Rayleigh-Bénard convection of a supercritical fluid

Abstract

Fluids above the critical point are widely used in industry. Chemical, pharmaceutical, food industry and energy production are some examples. In the energy production sector they are mainly used as cooling fluids, because they allow to increase the thermal efficiency of the power plants. However, the fundamentals of their heat transfer behavior are still unknown and current heat transfer models fail to predict it. Supercritical (SC) fluids are characterized by strongly varying fluid properties, which are responsible for their particular heat transfer behavior and make them very difficult to model, simulate and experimentally investigate. In past studies, buoyancy was identified as a key cause for the heat transfer deterioration observed in SC fluids. The aim of the research described in this thesis is to investigate the possibility of performing non-intrusive local velocity measurements with the optical technique PIV and to acquire global heat transfer measurements, with strongly changing fluid properties at SC conditions. The experiments were performed in a pure buoyancydriven flow: a Rayleigh-Bénard (RB) flow. The velocity fields of RB convection with strongly varying properties, beyond the so-called Oberbeck-Boussinesq (OB) approximation, were experimentally studied at atmospheric pressure first. An increase of the time-averaged velocity close to the bottom wall of the cell with respect to the top wall of about 13% was found. This finding confirmed experimentally a top-bottom ”broken symmetry” in the velocity field, which was observed in previous numerical and theoretical studies, but it was never experimentally demonstrated before. The heat transfer with strongly variable properties at SC conditions for constant Prandtl and Rayleigh numbers, specifically defined outside the validity range of the OB approximation, was experimentally studied. The measurements were performed at the Max Planck Institute of Dynamics and Self-Organization in Göttingen (Germany), with a European EuHIT project. It was observed that the measured Nusselt number defined for non-OB conditions was different from point to point, showing that merely the Rayleigh and Prandtl numbers are not sufficient to determine the heat transfer through the cell. It was also seen that the measured Nusselt number was 16% larger with respect to the one predicted by the Grossmann-Lohse theory (2000) for the same Rayleigh and Prandtl numbers at OB conditions. A feasibility study of particle image velocimetry (PIV) at SC conditions was done by using the background oriented schlieren technique (BOS). An estimation of the PIV experimental uncertainty at SC conditions was done with the statistical correlation method proposed by Wieneke et al., (2015). PIV was successfully performed at SC conditions. Main difficulties about its applicability were due to blurring and optical distortions in the boundary layer and thermal plumes regions. PIV measurements were performed at three different magnitudes of density difference between top and bottom of the cell. Two of the three experiments were done at similar Rayleigh and Prandtl numbers, defined for non-OB conditions: one towards the liquid phase and the other one towards the gas phase. The former showed a lower large scale circulation (LSC) velocity than the latter. All cases showed the presence of one asymmetric LSC roll, which is different from a typical RB convection flow at OB conditions.
Improvements in the accuracy of PIV measurements and the acquisition of more
heat transfer data at SC conditions, would help the study of the thermal and viscous boundary layer thicknesses and turbulence modifications that are responsible for different heat transfer regimes in SC fluids.