School of Engineering 2
Introduction and Objectives
Measurement of volume flow rate or velocity at a point is essential in many engineering applications
(both laboratory and real world environments). Examples include providing information for plant
process control, determining the airflow rate in an engine intake manifold for the Engine Control
Unit (ECU), or measurement of the airspeed of an aircraft. Usually a device called a “Flow Meter” is
used to determine the volume flow rate of a fluid. Flow meters can utilise many different principles
(vortex shedding, forced cooling from heated surfaces etc.) but the most common simple types are
based on the pressure change from either an obstruction in the flow or a change in duct crosssectional
area. Venturi meters and orifice flow meters are widely used.
For measurement of velocity at a point in a fluid the most common instrument is a pitot-static tube.
Point measurements can be integrated across a plane to give volume flow rates. Detail description
about these flow meters can be found in any standard fluid mechanics books. The purpose of this
laboratory class is to familiarise yourselves with measurement methods, including the use of
Bernoulli’s equation and the continuity equation.
The following characteristics for the use of any obstruction flow meters are desirable:
 Accurate, reliable and repeatable measurements
 Small energy loss in the system, so that the intrusive effect on the system being
measured is small
 Inexpensive
 Minimum space requirement
The venturi flow meter satisfies the first two requirements and the orifice flow meter satisfies the
last two requirements. Venturi meters are comprised of a reduction in the pipe area, followed by a
short section of straight (smaller diameter) pipe followed by a gradual expansion, see Figure 3. The
changes in duct area are gradual so that the flow does not separate (i.e. “break away” from the duct
walls). In contrast, orifice plates are plates with circular holes that are smaller than the duct
diameter and these are placed across the duct, forcing the flow to separate.
Orifices plates are one of the cheapest and easiest metering devices to install since the orifice can be
simply clamped between pipe flanges. As orifice flow meters induce a strong flow separation and
their energy losses are significant compared to venturi flow meters. In fact, due to the losses, the
actual volume flow rate passed for the pressure drop will be less than the theoretical value.
Therefore, a parameter called “Discharge Coefficient, CD” is introduced to account for this
discrepancy. The discharge coefficient for certain cases is standardised by the ISO, ASME and British
Standards. Compliance of an orifice flow meter with standard discharge coefficients is often
required. Therefore, in this experimental work, it is required to evaluate the flow discharge
coefficients of an orifice flow meter and compare with standard coefficients and also Venturi.
School of Engineering 3
Equipment
The Airflow Developments Rig – a duct with a conical intake, a centrifugal fan, a Pitot-static traverse,
an orifice plate and venturi meter (see Figure 1).
A Pitot-static tube with flexible plastic tubing (see Figure 2)
Inclinable Manometers each with a thermometer and barometer (to measure the atmospheric
pressure) (see Figure 3).
A flow controller (see Figure 4)
Figure 1 Experimental apparatus – the Airflow Developments Rig
Figure 2 Close up of pitot-static tube
School of Engineering 4
Figure 3 Venturi meter and manometers
Figure 4 Method of flow control and orifice plate
School of Engineering 5
Experimental Procedure
For the experiment, follow the below mentioned steps:
1. Measure and record the local values of atmospheric pressure and temperature.
2. Connect the manometers to measure the pressure differences across the Pitot static tube, the
orifice plate and the venturi meter.
This involves connecting the plastic tubes from each end of a manometer (one end goes from the
reservoir and the other from the top of the corresponding inclined tube) to the two pressure taps on
each instrument. Think carefully which end should be connected to which and why. If in any doubt
ask the supervisor since if you get it wrong fluid will get into the tubes and this will result in severe
delays whilst the tubes are cleared out and the manometers refilled with fluid.
3. Check bases of manometers are level using the spirit levels and levelling screws.
4. Check angle of manometer and record appropriate correction factor.
5. Check that manometers are correctly “zeroed”.
The manometer reading should be zero when there is zero pressure difference acting between the
two inputs.
6. Switch on the fan with controller (screw damper at the duct end) fully closed.
By unscrewing the damper you can vary the flow rate through the duct. Note that a linear movement
of the damper does not result in linear changes to the flow rate
7. Position the Pitot static tube in the centre of the duct and ensure it is reasonably aligned with
the flow (by eye is sufficient)
Test One: Determination of Discharge Coefficients
1. Estimate the volume flow rate for the fully open damper position.
You can do this by measuring the average flow speed across the rectangular duct using the Pitot
static tube. You should take readings at sixteen points which lie at the intersection of four equally
spaced vertical and horizontal lines across the duct. We can then use these sixteen measured points
to determine the average velocity in the duct and from this determine an estimate of the “Actual
Volume Flow Rate”, ̇Act.
School of Engineering 6
2. Evaluate the flow discharge coefficient of the orifice flow meter and the venturi flow meter, and
compare these with standard coefficients.
To do this you will need to measure the pressure difference across the flow meter in question, and
from this and given data determine the “Theoretical Volume Flow Rate”, ̇Th. Discharge coefficient is
then the ratio between ̇Act and ̇Th.
Test Two: Comparison of indicated volume flow rates
1. Move the pitot-static tube into the centre of the duct. Assuming that the velocity profile across
the duct will be the same for all flow rates we must find a correction factor to determine the
average velocity at the pitot-static tube from the velocity at the centre of the duct. As we have
determined the average velocity at “Fully Open” damper position in Test One we can use the
ratio of this and the velocity at fully open in the centre position to find the correction factor, f.
̅
Therefore, for subsequent measurements as we vary the damper position and change the flow
rate, we can find the average velocity with: ̅
2. Record pressure difference across each manometer for the Pitot static tube, the Venturi and the
Orifice Plate for eight damper positions that you think will give approximately equally spaced
flow rates (not eight equally spaced positions of the damper since the relationship between the
position and flow rate is not linear).
3. Using the discharge coefficients found in test one (above), determine and compare the volume
flow rates indicated by the three different flow meters, namely the Pitot-static tube, the Venturi,
and the Orifice Plate.
Note: to do this for the Pitot-static tube, you will need to use your data to estimate the ratio between
average flow speed across the section and the flow speed at the centre of the duct. You can easily do
this by leaving the fan running at the end of test one, and simply moving the Pitot-static tube to the
centre of the duct and measuring the pressure difference at that point.
Figure 5 Duct cross-section with measurement points
16 flow measurement points
School of Engineering 7
Table of Raw Data
Atmospheric Pressure Ambient Temperature
Test One
Varying
Pitot static
position
Pitot-static manometer
reading
Venturi manometer
reading
Orifice plate manometer
reading
Damper position
(Rotation)
Units = Units = Units =
Correction factor = Correction factor = Correction factor =
1 Fully Open
2 Fully Open
3 Fully Open
4 Fully Open
5 Fully Open
6 Fully Open
7 Fully Open
8 Fully Open
9 Fully Open
10 Fully Open
11 Fully Open
12 Fully Open
13 Fully Open
14 Fully Open
15 Fully Open
16 Fully Open
Test Two
Varying
damper
position
Pitot-static manometer
reading
Venturi manometer
reading
Orifice plate manometer
reading
Damper Position
(Rotation)
1 Fully Open
2
3
4
5
6
7
8 Closed
School of Engineering 8
Report content (Total marks on this report: 5)
Submit the report using MS Word format.
Lab report should include the following:
1. Abstract: A summary of your report including brief comments (eg. one sentence) on
each element: objective, method, results and findings.
(4% of total marks)
2. Introduction – A brief introduction to your report. Try to discuss the lab and avoid
generalised comments.
(6% of total marks)
3. Method – Explain the lab procedure, be concise but don’t just duplicate the lab brief.
(6% of total marks)
4. Results and sample calculations – Present your results. This should include: sample
calculations for one row of data, tables of raw data and processed results and
graphs.
(40% of total marks)
5. Discussion: Analysis and discussion of your results. Hint – refer to actual numerical
results in your discussion; think about how your results compare with what you are
expecting.
(30% of total marks)
6. Conclusion: Your conclusion on the outcome of this lab.
(6% of total marks)
7. References: References (such as text books, lecture notes, lab manual, technical
articles etc.) you have used to understand and perform this lab.
(4% of total marks)
8. Submission in time: within 7 days of completing the experiment.
(4% of total marks)
Please pay attention to general presentation and quality of written work – remember to set
aside time for proof reading and editing before submission (Please do not submit hand
written reports).
School of Engineering 9
Some Relevant Theory
Ambient air density
Ambient air density must be calculated using Ideal Gas Law. Air density at ambient temperature is
given by the equation:
With the gas constant, R, being equal to 287.4 J/kg-K for air, the absolute temperature, T , being
given in Kelvin (= temperature in °C + 273), and the atmospheric pressure given in Pa, the density will
be in kg/m3.
Bernoulli’s Equation (ignoring any losses)
Air Velocity
This may be calculated for any point in the flow by appropriate use of the Bernoulli equation.
This is the equation to be used when employing a Pitot-static tube to determine the flow speed.

( )
Continuity Equation
Mass flow rate across any section should be constant through the duct
Since the density is constant in this case:
(all along the duct)
Volume Flow Rate
The actual volume flow rate across a section of the duct is given by the equation
̇ ̅
Where ̅ is the average flow speed across the relevant section of the duct
School of Engineering 10
Theoretical Volume Flow Rate
Any flow meter (including the venturi and the orifice plate) will indicate a “theoretical volume flow
rate”, ̇th , based on a relevant measured pressure difference and areas.
̇ √
( )
( (
)
)
In this equation subscript “1” refers to a section in the duct upstream of the flow meter. For the
venturi and orifice plate, subscript “2” refers to the “throat” section, which is where the flow meter
has its minimum area.
The above equation is developed from the Bernoulli equation, and assumes that there is no energy
loss between sections “1” and “2”. However, the nature of the real flow in and around the flow
meter does lead to energy loss and so ̇th is not an accurate estimate of the actual volume flow rate.
However, ̇th calculated with the above equation can be used in practice to lead us to an estimate of
the actual volume flow rate if we know the fluid discharge coefficient, CD (defined below). That is,
the discharge coefficient is a “calibration factor” that allows us to obtain a true value from a
theoretical approach.
Fluid Discharge Coefficient
̇
̇
̇ ̇
Data you will require for this experiment:
Parameter Value
Duct internal diameter 140 mm
Venturi throat internal diameter 89 mm
Orifice plate internal diameter 108 mm
Duct dimensions (Perspex section) 115 mm (W) × 128 mm (H)