Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

mass flow rate through the nozzle. However, it does influence the char-

acter of the flow in the diverging section.

3.When PCPbPE, the fluid that achieved a sonic velocity at the

throat continues accelerating to supersonic velocities in the diverging

section as the pressure decreases. This acceleration comes to a sudden

stop, however, as a normal shockdevelops at a section between the

throat and the exit plane, which causes a sudden drop in velocity to sub-

sonic levels and a sudden increase in pressure. The fluid then continues

to decelerate further in the remaining part of the converging–diverging

nozzle. Flow through the shock is highly irreversible, and thus it cannot

be approximated as isentropic. The normal shock moves downstream

away from the throat as Pbis decreased, and it approaches the nozzle

exit plane as Pbapproaches PE.

When Pb
PE, the normal shock forms at the exit plane of the noz-

zle. The flow is supersonic through the entire diverging section in this

case, and it can be approximated as isentropic. However, the fluid

velocity drops to subsonic levels just before leaving the nozzle as it

Chapter 17 | 843

0

x

Subsonic flow

at nozzle exit

(shock in nozzle)

Exit

P*

Supersonic flow

at nozzle exit

(no shock in nozzle)

PA

A

Ma

Inlet Throat

B

C

D

}

}

}

Subsonic flow

at nozzle exit

(no shock)

Pe

x

Vi ≅ 0

Pb

P 0

E, F,

Shockin nozzle G

Sonic flow

at throat

Inlet Throat Exit

0

1

Sonic flow

at throat

Shock

in nozzle

E, (^) F,
G
x
Subsonic flow
at nozzle exit
(shock in nozzle)
Supersonic flow
at nozzle exit
(no shock in nozzle)
A
B
C
D
}
}
Subsonic flow
at nozzle exit
(no shock)
P 0
P
Throat
}
PB
PC
PD
PE
PG
PF
Pb
FIGURE 17–27
The effects of back pressure on the
flow through a converging–diverging
nozzle.
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