To study flow characteristics through venturi flume

Objectives:

1. To draw water surface profiles of flow through venturi flume
2. To study relationship between upstream discharge and depth
3. To study relationship between upstream discharge and depth at throat
4. To determine coefficient of discharge of venturi flume

Apparatus:

1. S6 tilting flume
2. Point gauge
3. Venturi flume

Related theory:

Venturi flume:

“It is an instrument used to measure the discharge of liquid flowing in open channels”

A venturi flume is a critical-flow open flume with a constricted flow which causes a drop in the hydraulic grade line, creating a critical depth.

It is used in flow measurement of very large flow rates, usually given in millions of cubic units.

Measurement of discharge with venturi flumes requires two measurements, one upstream and one at the throat (narrowest cross-section), if the flow passes in a subcritical state through the flume. If the flumes are designed so as to pass the flow from sub critical to supercritical state while passing through the flume, a single measurement at the throat (which in this case becomes a critical section) is sufficient for computation of discharge. To ensure the occurrence of critical depth at the throat, the flumes are usually designed in such way as to form a hydraulic jump on the downstream side of the structure. These flumes are called ‘standing wave flumes’

Difference between venturi meter and venturi flume:

A venturi meter is used to measure discharge in pipes while venturi flume is used to measure discharge in open channels.

A venturi meter would normally measure in millimetres, whereas a venturi flume measures in metres.

Venturi Flume                                                                                 Venturi meter

Parts of venturi flume:

1. Converging portion
2. Throat
3. Diverging portion

The length of divergent cone is made longer than that of converging cone in order to avoid tendency of breaking away the stream of liquid and to minimize frictional losses.

• The velocity of liquid is higher at the throat than that of inlet
• The pressure of liquid at throat is lower than that of inlet
• The velocity and pressure of liquid flowing through divergent portion decreases

Comparison with weirs

Venturi flumes have two advantages over weirs where the critical depth is created by a vertical constriction. First, the hydraulic head loss is smaller in flumes than in weirs. Second, there is no dead zone in flumes where sediment and debris can accumulate; such a dead zone exists upstream of the weirs.

Procedure:

1. Place venturi flume inside the tilting flume
2. Set the slope to 0
3. Fix some discharge and measure the depths at U/S, converging portion, throat, diverging portion and at D/S
4. Plot their water surface profiles
5. Take the average of depths at u/s side, denote it with “h”
6. Find area at u/s portion, denote it with “A”
7. Take average of depths at throat, denote it with “h1”
8. Find area at throat, denote it with “a”
9. Find ratio of area at throat and area at u/s
10. Find “C” by using following formula
11. Find “C_d” by using following formula
12. Take average of all results and that would be coefficient of discharge for specific venturi flume

Fluming at site:

Observations and calculations:

Water surface profiles:

Water surface profiles of all discharges:

Determination of coefficient of discharge:

Comments:

1. The coefficient of discharge for venturi flume is 1.09 while it should be less than 1. The literature range is 0.95-1. The possible reasons for our result greater than 1 may be:
2. The error in measurement of flow depth as a result of improper point gauge reading
3. Un-certainty in reading manometric levels.
4. Improper fixing of venturi flume in S6 tilting flume. As shown in figure below, some discharge is passing from ends of venturi flume due to improper plaster.
5. The graph of Q vs h shows that at u/s when discharge is increased, the depth of flow is also increased.
6. The graph of Q vs h₁ also shows that with increase in discharge, the depth of flow at throat is increased.
7. The depth of flow at throat is decreasing with gradually varied flow.
8. At divergent to d/s section hydraulic jump is formed which is an example of rapidly varied flow.

Figure: Improper plastering at sides of Venturi flume which is allowing discharge to pass through sides of Venturi flume

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