To keep up with the global efforts on the reduction of contaminant emissions and the development of more efficient energy conversion systems, gas turbine engineering strives constantly towards the improvement of each one of its components. The determinant role of turbomachinery blades on the thermodynamic behavior of the whole machine has captured for decades the attention of thermal and aerodynamics specialists that find in the optimization of airfoil profiles a promising and valuable way for the progress pointing to a cleaner energy industry.The present work is motivated by the need of an application for the automated generation of efficient choked turbine cascades, since this property is usually seen as advantageous for design purposes and intentional choking is actually a common practice in the first turbine stage. Also, the fundamental physics of those flow characteristics have been widely studied by a large number of authors; there is still a lack of open literature related to the implementation of this principle for design purposes and the development of intentionally choked blades.Therefore, a 3D model of a single stage close to choking conditions has been set up as reference model to prescribe the boundary conditions of the first vane and the conditions of the stage. Those conditions are used for a Blade-to-Blade (2D) optimization of the vane mid-section. The main concern of the 2D optimization is the realization of shock conditions at the throat section of the profile geometry, for the same boundary conditions. The philosophy of the optimization approach is related to an existing 2D design approach for subsonic flow, where the pressure distribution over the airfoil is characterized. A similar approach can be applied to choking blades, as the pressure distribution over an airfoil has a characteristic behavior. The comparison of the re-designed vane geometry shows a good agreement to the stage conditions of the reference model, while the shock at the throat section is more pronounced and the efficiency of the stage is kept on a high level.

Due to the worldwide increasing demand for energy and the simultaneous need in reduction of CO2 emissions in order to meet global climate goals, the development of clean and low emission energy conversion systems becomes an essential and challenging task within the future clean energy map. In this paper the design process of a highly efficient large scale USC steam turbine is presented. Thereby, automated design space exploration based on an optimization algorithm is applied to support the identification of optimal flow path parameters within the preliminary and the detailed design phases. The optimization algorithm is first integrated with a 1D-mean line design code to automatically identify the optimal major turbine layout in terms of number of stages, reaction degrees, flow path geometry and basic airfoil parameters. Based on a progressive multi-section optimization coupled with a parametric airfoil generator and a CFD code, the profile shapes of each airfoil row are adapted to local flow conditions and systematically optimized to minimize aerodynamic losses in each turbine stage. A final 3D flow simulation of one representative optimized stage confirms the achievement of a highly efficient steam turbine design that fulfills both climatic and economic requirements. KEYWORDS STEAM TURBINE, DESIGN, AUTOMATED DESIGN SPACE EXPLORATION, NOMENCLATURE 1D one dimensional 2D two dimensional 3D three dimensional BC boundary condition c chord length CFD computational fluid dynamics d diameter i incidence M Mach number r radius h blade height h static enthalpy u rotational speed USC ultra-supercritical Greek Symbols absolute flow angle relative flow angle isentropic efficiency (= Δℎ (Δℎ + Δℎ)) ⁄ stagger angle loss coefficient (= 2Δℎ / 2 2) ℎ degree of reaction (ℎ = Δℎ Δℎ ⁄) flow coefficient (= / 2) ℎ stage enthalpy coefficient (ℎ = 2Δℎ/ 2 2)