The primary devices used in open channel flow measurements are weirs and flumes. Unlike the flow through the closed channel, the height of the flow level is measured in relation to the flow of open channel flow.
Open channel flow meter working Principle:
Liquid flow is based on the principle of energy. Two types of energy govern flow in open channels, global and specific energy. According to the principle of specific energy, minimum energy occurs when the Froude number (Fr) is equal to 1.
Q = flowrate
A = cross-sectional area
T = top width of the flow
g = acceleration of gravity
This condition of minimum energy also produces a specific flow called critical flow. As can be seen in equation 1 when a critical flow occurs, the flow rate (Q) can be calculated through the cross-sectional area and the upper width of the flow. To accurately measure the flow in the open channel, it is necessary to create a critical flow condition that allows the flow to be measured simply by measuring the depth of the flow (producing values for T and A).
How to measure open channel flow?
To measure the flow of an open channel, we have to make the flow pass through obstructions. The flow is measured from this obstruction as weirs and flumes. A special method used to measure the flow is to use current meters other than the Pitot tube and measurement using float.
The height of the flowing liquid is measured using the any of the level meters such as ultrasonic level meters:
The ultrasonic sensor sends a sound signal to the flow channel and measures the elapsed time of echo return. The controller attached to the sensor calculates the flow according to the obstruction used, that is weirs and flumes.
Devices for open channel flow measurement:
The primary devices used in open channel flow measurements are weirs and flumes.
Weirs:
Weirs are openings in the upper part of a dam or reservoir that allow liquid flow and allow flow measurement. With the known landfill characteristics, the flow is usually determined by the height of the liquid in the weir.
There are rectangular and V-notch weirs
Advantages:
- Simple operation.
- Good Rangeability (for detecting high and low flow).
Disadvantages:
- Pressure loss.
- Accuracy of about 2%.
Flumes:
Flumes are a modification to the weir where the section of flow is reduced to maintain head pressure. A channel causes the liquid to enter a narrower channel and in doing so only incurs a pressure drop in the head of about 1/4 of that for a landfill of the same size. This process is similar, in principle, to a rectangular venturi tube.
Similar to the weirs, the water level in the canal is a function of the flow index. The channel provides a bit more precision by accurately channelling the flow. The channel is also independent of the fluid velocity as it enters, and as such, the application does not require dams or a buffer tank.
Advantages:
- Reliable and repeatable measurements.
- No erosion.
- Not sensitive to dirt and debris.
- Very low head pressure loss.
- Simple operation and maintenance
Disadvantages:
- High installation costs.
- Low accuracy.
- Expensive electronics
