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

Chapter 17 | 869

Discussion Note that the temperature and velocity increase and pressure

decreases during this subsonic Rayleigh flow with heating, as expected. This

problem can also be solved using appropriate relations instead of tabulated

values, which can likewise be coded for convenient computer solutions.

17–7 ■ STEAM NOZZLES

We have seen in Chapter 3 that water vapor at moderate or high pressures

deviates considerably from ideal-gas behavior, and thus most of the rela-

tions developed in this chapter are not applicable to the flow of steam

through the nozzles or blade passages encountered in steam turbines. Given

that the steam properties such as enthalpy are functions of pressure as well

as temperature and that no simple property relations exist, an accurate analy-

sis of steam flow through the nozzles is no easy matter. Often it becomes

necessary to use steam tables, an h-sdiagram, or a computer program for

the properties of steam.

A further complication in the expansion of steam through nozzles occurs

as the steam expands into the saturation region, as shown in Fig. 17–59. As

the steam expands in the nozzle, its pressure and temperature drop, and

The maximum value of stagnation temperature T* 0 occurs at Ma 
1, and its

value can be determined from Table A–34 or from Eq. 17–65. At Ma 1 


0.1702 we read T 0 /T* 0 
 0.1291. Therefore,

The stagnation temperature ratio at the exit state and the Mach number cor-

responding to it are, from Table A–34,

The Rayleigh flow relations corresponding to the inlet and exit Mach num-

bers are (Table A–34):

Ma 2
0.3142:¬

T 2

T*

0.4389 ¬

P 2

P*

2.1086 ¬

V 2

V*

0.2082

Ma 1
0.1702:¬

T 1

T*

0.1541 ¬

P 1

P*

2.3065 ¬

V 1

V*

0.0668

T 02

T* 0

1598 K

4285 K

0.3729SMa 2 
0.3142

T* 0 

T 01

0.1291

553.2 K

0.1291

4285 K

Then the exit temperature, pressure, and velocity are determined to be

V 2

V 1

V 2 >V*

V 1 >V*

0.2082

0.0668

3.117 S V 2 
3.117V 1 
3.117 1 80 m>s 2 
249 m/s

P 2

P 1

P 2 /P*

P 1 /P*

2.1086

2.3065

0.9142 S P 2 
0.9142P 1 
0.9142 1 480 kPa 2 
439 kPa

T 2

T 1

T 2 >T*

T 1 >T*

0.4389

0.1541

2.848 S T 2 
2.848T 1 
2.848 1 550 K 2 
1566 K

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