FlowThroughANozel

7/31/2019 FlowThroughANozel

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Faculty of Engineering and Technology

The University of Jordan, Amman-Jordan

Flow through a nozzle

By


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Abstract

It's needed to know the function of each nozzle type so in this experiment the

convergent-parallel nozzle is studied by the pressure distribution through a nozzle at

different ratios across it study.

To do this study three different values of inlet air pressure across the nozzle and the

pressure in each position of the nozzle is measured after that the function parameters

is calculated.

After doing this experiment it's shown that the pressure decreases slowly after the

throat and the mass flow rate is the most effected by the pressure difference.

List of symbols


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Critical pressureP* Pressure in the nozzle ChestPo

Velocity at the throatVt

specific heat ratio

Gas constantR

Pressure at the throatPt

Temperature in the nozzle ChestTt

Mass flow rate at the throatmt

Throat areaAt


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Objective

To understand how a nozzle works by study the pressure distribution through a nozzle

at different ratios across it.

Procedure1. The back pressure valve (inlet valve) is opened while the probe is being kept in no. 1

position.

2. The inlet pressure is set to 400 KN/m2 (the inlet pressure and the chest pressure are

noticed to be equal).

3. Chest pressure is to be observed throughout the experiment and re-adjusted to

initial setting if necessary.

4. The probe pressure is recorded at each of the stations shown on the nozzle replica.

5. Procedures are repeated for other back pressure values (pressure in the exit pipe).

Data observed


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The data collected by this experiment can be seen in table 1.

Table1: change of gage pressure for the probe with position no.

position no.

probe pressure [ Kpa ] for chestpressure =

400 [Kpa] 300 [Kpa] 200 [Kpa]

1 400 300 200

2 400 300 200

3 400 300 200

4 400 300 200

5 400 300 200

6 400 300 200

7 400 300 200

8 400 300 2009 399 300 200

10 380 282 190

11 330 242 160

12 290 218 141

13 280 210 136

14 270 202 130

15 260 200 129

16 250 190 123

17 240 185 119

18 240 182 118

19 240 180 11520 240 178 110

21 240 170 108

22 240 162 101

23 230 160 100

24 224 160 99

25 220 158 92

26 203 150 90

27 200 141 88

28 187 139 80

29 189 129 7930 165 120 71

31 150 110 61

32 117 78 38

33 0 0 0

Atmospheric pressure = 890 mbar

Atmospheric temperature = 20 C

** Throat at position no.11

Sample calculations


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For 300 KPa chest pressure

Abs Po = 300+89 =389 [KPa] Abs Pt = 242+89 = 331 [KPa]

P* = 0.528 Po = 0.528 * 389 = 205.392 [KPa]

Vt = (2* * R *To /( -1) *(1- (Pt/Po)^( (-1)/ ))^.5

( 2*1.4 * 287 *293 /( 1.4-1) *(1-(331/389)^( (1.4-1)/ 1.4))^.5 = 162.905 [m/s]

At = / 4( (throat diameter)2 (probe diameter)2 ) / 4( (4.77)2 (3.33)2 ) = 2.453 mm2

mt = At * Po * (Pt/Po)^(1/ ) * ( 2* / (R *To * ( -1) ) * (1- (Pt/Po)^( (-1)/ ))^.5

2.453 *10^-6 389*10^3 * (331/389) ^ (1/1.4) * ( 2*1.4 /( 287 *293 *( 1.4-1) ) * (1-

(331/389)^( (1.4-1)/ 1.4))^.5 = 0.0016 [Kg/s]

Pressure ratio = back pressure / Po

89 / 389 = 0.2288

Results and discussionsAfter calculate the parameters it can be seen in table 2.

Table 2 : change of critical pressure, velocity at the throat, mass flow rate and

Pressure ratio with chest pressure.

*Table 2 show that the critical pressure increases with chest pressure but it still bellow

the throat pressure this is important for condition of chocking.

*The velocity at the throat is much closed this may happened because it depends in the

ratio of the throat pressure to the chest pressure which is closed for each chest

pressure.

*The mass flow rate is increases with chest pressure because the increase in inlet

pressure increases the power to flow ( p*v = energy).

*The pressure ratio decreases because the bake pressure is constant and this changecan justify the relation in figure1 between pressure ratio and mass flow rate at the

throat.

Gage chestpressure[KPa]

Absolute chestpressure[KPa]

Criticalpressure[KPa]

Velocity at thethroat[m/s]

Mass flowrate[Kg/s]

Pressureratio[KPa/KPa]

200 289 152.592 156.617 0.0012 0.3080

300 389 205.392 162.905 0.0016 0.2288400 489 258.192 159.429 0.0020 0.1820


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Figure 1: Pressure ratio versus Mass flow rate at the throat

*Figure 1 show that the mass flow rate decreases with increasing of pressure ratio

because the pressure ratio represent the ratio of the bake pressure (a reversed pressureforce) to the inlet pressure so when it increase the resistance to flow increase and the

mass flow rate decreases.

*To focus a little in the collected data figure 2 shows the absolute pressure variation

with the distance along the nozzle.


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Figure 2: Position no. Versus Absolute probe pressure for each chest pressure

*Figure 2 shows a sudden drop in the probe pressure at position 11 which the throat

position when the area become small so the pressure decreases.

After that the decreases in pressure become very slow because the area become

constant in the convergent-parallel nozzle, then it sudden go to zero gage because the

probe go out the nozzle to the back pressure.

Summary and conclusion

This experiment show that the pressure decreases slowly after the throat and the mass

flow rate is the most effected by the pressure difference, this can help to select suitable

nozzle for specific function such control mass flow rate or velocity in awater jet, jet

engine etc.

Sources of errors

1. Human errors

In reading the Pressure Gauges whether in setting chest pressure or in recording

probe pressure.

2. Equipments errors

Like uncertainty in gauges and ability of leakage and other sources.

These errors can be decreased by using more precise equipments and more

careful in the work.

ReferencesThermodynamics Lab manual: by the Mechanical Engineering Department in theUniversity of Jordan.

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