Experiment Name: Determination of airflow velocity by using a Pitot static tube.
Objective:
(i) Measurement the air flow velocity through a closed duct by using a Pitot static tube.
Apparatus:
Pitot static tube, differential U-tube manometer, blower with pipe line assembly.
Theory:
The principle of flow measurement by Pitot tube was adopted by Henri Pitot in 1732. To measure the flow by a Pitot tube the necessity of separate piezometer and thus two readings make this arrangement a little awkward. Connecting the piezometer to a manometer would simplify things but there are still two tubes. For a fluid flowing through a closed duct, the Pitot tube measures only the stagnation pressure and so the static pressure must be measured separately. Measurement of static pressure in this case is made at the boundary of the wall by piezometer tube. The Pitot static tube combines the tubes recording the static and the stagnation pressure into one instrument which can then be easily connected to a differential manometer. The tube for sensing static pressure is known as static tube which surrounds the pitot tube that measures the stagnation pressure. A Pitot static tube is shown below in Figure. The holes on the side of the tube connect to one side of a manometer and record the static pressure, while the central hole is connected to the other side of the manometer to record, the stagnation pressure.
Figure:- Pitot static tube with differential manometer.
Now applying Bernoulli’s equation between point 1 and 2 we get,
where, ∆h = difference in manometric fluid level
Experimental procedure:
1. Start the flow of air through a closed duct by starting the blower.
2. Keep the flow control valve fully open and check the connection of the open end of Pitot static tube to the manometer.
3. Note down the reading of deflection of manometric fluid as the liquid in the manometer will be displaced due to differences between stagnation and static pressures.
4. Take reading in observation table.
5. Repeat above steps for different flow control valve opening.
Observations: (Sample reading shown below taken at room temperature)
- Temperature= 31°C
- Density of working fluid (ρ of air)= 1.1644 kg/m³
- Density of manometric fluid (ρ of H2O)= 995.2 kg/m³
FAQ :
1. List out the sources of error.
a. It requires high velocity flow.
b. It generates very low differential pressure which is difficult to measure.
c. Working fluid must not be dirty or sticky.
d. It block easily, particularly if there is debris in the flow line.
2. Draw the schematic diagram of the setup.
3. Provide sample calculation.
4.Describe- Static pressure, Dynamic pressure and Stagnation pressure.
Static pressure is the internal pressure of a flow if the pressure measuring device travels along with the flow. You can also measure it with a small hole in a surface along which the flow travels.
Dynamic pressure is the kinetic energy per unit volume of a fluid particle it is that pressure of fluid that results from its motion.
Stagnation pressure at a point in a fluid flow is the pressure which could result if the fluid were brought to rest isentropically. it implies that entire kinetic energy of fluid particle is utilised to increase its pressure only.
Stagnation pressure= static pressure + dynamic pressure.
Written by- SOUMIK.