No abstract

High asynchronous self-excited blade response was observed in a transonic first stage rotor during the evaluation of flutter stability in high forward speed conditions. This candidate baseline rotor stage is a highly loaded, snubber-less bladed-disc configuration mounted in an axial low pressure compressor with tip speed in the order of 400 m/s. During the tests, the high asynchronous blade response was measured by strain gages, tip timing system and unsteady blade pressure transducers, which were correlated with analytical predictions. To alleviate this problem, it was attempted to tailor the first rotor blade configuration alone by adhering to all the constraints such as geometric, aerodynamic matching, material selection and utilising the same dovetail root configuration in the existing disc configuration. While tailoring the rotor blade, the critical blade parameters such as axial chord, thickness to chord, stagger, camber, leading and trailing edge radius were iterated from hub to tip. In the tailored rotor blade, the first flexure mode frequency, 1F was improved by 45% whereas the separation between second flexure, 2F and torsion mode, 1T were improved by over 30% with 4.9% weight penalty. Using the one way fluid-structure interaction approach, the blade incidence variation for different inlet pressure conditions and aerodynamic damping were evaluated using energy method for both the configuration. Blade sets of the tailored configuration were manufactured and tested in a dedicated compressor test facility, where characteristics were generated from 70% to 100% corrected speeds. The rig tests confirmed the predicted compressor performance as well as the improvement of natural frequency using blade mounted strain gages for the tailored blade. Upon the verification in the test rig, the tailored rotor configuration was further fitted in the engine and tested up to 103.3% of its design speed. The blade experienced two different inlet total pressure conditions in the test rig and engine tests. The unsteady pressure transducers and blade tip timing sensors did not show any asynchronous response in the corrected speed range for the tailored configuration. Compared to the baseline rotor blade, this tailored rotor blade demonstrated the absence of asynchronous response in the fundamental flexure mode and also well correlated with the aerodynamic damping prediction by energy method. Using this correlation, it is further analytically demonstrated that the blade will have sufficient aerodynamic damping at higher forward speeds and also minimal incidence variation in these conditions.

Aero-elastic excitation can result in excessive blade vibration, which can cause blades to fail in high cycle fatigue (HCF). A severe aero-elastic failure can result in a complete blade separation and loss of thrust and loss of a blade can mean the loss of an aircraft. The primary aeromechanical design concerns are blade flutter and forced vibration that need to be quantified at the early part of engine tests. This paper details the experimental investigation carried out on a transonic shroudless low aspect ratio fan bladed disk that experienced subsonic/transonic stall flutter and forced vibration excitation. Experiments are performed on a full scale engine using tip timing sensors flush mounted on the fan casing to characterize the vibratory responses during flutter and forced vibration conditions during engine operation. Numerical simulations are performed using computational fluid dynamic (CFD) analysis. Blade natural frequencies and mode shapes are obtained from finite element (FE) modal analysis. The experimental data captured from engine tests are used to validate the predicted results.

No abstract