT FROM AUTHOR]