Analysis of radial-outflow turbine design for supercritical CO2 and comparison to radial-inflow turbines

Abstract

Cycles using supercritical carbon dioxide have been recognized as potential future power technology. However, the turbines in these systems tend to experience very high rotational speeds and entail small physical sizes, which can affect on the overall feasibility. One turbine type having potential to overcome some of the described challenges is the radial outflow turbine. However, its use in supercritical carbon dioxide power systems has not yet been extensively studied since typically axial or radial inflow turbines are considered instead. In this paper, the design of radial-outflow turbines with supercritical carbon dioxide is carried out and the results are compared to the respective radial inflow turbine designs. The analysis was carried out with turbine inlet temperature of 600 °C, inlet pressure of 200 bar and outlet pressure of 78 bar. Designs with four mass flow rates were investigated to study the effect of the power scale on the turbine design and losses. The geometry, efficiency, rotor stress and axial force were defined and analyzed for each design case. The results show that both the investigated turbine types can reach high isentropic efficiencies ranging from about 85% to over 90%. The radial outflow turbines can reach high efficiencies at wider specific speed range whereas the efficiency of radial inflow turbine is reduced more steeply, as the design speed is changed from its optimal value. Radial outflow turbines also reach high efficiencies with lower design speeds and that can be considered advantageous, since it helps to reduce the challenges related to turbomachinery rotordynamic and mechanical design. As a conclusion of the study, it is suggested that the use of radial outflow turbines can increase the feasibility of supercritical carbon dioxide power systems at the examined power scales.