Heat transfer and fluid flow characteristics of supercritical carbon dioxide flow

Abstract

The goal of this dissertation is to enhance the fundamental understanding of heat transfer phenomenon of sCO2 especially near the critical point and to investigate the thermal-hydraulic characteristics of printed circuit heat exchangers used in supercritical CO2 power cycles. To achieve these goals an experimental test facility was constructed to investigate the heat transfer and pressure drop characteristics of supercritical CO2 (sCO2) flow inside circular tubes and prototypic printed circuit heat exchangers. To achieve the first goal of the dissertation, the test facility was used to investigate the effect of variable fluid properties on the sCO2 flow inside heated circular tubes. Two circular tube test sections with inner diameters of 10.9 and 7.9 mm were selected for investigation. Wall temperatures and heat transfer coefficients were measured for a wide range of operating conditions by varying the fluid inlet temperature, mass flux, heat flux and system pressure. Three different test section orientations - horizontal, upward and downward flows were tested to investigate the effect of buoyancy on the heat transfer. Separate set of correlations are proposed for the horizontal, upward and downward flow test data. To achieve the second goal of the dissertation, the thermal-hydraulic characteristics of two discontinuous fin printed circuit heat exchangers (PCHEs) with offset rectangular and offset NACA0020 airfoil fin patterns were evaluated experimentally. The pressure drops and the heat transfer coefficients for both the PCHEs were measured over a wide range of conditions with Reynolds numbers in the range of 2,700-38,000 and Prandtl numbers in the range of 0.8-25. Based on the experimental data, friction factor and Nusselt number correlations were developed for both the PCHE test sections. Using the developed correlations, the impact of the tested discontinuous fin printed circuit heat exchangers (PCHEs) on the performance and the capital cost of supercritical CO2 Brayton cycle was investigated. A simulation model was developed for supercritical CO2 Brayton cycle and optimal flow regimes for several PCHEs were identified. The offset rectangular fin PCHE offered highest cycle efficiency and lowest capital cost (on $/KWe basis) followed by S-shaped fin, zigzag channel and the offset NACA0020 airfoil fin PCHEs.