Abstract

The excellent high-temperature properties of ceramics offer great potential for their application in gas turbines. However, ceramics lack the ability to reduce local stress concentrations by plastic deformation. As a result, stresses that are caused by different local thermal expansions can reach critical values, especially in the hot-section components. To improve the reliability of ceramic components, the temperature differences have to be reduced. At the Institut für Thermische Strömungsmaschinen (ITS) a systematic methodology for designing thermally high-loaded components has been developed. The principles of the design procedure include a segmentation of the parts according to the load and a three-layered construction of the component's wall. The inner hot-gas ducting layer consists of a high-temperature resistant ceramic material which is embedded into a metal containment by a flexible ceramic fibre insulation. By adjusting the individual thicknesses of the ceramic and the insulation layers according to the local boundary conditions on the hot-gas side, the local temperature differences in the ceramic can be considerably reduced. Finite element analyses of the temperature and stress distribution for first stage nozzle guide vanes and the vaneless scroll of a radial gas turbine are shown. Compared with conventional designs, the calculations clearly demonstrate that the hybrid wall construction and an ingenious segmentation of the components lead to a significant reduction in the stress level. The reliability improvement is documented by failure probability calculations performed using the ITS fracture statistics code CERITS.

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