Experimentally Verified Study of Regeneration-Induced Forced Response in Axial Turbines

Abstract

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.

Forced Response Excitation due to Stagger Angle Variation in a Multi-Stage Axial Turbine

“…All these studies have shown that the influence of several kinds of geometric variation in an upstream stator vane row is significant. These results were verified by experimental investigations in [7,9]. The aerodynamic excitation mechanisms occuring in a high pressure turbine stage were numerically investigated in [10].…”

“…The analyzed blade row is not adjacent to the varied vane row. The vibration amplitude is numerically predicted using a unidirectional fluid-structure approach validated experimentally in [9]. The results are compared with the reference case, and additionally with a variation in the vane row adjacent to the excited blade.…”

“…Further information, including the design process are presented in [8]. In contrast to the investigations in [3], [5], and [9] geometric variances were applied on the stator vanes one stage upstream of the excited blade row. Therefore, the stator vanes of the fourth stage were modified with a regeneration-specific geometric variation.…”

Aerodynamic and Aeroelastic Effects of Design-Based Geometry Variations on a Low-Pressure Compressor

“…This leads to the lower sensitivity on the vibration amplitude in Mode 3 than Mode 2, even the unsteady pressure fluctuation is suppressed more for Mode 3 than for Mode 2 with the 3D blading of the IGV. Previous studies concern parametric variations in adjacent blade rows [23,28]. This study shows that forced response of a target row can be considerably affected by varying aerodynamic design of blades remote from it.…”

“…In addition to those efforts to develop fast and accurate tools to predict forced response, many studies have been conducted to validate the prediction tools. Aschenbruck and Seume [23] presented the experimental verification of regeneration-induced forced response in a turbine. Bittner et al [24] showed experimental validation of the forced response analysis using a time-linearized method in a high speed compressor.…”