Vortex flowmeters

Vortex Flowmeter: Working Principle & Applications

1. Working Principle

The vortex flowmeter operates based on the Kármán vortex street phenomenon. When a fluid flows past a bluff body (vortex shedder), alternating vortices are generated downstream. The frequency of these vortices (F) is directly proportional to the fluid velocity (u), as described by the equation:

F = St cdot frac{u}{d}

F: Vortex shedding frequency (Hz)

St: Strouhal number (dimensionless, typically 0.17–0.22 for stable flow)

u: Fluid mean velocity (m/s)

d: Width of the bluff body (m)

The Strouhal number (St) remains constant within a specific Reynolds number (Re) range, ensuring linearity between frequency and flow rate.

2. Key Parameters

(1) Reynolds Number (Re) Range

Re = frac{rho u D}{mu}

Stable Measurement Range: 2×10⁴ ≤ Re ≤ 7×10⁶

Below Re < 2×10⁴: St varies, making measurement unreliable.

Above Re > 7×10⁶: Turbulence may cause St instability.

(2) Vortex Detection Methods

Piezoelectric Sensors: Detect pressure fluctuations caused by vortices.

Thermal Sensors: Measure temperature variations from vortex shedding.

Ultrasonic/Capacitive Sensors: Non-contact frequency detection.

3. Advantages

✅ No Moving Parts: Minimal wear, long service life.

✅ Wide Turndown Ratio (1:10 to 1:30): Suitable for varying flow rates.

✅ Low Pressure Drop: Energy-efficient compared to orifice plates.

✅ Versatile Fluid Compatibility: Works with liquids, gases, and steam.

4. Limitations

❌ Sensitivity to Vibration: External vibrations may interfere with signal accuracy.

❌ Minimum Flow Velocity Required: Typically >0.5 m/s for liquids, >3 m/s for gases.

❌ Pipe Straight-Run Requirements: Upstream/downstream straight sections needed for stable flow.

5. Applications

Steam Flow Measurement (e.g., boilers, HVAC systems)

Compressed Air/Gas Monitoring

Chemical & Petrochemical Processes

Water/Wastewater Treatment

6. Signal Output & Calibration

Pulse/Frequency Output: Directly proportional to flow rate.

4–20 mA Analog Output: For integration with control systems.

Calibration: Performed using water or air, with St adjusted for specific fluids.

Conclusion

Vortex flowmeters offer high reliability, low maintenance, and broad applicability for industrial flow measurement. Their performance depends on proper installation and operating within the stable Re range. Future advancements focus on vibration compensation and extended low-flow sensitivity.