UNUSUAL HEAT TRANSFER CHARACTERISTICS OF SUPERCRITICAL CARBON DIOXIDE
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Heat transfer mechanisms in supercritical fluids is quite different due to the fact that the thermophysical properties vary drastically within a span of few degrees Celsius near the critical point. A series of integral experiments were performed to investigate the unusual turbulent heat transfer characteristics of supercritical carbon dioxide flow in round tubes under heating conditions. Wall temperatures were measured over a range of experimental parameters that varied fluid inlet temperature from 20° C to 60° C, operating pressure from 7.5 to 10.2 MPa, mass flux from 100 to 1000 kg/m2-sec and a maximum heat flux of 100 KW/m2. Measurements were made for horizontal, upward, and downward flow to study the effects of buoyancy and flow acceleration caused by large variation in density. Existing criteria to predict the influence of buoyancy suggested that the experimental data can be classified into three regimes, namely normal, deteriorated, and enhanced heat transfer. Localized deterioration in heat transfer was characterized by a sharp increase in wall temperature and observed mainly in the case of upward flow due to reduction in the turbulent shear stress. Enhanced heat transfer regime was characterized by smooth variation in wall temperature and observed in the case of downward flow due to increase in the turbulent shear stress. Flow stratification occurred in horizontal flow resulting in a circumferential variation in wall temperature. Thermocouples mounted 180° apart on the tube revealed that wall temperatures on the top side are significantly higher than the bottom side of the tube. When the bulk temperature is much higher than the pseudocritical temperature, normal heat transfer was observed for all three tube orientations indicating that the buoyancy effects were negligible. Deterioration and enhancement in heat transfer were also observed in downward and upward cases respectively due to the flow acceleration effects. This occurred in the cases where outlet fluid density was much lower than the inlet fluid density causing the flow to accelerate. In the case of upward flow, this acceleration enhanced the turbulent shear stress and heat transfer. The large experimental database was used to evaluate the existing popular heat transfer correlations for supercritical fluids.
