Measuring Principle - Kármán vortex street
This variety of flowmeter operates on the principle of Kármán vortex street, whereas any medium passing through the pipeline and flows around the bluff body, sheds a series of alternating vortices on each side of the body. This phenomenon is referred to as Vortex Shedding. These vortices shed downstream of the bluff body and dissipate as they flow further. This pattern of vortices is called a Kármán vortex street (also called a Von Kármán vortex street).Operation pf Vortex Flowmeter
A Vortex flowmeter primarily consists of a bluff body, a sensor assembly, and a transmitter. A bluff body or a shedder is nothing but a non-streamlined object or a barrier placed perpendicular to the axis of the pipeline, around which the medium flows.
Calculation of the Mass Flow Rate
The frequency of the vortices, i.e. the number of vortices shed per second, is directly proportionate to the velocity of the medium. This Vortex Shedding Frequency is used to calculate the mass flow.The sensor assembly records the pressure and velocity oscillations generated on each side of the bluff body by the vortices and generate a digital linear output signal. The Vortex Shedding Frequency is calculated using the following formula:
f = St * V/d
Where:f = Frequency of Vortex Shedding St = Strouhal Number
V = Flow Velocity
d = Width of the Bluff Body
Strouhal Number St
The Strouhal Number in the above formula is also called as “reduced frequency”. It is a dimensionless param- eter that is a measure of the Vortex Shedding Frequency and the velocity of the flow medium. It is calculated using the formula:St = fd/U
Where:f = Frequency of Vortex Shedding d = Width of the Bluff Body
U = Velocity of the Flow Medium
The Strouhal Number is a function of the Reynold’s Number. Reynold’s Number is also a dimensionless parameter that is used to determine how the flow pattern of different fluids will change. The Strouhal Number should remain constant when the Reynold’s Number ranges from 2 × 104 to 7 × 106.
Calculation of Volume Flow Rate
When the Vortex Shedding Frequency is known, the volumetric flow rate can be calculated using the formula:q = f/k * d
Where:q = Volumetric Flow Rate
f = Vortex Shedding Frequency
k = k Factor, which is a ratio of the pulses transmitted to the unit volume.
You can download the TEK-TROL Tek-Vor 1300C Vortex Flowmeter Brochure from the Hile Controls of Alabama web site here.
For more information about vortex shedding flowmeters, contact Hile Controls of Alabama. Call them at 205-620-4000 or visit their web site at https://hilealabama.com.
