On the Assessment of CFD Assumptions for the Preliminary Design of a Two-Stage High-Pressure Turbine: Impact of Unsteady Effects on Thermal Loads

The continuous temperature and pressure increase to target thermodynamic efficiency as well as power density in modern gas turbines (GT) stretches the design space and calls for high-pressure-turbines (HPT) designed with very accurate design tools. Cooling, both internal and external, is necessary as the evolving fluid temperature is beyond the materials capability. Therefore, the design process of HPT is more and more based on computational fluid dynamics (CFD) that due to the growing computational power paired with very efficient numerical methods and improved models allows computer simulations in the framework of design iterations that were unthinkable couple of decades back. The design evolves from the conceptual phase, usually completed with the help of simple correlations and proprietary technology curves, not covered here, and proceeds with the preliminary and detailed phases where aero, thermal, mechanical and geometrical details are progressively defined. It is of paramount importance that design choices made in the preliminary phase are refined in the detailed phase without requiring excessive rework.

This paper concentrates on the CFD approach used in the preliminary phase where flow path and airfoils are defined, while disk cavities, squealer tips, seals, cooling and other details are not yet in their final form and will be defined at a later design phase. The paper describes the geometry and operating conditions simplifications typically adopted in the early design phase and continues discussing aero-performance and thermal loads. The paper discusses also steady and unsteady approaches as the adoption of computationally efficient CFD tools permit early multistage unsteady calculations. Unsteady effects are shown to have a strong impact on both momentum and enthalpy mixing. Unsteady effects should be carefully considered in the design process to guarantee an accurate prediction of gas temperature distributions as well as aerodynamic load that control the airfoil thermal load and ultimately the robustness of cooling and purge system design.