Venturi Flume Laboratory

Venturi Flume Lab

Objective

To collect flow rate and upstream depth data to allow the calculation of coefficient k and n for this particular flume, and then compare the experimental value with theoretical values contained in BS ISO 4359.

Introduction

A venturi flume is an open flow water channel that constricts flow and causes a drop in the hydraulic grade line, creating a critical depth. Venturi flumes are used in flow measurement of large flow rates. A flume is used to accelerate flow by converging sidewalls, or raising the bottom or both. There are many applications where the flow must be measured with a minimum of loss of energy and the Venturi flume has evolved for this purpose.

The venturi flume is often used in the measure of discharge. These have been extensively used in hydraulic mining and working placer deposits for gold, tin and other heavy minerals.

Apparatus

Stopwatch

Venturi flume

Theory

0.1m wide throat

0.3m wide horizontal channel

0.1m long throat

Tank size = 0.76m * 1.22m * 0.04m = 0.0371 m3

Equations

1.

Q=Khn

Q = Flow rate

K = Constant for particular flume

h = Upstream Energy (upstream depth)

n = Constant for particular flume

Constants K and h can be calculated using a graph of Q against h.

(BS ISO 4359:1983) Annex G

2.

K=1.7*CD*B

CD = Discharge coefficient = 0.9

B = Width of flume = 0.1

(BS ISO 4359:1983) Annex G

3.

n=1.5

(BS ISO 4359:1983) Annex G

Note: Equations 2 and 3 are used to calculate theoretical values of K and n.

4.

Q=Tank volumetime

Procedure

Begin the experiment by assigning members of the group to take control of the variables on the flume. One person should control the orange wheel that allows the flow to be adjusted and start/stop button.

The second person should measure the upstream depth by reading off the value on the gauge when the gauge is in slight contact with the flow of water.

The third person is needed to turn the big wheel anti – clockwise, this blocks the tank therefore allowing water to rise and fall in the tank so that measurements can be taken by use of a gauge and marker.

The fourth person should start the stop watch when the marker rises to 17 and then stop the clock when the marker reaches the 21 mark. The 4 cm difference should give a certain time.

Once time has been recorded, the big wheel should be turned clockwise to allow flow to continue as normal to prevent overflow of water.

This process should be carried out six times, the only change being that for each time the experiment is carried out, flow rate should be changed by turning the orange wheel a little each time.

Once the six upstream depths and times have been recorded, volumetric flow rate can be calculated and hence used to calculate values for K and h.

Results

time (s)

h (cm)

h (m)

Q (m3/sec)

log Q

log h

1

2.54

20.2

0.202

0.01460157

-1.8356

-0.69465

2

2.85

20

0.2

0.01301333

-1.88561

-0.69897

3

3.12

17.9

0.179

0.01188718

-1.92492

-0.74715

4

2.87

16.3

0.163

0.01292265

-1.88865

-0.78781

5

3.4

14.2

0.142

0.01090824

-1.96225

-0.84771

6

9.83

6.6

0.066

0.00377294

-2.42332

-1.18046

Theory value

K = 1.7 * Cd* B

K=1.7*0.9*0.1=0.153

For a rectangular throated flume, n = 1.5

Experimental values

K = 0.0945

n = 1.1696

Graph showing Q against h

Graph showing Q against h

Log Q against Log h

Log Q against Log h

Analysis

Example calculation at test run 3

Q=Tank volumetime

Q=0.76m * 1.22m * 0.04m 3.12 sec=0.01188718 m3sec

h=17.9100=0.179m

Q=Khn

LogQ=LogKhn

Log Q=nLogh+Log K

y=mx+c

y = Log Q

m = n

x = Log h

c = Log K

Therefore by plotting Log Q vs. Log h, we can attain values for n and Log K

From graph, equation of line;

Log Q=1.1696 Log h-1.0205

Therefore,

n = 1.1696

Log K = -1.0205

K = 10-1.0205 = 0.0954

Discussion

The venturi flume experiment is used to calculate coefficients K and n. The experimental value for K was 0.0954, theoretical value is 0.153. The experimental value for n was 1.1696, theoretical value is 1.5. The difference in experimental value for K and n from the theoretical values could be because of human error caused by the person reading the stopwatch.

The limitations of the venturi flume test are that the upstream depth is an approximate as it is down to human judgement. Secondly, the flow rate wheel does not give a set flow rate; therefore no experiment can be repeated again to give the same flow rates and upstream depths.

BS ISO 4359 states that the flume should be clean before an experiment takes place, this can cause alterations and errors in the results.

Overall, I believe that the results obtained are a good set of results with a strong positive correlation; there are not any anomalous results. If the experiment was to be repeated, I would carry out the experiment several times and then use an average K and n value. This would then reduce the error and hence allow further discussion to why there is a difference from theoretical values.

1

Log Q = 1.1696 * Logh - 1.0205

Log h

Log Q

Q = 0.0954 * h1.1696

h (metres)

Q (m3/s)

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