OpenChannel flow Lab Report

Open Channel Flow Experiment Lab Report 0904974

Abstract This experiment investigated the velocity-area method and turbine flowmeter method of calculating flow rate. Its aim was to compare accuracy and practical usage for each method. Initial hypotheses predicted that the turbine flowmeter would have a higher level of accuracy than the velocity-area method. This was confirmed by the results. It was concluded that for high accuracy a turbine flowmeter would be better suited however as it can only be installed within a pipe it would be unsuitable for many natural environments such as a river.

Introduction During this channel flow experiment the flow rate was measured in a tank using two methods; the velocity-area method and the turbine flowmeter method. The obtained results were used to compare the accuracy and analyse potential benefits and drawbacks of each method. Initial hypotheses predicted that the turbine flowmeter would be more accurate as there is fewer opportunity for human error.

Risk Assessment A risk assessment was completed before the experiment began see Appendix 1. This was discussed by all participants involved. Everyone was made aware of the health and safety protocols should any incidents have occurred.

Materials and Method See Appendix 2.

Data and Results Shown in Figure 1 are the results found by completing each experiment and completing the calculations to obtain the total flow rate. These are then plotted on the graph in Figure 2.

Velocity-Area

Velocity-Area

Turbine

1

2

Flowmeter

(Before Weir)

(After Weir)

(Direct Flow)

Flow

(m3/s)

(m3/s)

(m3/s)

1

0.0162

0.0185

0.0150

2

0.0144

0.0176

0.0133

3

0.0134

0.0159

0.0117

4

0.0118

0.0128

0.0100

5

0.0091

0.0119

0.0083

6

0.0085

0.0085

0.0067

7

0.0073

0.0064

0.0050

8

0.0071

0.0069

0.0033

Figure 1: Calculated Results of experiments Results

Figure

2:

Graphical Comparison

of

For full table of data - see Appendix 3 Discussion “The modern axial turbine flowmeter… is a reliable device capable of providing the highest accuracies attainable by any currently available flow sensor… Typical measurement repeatability is ±0.1% of reading”. (J.G. Webster, 1999) Assuming the turbine flowmeter is calibrated correctly it is likely to be highly accurate therefore it displays some inaccuracy in the velocity-area method. Although it does show a definite trend matching the turbine flowmeter reading. Some inaccuracy in the velocity-area experiment can be explained by the scale of the turbine velocity meter not being small enough. Consequently, the turbine used to measure the water velocity was not fully submerged during some parts of the experiment. This included all measurements after the weir and a few of the lower flow rates at 0.2 of the depth (minimum 0.0340m).

Analysis The turbine meter flow method would be more suited to installations in situations where there is a high demand for accuracy and a liquid flowing through a pipe where it can be installed. It can be used within a water supply or a in a factory however it could not be installed into a stream or river without severe environmental impacts. The velocity-area method would however be more suited to use in a natural environment without any major environmental impacts; providing a good representation of the flow rate and highlighting any changes occurring.

Conclusion Overall there are positive and negative points to each method of measuring open channel flow. The turbine flowmeter is highly accurate but requires installation into a pipe and the velocity-area can be easily set up and removed in a river or stream however it is less accurate than the turbine flowmeter.

References David Wadlow, Turbine and vane flowmeters, In: John G. Webster, 1999. The Measurement, Instrumentation and Sensors Handbook. CRC Press.

Appendix 1 Appendix 1

Appendix 2

UNIVERSITY OF ABERTAY DUNDEE Open Channel Flow Experiment (koko/watereng/ocflabo4) Object To measure and analyse flow in an open channel. To compare different methods of flow measurement.

Apparatus A long glass rectangular channel of constant cross-section width 305mm, fitted with a mechanism for varying the channel slope and with a downstream control weir is provided. A constant supply of water can be pumped through the channel, the flow rate being controlled by a valve and measured by a turbine flowmeter. The channel is fitted with depth measuring devices. A pre-calibrated propeller (or impeller) type meter is provided for the flow measurement exercise. Other instruments to be supplied are a stopwatch and a ruler. You must not change the channel bed configuration set up for your group.

Theory a) Velocity - area method

The flow cross-section can be divided into sub-sections so that velocity can be determined at 0.2 and 0.8 x depth of flow below the surface. Therefore, flow in each sub-section is given by:

Q (sub-section) = Area x Average velocity (in the sub-section) where average velocity in the sub-section is the average of the two velocities (i.e. at 0.2 and 0.8 x depth of flow below the surface).

Total flow through the cross-section is determined by adding the flows in all the subsections together. It must however be noted that average velocity in a sub-section can be taken as the velocity at 0.4 x depth of flow below the surface where the flow is shallow.

b) Turbine flowmeter This is a direct method of water flow measurement. The method gives the flow rate directly.

Procedure 1) With the flow condition established for you (i.e. slope of channel and valve at the inlet to the channel set at positions for delivering maximum flow), use the approaches in the theory section to determine or measure flow rate.

a) Velocity-area method

i) Divide the cross-sections before and after the weir into sub-sections and record the velocity values at 0.8 and 0.2 x depth of flow from the water surface in each of the sub-section.

ii) Determine the area of each of the sub-sections.

iii) Determine the flow rate of water through each of the sub-sections and hence the flow rates through the two cross-sections.

b) Turbine flowmeter

The value on the meter can be read directly.

2) Reduce the flow rate in stages, by closing the valve at the inlet to the channel and repeat (1) above at least six more times.

Analysis of Results Obtain/calculate the flow rate values from the two different approaches and compare the values in an appropriate manner.

Conclusions Conclude on the results obtained, particularly between the flow rates from the different approaches and on the accuracy of the methodology used.

Appendix 3