CHE241 - Lab Report Solteq Flow Over Weirs FM26 (2015)

TABLE OF CONTENT No 1 2 3 4 5 6 7 8 9 10 11 12 13

Title Abstract………………………………. Introduction………………………….. Objectives…………………………….. Theory………………………………... Material And Apparatus……………… Methodology…………………………. Data and Results……………………… Calculations…………………………... Discussion……………………………. Conclusion……………………………. Recommendations……………………. Reference……………………………... Appendix……………………………...

SOLTEQ® FLOW OVER WIRES (FM 26)

Page 2 3 4 5–6 7 8 9 – 11 12 – 15 16 – 18 19 20 21 22

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1.0 ABSTRACT The Flow over Weirs experiment was conducted to investigate the characteristics of flow over a rectangular notch and triangular notch. The difference in flow rate of water that flows into both of the channel was observed. Other than that, the experiment was capable to study the discharge coefficient of fluid flow that determined by calculation of this experiment. The experiment was started as follows the procedures, with the depth of water with different height was tested by recording the time taken to collect 3L of water, which later will be used to calculate the flow rate of the flow. The data obtained were further tabulated by calculating the discharge coefficient, using the equation provided. Then, graphs were constructed to analyze the characteristics of the flow. From the constructed graphs, rectangular notch graph shows the discharge coefficient decrease slowly before a constant value is reached. Meanwhile, the triangular notch graph shows the discharge decrease smoothly, but the values are higher than the rectangular notch. To conclude, triangular notch has a higher discharge coefficient than rectangular notch. The experiment was successfully accomplished as all the objectives were gained.


Page 2

2.0 INTRODUCTION The flow rate in pipes and ducts is controlled by various kinds of valves. Liquid flow in open channels, however, is not confined, and thus the flow rate is controlled by partially blocking the channel. This is done by either allowing the liquid to flow over the obstruction or under it. An obstruction that allows the liquid to flow over it is called a weir, and an obstruction with an adjustable opening at the bottom that allows the liquid to flow underneath it is called an underflow gate. Such devices can be used to control the flow rate through the channel as well as to measure it. A weir is a flow control device in which the water flows over the obstruction.

In this experiment, the rectangular weirs and triangular weirs are been used. Rectangular weirs and triangular or v-notch weirs are often used in water supply, wastewater and sewage systems. They consist of a sharp edged plate with a rectangular, triangular or v-notch profile for the water flow. Broad-crested weirs can be observed in dam spillways where the broad edge is beneath the water surface across the entire stream. Flow measurement installations with broadcrested weirs will meet accuracy requirements only if they are calibrated.


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3.0

OBJECTIVE The main objectives of this experiment is to observe the flow characteristics over a

rectangular notch and a V-notch. Besides, the other objectives is to determine the discharge coefficients of the fluid flow. Then, to teach student how to plot a graph of against Log H and of

against H on rectangular notch. While on V-notch, learn how to plot graph

against H. Next, to estimate an average value of

least, to compare

against H, Log Q

for the range of the test. Last but not

values between rectangular notch and V-notch.


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4.0 THEORY Flow over Weirs Weirs are hydraulic structures consisting of an obstruction placed across a water channel with a specially shaped opening or notch. The weir structure will increase the water level, which will be measured. Water level-discharge relationships are available for standard-shaped openings or notches. Two types of weirs are widely used: 1. Rectangular shape opening 2. V-notch. Stilling baffle is used to ensure minimum turbulence. The stilling baffle will act as a reservoir to collect water volume and slowly disperse the water from the opening at the bottom of the stilling baffle. Types of Weirs Rectangular Weir The rectangular weir is able to measure higher flows than the v-notch weir and over a wider operating range.

B, 33mm 89mm

Where

2

Q

= Cd

Cd B H Q

= Coefficient of discharge = Width of notch = Head above bottom of notch =Flow rate


3

B 2g H

3 2

eq. (1)

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V-Notch The V-notch weir is a notch with a V shape opening. V-notch weir is typically used to measure low flows within a narrow operating range. Typical Cd values for V-notch are in the range of 0.58 to 0.62. 90 ° 50mm

√ Where, Cd = Coefficient of discharge = Half the enclosed angle of the vee H = Head above bottom notch Q = Flow Rate


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5.0 MATERIAL AND APPARATUS MATERIAL 1) Water APPARATUS 1) 2) 3) 4) 5) 6)

F1-13 Stilling baffle Rectangular notch V- notch Stopwatch Spirit level F1-10 Hydraulics Bench

1

2

5

7

3 4

6

8

Figure 1: F1-10 Hydraulics Bench 1. Stilling Baffle

5. Hydraulic Bench (FM 110)

2. Vernier

6. Flow Control Valve

3. Hook

7. Water Channel

4. Weir Plate (V or rectangular)

8. Sump Tank


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6.0 METHODOLOGY Experimental Procedures 1) The hydraulic bench pump were confirmed is securely connected. 2) The collection tank drain valve is ensured left OPEN to allowed flow discharged back into sump tank. 3) The flow over weirs apparatus were set up on the hydraulic bench. 4) Thumb nuts were confirmed securing the rectangular notch weir plate is fully secured. General Shut Down Procedures 1) The hydraulic bench flow control valve were shut and water supply is closed. 2) The residue water from channel and sump tank was emptied by ensuring the drain tank valve is left open. Experiment 1: Flow Characteristics over Weirs 1) The weir apparatus on the hydraulic bench were levelled and the rectangular notch weir is installed. 2) The hydraulic bench flow control valve were slowly opened to admit water to the channel until the water discharged over the weir plate. The water level is ensured that it is even with the crest of the weir. 3) The flow control valve is closed and allowed water level to stabilized. 4) The Vernier Gauge was set to gives a datum reading using the top of the hook. 5) The gauge is installed about half way between the notch plate and stilling baffle. 6) Water is admitted to the channel. The water flow were adjusted by using the hydraulic bench flow control valve to obtain heads (H). 7) After water flow condition is stabilized, heads readings is recorded in every increasing of about 1 cm. 8) Step 4 and 5 were repeated for different flow rate. 9) The readings of volume and time were taking using the volumetric tank to determined flow rate. 10) The rectangular notch were replaced by the notch with v-notch. 11) The result was recorded in the tables.


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7.0 DATA AND RESULTS Rectangular Notch Volume (L) Rectangular Notch

Height (m)

3 3 3 3

0.0105 0.0197 0.0325 0.0434

T1 50 32 24 16

Q2/3

Log Q (

Rectangular Notch

-4.0988 -3.7700 -3.6140 -3.3010

Time (s) T2 T3 21 42 10 11 7 6 5 7

Average 37.6667 17.6667 12.3333 6.0000

Flow Rate, Q (m3/s) 7.9646 x 10-5 1.6981 x 10-4 2.4321 x 10-4 5.0000 x 10-4

Log H

-1.9788 -1.7055 -1.4881 -1.3625

Cd )

1.8512 x 10-3 3.0665 x 10-3 3.8963 x 10-3 6.2996 x 10-3

0.7600 0.6305 0.4262 0.5678

0.3182 0.5970 0.9848 1.3152

Graph of Q2/3 Against Head Above Bottom of Notch, H 0.007 0.006 Q2/3 (m2/s2/3)

0.005 0.004 0.003 0.002 0.001 0 0

0.01

0.02 0.03 Head Above Bottom of Notch (m)


0.04

0.05

Page 9

Graph of Log Q Against Log H 0 -2.5

-2

-1.5

-1

-0.5

-0.5

0

-1 -1.5 Log Q

-2 -2.5 -3 -3.5 -4 -4.5

Log H

Graph of Coefficient of Discharge, Cd Against Head Above Bottom of Notch, H 0.8 0.7 0.6

Cd

0.5 0.4

0.3 0.2 0.1 0 0

0.01

0.02 0.03 Head Above Bottom of Notch (m)


0.04

0.05

Page 10

V-Notch Volume (L) V-Notch

3 3 3 3

Height (m) T1 79 35 28 17

0.0158 0.0244 0.0297 0.0413 Q2/5 (

Time (s) T2 T3 27 31 11 10 6 8 4 4

Average 45.6667 18.6667 14.0000 8.3333

Flow Rate, Q (m3/s) 6.5693 x 10-5 1.6071 x 10-4 2.1429 x 10-4 3.6000 x 10-4 Cd

)

2.1233 x 10-2 3.0368 x 10-2 3.4072 x 10-2 4.1930 x 10-2

V-Notch

0.8866 0.7319 0.5970 0.4398

Graph of Q2/5 Against Head Above Bottom of Notch, H 0.045 0.04 0.035

Q2/5

0.03 0.025 0.02 0.015 0.01 0.005 0 0

0.005

0.01

0.015 0.02 0.025 0.03 0.035 Head Above Bottom of Notch (m)


0.04

0.045

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8.0

CALCULATIONS

Sample Calculation of Flow Rates , Q

Rectangular Notch

V-Notch

Q1

Q1

Q2

Q2

Q3

Q3

Q4

Q4

Sample Calculation of Log Q Rectangular Notch Log Q1 Log Q2 Log Q3 Log Q4


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Sample Calculation of Log H Rectangular Notch Log H1 Log H2 Log H3 Log H4

Sample Calculation of Rectangular Notch (

)

( )

(

)

( )

(

)

( )

(

)

( )


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Sample Calculation of Coefficient of Discharge, Cd Rectangular Notch √ (

) √

Where, Cd = Coefficient of discharge B = Width of notch H = Head above bottom notch Q = Flow Rate Cd1 (

) √

Cd2 (

) √

Cd3 (

) √

Cd4 (

) √


Page 14

V-Notch √ ( √

) ( )

Where, Cd = Coefficient of discharge = Half the enclosed angle of the vee H = Head above bottom notch Q = Flow Rate Cd1 ( √

) (

)

Cd2 ( √

) (

)

( √

(

)

Cd3 )

Cd4 ( √


) (

)

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9.0

DISCUSSION

The Cd value is not constant for the rectangular notch. This is because the value for the flow rates, Q and the head above bottom of notch, H obtained for both rectangular notch and V-notch are different in each experiment. However, the Cd values of rectangular notch is smaller compared to the Cd values of V-notch. Calculation below shows the average value of Cd for rectangular notch for the range of the test:Cd 0.7600 0.6305 0.4262 0.5678 Based on the calculations for Cd values of V-notch, the Cd values gain decreases as the flow rate decreases. Besides, as the head above bottom notch increases , the Cd values gain also decreases. This shows that, the Cd values is dependent on the value of flow rate, Q and the value of the head above the notch, H. To prove whether Q and H relationship described by an empirical formula graph of against is plotted, the derivation from the empirical is as follow:-

, the

Where, when the equation is ploted on a graph, log Q = value on y-axis log H = value on x-axis log k = y-intercept n = the gradient of the graph


Page 16

Graph of Log Q Against Log H 0 -2.5

-2

-1.5

-1

-0.5

-0.5

0

-1 -1.5 Log Q

-2 -2.5 -3 -3.5 -4 Log H

-4.5

Based on the graph, The gradient (n) of the graph is :-

The value k is :-

Thus, this shows that Q and H does relate and can be describe using the empirical formula .


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Graph of Q2/5 Against Head Above Bottom of Notch, H 0.045 0.04 0.035

Q2/5

0.03 0.025 0.02 0.015 0.01 0.005 0 0

0.005

0.01

0.015 0.02 0.025 0.03 0.035 Head Above Bottom of Notch (m)

0.04

0.045

Based on the graph , the values of increases as the head above the bottom of notch, H increases. The Cd values can be obtained from the tangent of the graph on each point plotted. Cd Calculated Using Formula

Cd Gain From Graph

Cd1 = 0.8866

Cd2 = 0.7319

Cd3 = 0.5970 Cd4 = 0.4398

From the data, there is difference between Cd calculated from formula and Cd calculated using the plotted graph. The reason of this error could be because of the reading from the graph is not as accurate as the calculation using formula which lead to about 0.2 – 0.4 result error.


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10.0 CONCLUSION The smooth flow to and over the weir is essential to the determination of accurate rates of flow since the distribution of velocities on the approach flow has a definite influence on the discharge over the weir. As the flow rate increases, the discharge coefficient becomes more accurate to the theoretical value. When the flow rate is to low it clings to the notch and flows down it. This changes the coefficient of discharge because now the water is not only being affected by gravity it is having to resist viscosity and the friction of the surface of the notch. The limitations of the theory is it has to be level so the only force on the water is gravity, there has to be a constant flow, and constant pressure.The theory behind this experiment makes an assumption that there is a minimum height of water above the notch and any heights below this start to deviate from theory at an increasing rate.The relationship between the head of the weir and the discharge of the water over the weir is directly proportional. The lower flow rates produce lower heights above the notch creating larger changes from the theoretical equations


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11.0 RECOMMENDATION A weir is a barrier across a weirs designed to modify its flow characteristics. Weirs are commonly used to alter the flow of rivers to prevent flooding, measure discharge, and help render rivers navigable. There are several of recommendations to estimate discharge when using a weirs because it is very important to ensure that all flow enters by travelling over the weir and not around the weir or under the weir. It must be noted that the weir should be extended into the ground to minimize groundwater to pass under the weir. To ensure critical flow over the crest of the weir, it is important to maintain a ‘free outfall’. As long as the flow conditions downstream of the weir do not affect the flow over the weir, a free outfall is maintained. First of all, before doing an experiment it is better to learn and understand first on how to conduct the experiment. Moreover, the result of rectangular-notch and v-notch obtained must be taken in 4 decimal points to get an accurate values. Errors can never be ignored when it comes to laboratory work. The aim is to reduce the error as much as possible to obtain accuracy in work. Ways to reduce the error are by repeating the experiment for three times or more and then taking the average readings, by being extra cautious during the experiment, by asking more than one person to record the readings and carry out the experiment. It is important to keep the voice to a minimum while in a laboratory and always listen to the instructor. If any guidelines are needed, then refer to the supervisor.


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12.0 REFERENCES Books 1) Kundu, Pijush K.; Cohen, Ira M. (2008), Fluid Mechanics (4th revised ed.), Academic Press 2) Hamilton Smith, 1886, Hydraulics, the Flow of Water Through Orifices, Over Weirs, and Through Open Conduits and Pipes

Web 1) http://discoverarmfield.com/en/products/view/f1-13/flow-over-weirs , retrieve in August 2015. 2) http://www.codecogs.com/library/engineering/fluid_mechanics/weirs/index.php , retrieve in August 2015. 3) https://en.wikipedia.org/wiki/Fluid_mechanics , retrieve in August 2015. 4) http://www.engineeringtoolbox.com/weirs-flow-rate-d_592.html , retrieve in August 2015. 5) http://www.lmnoeng.com/Weirs/RectangularWeir.php , retrieve in August 2015. 6) http://www.aquatext.com/calcs/weir%20flow.html , retrieve in August 2015. 7) http://accessengineeringlibrary.com/browse/applied-fluid-mechanics-for-engineers , retrieve in August 2015. 8) https://www.scribd.com/doc/34695544/Flow-of-Water-Over-Weirs , retrieve in August 2015.


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13 APPENDIX


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CHE241 - Lab Report Solteq Flow Over Weirs FM26 (2015)

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