In the industrial production system, flow measurement is a core link to ensure production stability, product quality and cost control. Whether it is the precise transportation of raw materials and products in the petrochemical field, the direct impact of steam and water flow on power generation efficiency in the power industry, or the monitoring of wastewater and waste gas discharge in the environmental protection industry, the accuracy of flow data directly determines the operation quality of the production process.
Among various flow measurement equipment, vortex flow meters have become the core equipment for industrial flow measurement due to their unique technical advantages. They are widely used in industrial scenarios across multiple fields and provide key data support for production operations. The following article will conduct a detailed analysis around the working principle, technical characteristics, usage specifications and application scenarios of vortex flow meters, revealing the technical essence of their efficient measurement.
The operation of vortex flow meters is based on the Karman Vortex Street phenomenon in fluid mechanics. When fluid flows through the built-in vortex generator (usually a non-streamlined bluff body) of the flow meter at a specific velocity, alternating rotating vortices are formed on both sides of the bluff body due to the effects of fluid viscosity and inertia. These vortices form two regular rows of vortices with opposite rotation directions downstream, known as the "Karman Vortex Street".
The formation of this phenomenon is an inevitable result of the interaction between boundary layer separation and fluid stability. When fluid approaches the bluff body, a boundary layer forms on its surface. As the fluid flows, the boundary layer separates at a specific position on the bluff body, forming independent vortices that alternately shed downstream, eventually forming the Karman Vortex Street.
The core law of the Karman Vortex Street phenomenon is that the vortex shedding frequency is positively correlated with the fluid velocity. The higher the fluid velocity, the more vortices shed from both sides of the bluff body per unit time, and the higher the vortex shedding frequency. By accurately measuring this frequency, the fluid velocity can be calculated inversely.
The above relationship is quantitatively described by the formula: f = StV/d. In the formula:
- f is the Karman vortex frequency on one side of the vortex generator (unit: Hz), which reflects the vortex shedding rate;
- V is the average fluid velocity (unit: m/s), which is the core measurement parameter;
- d is the width of the vortex generator (unit: m), a key geometric parameter of the bluff body;
- St is the Strouhal number (dimensionless), which remains constant within a specific Reynolds number (Re) range.
For conventional vortex flow meters, when Re is in the range of 10²~10⁵, the value of St is approximately 0.2. Within this range, if the vortex frequency f and the width of the vortex generator d are known, the fluid velocity V can be derived through the formula.
After obtaining the fluid velocity V, the volume flow rate or mass flow rate can be further calculated by combining it with the cross-sectional area of the pipeline.
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Volume Flow Rate Calculation: If the cross-sectional area of the pipeline is A (unit: m²), the calculation formula for the volume flow rate Qv (unit: m³/s) is Qv = V×A. This formula indicates that when the flow velocity is constant, the larger the cross-sectional area of the pipeline, the greater the volume of fluid passing through per unit time; when the cross-sectional area of the pipeline is constant, the higher the flow velocity, the greater the volume flow rate.
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Mass Flow Rate Calculation: For scenarios where mass flow rate measurement is required, the volume flow rate is multiplied by the fluid density ρ (unit: kg/m³). The calculation formula is Qm = Qv×ρ = V×A×ρ (unit of Qm: kg/s).
Through the above calculations, the vortex flow meter can convert the vortex shedding frequency signal into the flow data required for industrial production, providing data support for production monitoring and optimization.
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