Thermal Effects of Turbulent Mixed Convection in Horizontal Pipes under Uniform Wall Heat Flux

Direct numerical simulations - DNS hereafter - of a fully developed, turbulent horizontal pipe flow in the presence of buoyancy forces were carried out in order to investigate the interactive shear and buoyancy effects on the turbulent momentum and heat transport. Similar studies were carried out for the channel flow configuration [1,2], while, to the authors best knowledge, no investigation has been reported so far for the pipe flow. The isoflux wall thermal conditions were imposed, which generated a sort of stable stratification and a complex secondary flow pattern, affecting both the hydraulic and the thermal efficiency. While knowledge of synthetic parameters, like the friction factor and the Nusselt number, is valuable for practical applications, DNS studies produce a wide variety of higher-order statistics, which can be used to develop a better physical comprehension of the phenomenon, or even to improve the existing turbulence models. The modest influence of buoyancy on both the Nusselt number and the friction factor does not reflect the relevant modifications, which, at the considered values of the Richardson number, characterize the velocity and the temperature fields