This paper presents a design concept for thermally high loaded ceramic shell components, that is developed at the Institute for Thermal Turbomachinery (ITS, University of Karlsruhe). The aim is to obtain homogenous temperature distribution in the ceramic structure for the whole operating cycle of the engine.
Based on simple flat wall considerations design guidelines are introduced to reduce the dominant thermally induced stresses in ceramic components. The concept includes a systematic adjustment of the local cooling configuration and the local wall thicknesses according to the local thermal boundary conditions.
The concept is applied to increase the reliability of a directly cooled Sintered Silicon Carbide first stage nozzle vane with trailing edge ejection for a 70MW stationary gas turbine. Finite element analyses demonstrate that the application of this methodology leads to a rather uniform temperature distribution in the ceramic structure and therefore to low stress levels for steady state and for tripping conditions.
A transformation of the theoretically derived optimum cooling configuration to a technically feasible cooling setup is performed in the purpose of real engine application. It is found that some differences between optimum and technically feasible cooling configuration (mainly located at the trailing edge region) are of minor importance. The results of failure probability calculations (ITS fracture statistics code CERITS) confirm significantly improved reliability of the ceramic nozzle vane.