Question: What data can be obtained from the experiment? What are we looking for, and how do we get there?

For Venturi meter

Objective of lab

To determine the pressure drop due to the flow of the fluid through the Venturi tube.

Theory Governing experiment

A Venturi tube consists of three main parts:

• Pressure in the upstream section that has the same diameter as the pipe with an opening that allows to place an obstacle such as a pressure sensor to measure the static pressure p1 in that section.

• A convergent conical inlet section where the transverse section decreases as the velocity of the fluid increases as the pressure decreases.

• A cylindrical section where the low pressure tap p2 is located and the velocity of the fluid is practically constant

• Divergent conical outlet section where the transverse section increases until it reaches the original diameter of the tube which will decrease the velocity and increase the pressure. This outlet section allows for most of the differential pressure to be generated and therefore saves energy.

The Venturi tube must be guided by a length of at least 10 times the diameter of the pipe to obtain accurate measurements.

The non-turbulent upstream has a pressure difference of P between point (1) and the throat point (2). The radii relationship is 21=0.5

.

There are also losses of flow due to friction in the pipe which generates a constant loss known as

that can range from a 10-20% change in

Pressure in the upstream section and the throat are actual pressures and have losses that must be considered in the energy conservation equation.

The Bernoulli equation between points 1 (upstream section) and 2(throat):

1+122+1h=2+222+2

Where (12

) are at the same height and therefore the difference in height is negligible,

1+122+1h=2+222+2

Where 1

is the velocity of the fluid section in /

, 2

is the velocity of the fluid section 2, h

is the energy loss, P1 is the pressure in section 1, P2 is the pressure in section 2,

is the acceleration due to gravity,

is the density of the working fluid.

When the flow continuity equation is applied:

111=222

Where 1

is the velocity of the fluid in section 1, 2

is the velocity of the fluid in section 2,

is the density of the fluid, 1

is the area of section 1, 2

is the area of section 2,

Transforming the equation:

=1212222

Where 1

is the are in section 1, 2

is the are in section 2,

H is the level difference of the manometric tubes between tap 1 and 2,

is the acceleration due to gravity

In an actual flow with actual losses:

=

Where

is the discharge coefficient which is obtained under calibration. The figure below demonstrates how

varies

If

is close to 1 the losses are small. If

is approximately constant then the Reynolds number is above the critical value and it is considered to be within the normal working range of the meter.

varies when the Reynolds number is below the critical value where viscous are considered important. The

is relatively independent of the area's relation on the venturi tube size.

Experimental set up

• FME18.

• Hydraulic Bench

• Graduated test tube

• Chronometer

Experimental procedure

Connect the Hydraulic bench to the unit and switch on the hydraulic bench. Fill the manometric tubes. Vary the flow of 5 1/min. Take the measurements of the manometric tubes 1,2, and 3. Switch off the hydraulic bench.