Natural convection driven heat transfer in fluids with strongly variable properties

Abstract

The High Performance Light Water Reactor (HPLWR) is one of the six innovative nuclear energy systems proposed by the Generation IV International Forum. The use of water at supercritical pressure as the coolant in the HPLWR allows a significative increase of the thermal efficiency of the power plant, a reduction in size and complexity of the system and a safety improvement with respect to the use of two phase flows. Fluids at supercritical pressure are characterized by a sharp change of properties, which may lead to an enhancement or deterioration of their heat transfer properties, whose underlying mechanisms are mainly driven by buoyancy and acceleration effects. The motivation of this research is therefore to understand the effect of the sharp change of properties in the fluid flow structure and turbulence production. This work focuses on the influence of buoyancy in particular. The effect of the strongly varying properties, which are far beyond the so-called Boussinesq approximation, was experimentally studied in a water-filled, cubical Rayleigh-Benard cell using Particle Image Velocimetry (PIV). A temperature difference of 40 K is imposed between the bottom and top plate of the cell, ensuring non-Boussinesq conditions. These experiments were conducted at Rayleigh and Prandtl numbers of 6.8 x 108 and 4.4, respectively. For the first time in literature, the instantaneous and averaged flow structures under non-Bousinesq conditions have been experimentally determined on a cross section of the whole domain. Results reveal a slight asymmetric behavior of the fluid due to the large temperature difference between the bottom arid the top plates of the cell, which is a sign of non-Boussinesq effects. The data provided in this study can be used to gain a more in depth understanding of the effect of the strongly varying proprieties of supercritical fluids on natural convection phenomena in supercritical water cooled reactors.