Turbine Vane Testing in Twisting, Radial & Longitudinal Bending Modes
Frequency response testing was conducted acoustically on compressor turbine vanes so as to prevent mass loading and external damping effects.
The tested vane was set upon a small piece of foam to create a free-free boundary condition. Force input was furnished by a small instrumented hammer. The impact location is on the “thicker” flange at the trailing edge. Acoustical response was measured by a microphone located directly above the vane.
A series of frequency response measurements were acquired to assess patterns of modal deformation associated with the first three free-free natural frequencies.
- Magnitude and phase relationships indicate that the first mode involves twisting of the vane, where opposite ends of each flange respectively experience out-of-phase motion.
- The mode of primary interest is the second frequency, found to be bending across the face of the vane (radial direction). This mode is judged to most represent comparative strength between the OEM and New vanes because it is dictated by vane stiffness.
- The third frequency is found to be bending along the length of the vane (flow direction). This mode is judged to not be representative of blade strength because it is dictated by flange stiffness.
A typical force time history was presented. A total of 32,768 time history points was used within a 0.800 second analysis window to produce a maximum frequency range of 18,000 Hz with 1.25 Hz resolution. A pre-trigger was used to assure capture of the entire transient.
The first 20,000 time points were recorded unaltered, after which a 5% damping factor was applied so as to force the data to be periodic within the analysis window and reduce frequency “leakage” errors.
The test results show that in the lowest free-free mode of vibration, which is twisting, the New vanes have a natural frequency of 4,683 Hz, which is 10.4% below the OEM natural frequency of 5,224 Hz. However, in the actual installed condition in service, this twisting mode is restrained. The second frequency was found to be bending across the face of the vane (radial bending). The new vanes have a natural frequency of 11,009 Hz in this mode, which is only 5.4 percent lower than the OEM frequency of 11,620 Hz.
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