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

17–1 ■ STAGNATION PROPERTIES

When analyzing control volumes, we find it very convenient to combine the

internal energyand the flow energyof a fluid into a single term,enthalpy,

defined per unit mass as h
uPv. Whenever the kinetic and potential

energies of the fluid are negligible, as is often the case, the enthalpy repre-

sents the total energy of a fluid. For high-speed flows, such as those

encountered in jet engines (Fig. 17–1), the potential energy of the fluid is

still negligible, but the kinetic energy is not. In such cases, it is convenient

to combine the enthalpy and the kinetic energy of the fluid into a single

term called stagnation(or total) enthalpyh 0 , defined per unit mass as

(17–1)

When the potential energy of the fluid is negligible, the stagnation enthalpy

represents the total energy of a flowing fluid streamper unit mass. Thus it

simplifies the thermodynamic analysis of high-speed flows.

Throughout this chapter the ordinary enthalpy his referred to as the static

enthalpy, whenever necessary, to distinguish it from the stagnation

enthalpy. Notice that the stagnation enthalpy is a combination property of a

fluid, just like the static enthalpy, and these two enthalpies become identical

when the kinetic energy of the fluid is negligible.

Consider the steady flow of a fluid through a duct such as a nozzle, dif-

fuser, or some other flow passage where the flow takes place adiabatically

and with no shaft or electrical work, as shown in Fig. 17–2. Assuming the

fluid experiences little or no change in its elevation and its potential energy,

the energy balance relation (E

.

in
E

.

out) for this single-stream steady-flow

system reduces to

(17–2)

or

(17–3)

That is, in the absence of any heat and work interactions and any changes in

potential energy, the stagnation enthalpy of a fluid remains constant during

a steady-flow process. Flows through nozzles and diffusers usually satisfy

these conditions, and any increase in fluid velocity in these devices creates

an equivalent decrease in the static enthalpy of the fluid.

If the fluid were brought to a complete stop, then the velocity at state 2

would be zero and Eq. 17–2 would become

Thus the stagnation enthalpyrepresents the enthalpy of a fluid when it is

brought to rest adiabatically.

During a stagnation process, the kinetic energy of a fluid is converted to

enthalpy (internal energy flow energy), which results in an increase in the

fluid temperature and pressure (Fig. 17–3). The properties of a fluid at the

stagnation state are called stagnation properties(stagnation temperature,

h 1 

V^21

2

h 2 
h 02

h 01 
h 02

h 1 

V^21

2

h 2 

V^22

2

h 0 
h

V^2

2

¬¬ 1 kJ>kg 2

824 | Thermodynamics

FIGURE 17–1

Aircraft and jet engines involve high

speeds, and thus the kinetic energy

term should always be considered

when analyzing them.

(a) Photo courtesy of NASA,

http://lisar.larc.nasa.gov/IMAGES/SMALL/EL-

1999-00108.jpeg, and (b) Figure courtesy of Pratt

and Whitney. Used by permission.

Control

volume h

02 =

h 1

V 1 V 2

h 01 h 01

h 2

FIGURE 17–2

Steady flow of a fluid through an

adiabatic duct.

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