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en KC)
ای
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FIGURE 7-1 my
A system that is in
equilibrium with its
environment is said to
be at the dead state.
Ty = 25°C
Py = 101 kPa
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display.
FIGURE 7-3
The immediate
surroundings
ofahotpotato = =~——-———-~— ae
are simply the ee 55
temperature
gradient 2086
of the air next /
to the potato. /
POTATO
25°C
72 Environment
surroundings
صفحه 4:
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FIGURE 7-8 aisplay.
As a closed system
expands, some work
needs to be done to push
the atmospheric air out of
the way مغ اجب لسن الام
| \
: Atmospheric |
SYSTEM
SYSTEM Vy
۳"
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FIGURE 7-10 Sq
The difference Initial
between
reversible work state
and actual useful i Actual process
work is the 9
~
irreversibility. من < Wrev
Reversible Final state
process
W,
16۷
-W,
7 < ۷,
7-4 Trev
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displav.
one >15
FIGURE 7-15
Two heat
engines that
have the
same
thermal
efficiency,
but different
maximum
thermal
efficiencies.
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FIGURE 7-16 display.
Second-law
efficiency is a
measure of the
performance of a
device relative to
its performance
under reversible
conditions.
11
صفحه 8:
a2 Rbpytight’© The McGraw-Hill Companies, Inc. Permission requi
The exergy of displav.
a specified
mass ata
specified state
is the useful
work that can
be produced
as the mass
undergoes a
reversible
process to the
state of the
environment.
d for reproduction or
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or
FIGURE 7-21 displav.
The exergy of Atmosphere
a cold medium _ 4K6,
is alsoa Ty =25 6
positive
quantity since
work can be
produced by
transferring
heat to it. Work
output
Cold medium
=e
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‘display.
FIGURE 7-22 my ۱ ۱
The exergy of flow Imaginary piston
energy is the useful
work that would be (represents the
delivered by an
imaginary piston in
the flow section.
fluid downstream)
shaft
Atmospheric
air displaced
Pv = Pov + Wea
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Copyright © The McGraw-Hill Companies, Inc. Permission require
FIGURE 7-23 a
The energy and Energy:
exergy contents 1
of (a) a fixed mass onus عو Fixed
and (b) a fluid 1 mass
stream.
Exergy:
=U — up) + Pov — vy) — Tols 39) +
(a) A fixed mass (nonflowing)
Exergy: A
W=(h—hg) ~ Tos) + + gz
(b) A tluid stream (flowing)
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<2 28) tbpytighPo The McGraw-Hill Companies, Inc. Permission required for reproduction or
The Carnot display.
efficiency n, = 1 -
T, /T represents
the fraction of
the energy of a
heat 1
temperature T HEAT SOURCE
that can be
converted to work
enviroainGhbat Temperature: T
temperature T, .
Energy content: ۶
1
Exergy = ( - PE
10
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isplav.
MEDIUM I MEDIUM 2
Wall
qt
2
Heat
transfer
Entropy
generated
Entropy
transfer
Exergy
destroyed
Exergy
transfer
FIGURE 7-27
The transfer
and
destruction of
exergy during
a heat transfer
process
through a finite
temperature
difference.
صفحه 14:
FIGU Rapyrig8® The McGraw-Hill Companies, Inc. Permission required for reproduction or
Mass contains display.
energy,
entropy, and
exergy, and a
thus mass flow
into or out of a Control volume
system is
accompanied mh
by energy, .
entropy, and
exergy 5 ms
transfer. y
may
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FIGURE 7-30 display.
The isolated system
considered in the No heat, work
development of the yf 1
decrease of exergy / or mass transfer
principle.
Isolated system
AX isolated s0
(or x تن 0)
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or
FIGURE 7-32 display.
Mechanisms
of exergy
transfer fora
general
system.
2
1
تسم
Mass
Heat
Work
System
AX
system
Serre
صفحه 17:
CePysiagt,© The McGraw-Hill Companies, Inc. Permission required for reproduction or
FIGUR! displav.
Exergy balance for a X
closed system when work
the direction of heat {
transfer is taken to 2
be to the system
and the direction of
work from the
system.
destroyed
Q
رگ
7-16 میرک ~ 00006 = AX, system
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or
FIGURE 7-34 displav.
Exergy destroyed
outside system
boundaries can Outer تن 7
be accounted for di م 8 0
by writing an surroundings
exergy balance (environment), Immediate
on the extended 7 / | surroundings \
system that 9
includes the
system and its
immediate
surroundings.
صفحه 19:
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displav.
Surroundings
Control
volume.
X,
cv
Copyright © The McGraw-Hill Companies, Inc. Permission requi
FIGURE 7-42
Exergy is transferred
into or out of a control
volume by mass as
well as heat and work
transfer.
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display.
FIGURE 7-49
The irreversibility associated
h a student studying and
watching a movie on
television, each for two hours.
Mental Mental
alertness alertness:
_Naxiation of mental
alertness with time
Variation of mental
alertness with time
_-Wasted alertness
(irreversibility)
_- Wasted alertness
(irreversibility) a
۱
Alertness Alertness
required for 1 required for
studying Alertness Alertness studying
| required for required for
watching TV watching TV
0 5 3 + 5
0 2 4 Time (h) 0 2 4 Time (h)
7-19 (a) Studying first (b) Watching a movie first
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Thave only just a minute,
Only 60 seconds in it,
Forced upon me — can’t refuse it
Didn’t seek it, didn’t choose it.
But it is up to me to use it.
I must suffer if I lose it.
Give account if I abuse it,
Just a tiny littke minute —
But eternity is in it.
(anonymous)
FIGURE 7-!
A poetic
expression
of exergy
and exergy
destructior
7-20
