Impact of Unsteady Full-Annulus CFD on the Predictions of High-Pressure Turbine Thermal Loads: A Comparative Study With Steady State Mixing-Plane Approach

Field evidence has suggested that numerical CFD predictions based on the steady state approach, commonly used in the preliminary design of turbomachinery due to their ability to provide rapid answers, systematically overestimate the gas-side temperatures experienced by high-pressure turbine blades. To have a deep insight into this aspect, a comprehensive numerical investigation based on steady and unsteady predictions has been carried out, focusing on the radial mixing effects along a four-stage high-pressure turbine. The introduction of a mixing plane prevents the transmission of any circumferential flow distortion between the fixed and rotating rows. The result is a reduction of both enthalpy and momentum mixing in the radial direction, with a potential impact on thermal loads prediction. To improve model fidelity, a full-annulus setup, from the combustion chamber exit to the turbine exhaust was considered. All key turbine features, including leakages, tip clearances, and film-cooling holes that can affect the mixing have been retained. The study shows how the time-accurate approach allows for better reproduction of the flow evolution along the machine and, in particular, provides more accurate prediction of gas temperature distributions and hot streaks migration. Due to the high computational resources needed for unsteady simulations, such an approach is usually not compatible with the design iterations. However, the results of unsteady simulation can be used to inform radial mixing models able to correct the lack of mixing typical of adopting a standard mixing-plane approach. These models are conceived to assist the designer in the preliminary phase, reducing the need for subsequent corrections in the detailed phases in which the aerothermal design is finalized. Finally, a comprehensive comparison of the results obtained using the different methodologies is presented and discussed in detail.