<p>Thermal protection is key for enhancing material reusability and efficiency in gas&nbsp;turbine applications, by allowing metals to be exposed to increasingly aggressive temperatures. Double wall transpiration cooling systems display enormous cooling benefits thanks to the combination of external film cooling and internal impingement cooling. However, thee structural integrity of these systems is still raising concerns. Although external film holes offer a remarkable reduction in heat flux into metallic components by enabling the formation a cool buffer layer between the hot fluid and the component, they can compromise structural integrity due to local stress raising effects. We specifically find that increasing the angle of film holes up to &nbsp;(degrees) to the surface normal of a thin plate, can locally magnify five times the stresses that would exist in the absence of the hole, well exceeding the yield stress of Nickel superalloys, especially at high temperatures of 1000 °C. For these conditions, we show that significant plastic and creep deformation can occur within large material volumes of turbine blade walls, such that incremental plastic strain growth (ratcheting) may occur locally, only due to thermal loading. The mechanism which can trigger such local ratcheting phenomena in double wall components, is explained by an elementary 3-bar system where the yield stress varies with temperature. First order analytical calculations and FE step-by-step simulations are used to determine the regime of structural behaviour for given temperature loading conditions and geometric characteristics of the double wall cooling system, as well as to compute the inelastic strain range applied at the critical film hole location, based on which low-cycle fatigue life is evaluated. The analytical calculation is based on a local approach and includes a material relaxation prediction during the hot part of the thermal cycle according to a creep power law and a Neuber scheme that utilises the local stress information from thermo-elastic analysis. This simplified approach is found to provide a reasonable conservative life estimate compared to the results of the detailed step-by-step FE analysis. The analytical solution also provides a straightforward evaluation of the role of creep for the application, specifically indicating that creep becomes particularly important for long thermal cycle durations and for high maximum metal temperatures. This is because compressive creep deformation accumulating during the hot part of the thermal cycle needs to be compensated by tensile plastic strain at the cold part of the cycle, which eventually increases the total inelastic strain range and thus degrades fatigue life. Firstly, our analysis primarily indicates that local creep-plasticity at largely inclined film holes can be detrimental to creep-fatigue life and highlights the need for reducing the geometric ellipticity of the currently implemented film hole shapes in order to reduce the stress raising effect. This may be addressed by emerging manufacturing methods that provide freedom in producing hole shapes for better mechanical performance. Secondly, our current analysis establishes the foundation for studying more detailed effects of the transient temperature field and thermal cycle characteristics on the creep-fatigue performance at critical film holes in a wide range of gas turbine blades. These results also form the basis for investigating the more complex scenario of combined thermal and inertial loading, that occurs in practise.</p>

Creep-Fatigue Calculations for Effusion Holes in Transpiration Cooled Gas Turbine Blades

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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