EXPERIMENTAL INVESTIGATION OF THE EFFECT OF FLOW DIRECTION ON HEAT TRANSFER OF SUPERCRITICAL CARBON DIOXIDE FLOWING IN A COOLED VERTICAL TUBE

摘要

Heat transfer coefficient of turbulent flows of supercritical carbon dioxide was experimentally investigated for vertical flows. The test section consists of two subsections connected in series and forming a U-tube. Each subsection is a 0.75 m-long vertical tube-in-tube counter-flow heat exchanger, with CO{sub}2 flowing in the inner copper tube of 6 mm diameter and cooling water flowing in the annulus. Experimental results indicate that strong differences on heat transfer coefficients appear with flow direction, indicating an influence of buoyancy forces on heat transfer process. Results are presented on a dimensionless form commonly used in the studies of mixed convection in heating but not used in studies of cooling of supercritical carbon dioxide. Based on the experimental data collected in the present work, correlations have been developed for upward and downward turbulent flows, respectively. The present results complete the missing data of the literature on turbulent vertical mixed convection in cooling conditions.