UNIVERSITI TEKNOLOGI MARA FACULTY OF CHEMICAL ENGINEERING CPE 453 No. Title Allocated Marks (%) Marks 1 Abstract/Summary 5 2 Introduction 5 3 Aims 5 4 Theory 5 5 Apparatus 5 6 Methodology/Procedure 10 7 Results 10 8 Calculations 10 9 Discussion 20 10 Conclusion 10 11 Recommendations 5 12 Reference 5 13 Appendix 5 TOTAL MARKS 100 Remarks: NAME : MOHAMMAD AKMALHAKIM BIN ZAKARIA STUDENT NO. : 2010229424 GROUP : EH2213B EXPERIMENT : FLOW OVER WEIRS DATE PERFORMED : 27 OCTOBER 2011 SEMESTER : SEMESTER 3 PROGRAMME / CODE : CPE 453 SUBMIT TO : PUAN SUHAIZA HANIM HANIPAH
Transcript
UNIVERSITI TEKNOLOGI MARAFACULTY OF CHEMICAL ENGINEERING
CPE 453
No. Title Allocated Marks (%) Marks1 Abstract/Summary 52 Introduction 53 Aims 54 Theory 55 Apparatus 56 Methodology/Procedure 107 Results 108 Calculations 109 Discussion 20
NAME : MOHAMMAD AKMALHAKIM BIN ZAKARIASTUDENT NO. : 2010229424GROUP : EH2213BEXPERIMENT : FLOW OVER WEIRSDATE PERFORMED : 27 OCTOBER 2011SEMESTER : SEMESTER 3PROGRAMME / CODE : CPE 453SUBMIT TO : PUAN SUHAIZA HANIM HANIPAH
1. Abstract This report will discuss about the flow characteristics over a rectangular notch and a
triangular(V) notch. The discharge coefficient of the fluid flow are also have been defined through this experiment. In order to achieve the objective, every depth of water with different height and two different notches was recorded in conducting experiment. The amount of volume is set constant for easier calculation.
Graphs have been plotted from the data obtained for analyzing the flow characteristics. For the rectangular graphs, the discharge coefficient decrease slowly, then it reaches almost constant value for the rest. Very differ from triangular notch which is, the discharge decrease smoothly but the values are way higher than rectangular notch.
From the result, it can be shown that the triangular notch have more discharge coefficient rather than the rectangular notch.
2. IntroductionFluids mechanics has develop as an analytical discipline from the application of the
classical laws of statistics, dynamics and thermodynamics, to situations in which fluids can be treated as continuous media. The particular laws involved are those of the conservation of mass, energy and momentum and, in each application, these laws can be simplified in an attempt to describe quantitatively the behaviour of the fluid.
A weir is an opening in the sidewall of a tank at top. The stream of liquid coming out the weir is known as a nappe, sheet, or vein. There is no difference between a notch and weir except that the former is a small structure and has sharp edges. A weir is generally an overflow structure, with a broad crest, built across an open channel. The terms air and weirs are used synonymously in general. The top of weir wall over which the liquid flows is known as the sill or crest. The head under which the weir is discharging is measured from the crest to the free surface. A weir or notch is generally used for measuring the flow of liquids.
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 broad-crested weirs will meet accuracy requirements only if they are calibrated.
3. Objectives of the Experiment To observe the flow characteristics over a rectangular notch and a vee (V) notch. To determine the discharge coefficients of the fluid flow.
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4. TheoryWith different type of notch, the method of calculating the discharge coefficients of the fluid flow is also different. These are some theories that discuss about the reasoning of two different notches, which are:
1. Rectangular NotchA rectangular notch in a thin square edged weir plate installed in a weir channel as shown in figure 2.
Figure 2: Rectangular Notch
Consider the flow in an element of height H at a depth h below the surface. Assuming that the flow is everywhere normal to the plane of the weir and that the free surface remains horizontal up to the plane of the weir.
In practice the flow through the notch will not be parallel and therefore will not be normal to the plane of the weir. The free surface is not horizontal and viscosity and surface tension will have an effect. There will be a considerable change in the shape of the nappe as it passes through the notch with curvature of the stream lines in both vertical and horizontal planes as indicated in Figure 3, in particular the width of the nappe is reduced by the contractions at each end.
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Figure 3: Shape of a Nappe
Qt=Cd23b√(2g)H
32
Where;
Qt = volume flow rate ( m/s )
H = height above notch base (m)
b = width of rectangular notch ( 0.03 m)
Cd = the discharge coefficient, which has to be determined by experiment
The discharge from a rectangular notch will be considerably less, approximately 60%, of the theoretical analysis due to these curvature effects. A coefficient of discharge Cd is therefore introduced so that
Cd=32
Q t
b√(2g)H32
However, Cd is not a true constant tending towards a constant only for large heads and a low velocity of approach in the weir channel.
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2. Triangular ( vee ) Notch
Figure 4: Triangular or V Notch
Qt=Cd8
15tan( θ2 )√(2g)H
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Where;
Qt = volume flow rate
H = height above notch base
B = width of rectangular notch
θ = angle of the Vee in the triangular notch
Cd = the discharge coefficient, which has to be determined by experiment
Thus,
Cd=158
Qt
tan( θ2 )√ (2g )H52
For a rectangular notch the rate of discharged is proportional to the liquid depth raised to power 1.5 and for the triangular notch to a power of 2.5. A triangular notch will therefore handle a wider range of flowrates. It can be shown that the notch must have curved walls giving a large width to the bottom of the notch and a comparatively small width towards the top. The weir is frequently installed for controlling the flow within the unit itself, for instance in a distillation column or reactor.
5. Apparatus and Material SOLTEQ Flow Over Weirs (Model: FM 26) - Appendix
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Water
6. Methodology1. The hydraulic bench is placed in a way that its surface is horizontal. This is important as
the flow over notch is driven by gravitational force.
2. Initial reading on the wall of the water tank was noted and recorded.
3. The stopwatch is set to zero before the experiment started.
4. The rectangular notch is mounted into the flow channel and the stilling baffle as shown
in diagram 1.
5. The main valve is opened.
6. The pump is opened until the level of the water is just above of the weir crest by locking
the coarse adjustment screw.
7. To take an accurate height reading, the fine adjustment is used to lower the gauge until
it almost touched the surface.
8. The general features of the flow of water over the weir is being observed and recorded.
9. The volume flow rate is being determined by measuring the time with the aid of
stopwatch for the water to flow into the container or volumetric tank in a known
volume.
10. The ball valve is used to close the tank outflow and the volume collected will be
recorded.
11. The valve is then opened again, this time the bench valve is opened further to produce
an increase in depth of approximately 10 mm.
12. This will shows an increase in the reading of the flow rates and hence being recorded.
This continued until the level reached the top of the notch.
13. The procedure is repeated by replacing the rectangular notch plate with the triangular
(Vee) notch plate.
7. Results and CalculationsType of notch Volume(L) Water height
8. DiscussionIn this experiment, the objectives are to observe the flow characteristics over a
rectangular notch and a triangular(V) notch, and to determine the discharge coefficients of the fluid flow. The objectives of this experiment were achieved since the analysis of the flow characteristics can be done by plotting graphs related to the experiment. The data of discharge coefficient also was determined by calculation using the formula.
The discharge coefficient, Cd originally derived by dividing the actual volume flowrate with the Ideal volume flowrate. Then, the final formula of Cd is obtained as shown in the theory section. In this experiment, the formula was used for calculation and tabulation. The Cd
formulas are also derived depending on the type of notch. In this experiment, only two types of notch were used. Thus, only two different formula of Cd is used for calculation.
From the rectangular notch, the graph shows that the depth of water, H increases as volume flowrate, Qt increase. The discharge coefficient, Cd decrease slowly, then it reaches almost constant value for the rest of the depth. This could due to the water surface tension that disturbs the flow of the water toward the notch. When Q has reached high value that can overcome the tension, the Cd becomes almost contant value in the flow. The space for the water to flow, B is also enhancing the degeneration of Cd.
As for the triangular notch, the graph shows the same, the depth of water, H increases as volume flowrate, Qt increase. But the only differ is, the discharge, Cd decrease smoothly and the value of discharge are way higher that the rectangular notch. The water surface tensions also exist in this situation but the high magnitude of Cd occurs due to the angle of the triangular notch. More higher the depth, the larger space for the water to flow(value of B). It causes the value of Cd different from the rectangular notch.
9. Conclusion From this experiment, it can be concluded that the discharge coefficient, Cd for the
triangular notch are higher compare to the rectangular notch. It is due to the value of B, which is the space for the water to flow through the notch. For rectangular notch, it is constant for every depth. But, for triangular notch the value of B will increase as the depth, H increase related to the angle of notch.
10. Experimental Precaution Experiment must be carried at steady place to prevent existence of large waves on the
water. Ensure that the scale of the needle is in zero positioning before setting the initial depth
of water.
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Ensure that the surface of the water touches the tip of the needle before recording the volume of the water flow.
In installing the notch, make sure that the screws are tight before start the experiment to prevent leakage of water below the notch.
This experiment involves large volume of liquid. Thus, ensure that the water flows accordingly in the container/apparatus to prevent flood occur in the laboratory.
In collecting volume, make sure that the water is fully secure before collecting data.
11. References Fluid Flow, Heat Transfer and Mass Transfer Volume 1, Sixth Edition, Coulson &
Richardson’s Chemical Engineering by J M Coulson & J F Richardson with J R Backhurst and J H Harker.
Operating and Experiment Manual for SOLTEQ Flow Over Weirs (Model: FM 26). Laboratory Experiment Manual of CPE453 – Provided by Puan Suhaiza Hanim Hanipah. Trebal R. E., Mass Transfer Operation, McGraw-Hill Book Company, New York, 1990. CHE 493 Lecturer Notes – Puan Sunita Jobli, Semester 2 (EH2212B). http://www.cussons.co.uk/SOFTWARE/Part5/PART5.HTM http://mysite.du.edu/~jcalvert/tech/fluids/orifice.htm#Expt
