Abstract

During the development of a sCO2 turbine for the STEP program under DOE funding, radial inlet and exit designs were completed under mechanical and thermal constraints while seeking to maximize aerodynamic performance [1]. The flow path geometries of the designs were optimized at a single steady-state, design condition to minimize the total pressure loss. In order to efficiently characterize changes in performance across a range of operating points of the closed-loop cycle, off-design loss models for the inlet and exit diffuser have been created from CFD simulation results. To generate boundary conditions for the exit diffuser, numerous 1D exit profiles were selected from a regression analysis of turbine map points as a function of pressure ratio and corrected speed; radial inlet conditions emanated from planned cycle operating points. From base equation forms that include dynamic pressure and swirl components, equation constants were optimized to provide a best fit to CFD results. The equations generated were able to be integrated into existing STEP cycle modeling tools to enhance the prediction of turbine response as the operating point deviates significantly from design values. The methods outlined in this paper can provide a guide for evaluating the off-design performance of turbine inlet and exit designs dissimilar from conventional designs for which correlations are available.

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