Development and characterization of an airflow vortex-shedding flowmeter with pvdf piezoelectric film sensors

摘要

The vortex-shedding flowmeter shows good accuracy, a high range of measurement and small intrusively in flow measurements. The systems presently in commerce have very good throughputs but a high costs which discourage their employment in many industrial applications. In particular, high costs depend on the presence of a pressure's sensor. In fact, the mechanic construction results very simple. The flow measurement is obtained by inserting a special shedder along a pipeline's diameter. When a flow invests the shedder, the vortex (the von Karman vortices) produce alternating forces or local pressures on the shedder. The vortex frequencies, for a wide range of N_r (Reynolds number), linearly depend on the flow speed. The Strouhal number, that depends on the shedder shapes, connects the vortex frequencies with the speed. In this work a new vortex-shedding flowmeter for airflow fluid has been built and characterized. The instrument uses pressure sensors realized by piezoelectric film PVDF to measure the vortex frequency and therefore the flow velocity. The PVDF films are piezoelectric materials with a good pressure sensitivity and very high frequency response. Moreover, they are not expensive and it is easy to set them up. For this reason it is possible to use the vortex shedding for a lot of different applications. Unfortunately the temperature influence and the surface deformation, that the shedder can have, mitigates the signal-to-noise ratio in the measurement. The instrument realized has been installed in a wind runnel and has been calibrated in large flow velocity range. Hot wire anemometers and Pitot tubes have been used as reference sensors for the calibration. We have studied the interfering and modifying input in the chain of measurement and the effects of the pipe vibrations. A specific low-cost electronic circuit for the identification and analysis of the signal, which is generated by the PVDF pressure sensors, has also been developed. In mis way, the flow velocity on the liquid crystals display can be directly read. The low measurement uncertainty and the low cost of the system encourage further developments of the present work, in order to refine the construction techniques.