Effect of Buoyancy on Heat Transfer Characteristics of Supercritical Carbon Dioxide in the Heating Mode

Abstract

The Supercritical carbon dioxide Brayton cycle is a leading competitor for the Generation IV nuclear power plants due to the minimal initial cost, increase in efficiency at higher outlet temperatures, and reduction in the plant footprint. However, due to the complex behavior of thermophysical properties in the critical region, a better understanding of the turbulent heat transfer characteristics needs to be explored. Experiments were performed with the carbon dioxide under heating conditions at the Texas A&M University Supercritical Fluids Facility. Turbulent flows with Reynolds numbers up to 60,000, at operating pressures of 7.5, 8.1, and 10.2 MPa were tested in a 10.9 mm inner diameter, 1 m long 316 stainless steel round tube. Local heat transfer values were obtained using measured wall temperatures over a large set of experimental parameters that varied inlet temperature from 20 0 C to 55 0 C, mass flux from 150 to 350 Kg/m 2 -sec and maximum heat flux of 65 KW/m 2 . Results were also obtained for 90 o upward and downward flow to understand the unusual heat transfer characteristics due to the effect of buoyancy and flow acceleration caused by large variation in density. Buoyancy factor calculations for all the test cases indicated that buoyancy effects cannot be neglected even for horizontal flow at Reynolds number as high as 20,000. Experimentally determined Nusselt numbers are compared to existing correlations available in the literature. Existing correlations predicted the experimental data within 30% with some deviation around the pseudo-critical temperature.