A Metering flume or flow measuring flume is an artificial channel section which is used to measure the discharge of a channel. Flumes are of different types and can be made up of materials such as aluminum, fiberglass, steel, wood etc. Different types of metering flumes and their working is explained in this article.
Types of Metering Flumes
Metering flumes are classified into following types :
Submerged Venturi Flume
Free flow Venturi Flume
1. Submerged Venturi Flume
Submerged Venturi flume consists of gradual wrinkle on its upstream throat section in the middle and the gradual expanding section from the throat. Here, the downstream level of flume is kept equal to the level of throat section hence, there may be wrong-side-up spritz into the throat section. So, it is said to be submerged Venturi flume.
To measure venting in submerged Venturi flume, two throne measurements are required, one at the archway and one at the throat. These measurements are taken by providing stilling wells at the archway and throat. The venting through submerged Venturi flume can be calculated by the pursuit formula :
Where, a1 = Area at the entrance
a2 = Area at throat
g = acceleration due to gravity
h = head difference between two stilling wells
Cd = Coefficient of discharge of Venturi flume (generally varies from 0.92 to 1.0)
2. Free flow Venturi Flume
Free flow Venturi flume is moreover known as standing wave flume since it is designed in such a way as to form a hydraulic jump or standing wave on the downstream portion of the flume. In this case, the level of the downstream portion of flume is kept lower than throat level to form hydraulic jump. Hence, the venting through the throat section depends only on the upstream head.
In this case, single throne measurement at the throat section is unbearable to summate the venting of channel. Stilling well is provided at the throat to measure the head. Venting through self-ruling spritz Venturi flame is calculated by using the pursuit expression :
Where, Cd = Coefficient of discharge of Venturi flume (generally varies from 0.92 to 1.0)
B = width of throat section
H = Head measured in stilling well.
3. Parshall Flume
Parshall flume is a modified version of venturi flume. Some modifications are made in venturi flume to change the flow conditions from sub critical to supercritical and those modifications are as follows :
Increase in throat length
Reduction in angle of convergence of inlet walls
Reduction in angle of divergence of outlet walls
Drop in elevation through the throat of the flume
Design of Parshall flume can be done as either free flow parshall flume or submerged parshall flume like in venturi flume.
4. Cut-throat Flume
A cut-throat flume consists of a gradually contracting channel section followed by a gradually expanding channel section. Throat section is eliminated in case of cut-throat flumes. The bottom surface of a cut-throat flume is flat and horizontal.
The construction of cut-throat flume is easier compared to other types of flume since it requires horizontal floor and flat metal sheets and, also, there is no need of throat section.
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