Vibration Characteristics of Segmented Stator Assemblies in Axial Compressors

Abstract

A common component in axial compressors used in turbo machinery applications is the segmented stator assembly with fixed airfoils attached to inner and outer bands forming a partial ring. Understanding the potential for vibratory responses of these components during engine operation is vital to avoid harmful vibrations. However, an accurate prediction of segmented stator assembly vibratory characteristics is difficult due to the partial nature of their cyclic symmetry and high part to part manufacturing variation. Numerous analyses related to laboratory tests, engine tests and theoretical studies have been performed to advance the understanding and better quantify the segmented stator assembly dynamic behavior. A summary of the component dynamic behavior is provided through a series of lessons learned from the various analyses completed. The segmented stator assemblies can be thought of as cyclically symmetric parts with properties of extreme mistuning. These properties are realized when one attempts to evaluate the components using experimental and theoretical approaches traditionally reserved for cyclically symmetric components. Specifically, harmonic indices are often employed to identify modes of interest in a cyclically symmetric system with reference to known engine order excitations. However, with segmented stator assemblies, there is a high density of modal frequency distribution across the engine operating range and mode shapes can suffer from significant contamination of the diametral patterns. These observations complicate linking specific modes to known engine order excitations. This study implements traditional FEA approaches and reduced order models in an attempt to predict the key vibratory modes of concern and to show which component features most significantly affect the vibratory characteristics of segmented stator assemblies. Observations made during bench tests are compared to theoretical analyses and confirm the challenges of assessing the dynamic properties of segmented stator assemblies. This work supports emerging segmented stator design evaluation techniques, an area that is critical to successful integration of these components into turbo machinery applications.