EXPERIMENTAL STUDY ON FLUTTER STABILITY OF TRANSONIC COOLED TURBINE BLADE STAGE WITH 3-DIMENSIONAL MODE SHAPE EXCITATION

It is well known that mode shape plays very important role in stability of turbine blade since the aerodynamic work per cycle and aerodynamic damping depend on mode shape. With the advancements of theoretical formulation with influence coefficient method, experimental studies with rigid body blade motion have significantly improved understanding of turbine flutter mechanisms and design parameters. However rigid body motion cannot accurately match the complex mode shapes of modern turbine blade, so there are limitations of accuracy on experimental evaluation of flutter stability with rigid body blade motion. This study utilized 3-dimensional mode shapes for evaluating aerodynamic work per cycle and stability of turbine blade through experimental method. A transonic annulus turbine cascade rig was built at Notre Dame Turbomachinery Laboratory. Three center blades with modern cooled 3-dimensional aero design were instrumented with 144 EA of ultraminiature fast-response pressure transducers on the blade surface at 50%, 75%, and 95% of blade spans. Center blade and adjacent blades were designed to have the same blade mode shape as a reference turbine blade and the center blade was actuated with an electromagnetic shaker at natural frequencies of 1st bending and 1st torsional modes to simulate the same level of reduced frequencies under engine operating condition. Mode shape scanning of test blade through laser doppler vibrometer confirmed the design intent of blade bending and torsional mode shapes and their frequencies. All the dynamic pressure measurements on the three center blades were synchronized with blade position measurement and influence coefficient method was applied to calculate aerodynamic work per cycle and damping parameter. It was found that pressure side generally stabilizes the blade and that there was strong stable zone from leading edge to about 20% in arcwise coordinate torward suction side. After this zone, a destabilizing zone follows and this can be strong enough to destabilize the blade in some range of nodal diameter.