Turbulent heat transfer and secondary flows in a non-uniformly heated pipe with temperature-dependent fluid properties

Abstract

The influence of temperature-dependent fluid properties in a turbulent pipe flow with sinusoidal heat flux boundary conditions is studied. Four cases with increasing sensitivity to temperature variations, representative of molten salts in solar heat receivers, are calculated by means of direct numerical simulation. The computations have been performed with a reference friction Reynolds number equal to 180 and a reference Prandtl number equal to 0.7. It is found that the fluid properties variations result in an enhancement (damping) of the flow and temperature fluctuations on the cold (hot) side of the pipe. Small secondary motions of Prandtl second kind are found to occur with a significant impact on the vertical heat flux, accounting for one third of the total heat flux in the most sensitive case. Finally, the effect of the variable fluid properties in integral quantities like friction coefficient and Nusselt number is quantified.