Experimental Subsonic Flutter of a Linear Turbine Blade Cascade With Various Mode Shapes and Chordwise Torsion Axis Locations

Abstract

Axial flow turbomachinery is susceptible to self-excited vibrations known as flutter, primarily affecting high-aspect ratio rotor blades. Experimental studies of blade flutter are essential to understanding the phenomenon. The existing research has shown that the most critical factor in determining the aerodynamic stability of the blade is the mode shape. In this work, measurements of subsonic flutter of a linear turbine blade cascade with various mode shapes and torsion axis locations in a chordwise direction are carried out. The cascade consists of eight blades representing reduced tip sections of the last stage blade of the steam turbine rotor, and central four flexibly mounted blades are forced to vibrate in pure bending and pure torsion modes and a combined motion. Both the travelling wave mode approach and the influence coefficient method are tested. The results show that the pure torsion mode is insensitive to increasing reduced frequency and is aerodynamically unstable for each positive angle of incidence tested, while pure bending and combined modes become less stable with the increased angle of incidence. In addition, high sensitivity of blade behaviour to changes in torsion axis position and phase shift between bending and torsion motions is demonstrated.

Enhanced Flutter Test Rig for a Transonic Flow in a Linear Turbine Blade Cascade and Numerical Data Validation

“…The measurement technique utilizes high-accuracy control of blade cascade vibration with and without fluid flow [10,11]. By assuming the system motion is the same with and without flow, the methodology allows the estimation of the aerodynamic forces and moments on the blades for different types of oscillations (a blade torsion α and a blade bending y).…”

“…2). More details about the system instrumentation, data acquisition and results for subsonic flow are described in [11].…”

“…high aerostatic and aerodynamic forces and moments). The blade length and chord are the same as the previously tested nonmetallic blades [11]. The usage of heavier steel blades causes a drop in the shaker flexible suspension tuning by approximately 10 Hz, so the flutter excitation frequency is set to 72.8 Hz.…”

“…The experimental method utilizes no instrumented blades with pressure taps, and electromagnetic shakers are used to measure the aerodynamic load. One of the major advantages of this setup over the available testing facilities is that coupled bending-torsion modes can be easily achieved [10,11]. Before the test rig enhancement, measurements in the controlled flutter test rig were carried out on subsonic flow field cases, and datasets were exploited for CFD validations [12][13][14][15], where a close match between the experimental data and the numerical results was achieved.…”

Flutter Measurement of a Linear Turbine Blade Cascade with an Angular Position Deviation of One Blade

“…3. Schematic of the electromagnetic shaker with a flexibly mounted blade, taken from [31]: 1 -main elastic element; 2 -flexible element of base; 3 -main body; 4 -base; 5moving coil; 6 -magnetic core; 7 -mass of electromagnetic shaker; 8 -permanent magnet solenoid; 9 -a pair of eddy current displacement sensors Schenck IN-085, 10 -crossbar; 11sealing cover; 12 -blade profile; 13 -additional mass for tuning; 14 -auxiliary elastic element Fig. 4 shows the oscillation modes of the flexible suspension with a blade profile as analysed using FEM.…”