Fluid flow and convective heat transfer to fluids at supercritical pressure

The feature of fluids at pressures just above the critical value which makes them of particular interest is that they change in a continuous manner from being liquid-like to gas-like with increase of temperature at constant pressure. As a consequence of the extreme dependence of fluid properties on temperature, non-uniformity of density can lead to important effects on the mean flow and turbulence fields and heat transfer effectiveness. When the author and his colleagues first commenced research on supercritical pressure fluids many years ago it was decided to begin with a novel experiment specifically designed to include effects of strong non-uniformity of fluid properties whilst avoiding other complications associated with the temperature dependence of density. This fundamental experiment on stably stratified turbulent flow of supercritical pressure carbon dioxide between two horizontal planes, with the upper one heated and the lower one cooled, in such a way that there was no net heat transfer to the fluid, yielded evidence of a special mechanism for enhancement of turbulent mixing. Later, experiments with uniformly heated vertical tubes using carbon dioxide at pressures very near to the critical value gave results, which exhibited further striking features. Severe localized non-uniformity of heat transfer developed in the case of upward flow, but was not found with downward flow. Gravitationally induced motion caused effects on heat transfer which could only be explained by postulating drastic modification of turbulence. Such results stimulated the development of physically based ideas concerning the mechanisms which might be involved and led to the development of a semi-empirical model of buoyancy-influenced turbulent flow and heat transfer. The main aim of this paper is to show how such early work is now providing a basis for correlating experimental data and enabling the complicated phenomena encountered in those early experiments to be properly accounted for in thermal design procedures.