Experimental Study On The Response Of Mistuned Bladed Disk

Abstract

Gas turbine vibrations can be caused by several mechanisms, but some blade failures cannot be explained by the more commonly known mechanisms and theories. These blades are conveniently regarded as rogue “mistuned� blades that had failed from abnormally high stresses. This led to extensive studies of bladed disk vibration characteristics. There are currently no theoretical predictions which can fully explain the blade vibration response in the presence of airflow due to the complicated aerodynamic structural interaction. A literature review is presented on mistuned blades research. This work involved the experimental study of forced response amplitude of model blades due to structural mistuning and inlet flow distortion in the presence of an air flow. This controlled study of blade mistuning with inlet flow distortion therefore represents a nearly realistic environment involving rotating blades in the presence of airflow. Previous work by others were usually based on a non-rotating blade. The presence of airflow which introduced effects of fluid structural interaction was not considered in previous works on mistuned blades. The primary intent of this work was to acquire the data while the blade is rotating in a situation that almost replicates the actual situation. A test rig was fabricated consisting of a rotating bladed assembly, an inlet flow section (where flow could be controlled or distorted in an incremental manner), flow conditioning module and an aerodynamic flow generator (air suction module with an intake fan) for investigations under laboratory conditions. Instrumentations included ultra-lightweight surface mounted pressure sensor on a rotating blade and vibration accelerometer on typical blades with signal routed through a telemetry system from the rotating shaft. These then allowed studies under a nearly realistic environment with rotating blades vibrations and pressure distributions measurements in the presence of airflow for the study of blade mistuning and inlet flow distortion with structural and aerodynamic interaction. Computational studies for vibration response of the blades and computational fluid dynamics of the inlet flow distortion were also undertaken to support the experimental studies. Tests were undertaken for a combination of different air-flow velocities and blade rotational speeds. The experimental results showed that the vibration responses of a mistuned blade (in a single stage of 12 bladed rotor assembly) were greatly influenced by the flow velocity, flow-induced frequency and blade/vane count. When the inlet flow was distorted, additional frequencies were excited and the amplitudes of these excited frequencies increased with increase in flow velocity.