A.SUMMARY: The topic investigated in this experiment was the venturi Flow Rig. This is very significant in chemical engineering because the venturi meter offers the best means of measuring the flowrate of fluids out of all the other means. The major objectives of this lab was to; compare the behaviour of real and ideal fluids using a venture meter, to record the head loss profile through a venturi meter at different flowrates and to estimate the coefficient of discharge of a venturi meter. In the course of the experiment, the following equipments were used; a Perspex venture meter having eleven (11) manometric tubes to measure the static head of the fluid, a water supply tank, a pump, a level control device, a stop-clock to take readings of the time it took the water to discharge and a lever system to measure the weight of water discharge. From the experiment, it was found out that the Bernoulli's theorem only applies to ideal fluids. That is fluids that are incompressible (constant density), irrotational (smooth flow or laminar flow), non- viscous (fluid without internal friction) and finally a steady flow (one with constant velocity at each point. It was also found out that the head loss across different sections of the Venturi meter were minimal when compared to that involved in other flow-measuring device such as an orifice. From the plots of various graphs at different flowrates, it was also found out that the static head, the velocity head and the total head are directly related (direct proportionally) to the flowrates. Also there is a direct relationship between pressure drop and flowrate. Page 1 of 26
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A. SUMMARY:
The topic investigated in this experiment was the venturi Flow Rig. This is very significant in chemical
engineering because the venturi meter offers the best means of measuring the flowrate of fluids out of all
the other means. The major objectives of this lab was to; compare the behaviour of real and ideal fluids
using a venture meter, to record the head loss profile through a venturi meter at different flowrates and to
estimate the coefficient of discharge of a venturi meter.
In the course of the experiment, the following equipments were used; a Perspex venture meter having
eleven (11) manometric tubes to measure the static head of the fluid, a water supply tank, a pump, a level
control device, a stop-clock to take readings of the time it took the water to discharge and a lever system to
measure the weight of water discharge.
From the experiment, it was found out that the Bernoulli's theorem only applies to ideal fluids. That is
fluids that are incompressible (constant density), irrotational (smooth flow or laminar flow), non-viscous
(fluid without internal friction) and finally a steady flow (one with constant velocity at each point. It was
also found out that the head loss across different sections of the Venturi meter were minimal when
compared to that involved in other flow-measuring device such as an orifice. From the plots of various
graphs at different flowrates, it was also found out that the static head, the velocity head and the total head
are directly related (direct proportionally) to the flowrates. Also there is a direct relationship between
pressure drop and flowrate.
One other important finding from the experiment was that the coefficient of discharge of a venturi was
higher due to the fact that the head loss is small. Therefore, it can be inferred that the smaller the head loss,
the greater the coefficient of discharge when measuring the flow of fluids. It was found out also that the
theoretical flow of fluid through venture is slightly higher than the actual value. Also the coefficient of
discharge for the venture was estimated to be 0.95 ± 0.03.
From the experiment, it can be concluded that the ventrui meter is suitable for both compressible and
incompressible fluids. This is true because when it comes to measuring the flow of compressible fluids or
real fluids, the mass flow rate for a compressible fluid can increase with increased upstream pressure,
which will increase the density of the fluid through the constriction even though the velocity will remain
constant (Holland, 1986).
Finally, it can also be concluded that for higher flow rates where power loss could become significant; the
venturi meter could be very suitable.
Page 1 of 20
B. INTRODUCTION:
There were three major objectives of this experiment. These are; to compare the behaviour of real and ideal
fluids using a venture meter, to record the head loss profile through a venturi meter at different flowrates
and to estimate the coefficient of discharge of a venturi meter.
Venturi meters are used to accurately measure fluid flow in a pipe for both compressible and
incompressible fluid. This is true because the head loss when using a venture meter is minimal when
compared to other devices for measuring fluid flow. For an incompressible fluid, its behaviour may be
explained or predicted by Bernoulli’s equation. This can be done by considering it in terms of the total
head. The total head can be calculated as:
pρg
+ v2
2 g + z = H (1)
Where:
pρg
is the static or pressure head (m)
v2
2 g is the velocity head (m)
z is the elevation head (m)
H is the total head (m)
For a horizontal venture (that is one without height), the equation becomes:
pρg
+ v2
2 g = H
(2)
This means that for an incompressible fluid, the pressure head or static head plus the velocity head is
constant or equal to the total head.
It is noteworthy to mention here that the total head (H) should have remained constant at all points along
the tube, but due to the effect of friction, some head losses were experienced. Though, these head losses
were minimal in this case since we are using a venturi meter.
Page 2 of 20
If the flow rate (Q) was measured, then from the continuity equation, the velocity of at each position along
the duct can be calculated.
The continuity equation is:
Q=A1V 1 = A2 V 2 (3)
Where:
Q is the flow rate (m3/s)
A1is the area at the inlet section of the venturi (m2)
V 1 is the velocity at the inlet section of the venturi (m/s)
A2 is the area at the throat of the venturi (m2)
V 2 is the velocity at the throat of the venturi (m/s)
Note: the above equation can be extended to other sections of the tube
The above equation can be transposed for the velocity at each section as:
V n = Qan
(4)
Where:
V n is the velocity at each section of the tube (m/s)
Q is the flow rate (m3/s)
an is the area at each section of the tube (m2)
By determining the velocities at each sections of the duct together with the peizometric heads, the
behaviour of real fluids (compressible fluids) can be compared with that of ideal fluids (incompressible
fluids) as predicted by Bernoulli. For the latter (that is for compressible fluids), the density would not be
constant. It will vary from the density ρ1 at the venturi inlet to the density ρ2 at the venturi throat. (Douglas,
Gasiorek and Swaffield 2001). As mentioned earlier, for a compressible fluid the mass flow rate would
increase with increased upstream pressure, which will increase the density of the fluid through the
Page 3 of 20
constriction (though the velocity will remain constant). The reverse is the case for an incompressible fluid
or ideal fluid.
By measuring the pressure drop across the venturi meter, we can also calculate the value of the coefficient
of discharge ( Cd ).
This can be expressed in terms of the flow rate (Q) as:
Q=Cd*a t * √¿¿) (5)
Where:
Q is the flowrate (m3/s)
Cd is the coefficient of discharge (dimensionless)
a t is the area of the throat of the venturi (m2)
∆ p is the pressure drop across the venturi (Nm-2)
∆ p is the density of water (100 kgm-3)
m is the ratio of the area of the throat of the venturi (a t ¿ to the area at the approach duct a p
Thus, m=a t
ap (6)
Where:
a t is the area of the throat of the venturi (m2)
a p is the area at the approach duct (m2)
The coefficient of discharge is introduced to account for the fact that the theoretical flow of a fluid is
slightly greater than the actual or measured flow. Equation (5) above can be transposed for Cd as:
Cd= Q
at∗√ 2∆ pρ (1−m2)
(7)
Note: all the parameters are as explained above in the previous equations (5) and (6).
Page 4 of 20
Having known the coefficient of discharge Cd, the theoretical flow can also be estimated and compared
with the actual flow using:
Qactual=¿Cd¿ * Qtheoritical (8)
Where:
Qactual is the actual or measured flow (m3/s)
Cd isthe discharge coefficient (dimensionless)
Qtheoritical is the theoretical flow (m3/s)
The theoretical values of the static heads can also be compared with its experimental static heads by
calculating the theoretical heads as:
hn= h1 - [vn
2
2 g -
v12
2 g] (9)
Where
hnis the theoretical static head at various sections of the tube (m)
h1 is the static head at the inlet of the tube (m)
Vn is the velocity at various sections of the tube (m/s)
V1 is the velocity at the inlet of the tube (m/s)
One important relationship that can be shown when using a venturi meter to measure fluid flow is the direct
relationship between the pressure drop and the flowrate. This can be shown by calculating the pressure
drop across the venturi and then plotting a graph of the square-root of the pressure drop against the
flowrate. The pressure drop can be calculated as:
∆ p=ρg ∆ h (10)
Page 5 of 20
Where:
∆ p is the pressure drop across the venturi (Nm-2)
ρ is the density of water = 1000 kgm3
∆ h is the head loss across the venturi (m)
The energy loss can also be calculated in terms of the power consumption as:
Power consumption=Q∗∆ P (11)
Where: Q is flowrate (m3/s)
∆ P is the pressure drop across the venturi (Nm-2)
C. EXPERIMENTAL EQUIPMENT:
The main experimental equipment used in this experiment is a Perspex venturi meter having eleven
manometric tubes to measure the head loss across the venturi. Other equipments include; a lever system to
measure the weight of the water discharged, a stopwatch to measure the time needed for a certain volume
of water to be supplied; in order to then calculate the actual rate of flow. Other equipments are; water
supply tank, a pump and a level control device.
A Venturi Meter consists of a short converging conical tube leading to a cylindrical portion, called the
throat, of smaller diameter of that of the pipeline, which is followed by a diverging section in which the
diameter increases again to that of the main pipeline (Douglas, Gasiorek and Swaffield 2001). The
geometry of the venturi meter is designed to reduce head losses to a minimum. This is accomplished by
providing a relatively streamlined contraction (which eliminates separation ahead of the throat) and a very
gradual expansion downstream of the throat (which eliminates separation in the decelerating portion of the
device. Thus most of the head losses that occurs in a well-designed venturi meter is due to friction losses
along the walls rather than losses associated with separated flows ( Munson, Young and Okiishi 2002). The
diagram of a venturi meter used in this experiment can be seen in figure 1 below.
Page 6 of 20
Taken from http://www.cs.cdu.edu.au/homepages/jmitroy/eng243/VenturiMeter.pdf
The venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone. It has
no projections into the fluid, no sharp corners, and no sudden changes in contour. In Figure 3 showing
the venturi tube, the inlet section decreases the area of the fluid stream, causing the velocity to increase and
the head loss to decrease. The low pressure is measured in the centre of the cylindrical throat since the
pressure will be at its lowest value, and neither the pressure nor the velocity is changing. The
recovery cone allows for the recovery of head such that total head loss is minimal only about 10%.
The working principle of the venturi meter can be explained using a schematic diagram of the venturi
metre on the next page:
Page 7 of 20
Figure 1: Diagram of a venturi meter
manometric tubes
Venturi tube
Taken from www.icaen.uiowa.edu/~fluids/Posting/Home/EFD/EFD.../lab2_lecture.ppt
For ideal fluids, the velocity V1 and V2 are uniform and steady at the points A1 and A2. For real fluids, this
would not be the case. The pressure would fall where the liquid velocity increases in the constriction, thus
the greater the velocity change the greater the pressure drop. As the fluid moves from point 2 to point 3, the
velocity is decreased and the pressure is increased. The slow rate of change creates a small energy loss;
most of the energy is changed back to pressure. It is this relatively small loss that allows the venturi
meter’s use in low pressure pipes. This small loss is also what makes the use of the Venturi meter
attractive for flow measurements. (Watson 1998).
There is a gain in kinetic energy resulting from the increased linear velocity in the throat. This is been
balanced by the decrease of pressure in the throat. This reduction in pressure which occurs when the fluid
flows through the throat is called the venturi effect (Coulson & Richardson, 1999).
D. EXPERIMENTAL PROCEDURE AND OBSERVATIONS:
The procedures undertaken in this experiment was in two (2) stages. First, the pump was switched on, and
the rotameter adjusted to ensure that water flows through the venturi. During this process, it was ensured
that no air was induced into the system via the throat of the venturi.
After establishing a steady flow, the static heads at each manometric tube were measured. Also with the
weigh bench, the weights of water discharged by the system per unit time were measured. This together
with the volume of water discharged was then used to calculate the flowrate (Q). With the dimensions of
the essential parts of the venturi meter that were given in a table behind the venturi tube, the velocity and
the velocity heads were also calculated.
Page 8 of 20
Figure 2: Schematic diagram of a venturi meter
Using the head difference between the manometer tube 1 h1 (approach duct) and the manometer tube in the
venturi throat position ht, the pressure drop across the venturi for the different flow rates were also
calculated. The experiment was repeated for two other flowrates. For full list of measurements taken and
for calculated data, see table 1-6. For graphs showing relationship between different parameters (both
experimental and calculated), see figures 3-6
E. RESULTS AND CALCULATIONS:
The results (both experimental and calculated results) obtained in the experiment are represented in
table 1-4 below. The graphs of static head h, the velocity head hv and the total head H across the