The degradation of gas turbine parts due to aging leads to changes in airfoil shape and often causes performance loss. Although the degradation mechanisms and their effects on performance are understood in general (e.g. it is well known that fouling of compressor airfoils reduces mass flow and efficiency), the first quantitative relationships between specific types of part degradation and performance characteristics have only recently been published.
In this paper the degradation of turbine blades with aft-loaded airfoils is considered. The typical deviations of shape were identified based on field experience. The effects of these deviations on turbine performance were assessed using different calculation methods, including 3D Navier-Stokes calculations and methods based on empirical correlations. The effect of blades-length reduction, chord-length reduction, changes in trailing-edge thickness and shape, and variation of stagger angle were analysed. The analysis showed that for aft-loaded airfoils without shrouds, the major influence on turbine performance is the degradation of radial clearances.
A simplified engineering procedure allowing estimation of turbine performance loss due to degradation has been developed. This paper demonstrates how this simplified procedure, can be applied to the estimation of turbine recovery potential during a typical engine overhaul.
