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INTRODUCTION :
Flow rate is a common measurement which often needs to be performed. A venturi meter
allows the flow rate in a pipe to be determined from a pressure differential. A venturi
narrows the diameter of the pipe for a short duration, converting pressure head to
velocity head. Through this pressure differential, Bernoulli’s equation, and the known
dimensions of the venturi, the flow rate of the incompressible fluid can be determined.
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.
Theory: Venturi meter and orifice meter are the commonly used flow meters for measuring mass/volumetric flow rate or velocity of the flowing fluid. These flow meters are also known as variable head meters. They are categorized as full-bore meter as measurement of the fluid takes place when it flows through a conduit or channel. Venturi meter: The venturi meter 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. The following figure shows the venturi meter with uniform cylindrical section before converging entrance, a throat and divergent outlet. D (^) d Pressure taps The converging inlet section decreases the area of the fluid stream, causing the velocity to increase and the pressure to decrease. The low pressure is measured in the center of the cylindrical throat as the pressure will be at its lowest value, where neither the pressure nor the velocity will be changing. As the fluid enters the diverging section the pressure is largely
The small effect of the kinetic energy factors α 1 and α 2 are also taken into account in the definition of Cv. Volumetric flow rate Qa can be calculated as: Qa = V 2 S 2 = ……………………… (5) where, S 2 is the cross sectional area of throat in m^2. Substituting (P 1 – P 2 ) = ρgH in above equation (5) we get, Qa = V 2 S 2 = …………………………. (6) where H is the manometric height difference * (specific gravity of manometric fluid – specific gravity of manometric fluid of water).
Aim:
To determine the discharge coefficient of given venture meter and its
variation with Reynolds number.
Apparatus:
A venturi meter,
Suitable manometer (with mercury as the manometric fluid) for measuring
pressured drop across the meters.
Water circulation system consisting of water reservoir, a centrifugal pump
having delivery line with by pass. Flow control valves are provided on
delivery line as well as by pass line.
Calibrated tank for measuring the flow rate of water through orifice meter.
Stop watch
Procedure:
1. Check all the clamps for tightness.
2. Check whether the water level in the tank is sufficient such that the suction pipe
of pump is completely immersed.
3. For measurement through venturi, open the outlet valve of the venturi meter.
4. For a good amount of variation in discharge also close the by-pass valve of
pump.
5. Now switch on the pump.
6. Open the gate valve and start the flow.
7. If any air bubbles exist in U-tube manometer remove them through air cock
valve. Operate the air cock valve slowly and cautiously to avoid mercury run away
through water.
Results and Discussion
The venturi meter constant k , calculated from equation 1, is a reformulation of
Bernoulli’s equation, taking into account the change in velocity head between the inlet
and the throat of the venturi.
(1) Venturi meter constant
This constant is then multiplied by the square root of the differential head observed on
the manometers (Equation 2). This will give the theoretical discharge, which assumes
an ideal fluid with no energy loss due to viscosity/friction. In order to determine a
more realistic value, a coefficient determined either empirically or from charts of
Reynolds Numbers and venturi materials must be used. The coefficient will always be
less than 1, as in the real world energy is always lost. This coefficient is multiplied
(Equation 3).
(2) Theoretical Discharge
(3) Actual Discharge
The flow rate is related logarithmically to the differential head (See Figure 2). In the
case of this experiment, the discharge coefficient C is calculated graphically using
empirical data – measured volumetric flow rate and change in head. By plotting the
logarithmic measured discharge and measured head loss data points (see Figure 3) and
determining the linear trend line equation, two constants – α and β – can be found.
Rearrangement of Equation 3 can also be used to determine C:
Determination of C from measured flow rate
Given data:
Cross sectional area of throat in venturi meter :
APPLICATIONS :
Measurement Of Blood In Vessels It is used for the measurement of volume flow of blood through vessels. It can be done by two ways .One way is to insert an accurately calibrated Venturi meter made of glass into the circulation. This method requires an anticoagulant, but is accurate method and is sensitive to even slight changes in flow. Other way is by producing a constriction (making it narrower ) in a vessel by means of a ligature near a branch, which can be used as a side tube, thus transforming the vessel itself into a Venturi meter. Second method is subject to greater error in calibration for absolute flows but its advantage is that it does not require any type of anticoagulant. The Flow of Chemicals in Pipelines One of the advantage of using Venturimeter is that temperatures and pressures of chemicals in a pipeline do not affect the accuracy of this device .Due to this reason we useVenturiflowmeter in crude oil pipelines. Crude oil pipelines, such as the ones in Alaska, are exposed to extreme temperatures during winter. Another great advantage of using thisflowmeter in volatile and cold environments is that it does not contain any moving part so there is no chance of freezing and breaking due to thermal expansion.
