Novel Approaches for sCO2 Axial Turbine Design

The axial sCO2 turbine design for shaft power above 10 MW can be approached in a manner similar to the High Pressure (HP), backpressure steam turbine. Starting from the overall performance specification, the detailed turbine design is carried out in steps; 1-Dimensional (1D) meanline design, Quasi 3D (Q3D) throughflow design, cascade blade-to-blade design, 3-Dimensional (3D) blade design, stress and vibration analysis. These design steps are well established and validated, using dedicated test rigs and field performance measurements, for the steam turbines. Even though detailed validation tests are not available for axial sCO2 turbines, there exists a scope to utilize the established steam turbine design principles. This paper highlights sCO2 turbine design procedure through a 10 MW turbine design case study for Waste Heat Recovery (WHR) power plant. The focus areas are blade-to-blade design and stress analysis for which the challenges and novel approaches to design are elucidated. Classical blade design typically relies on expert knowledge where the 2D blade profile geometry is successively iterated to minimize the profile loss. Automated optimization routines are also employed by geometry parametrization techniques such as Bezier or B-Spline control points. This paper introduces a novel approach to 2D blade design as applied to a sCO2 turbine through a combination of Kulfan Class Shape Transformation (CST) for blade parametrization and unique optimization constraints to mimic the expert knowledge. The high power density of sCO2 turbomachinery while advantageous for weight and footprint reduction poses significant challenge in mechanical design. The overall power is distributed among few stages resulting in higher blade stress compared to an equivalent steam turbine. Increasing the blade chord, alternative root design are some of the mitigation methods to deal with the increased stress. They however lead to compromise in aerodynamic performance due to reduced blade aspect ratio. This necessitates novel approaches to balance mechanical and aerodynamic design, which are considered in this paper. Through a 10 MW sCO2 axial turbine design case study, this paper brings to the fore certain design challenges as compared to a conventional steam turbine and puts forth novel approaches to overcome the identified challenges.