Progress in Developing an Improved Empirical Heat Transfer Equation for Use in Connection With Advanced Nuclear Reactors Cooled by Water at Supercritical Pressure

Consideration of advanced power plant such as nuclear reactors cooled by water at pressures above the critical value has stimulated a renewed interest in heat transfer to supercritical pressure fluids. Severe deterioration in the effectiveness of heat transfer can be encountered as a result of the extreme dependence on temperature of the physical properties of such fluids, particularly near the pseudocritical temperature where their molecular structure changes from being liquid-like to gaseous. This deterioration arises mainly as a result of the non-uniformity of density, which can lead to significant influences of bulk flow acceleration and fluid buoyancy. A good physical understanding has been arrived at of the mechanisms by means of which such influences can modify the mean flow and turbulence fields and thereby the advection and turbulent diffusion of heat and effectiveness of heat transfer. However, this progress in understanding the physics has so far not resulted in such effects being reliably accounted for in the empirical equations which are available for thermal design. With a view to addressing this matter, the author has recently attempted to update and improve an existing physically-based semi-empirical model of variable property heat transfer. The aim has been to combine it with a soundly-based empirical forced convection equation to extend the applicability and reliability of currently available thermal design procedures. In the present paper, progress in validating this approach and optimising the performance of the extended equation is reported.