Develop Understanding Of Flow Measurement Methods Biology Essay

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Air flow measurement techniques such as volume flow rate, the instantaneous velocity of a fluid at a point are very important in the engineering field. Applications that use these techniques include acquiring data for plant process controls, determining the airflow rate in an engine intake manifold for the Engine Control Unit(ECU), measuring the airspeed of aircrafts etc.

To carry out measurements of volume flow rates in fluids, a Flow Meter is typically used. This device functions on different principles such as vortex shedding, forced cooling from heated surfaces etc., but the most common simple types work on the basis of pressure changes due to an obstruction in the steady flow, or a change in the duct cross-sectional area.

Venturi meters and orifice flow meters are extensively used.

For measurements of the velocity at a point in a fluid, the most commonly used instrument is a Pitot-static tube. This can be done by integrating point measurements across a plane to give the volume flow rates.

Properties that are desired for ideal obstruction flow meters:

Reliable and repeatable measurements.

Small or negligible energy losses in the system, so that the intrusive effect on the system is minimal and may be ignored.

Inexpensive.

Has low volume, so requires minimal space.

The venturi flow meter satisfies A. and B., whereas the orifice plate meter satisfies C. and D.

Venturi meters constitute of a reduction in the pipe area, followed by a short straight section of pipe with a smaller diameter, which leads to a gradual expansion. The flow in a venturi meter doesn't separate or break away from the duct walls, as the changes in the duct area is gradual.

Orifice plates, on the other hand, are plates placed across the duct, with circular holes that are smaller than the diameter of the duct, and these force the flow to separate.

They are the cheapest and easiest metering devices, but since a strong flow separation is caused by it, there are significant energy losses in the flow as compared to the venturi flow meters. This causes the actual flow rate passed for the pressure drop to be higher than the theoretical value, so a parameter called Discharge Coefficient, CD, is used to account for this inconsistency.

Apparatus:

The Airflow Developments Rig ( A duct with a conical intake, a centrifugal fan, a Pitot-static traverse, an orifice plate, venturi meter and a flow controller)

A Pitot-static tube with flexible plastic tubing

Inclinable Manometers each with a thermometer and a barometer(to measure the ambient/atmospheric pressure)

Methodology:

These are the basic steps carried out for each of the tests:

The atmospheric pressure and temperature are measured and recorded.

Plumb the manometers to measure the pressure differences across the Pitot-static tube, the orifice plate and the venturi meter.

Ensure the bases of manometer are level, using the spirit levels and levelling screws.

Ensure the manometers are correctly zeroed.

The fan is then turned on with the controller/screw damper at the duct end fully closed.

By unscrewing the damper, the flow rate of the fluid through the duct can be varied.

Position the Pitot-static tube in the centre of the duct, with a visual check that it is aligned with the flow.

Test One: Determination of the Discharge coefficients

Estimate the volume flow rate for the fully open damper position

This is done by using the Pitot-static tube to measure the flow speed across the rectangular duct. These readings will give the estimate for the Actual Volume Flow Rate, QAct.

Evaluate the flow discharge coefficient of the orifice flow meter and the venturi flow meter, and compare these with standard coefficients.

This is done by measuring the pressure difference across the flow meter being used, and using this data collected, along with the given data, to determine the Theoretical Volume Flow Rate, Qth. The discharge coefficient is then the ration between QAct and QTh.

Test Two: Comparison of indicated volume flow rates

Record the pressure differences across each of the manometers for the Pitot-static tube, the venturi, and the orifice plate.

Using the discharge coefficients found from Test One, determine and compare the volume flow rates indicated by the Pitot-static tube, the Venturi and the Orifice plate.

Measured Results and Calculation Procedure

Test One

Raw Data

Test One

Pitot-Static Tube

Venturi meter

Orifice Plate

Unit

mm H2O

mm H2O

mm H2O

Initial

13

-11.0

-4

1

94

72.5

75.5

2

116

72.5

75.5

3

116

72.5

75

4

112

72.5

74

5

109

72.5

75

6

105

73

75

7

104

73.5

76

8

102

72.5

75.5

9

105

72.5

76.5

10

110

72.5

76

11

88

73.0

75.5

Damper Position: Fully Open for all the recorded values, except initial.

Average Pressure in Pitot-Static Tube, PP.T.

e.g. (94 - 13)sin11 = 15.46 mm H2O PP.T. = Σ[(P2 - P1) x sin11]/11

Average Pressure in Venturi Meter, PV.M.

e.g. [72.5 - (-11)] = 83.5 mm H2O PV.M. = Σ (P2 - P1)/11

Average Pressure in Orifice Plate, PO.F.

e.g. [75.5 - (-4)] = 79.5 mm H2O PO.F. = Σ (P2 - P1)/11

Ambient Pressure (Patm): 764 mm HG 764 x 133.322 = 101858.008 Pa

Temperature: 21°C 21 + 273 = 294K

Results

Test One

Pressure difference across

Pitot static tube

[(P2 - P1) *sin 11°

Venturi Pressure

Difference

(P2 - P1)

Orifice Pressure

difference

(P2 - P1)

Unit

mm H2O

mm H2O

mm H2O

1

15.45553

83.5

79.5

2

19.65333

83.5

79.5

3

19.65333

83.5

79

4

18.89009

83.5

78

5

18.31766

83.5

79

6

17.55443

84

79

7

17.36362

84.5

80

8

16.982

83.5

79.5

9

17.55443

83.5

80.5

10

18.50847

83.5

80

11

14.31067

73

79.5

PP.T. = 17.65851

PV.M. = 82.683

PO.F. = 79.4091

Pitot-Static Tube Calculations

Venturi Meter Calculations

The result of 1.046 obtained is impossible, as the CD can never be more than 1. The standard CD for a venturi meter is 0.99 - 0.9, and averagely 0.95, so the higher result calculated may be due to wrong data being recorded during the experiment.

Orifice Plate Calculations

The standard CD of the orifice plate is 0.5 - 0.7, averagely 0.6. The CD calculated, 0.637, is consistent with the standard discharge coefficient value.

Test Two

Test Two

Unit

mm H2O

mm H2O

mm H2O

1

106

72

75

2

103

69.5

72

3

98

63

65

4

89

53

55

5

66

34

36

6

39

9.5

15

7

16

-8.5

-2

8

13

-11

-4

Damper Position: Fully open, except for reading 8, for which it was closed.

Average Pressure in Pitot-Static Tube, PP.T.

e.g. (106 - 13)sin11 = 17.75 mm H2O PP.T. = [(P2 - P1) x sin11]/8

Average Pressure in Venturi Meter, PV.M.

e.g. [72 - (-11)] = 83 mm H2O PV.M. = Σ(P2 - P1)/8

Average Pressure in Orifice Plate, PO.F.

e.g. [75 - (-4)] = 79 mm H2O PO.F. = Σ (P2 - P1)/8

Test Two

Pressure difference across

Pitot static tube

[(P2 - P1) *sin 11°

Venturi Pressure

Difference

(P2 - P1)

Orifice Pressure

difference

(P2 - P1)

Unit

mm H2O

mm H2O

mm H2O

1

17.74524

83

79

2

17.17281

80.5

76

3

16.21876

74

69

4

14.50148

64

59

5

10.11288

45

40

6

4.961034

20.5

19

7

0.572427

2.5

2

8

0

0

0

PP.T. = 10.16058

PV.M. = 46.1875

PO.F. = 31.27273

Pitot Static Tube:

Venturi Meter:

Orifice Plate:

Pitot Static Tube Graph

Venturi Meter Graph

Orifice Plate Graph

Discussion

According to the calculations for Test one, an error can be seen in the fact that the actual Volumetric flow rate in the pitot-static tube is higher than the theoretical volume flow rate in the venturi meter, and this results in the discharge coefficient of the venturi meter being higher than 1. However, the difference between the calculated discharge coefficient, 1.046, and the maximum standard coefficient, 0.99, is 0.056, which is just a 5.65% error from the standard CD.

This slight inconsistency may have been caused due to human error in reading and recording the pressure values wrongly, for example, not taking the reading at eye level or reading an 8 as 6 etc. Other errors may have occurred in the apparatus such as systematic errors, or wrong operation of the apparatus at some point in the experiment.

Although this error percentage is really small, it still makes a big difference as a CD above 1 is invalid for the venturi meter, and the experiment should be repeated carefully for more accurate values.

The discharge coefficient for the Orifice plate matches the standard discharge coefficient, which suggests that the experiment was done correctly there was minimal error done during the experiment for the Orifice Plate.

The discharge coefficient for the orifice plate is much lower than for the venturi meter, due to the reduced losses when using the venturi meter. The venture tube changes shape in a smooth manner, so the conversion between pressure, or potential energy, and the velocity, or kinetic energy, can be done over a long range with very little loss of energy. This is shown in its high discharge coefficient.

The orifice plate, on the other hand, has a sharp edged plate with holes, and this causes pressure drop and losses, as well as energy loss due to heat losses.

Although the discharge coefficient of the venturi meter is higher than that of orifice plates, orifice plates are more widely used. This is mostly because venturi meters are more expensive to build, to install, and have higher maintenance costs, unlike orifice plates, so orifice plates are preferred.

For test two, it can be seen from the graph that the relationship between actual volumetric flow rate and the Pressure difference due to the varying damper position is not linear, but in fact seems to be parabolic.

Comparison between the theoretical and actual volumetric flow rate for the three flow meters does show some difference in these values. For example, for the venturi meter, Qth = 0.0796m3/s while QAct = 0.0833 m3/s, and for the orifice plate, Qth = 0.131m3/s while QAct = 0.137 m3/s. The differences between QTh and QAct for both meters are less than 5%. Qth is the theoretical estimate, and the slight discrepancy between the values could be due to human error when taking down raw data, and/or due to the changing pressure difference caused by the closing damper in reading 8. In reality, the QAct should be lower than the QTh due to energy losses in the apparatus, especially in orifice plate.

Conclusion

Due to the improbable value of CD for the venturi meter, and the inappreciable difference in the QTh and QAct values of the meters, the experiment data show that there were errors during the experiment.

However, since the percentage value of these errors are very small, it can be established that it was caused by human error and repeating the experiment should reduce these errors.

The calculated results also help show the parabolic relationship present between QAct and Pressure difference when the damper is being closed during test two.

Furthermore, the CDs calculated prove that venturi meters are more efficient than orifice plate meters.

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