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[answered] MET 335: Fluid Mechanics Laboratory Old Dominion University

Please help with the calculations, data tables, and the graph?that are asked for on pages 4 and 5.

MET 335: Fluid Mechanics Laboratory Old Dominion University

Mechanical Engineering Technology

MET 335 ? Fluid Mechanics Laboratory

Title: Pitometers as Flow Measuring Devices (Rev. 8/25/16, GRC)

Purpose:

1. The student shall understand the typical velocity profile for air flow in a circular duct.

2. The student shall understand the operation of the pitometer.

Objectives:

1. Use the pitometer in measuring the velocity profile in an duct.

2. Use the velocity profile to determine the average velocity and flow rate.

3. Tabulate the results obtained in this experiment and plot a graph of pitometer position versus

velocity.

4. Determine which pitometer placements yield the average velocity.

Theory:

A pitometer is a device used for measuring the velocity of a fluid stream. It is basically a tube within a

tube. 1 2

3 V

1 The inner tube inlet is placed in a fluid stream so that it directly faces the fluid velocity. This places the

outer tube inlets perpendicular to the velocity. Upstream of the pitometer the energy possessed by the

fluid is

2 P1 V1 Z1 2g Since just as it tries to enter the tube, its velocity decreases to zero, we have 7. Pitometer as Flow Measuring Devices Lab Instructions

Page 1 of 6 MET 335: Fluid Mechanics Laboratory

2 P1 V1

P 2 2g or

P2 P1 Where P1 = pressure at point 1

P2 = pressure at point 2

V1 = velocity at point 1

? = specific weight of a flowing fluid

g = acceleration of gravity, 9.81 m/s 2 V1 2

2g If the pitometer is very small as compared to the system in which the fluid is flowing, we can assume

that the velocity at the entrance to the outer tube is the same as upstream velocity and if we hook a

differential manometer to the inner and outer tubes, we can now determine what this manometer will

read. 3

2 x

1 x hm hm m 2 V P2 P3 P1 1 P1 x m hm m x hm 2g 2 and V1

hm m 2g where specific weight of flowing fluid kN m 3 m specific weight of manometer fluid, water kN m 3 hm = height of manometer (m)

7. Pitometer as Flow Measuring Devices Lab Instructions

Page 2 of 6 MET 335: Fluid Mechanics Laboratory V1 2 ghm m 1 then air Patm

g RTatm g c where: Patm = atmospheric pressure kN m 3 Tatm = atmospheric temperature (oK)

R = air gas constant, (0.287 KJ/kg?K)

g = acceleration of gravity, (9.81 m/s2)

gc = gravitational constant, (1 kg ?m /n-s2)

For a gas m >>> V 2 ghm m m

>>>1 then V = velocity in m/s Figure 1

If we move the pitometer across the duct we can find the velocity at different points and get a velocity

profile. From this velocity profile we can determine the flow rate and the average velocity The

procedure for doing this is to first obtain a velocity profile across the duct, say about 15 readings and

plot a graph of velocity versus duct diameter. A typical curve for turbulent flow is shown at the left of

Figure 1. 7. Pitometer as Flow Measuring Devices Lab Instructions

Page 3 of 6 MET 335: Fluid Mechanics Laboratory D 2

). It has been found from

40

experiment that the average velocity Vn in each area occurs at the points shown in the right half of

Figure 1. Then the flow rate through each area can be found from Then divide the duct into 10 equal semi-concentric areas (each area = Qn AnVn D 2

Vn

40 The total flow then is Q1 Q2 Q3 ............. Q10

Qt D 2

D 2

D 2

V1 V2 ........

V1 V2 ...... 40

40

40 The average velocity can then be found from

V = Qt / A = Qt / ( D2 / 4)

Description of Apparatus:

A circular duct, 140 mm in diameter, which has a Pitometer installed upstream of the suction of a

centrifugal fan, is used for this experiment. The fan is started to produce the air flow in the duct. The

pitometer should be placed so that the opening is facing the air flow. The pitometer must be parallel to

the flow to get accurate readings. An inclined manometer is used to measure the pressure drop.

Experimental Procedure:

1. Place the pitometer in position in the duct as directed by the instructor.

2. Connect the manometer differentially across the pitometer.

3. Start the fan and produce a flow through the duct.

4. Place the pitometer at the bottom of the duct. This will be 68.5 mm below the center line, or 1.5

mm (half the thickness of the pitot tube) from the lower wall. Record the position and manometer

reading. (Note that the manometer reading must be multiplied by the sine at the of the manometer

angle as shown in the video.)

5. Using the scale provided and some reference point, move the pitometer 8.5 mm upward or 60 mm

below the center line. Record the position and manometer reading.

6. Repeat step 5 in 10 mm increments until the pitometer is 60 mm above the center line, then move it

to its highest position and record this distance. Record all manometer readings.

7. Secure the fan.

8. Note that the pitometer cannot read the velocity at the exact wall?s edge. Theory says this velocity

is zero, so record it as such.

Results:

1. Place the manometer readings, pitometer position, and calculated velocity, using equation (l),

results in tabulated form.

2. Make a best fit smooth curve plot of pitometer position (ordinate)versus velocity (abscissa),

assuming V = 0 at the walls.

3. Using the procedure outlined above obtain the ten velocities at the ten specified points from the

graph, in Figure 1.

7. Pitometer as Flow Measuring Devices Lab Instructions

Page 4 of 6 MET 335: Fluid Mechanics Laboratory 4.

5.

6.

7. Calculate the flow rate and average velocity using these ten values.

Determine which pitometer placements yield the average velocity.

Discuss the shape of your graph and results.

Discuss possible sources of error. MET 335 Fluid Mechanics Laboratory

Experiment: Pitometers as Flow Measuring Devices

Date:_________________

Probe Position (mm)

Bottom most +Topmost -70.0

-68.5

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

+10.0

+20.0

+30.0

+40.0

+50.0

+60.0

(+70.0) Room Temperature/Pressure: *__25?C / 760mmHg__

Manometer Reading

(mmH20)

0

129

182

195

209

210

202

196

197

197

201

205

211

217

193

0 Velocity (ft/sec) *If the room temperature/pressure is not obtained during the experiment, use 25oC/760 mm Hg. 7. Pitometer as Flow Measuring Devices Lab Instructions

Page 5 of 6 MET 335: Fluid Mechanics Laboratory 7. Pitometer as Flow Measuring Devices Lab Instructions

Page 6 of 6

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