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

Substituting the known quantities, the exit temperature of the air is deter-

mined to be

Discussion Note that heat loss from the duct reduces the exit temperature

of air.

5–5 ■ ENERGY ANALYSIS OF UNSTEADY-FLOW

PROCESSES

During a steady-flow process, no changes occur within the control volume;

thus, one does not need to be concerned about what is going on within the

boundaries. Not having to worry about any changes within the control vol-

ume with time greatly simplifies the analysis.

Many processes of interest, however, involve changeswithin the control

volume with time. Such processes are called unsteady-flow,or transient-

flow,processes. The steady-flow relations developed earlier are obviously

not applicable to these processes. When an unsteady-flow process is ana-

lyzed, it is important to keep track of the mass and energy contents of the

control volume as well as the energy interactions across the boundary.

Some familiar unsteady-flow processes are the charging of rigid vessels

from supply lines (Fig. 5–43), discharging a fluid from a pressurized vessel,

driving a gas turbine with pressurized air stored in a large container, inflat-

ing tires or balloons, and even cooking with an ordinary pressure cooker.

Unlike steady-flow processes, unsteady-flow processes start and end over

some finite time period instead of continuing indefinitely. Therefore in this

section, we deal with changes that occur over some time interval tinstead

of with the rate of changes (changes per unit time). An unsteady-flow sys-

tem, in some respects, is similar to a closed system, except that the mass

within the system boundaries does not remain constant during a process.

Another difference between steady- and unsteady-flow systems is that

steady-flow systems are fixed in space, size, and shape. Unsteady-flow

systems, however, are not (Fig. 5–44). They are usually stationary; that is,

they are fixed in space, but they may involve moving boundaries and thus

boundary work.

The mass balancefor any system undergoing any process can be expressed

as (seeSec. 5–1)

(5–42)

where msystemmfinal
minitialis the change in the mass of the system.

For control volumes, it can also be expressed more explicitly as

(5–43)

where iinlet,eexit, 1 initial state, and 2 final state of the control

volume. Often one or more terms in the equation above are zero. For exam-

mi
me 1 m 2 
m 12 CV

min
mout¢msystem¬¬ 1 kg 2

T 2 21.9°C

1 15 kJ/s 2
1 0.2 kJ/s 2  1 3 kg/s 21 1.005 kJ/kg#°C 21 T 2
172 °C

246 | Thermodynamics

Supply line

Control

volume

CV boundary

FIGURE 5–43

Charging of a rigid tank from a supply
line is an unsteady-flow process since
it involves changes within the control
volume.

Control

volume

CV boundary

FIGURE 5–44

The shape and size of a control
volume may change during an
unsteady-flow process.

SEE TUTORIAL CH. 5, SEC. 5 ON THE DVD.

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