CE 415L
Applied Fluid Mechanics Laboratory
Experiment: No. 1 - Venturi Flow Meter
Learning Objective
Following the completion of this experiment and the analysis of the data, you should
be able to
1. describe how Bernoulli’s equation is used with the venturi flow meter to
measure the flow rate of water,
2. explain the process used to calibrate the venturi flow meter,
3. recommend the flow rate range over which a predefined measurement
accuracy may be expected for the venturi flow meter, and
Introduction
There are several types of flow meters commonly used to measure the flow rate in
pipes flowing full. Three of these are the orifice, nozzle, and venturi flowmeters. All
three flow meters operate on the principle that a decrease in the flow area in a pipe
carrying a fluid under pressure causes an increase in velocity accompanied by a
decrease in pressure. In this experiment, you will calibrate a venturi flowmeter.
The venturi flowmeter experiment demonstrates an effective way to measure the flow
rate through a pipe. This can be done by placing a tapered constriction within the
pipe and measuring the pressure difference between the low-velocity, high-pressure
upstream section (1), and the high-velocity, low pressure downstream section (2)(see
Figure 1.1)
Manometer tubes
P1
h=HL
P2
h1
h2
FLOW
(Cross-sectional
area of throat)
V2
V1
Figure 1.1 - Venturi Meter Section Diagram
By assuming the velocity profiles are uniform at sections (1) and (2), then the
Bernoulli equation can be written as:
P1 /   z1  V1 / 2 g  P2 /   z2  V2 / 2 g  H L
2
2
(Eq. 1.1)
and
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Experiment: No. 1 - Venturi Flow Meter
CE 415L
Q  A1 *V1  A2 *V2
(Eq. 1.2)
P1 , V1 and z1 refer to the head pressure, velocity and elevation, respectively, of the
flow upstream of the constriction (section 1). P2 , V2 and z2 refer to the head
pressure, velocity and elevation, respectively, of the flow at or immediately
downstream of the constriction (section 2). HL is the headloss from area (1) to (2) that
accounts for energy losses associated with the flow constriction. Note that the unit
weight of water,  = ρg, where ρ is the density of water and g is the acceleration due
to gravity.
The ideal situation has HL=0, i.e., no headloss, meaning no energy in the fluid stream
is lost. The difficulty in including the headloss is that there is no accurate expression
to account for it. The result is that empirical coefficients are used in the flow rate
equations to account for the headloss. The equation typically used for a venturi flow
meter is:
Q  KA 2 gh
(Eq. 1.3)
Where
Q = Flowrate (ft3/s)
A = Venturi throat cross-sectional area (ft2)
g = Acceleration due to gravity (32.2 ft/s2)
h = Pressure head difference due to the venturi (ft of water)
K = Venturi flow coefficient (dimensionless)
Note that h is not the total energy headloss, just the pressure head difference due to
the change in pressure head across the venturi section. Effects of the venturi flow
meter on the system energy grade line (EGL) and hydraulic grade line (HGL) are
illustrated in Figure 1.2.
1
2
EGL
h
HGL
v2/2g
FLOW
Figure 1.2 - Venturi Flow Meter Energy Effects in a System
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Purpose
The purpose of this experiment is to determine the value of K and compare it to the
values documented in literature. (Note the value of K is a function of =d/D and the
Reynolds Number, Re=VD/)
Equipment
o Hampden Model H-6925 Fluid Circuits Demonstrator (with venturi flow meter
section installed, inlet diameter = 1.049 inch, throat diameter = 0.750 inch)
o Differential manometer.
Procedure
To reduce stresses on the pump, the main flow control valve, Globe Valve should be
closed (or nearly so) before turning the pump either on or off.
1. Close all the valves on the demonstrator.
2. Attach the plastic tubes from the manometer to the pressure taps next to the
venturi throat. This will measure the head difference before and after the venturi
throat, or (h = h1 – h2). Bleed the air trapped in the lines to the manometer. Adjust
the water levels in the manometer tubes to within 0.1 inch of each other.
3. Open valves 6, 7, 8, 9,13 and the Gate Valve (in the horizontal 1-inch pipe).
Turn the pump power ON and then open the Globe Valve.
4. Adjust the Globe Valve to obtain a flowrate of 2 gallons per minute (gpm) as
indicated by the Omega flowmeter (observed flow rate.)
5. Record the measured flowrate along with the differential headloss in the data table
below. Take the representative (average) pressure reading at the current flowrate and
enter in the data table.
NOTE: Be sure to re-zero (tare) the digital manometer just before each reading. Note
that because of the averaging algorithm the manometer display uses it may take
several (typically about 5) seconds before it settles to the actual value.
6. Repeat the pressure measurements for a total of six (6) flowrate settings by
adjusting the Globe Valve for additional flow rates of approximately 4, 8, 12 16 and 20
gpm (or the highest flowrate that can be achieved up to the flowmeter capacity).
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Table 1.1 - Experimental data.
Trial
Qobs
(gpm)
h
(inch of H2O)
1
2
3
4
5
6
Note: 1 gpm = 0.002228 ft3/s
Analysis
1. Determine the venturi flow coefficient K from the equation Qobs  KA 2 gh
by linear regression (i.e. trendline). In this equation Qobs is the observed flow
rate for each trial. Plot Qobs (y-axis) vs. A 2 gh (x-axis). The slope of that
line is the value of K.
2. Compare the computed K to the typical value listed in a suitable reference.
3. Take the K value and insert it into the formula KA 2 gh to find Qcalc . Enter
these numbers in a table like shown in Table 1.2 below.
4. Plot Qobs vs. Qcalc data points and a linear trendline to see how well the
observed results compare with the results using your computed K value. The
slope of this line should be close to one. Also, display the R 2 for the linear
trendline.
5. What could you do to Eq. 1.3 to improve the match between your experimental
results and the results calculated by Eq. 1.3? Write the improved equation and
demonstrate the improved accuracy of the resulting equation compared to the
flow rate indicated by the Omega flow meter.
Table 1.2 – Suggested data table header
Trial
h
(inch H2O)
h
(ft H2O)
Qobs
(gpm)
Qobs
(ft3/sec)
A*(2gh)^0.5
(ft3/sec)
K
Qcalc=KA*(2gh)^0.5
(ft3/sec)
Report
Please refer to the report requirements included in the syllabus and posted on the
course website.
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