If a non-streamline vortex generator (bluff body) is installed in the fluid, two rows of regular vortices are alternately generated from both sides of the vortex generator. This type of vortex is called the Karman vortex street, as shown in Figure (1).
vortex array is arranged asymmetrically downstream of the vortex generator. Suppose the frequency of the vortex is f, the average velocity of the measured medium is V, the width of the front face of the vortex generator is d, and the diameter of the surface body is D. According to the principle of Karman vortex street, the following relationship is expressed:
Where: f－Carman vortex frequency generated on one side of the generator
St-Strohal number (dimensionless number)
V－The average flow velocity of the fluid
D－The width of the vortex generator
It can be seen that the instantaneous flow can be calculated by measuring the separation frequency of the Karman vortex street. Among them, the Stroghal number (St) is a dimensionless unknown number,
Figure (2) shows the relationship between St. Rohal number (St) and Reynolds number (Re).
In the straight part of the curve table with St=0.17, the release frequency of the vortex is proportional to the flow velocity, which is the measurement range of the vortex flow sensor. As long as the frequency f is detected, the flow velocity of the fluid in the tube can be obtained, and the volume flow rate can be obtained from the flow velocity V. The ratio of the measured pulse number to the volume is called the meter constant (K), see formula (2)
Where: K = instrument constant (1/m³).
N = number of pulses
Q=Volume flow rate (m³)
1. No moving parts, long-term stability, simple structure, easy to install and maintain
2. The output of the sensor is pulse frequency, its frequency is linear with the actual flow of the fluid being measured, the zero point has no drift, the performance is very stable, the structure is diverse, and there are pipeline plug-in flow sensors.
3. The accuracy is high, usually the measurement accuracy of liquid is ±1%; the measurement accuracy of gas is ±1.5%
4. Wide measuring range, up to 1:20 within the Reynolds number of 2×10⁴~7×10⁶
5. The pressure loss is small (about 1/4~1/2 of the orifice flow meter), which is an energy-saving flow meter
6. The installation method is flexible, according to the different process pipelines on site, it can be installed horizontally, vertically and inclinedly at different angles
7. It adopts noise cancellation circuit and anti-vibration sensor head, which has certain anti-environmental vibration performance
8. Using ultra-low power consumption single-chip microcomputer technology, one 3V10AH lithium battery can be used for more than 5 years
9. The non-linearity of the meter coefficient is corrected by the software to improve the measurement accuracy
10. Use EEPROM to protect the accumulated flow from power failure, and the protection time is more than 10 years
Vortex flowmeters can be widely used in large, medium and small pipelines for water supply and drainage, industrial circulation, sewage treatment, oil and chemical reagents, as well as compressed air, saturated and superheated steam, natural gas and various medium flow measurement.
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