Velocity Distribution of a Circular Air Jet - Lab Report Example

The velocity distribution in the jet is measured using a pitot-static. The Pitot-static tube can be traversed across the jet diameter and also across its length by sliding the support against a millimeter scale to record its position. The pitot-static is connected to an inclined manometer.

Procedure:

7.1 Measure the velocity distribution along the centerline of the jet and note the value of V0 at a distance of approximately x =1D where D is the diameter of the exit of the nozzle.

7.2 Measure the air velocity at points along the horizontal diameter at x=2D. Plot the variation of v with y as you take your readings. Use your judgment over how many points you need, ensuring that you locate the endpoints at the edge of the jet

7.3 Repeat the procedure for x=10D

7.4 The graphs should be symmetrical about the centerline. Decide for which graph, which values for y is at the center so that by subtraction, it is possible to plot V against radius r, measured from the centerline of the jet.

8.1 Replot small versions of the three v, y curves on a plan view, identify and label

(i) the edge of the jet

(ii) the core, in which the air velocity is v0

(iii) the mixing region, where v is less than v0

8.2 From the plan view measure the divergence angle of the jet

8.3 Plot the centerline velocity distribution

8.4 Calculate q0, the volume flow rate leaving the nozzle, in m3s-3. I west reasonable to assume that v0 is constant in the nozzle exit plane.

8.5 Calculate q10, the volume flow rate at x = 10D. In this case, the air velocity is not constant thus it is necessary to integrate to find the volume flow rate.

9. Results of the air jet experiment:

Air Jet

v distribution

x=1D

pitot position (mm)

inches

mm

0

0.875

22.225

20

0.87

22.098

40

0.875

22.225

60

0.875

22.225

80

0.86

21.884

100

0.8

20.32

150

0.61

15.494

200

0.44

11.176

250

0.24

6.096

300

0.24

6.096

350

0.18

4.572

400

0.145

3.683

450

0.11

2.794

Air Jet

v distribution

x=2D

pitot position (mm)

inches

mm

0

0

0

10

0.01

2.254

15

0.09

2.286

20

0.51

12.954

25

0.84

21.336

30

0.84

21.336

35

0.795

20.193

40

0.765

19.431

45

0.48

12.192

50

0.12

3.048

55

0.05

1.27

60

0

0

Air Jet

v distribution

x=10D

pitot position (mm)

inches

mm

0

0

0

5

0

0

10

0.01

0.254

15

0.05

0.254

20

0.025

0.635

25

0.04

1.016

30

0.07

1.778

35

0.11

2.794

40

0.22

5.588

50

0.3

7.62

55

0.37

9.398

60

0.38

9.652

65

0.36

9.144

70

0.275

6.985

75

0.2

5.08

80

0.14

3.556

85

0.1

2.54

90

0.065

1.651

95

0.04

1.016

100

0.03

0.762

105

0.01

0.254

110

0

0

10. Discussion

10.1 Describe the centerline velocity distribution

The centerline velocity is highest at the point of origin and subsides due to friction further away from the nozzle. It is significantly reduced towards the outer radius of the flow of air.

10.2 The flow rate at x=10 D should be significantly increased compared with that at x0, where has the additional volume flow rate come from?

Since at 10D, the jet is wider and displaces more air than it has absorbed through the fan, there is a higher volume flow rate than at X0.

10.3 Explain why the jet spreads out as it flows downstream

The pressure in the center of the jet is higher at the point of origin. The pressure stabilizes further away to the point of origin to match the surrounding pressure and hence the jet spreads out as it flows to reduce its density.