計測と制御

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A new flowmeter which operates on Kármán vortex principle is described. The frequency with which vortices are shed is proportional to the flow velocity.
Various techniques are tested for sensing the alternating lift or circulation due to the vortex shedding from a circular cylinder held normal to the flow. Stability and regularity of the wake behind the cylinder are also investigated assuming a three-dimensional flow in the circular pipe.
A circular cylindrical detector is developed. Plural sensing ports are bored through the cylindrical detector rod, and those ports are opened on the rod's surface near the separation point of the boundary layer. The fluid in the ports is moved periodically by the alternating lift.
A sensing wire which is heated by electric current is located in a port. The flow-rate is measured by counting the frequency of fluctuation of heating current. The detector functions also as a controller of the boundary layer to stabilize formation and shedding of vortices. Noise due to turbulence is eliminated by an automatic tracking filter in the circuit.
The detector is tested in rectangular ducts circular pipes of various sizes ranging from 50mm to 1100mm in diameter.
The confirmed operating range is 2000-100000 Reynolds number (detector diameter) for gas, and 4 000-140000 for liquid. The linearity is within ±1% of the full scale.
The equipments for flow calibration is described, and several factors influencing the accuracy of the flowmeter are given.
Experiences and results of field tests in various industries are discussed.
Merits of this flow meter are as follows:
(1) No effect of density, temperature, pressure, composition, viscosity.
(2) No moving part.
(3) Linear frequency output with respect to the measured flow.
(4) Low pressure loss.
(5) Applicability for gas and liquid.
(6) Easy installation during plant operation.

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