Interpreting and Categorising Experimental Data on Forced and Mixed Convection Heat Transfer to Supercritical Pressure Fluids Using Physically-based Models

Abstract

The need to improve the reliability of thermal design procedures for advanced pressurised water reactors has stimulated a renewed interest in heat transfer to fluids at supercritical pressure. Deterioration of heat transfer is sometimes encountered with such fluids, mainly as a result of the strong dependence of density on temperature. Two physically-based models of turbulent heat transfer in tubes are described here which can be used for categorising and interpreting experimental data on heat transfer to fluids at supercritical pressure.    The first is a model of variable property mixed convection in vertical heated tubes. This predicts reduced or increased turbulence and, therefore, impaired or enhanced heat transfer, respectively, due to the influence of buoyancy on the flow. The second is a model of variable property forced convection with thermally-induced bulk flow acceleration due to high heat flux and low mass flux. This predicts reduced turbulence due to acceleration of the flow, and consequently, impaired heat transfer. Used together for conditions where influences of buoyancy and acceleration are both significant, the models enable the resulting complex heat transfer behaviour to be interpreted and better understood. Some recent experimental data on heat transfer to supercritical pressure carbon-dioxide in heated tubes of small diameter in the mini and micro ranges have been categorised and interpreted in a preliminary manner with the aid of the models and the conclusions arrived at are presented here. The work is ongoing and a more detailed demonstration of the usefulness of the models will be completed shortly.