REALprojek-BACK PRESSURE

8/7/2019 REALprojek-BACK PRESSURE

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PRESSURE DISTRIBUTION IN

CONVERGING-DIVERGING NOZZLE

OBJECTIVE: to observe the pressure

distribution in the

converging-divergingnozzle,

to observe the backpressure and the effect of

it. To investigate pressure

distribution and mass flowrate in nozzles.



PRESSURE DISTRIBUTION INCONVERGING-DIVERGINGNOZZLE

The usual configuration for a converging diverging(CD) nozzle is shown in the figure. Gas flows throughthe nozzle from a region of high pressure (usuallyreferred to as the chamber) to one of low pressure(referred to as the ambient or tank). The chamber isusually big enough so that any flow velocities here

are negligible. The pressure here is denoted by thesymbol pc. Gas flows from the chamber into theconverging portion of the nozzle, past the throat,through the diverging portion and then exhausts intothe ambient as a jet. The pressure of the ambient isreferred to as the 'back pressure' and given thesymbol pb.



RESULT Po - Inlet Pressure Pb - Back Pressure



continue by asnorizam…



BACK PRESSURE EXPERIMENT



OBJECTIVE

The aim of the experiment is to observethe pressure distribution in the

converging-diverging nozzle with three

different back pressure. The results andthe graphs plotted will be compared withthe theory. The experiment was beenperformed in under design,underexpended and over expendedconditions.



CONCEPT IN BACK PRESSURE



In this section we will explain the flow

characterictics and mechanisms throughout theconvergent-divergent nozzle that were used in

this experiment. First of all we assumed that

the nozzle is divided into three parts; the

chamber (inlet), throat, and the ambient (exit).



Figure shows the flow through the nozzle when

it is completely subsonic.

The flow accelerates out of the chamber.

Its maximum (subsonic) speed at the throat.

The flow then decelerates through the divergingsection and exhausts into the ambient as a

subsonic jet.

Lowering the back pressure in this stateincreases the flow speed everywhere in the

nozzle.



Lower it far enough and we eventually get to

the situation shown in figure 3.

The flow pattern is exactly the same as in

subsonic flow

Except that the flow speed at the throat has just

reached Mach 1.

Flow through the nozzle is now choked since

further reductions in the back pressure can't

move the point of M=1 away from the throat.

However, the flow pattern in the diverging

section does change as you lower the back

pressure further.



A b i l d th fl i f i fl



As pb is lowered the flow a region of supersonic flowforms just downstream of the throat.

Unlike a subsonic flow, the supersonic flow

accelerates as the area gets bigger.This region of supersonic acceleration is terminated by

a normal shock wave.The shock wave produces a near-instantaneous

deceleration of the flow to subsonic speed.This subsonic flow then decelerates through the

remainder of the diverging section and exhausts as asubsonic jet.

In this regime if we lower or raise the back pressurewe increase or decrease the length of supersonic flowin the diverging section before the shock wave.



If we lower pb enough we can extend the

supersonic region all the way down the nozzle

until the shock is sitting at the nozzle exit

(figure 5).

Because we have a very long region of

acceleration (the entire nozzle length) in this

case the flow speed just before the shock willbe very large in this case.

However, after the shock the flow in the jet will

still be subsonic.



L i th b k f th th h k



Lowering the back pressure further causes the shock

to bend out into the jet.

A complex pattern of shocks and reflections is set up

in the jet which will now involve a mixture of subsonicand supersonic flow.

Or (if the back pressure is low enough) just supersonic

flow.

Because the shock is no longer perpendicular to the

flow near the nozzle walls, it deflects it inward as it

leaves the exit producing an initially contracting jet.

We refer to this as overexpanded flow because in thiscase the pressure at the nozzle exit is lower than that

in the ambient (the back pressure)- i.e. the flow has

been expanded by the nozzle to much.



A f th l i f th b k



A further lowering of the back pressure

changes and weakens the wave pattern in the

jet.

Eventually we will have lowered the back

pressure enough so that it is now equal to the

pressure at the nozzle exit.

In this case, the waves in the jet disappear

altogether

The jet will be uniformly supersonic.

This situation, since it is often desirable, is

referred to as the 'design condition'.



Fi ll if l th b k



Finally, if we lower the back pressure even

further we will create a new imbalance between

the exit and back pressures.

Exit pressure greater than back pressure

In this situation (called 'underexpanded')

what we call expansion waves (that producegradual turning and acceleration in the jet) form

at the nozzle exit.

Initially turning the flow at the jet edges outward

in a plume and setting up a different type of

complex wave pattern.



Po

Pb

P1

P2

P3

P4

P5

P6

P7

P8

Air FlowRate

(kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (gs-1)

400 400 400 380 380 380 380 390 390 380 2

400 300 390 180 240 260 270 280 290 280 3.6

400 200 400 180 160 120 160 160 180 180 3.6

400 100 380 180 60 20 0 60 70 80 3.6

400 0 380 180 60 20 0 -30 -40 -60 3.6

Po

Pb/ P

oP

1/ P

oP

2/ P

oP

3/ P

oP

4/ P

oP

5/ P

oP

6/ P

oP

7/ P

oP

8/ P

o

(kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2)

400 1 1 0.95 0.95 0.95 0.95 0.975 0.975 0.95

400 0.75 0.975 0.45 0.6 0.65 0.675 0.7 0.725 0.7

400 0.5 1 0.45 0.4 0.3 0.4 0.4 0.45 0.45

400 0.25 0.95 0.45 0.15 0.05 0 0.15 0.175 0.2

400 0 0.95 0.45 0.15 0.05 0 -0.075 -0.1 -0.15



1 2 3 4 5 6 7 8-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Position Of Pressure Gauges Along Nozzle

Px/P

in

Effect Of Overall Pressure Ratio On

The "Pressure Profile" In Nozzle

Inlet Pressure = 400 kN/m

Inlet Temperature = 27Pb/Pi = 1.0

Pb/Pi = 0.75

Pb/Pi = 0.5

Pb/Pi = 0.25

Pb/Pi = 0.0

Critical pressure

ratio

Throat

Over

expe

ndi n

g

Designc

ondition

Under expended



Po

Pb

P1

P2

P3

P4

P5

P6

P7

P8

Air FlowRate


500 500 510 480 490 480 480 490 500 480 1.9

500 400 480 280 340 350 360 380 390 340 4.2

500 300 480 230 130 220 240 260 270 280 4.3

500 200 480 230 100 40 120 150 160 170 4.3

500 100 480 230 100 40 20 -20 40 60 4.3

500 0 480 230 100 50 20 -20 -30 -40 4.3

Po

Pb/ P

oP

1/ P

oP

2/ P

oP

3/ P

oP

4/ P

oP

5/ P

oP

6/ P

oP

7/ P

oP

8/ P

o


500 1 1.02 0.96 0.98 0.96 0.96 0.98 1 0.96

500 0.8 0.96 0.56 0.68 0.7 0.72 0.76 0.78 0.68500 0.6 0.96 0.46 0.26 0.44 0.48 0.52 0.54 0.56

500 0.4 0.96 0.46 0.2 0.08 0.24 0.3 0.32 0.34

500 0.2 0.96 0.46 0.2 0.08 0.04 -0.04 0.08 0.12

500 0 0.96 0.46 0.2 0.1 0.04 -0.04 -0.06 -0.08



1 2 3 4 5 6 7 8-0.2

0

0.2

0.4

0.6

0.8

1

1.2


Px/P

in




Inlet Temperature = 27

Px/Pin = 1.0

Px/Pin = 0.8

Px/Pin = 0.6

Px/Pin = 0.4

Px/Pin = 0.2

Px/Pin = 0.0

Critical pressure

ratio

Throat

Over

expe

ndi n

g

Designc

ondition

Under expended



Po

Pb

P1

P2

P3

P4

P5

P6

P7

P8

Air FlowRate


600 600 610 600 600 590 598 600 605 590 1.0600 500 580 380 430 440 460 480 480 480 5.79

600 400 570 280 280 320 340 360 370 380 5

600 300 570 280 130 150 220 240 260 270 5

600 200 570 280 130 70 70 140 150 160 5.1

600 100 570 280 130 60 20 0 0 50 5.1

600 0 570 280 130 70 20 0 0 -40 5

Po

Pb/ P

oP

1/ P

oP

2/ P

oP

3/ P

oP

4/ P

oP

5/ P

oP

6/ P

oP

7/ P

oP

8/ P

o


600 1.00 1.017 1.000 1.000 0.983 0.997 1.000 1.008 0.983

600 0.83 0.967 0.633 0.717 0.733 0.767 0.800 0.800 0.800600 0.67 0.950 0.467 0.467 0.533 0.567 0.600 0.617 0.633

600 0.50 0.950 0.467 0.217 0.250 0.367 0.400 0.433 0.450

600 0.33 0.950 0.467 0.217 0.117 0.117 0.233 0.250 0.267

600 0.17 0.950 0.467 0.217 0.100 0.033 0.000 0.000 0.083

600 0.00 0.950 0.467 0.217 0.117 0.033 0.000 0.000 -0.067



1 2 3 4 5 6 7 8-0.2

0

0.2

0.4

0.6

0.8

1

1.2


Px/P

in





Pb/Pi = 0.83

Pb/Pi = 0.67

Pb/Pi = 0.50

Pb/Pi = 0.33

Pb/Pi = 0.17

Pb/Pi = 0.00

Critical pressure

ratio

Throat

Over

expendi n

g

Designc

onditio

n

Under expended



a.) Subsonic flow

b.) Flow just choked

c.) Shock in nozzled.) Shock at exit

e.) Overexpanded

f.) Design condition

g.) Underexpanded



1 2 3 4 5 6 7-0.2

0

0.2

0.4

0.6

0.8

1

1.2


Px/Pin





Pb/Pi = 0.83

Pb/Pi = 0.67

Pb/Pi = 0.50

Pb/Pi = 0.33

Pb/Pi = 0.17

Pb/Pi = 0.00

Critical pressure

ratio

Throat

a.) Subsonic flow

b.) Flow just choked

c.) Shock in nozzle

d.) Shock at exit

e.) Overexpanded

f.) Design condition

g.) Underexpanded

COMPARISON WITH THEORY



CONCLUSONWe can see that our graphs obtained agrees with the theoretical

graphs in terms of several consitions :-

•Subsonic flow

•Flow just choked

•Shock in nozzle

•Shock at exit

•Overexpanded

•Design condition

•Unserexpanded

REALprojek-BACK PRESSURE

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