Geometrical variations occur in highly loaded turbine blades due to operation and regeneration. To determine the influence of such regeneration-induced variances of turbine blades on the aerodynamic excitation, a typical stagger angle variation of overhauled turbine blades is applied to stator vanes of an air turbine. This varied turbine stage is numerically and experimentally investigated. For the aerodynamic investigation of the vane wake, computational fluid dynamics (CFD) simulations are conducted. It is shown that the wake is changed due to the stagger angle variation. These results are confirmed by aerodynamic probe measurements in the air turbine. The vibration amplitude of the downstream rotor blades has been determined by a computational forced response analysis using a unidirectional fluid–structure interaction (FSI) approach and is experimentally verified here by tip-timing measurements. The results of the simulations and the measurements both show significantly higher amplitudes at certain operating points (OPs) due to the additional wake excitation. For typical regeneration-induced variations in stagger angle, the vibration amplitude is up to five times higher than in the reference case of uniform upstream stators. Based upon the present results, the influence of these variations and of the vane patterns on the vibration amplitude of the downstream rotor blade can and should be estimated in the regeneration process to minimize the dynamic stresses of the blades.

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The effect on non-uniform surface roughness on the aerodynamics of a turbine blade is investigated. Surface roughness on airfoils has a significant impact on total energy loss due to skin friction and typically leads to an increased thermal loading. In the present research project, investigations are supposed to be carried out experimentally. For this a blade must be designed, which accommodates the contradictory requirements of aerodynamics and manufacturing the sections of surface roughness. A fully automatic design process based on a genetic algorithm is developed and results are shown. The designed blade sufficiently fulfills the given requirements. A numerical study, using a low-Reynolds approach, is performed to investigate the influence of non-uniform roughness applied to different positions on the suction side of a high pressure turbine blade. It is shown that roughness applied at the leading and trailing edge does not significantly influence the flow whereas roughness at 20% cord length and at midchord induce transition. Especially surface roughness at 20% chord length shows a strong correlation to the change of total pressure loss.

The regeneration of highly loaded turbine blades causes small variations of their geometrical parameters. To determine the influence of such regeneration-induced variances of turbine blades on the nozzle excitation, an existing air turbine is extended by a newly designed stage. The aerodynamic and the structural dynamic behavior of the new turbine stage are analyzed. The calculated eigenfrequencies are verified by an experimental modal analysis and are found to be in good agreement. Typical geometric variances of overhauled turbine blades are then applied to stator vanes of the newly designed turbine stage. A forced response analysis of these vanes is conducted using a uni-directional fluid-structure interaction approach. The effects of geometric variances on the forced response of the rotor blade are evaluated. It is shown that the vibration amplitudes of the response are significantly higher for some modes due to the additional wake excitation that is introduced by the geometrical variances e.g. 56 times higher for typical MRO-induced variations in stagger-angle.

Due to manufacturing tolerances, wear during operation or regeneration processes like maintenance operation, the structural properties of turbine blades deviate from design condition to reference blades. This deviation usually causes higher vibration amplitudes and as a consequence a lower service life expectation. Many different calculation methods can be used to simulate these increased amplitudes of mistuned blades. The major resulting problem is on the one hand to capture the occurring deviation of the eigenfrequencies from the reference blade and on the other hand to incorporate these real deviations in simulations. Solving these problems with a simplified experimental setup will make it possible to predict the maximum amplitude and to avoid costly experiments in a rotating turbine. The aim of the paper is to verify a simulation of the vibration amplitude by experiments using a reduction method to calculate a mistuned system in reasonable time. The results of the chosen simulation are compared to experiments in a rotating turbine. To reduce the number of degrees of freedom of the full finite-element model and the computational effort, a multi-step reduction method is used. In the simulation, the centrifugal force, the structural damping, the steady static pressure on the blades, and the mistuning are considered. To find the occurring deviations of each manufactured blade, an experimental modal analysis is performed for every single blade in a non-rotating setup with the eigenfrequencies of every single blade as an output. The single-stage results of the simulation are subsequently compared to experiments in a 5-stage air turbine in which the vibration amplitudes and the eigenfrequencies of every blade in the last rotor blade row are measured by a tip-timing system.