Venturimeter Expt

Aim of the Experiment:
To calibrate venturimeter with different * ratios. Determine the flow head relationship for this
flow meter.Determine the effect of Reynolds number on the coefficient of discharge (C D).
* is the ratio of throat diameter to pipe diameter
Apparatus required: CCL4manometer, Hg manometer
Theory:
Flow meters are used in the industry to measure the volumetric flow rate of fluids. Differential
pressure type flow meters ( Head flow meters)measure flow rate by introducing a constriction in the
flow. The pressure difference caused by the constriction is correlated to the flow rate using
Bernoulli’s theorem.
If a constriction is placed in a pipe carrying a stream of fuid,there will be an increase in velocity,and

hence an increase in kinetic energy ,at the point of constriction.From an energy balance as given by
Bernoulli’s theorem,there must be a corresponding reduction in pressure.Rate of discharge from the
constriction can be calculated by knowing this pressure reduction,the area available for flow at the
constriction ,the density of the fluid and the coefficient of discharge C d. Coefficient of discharge is
the ratio of actual flow to the theoretical flow and makes allowances for stream contraction and
frictional effects. Venturi meter, orifice meter, and Pitot tube are widely used head flow meters in
the industry. The Pitot-static is often used for measuring the local velocity in pipes or ducts. For
measuring flow in enclosed ducts or channels, the Venturi meter and orifice meters are more
convenient and more frequently used. The Venturi is widely used particularly for large volume liquid
and gas flows since it exhibits little pressure loss. However, for smaller pipes orifice meter is a
suitable choice. In order to use any of these devices for measurement it is necessary to empirically
calibrate them. That is, pass a known volume through the meter and note the reading in order to
provide a standard for measuring other quantities.
Venturi meter:
One of the disadvantages of orifice meters is the large irreversible pressure loss across the orifice,
which results in substantial pumping costs in case of large diameter pipes. However, the same
principle can be exploited with only minimal pressure loss with the use of a Venturi meter. In this
case, the meter consists of a section with both a smooth contraction and a smooth expansion.
Because of the smoothness of the contraction and expansion, the irreversible pressure loss is low.
However, in order to obtain a significant measurable pressure drop, the downstream pressure tap is
placed at the “throat” of the meter; i.e., at the point of the smallest diameter. Venturimeter is used
to measure the rate of flow through a pipe. Venturimeter consists of a converging portion, throat
and a diverging portion. The function of the converging portion is to increase the velocity of the fluid
and temporarily lower its static pressure. The pressure difference between inlet and throat is
developed. This pressure difference is correlated to the rate of flow. The expression for theoretical
flow rate is obtained by applying the continuity equation and energy equation at inlet and throat
section.
For measuring discharge we should apply Bernoulli’s equation at point 1 and at point 2.The following
treatment is limited to incompressible fluids. Friction is neglected , the meter is assumed to be
horizontal and there is no pump. If v 1 and v2 are the average velocities at point 1 and point 2
respectively and is the density of fluid.
Since
------(1)
Now applying the equation of continuity at both points, we have
------------(2)
--------(3)
--------- (4)
where d1 and d2 are the diameters at point 1(pipe) and at point 2(throat) respectively.
Now putting the value of V2 in the above expression (1) and if =d2/d1, we have
-------- (5)
Q=A2V2-----------(6)
-------- (6)
Qth is the theoretical flow rate as computed from Eq.(6) and applies to frictionless flow of
incompressible fluids. Actual flow includes frictional loss between point 1 and 2.So to account for
small friction between points 1 and 2,
-------- (6)
Where CD is called as co-efficient of discharge and it depends upon the type of flow, type of fluid and
dimensions of venture tube and pipe.
-------- (6)
It is the ratio of actual flow rate to the theoretical flow rate.
For a well designed venture the constant CD is about 0.98 for pipe diameters of 2 to 8inches and
about 0.99 fro larger sizes.
The equation relating flow rate to pressure drop is
Where CD is the coefficient of discharge for venturimeter.

At is the crosssectional area of the throat and is the ratio of throat diameter to pipe diameter.
For a Venturi,Cd=0.99 for 105 < NRe < 107 is a useful approximation.
The pressure recovery is much better for the venturi meter than for the orifice plate.
The main advantages of the Venturi over the orifice plate are :
 low head loss.Around 90% of the pressure is recovered.
 less affected by upstream flow disturbance
 good performance at higher
 even more robust
 The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services
 self-cleaning
 less affected by erosion
The disadvantages compared to the orifice are :
 occupies longer length of pipe
 more expensive (manufacture and installation)

Discharge Coefficients against Reynold’s Number (QD tVt/ ) Dt and Vt are the thoat diameter and
velocity at the throat respectevily.
Comparison of permanent head loss caused by different head meters Venturimeter
Venturimeter
 Throat to diameter ratio 0.25 to 0.75
 Discharge co-efficient - 0.9 to 1.0
 Made of cast iron, gun metal, stainless steel
Procedure:
 Keep the bypass valve completely open and the main valve completely closed. Switch on the
pump.
 Connect a CCL4 manometer,Hgmanometer across the venturi.
 Open the main valve and set a flow rate of water using the rotameter.
 Note down the rotameter reading and manometer reading after a steady state is attained.
 Increase the flow rate by opening the main valve and throttling the bypass valve suitably and
repeat step 4.
 Use mercury manometer for higher flow rates.
 Take readings with CCL4 manometer, Hg manometer as well as with Hg manometer.
 Repeat the experiment with different ratios and with different fluids.
Data:
Diameter of the pipe = d = cm
Diameter of the venture throat= d t= cm
Density of CCL4 = CCL4 = kg/m3
Density of Hg = Hg = kg/m3

Density of fluid = = kg/m3
Viscosity of fluid = = cp
Observations :
Calculations :
Flow rate: Hm= cm of manometric fluid.
Volumetric flow rate = Q = m3/s
= m of H2o
m= density of manometric fluid.
Average velocity through the throat =V t=Q/At= m/s
At = Cross sectional area of throat =

=dt/d=
Reynolds Number at the throat = =
Coefficient of discharge = Cd =
Plot Q vs and Q vs on ordinary graph(calibration).

Plot Cd vs NRet on a semilog graph sheet (with NReton log scale).
Plot log Q vs log .Find the slope and the intercept. Find the coefficient and power of .
Comment on the nonlinear flow head relationship. Obtain C d from the plot.
Results:
 Report on calibration
 Comment on Cd vs NRet
 Report the Cd
 Comment on flow head relationship
 compare Cd for orifice and venturimeter

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Aim of the Experiment:

If a constriction is placed in a pipe carrying a stream of fuid,there will be an increase in velocity,and

Now applying the equation of continuity at both points, we have

-------- (5)

-------- (6)

-------- (6)

-------- (6)

It is the ratio of actual flow rate to the theoretical flow rate.

The equation relating flow rate to pressure drop is

Where CD is the coefficient of discharge for venturimeter.

For a Venturi,Cd=0.99 for 105 < NRe < 107 is a useful approximation.

 low head loss.Around 90% of the pressure is recovered.

 less affected by upstream flow disturbance

 good performance at higher

 even more robust

 less affected by erosion

The disadvantages compared to the orifice are :

 occupies longer length of pipe

 more expensive (manufacture and installation)

 Throat to diameter ratio 0.25 to 0.75

 Discharge co-efficient - 0.9 to 1.0

 Made of cast iron, gun metal, stainless steel

 Connect a CCL4 manometer,Hgmanometer across the venturi.

 Use mercury manometer for higher flow rates.

 Take readings with CCL4 manometer, Hg manometer as well as with Hg manometer.

Density of Hg = Hg = kg/m3

Average velocity through the throat =V t=Q/At= m/s

At = Cross sectional area of throat =

Coefficient of discharge = Cd =

Plot Q vs and Q vs on ordinary graph(calibration).

 Comment on Cd vs NRet

 Comment on flow head relationship

 compare Cd for orifice and venturimeter

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