Tip deflections of a thermowell can be used for diagnosing the flow-induced vibration of a thermowell by Kármán vortex street. In the literature, the tip deflections are visualized by means of a high-speed camera. In this visualization technique, locations of the thermowell are recorded as images at each frame. The location of the thermowell is influenced by image resolutions, which limits the accuracy of visualization technique by the high-speed camera.Laser Doppler vibrometry would be one of solutions to improve the accuracy of tip deflection measurement. The laser Doppler vibrometry has an accuracy of ±0.2 % (k = 2), such that tip deflection can be reliably measured in a condition that the laser Doppler vibrometry is well aligned with the round surface of the thermowell. Although the laser Doppler vibrometry is pointwise measurement device, the laser Doppler vibrometry is comparable with the strain rate measurement. The strain rate measurement becomes the standard measurement technique to evaluate the tip deflection of a thermowell. It is because the tip deflection can be evaluated by flexural motion of the Bernoulli-Euler beam theory.In this study, the tip deflection of a thermowell was measured by using the laser Doppler vibrometry and the strain gauges. Direct measurement of tip deflection by the laser Doppler vibrometry was compared with indirect measurement of the tip deflection by the strain gauges. At a point where the shedding frequency due to the Kármán vortex street coincides with the natural frequency of a standard thermowell, the two measurement techniques provided an equivalent result.

Use of Laser Doppler Vibrometry for Measuring Flow-Induced Vibration of a Thermowell in a Pipe Flow

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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