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Specific Speed Considerations for Inward Flow Radial Supercritical CO2 Turbines

Hoque, SJ and Kumar, P and Gopi, PC (2022) Specific Speed Considerations for Inward Flow Radial Supercritical CO2 Turbines. In: ASME Turbo Expo 2022: Turbomachinery Technical Conference and Exposition, GT 2022, 13 June 2022 - 17 June 2022, Rotterdam.

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Official URL: https://doi.org/10.1115/GT2022-82090


Inward flow radial supercritical CO2 turbines require higher operating speeds than steam or gas turbines making only low specific speed designs practically realizable. A theoretical model is presented to estimate the low specific speed design regime. The model incorporates boundary conditions across the turbine and engineering limitations such as blade height, Mach number, and flow angle at the rotor inlet to predict the minimum allowable rotational speed. A CFD study is subsequently performed to review the applicability of gas turbine design principles on sCO2 turbines for low specific speed designs. The effect of specific speed and velocity ratio on turbine efficiency and flow physics is studied for power output ranging from 100 kW to 5 MW. The results show a significant deviation in the optimal specific speed, velocity ratio, and incidence angle values than traditional gas turbine designs. It is found that viscous losses dominate low specific speeds, while Coriolis effects dominate the high specific speed designs. High specific speed designs require higher negative incidence angles ∼ -50° to -55° to overcome the flow stagnation at blade pressure surface arising from the Coriolis effect. Maximum turbine efficiencies (∼ 83%) are achieved at lower specific speeds of ∼ 0.4 than gas turbines' optimal specific speed of ∼ 0.55 to 0.65. Variations of stator, rotor, and exit kinetic energy losses with specific speeds are also presented. Finally, the results are superposed on Balje's Ns-Ds diagram and compared with gas turbine designs.

Item Type: Conference Paper
Publication: Proceedings of the ASME Turbo Expo
Publisher: American Society of Mechanical Engineers (ASME)
Additional Information: The copyright for this article belongs to American Society of Mechanical Engineers (ASME)
Keywords: Computational fluid dynamics; Energy dissipation; Gases; Kinetic energy; Kinetics; Turbine components; Velocity, Coriolis effect; Gas turbine design; Low specific-speed; Meanline design; Ns-ds diagram; Specific speed; Speed design; Turbo machines; Turbomachine loss; Velocity ratio, Gas turbines
Department/Centre: Division of Mechanical Sciences > Mechanical Engineering
Date Deposited: 04 Jan 2023 07:09
Last Modified: 04 Jan 2023 07:09
URI: https://eprints.iisc.ac.in/id/eprint/78727

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