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CHAPTER
4
The First Law of
Thermodynamics –
Steady flow systems
(steady means no
change with time)
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FIGURE 4-22
Under steady-flow
conditions, the mass and
energy contents of a
control volume remain
constant.
4-2
Steady flow systems
•
•
•
•
•
Wb = 0 (constant volume)
min = mout (rates also)
Ein = Eout (rates also)
W and Q are constant with time
Properties of mass in and out are generally
taken to be constant with time.
Steady State Systems
• Mass balance: min = mout
• Energy balance
–
Ein – Eout = ΔEsystem = 0


 



Vi 2
Ve2
Qin  Win   mi  hi 
 gzi   Qout  Wout   me  he 
 gze 
2
2







 



Ve2
Vi 2
Q  W   me  he 
 gze    mi  hi 
 gzi 
2
2







– or q – w = hout – hin – Δke - Δpe
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FIGURE 4-28
At very high
velocities, even
small changes in
velocities may
cause significant
changes in the
kinetic energy of the
fluid.
4-3
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FIGURE 4-30
Nozzles and diffusers
are shaped so that
they cause large
changes in fluid
velocities and thus
kinetic energies.
Q=0
W=0
ΔPE = 0
so
ΔKE = ΔH
4-4
Turbines, Compressors, Pumps and
Fans
•
•
•
•
Usually Q = 0
ΔPE ≈ 0
ΔKE ≈ 0
So w = Δh
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FIGURE 4-35
Throttling valves are
devices that cause
large pressure drops in
the fluid.
Often, temp drops.
h1 ≈ h2 so:
u1 + P1v1 ≈ u2 + P2v2
Therefore, u is usually
exchanged for Pv or
flow energy and T drops
Isenthalpic process.
4-5
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FIGURE 4-37
During a throttling process, the
enthalpy (flow energy + internal
energy) of a fluid remains
constant.
But internal and flow energies
may be
converted to each other.
How about an ideal gas? Will Temperature change?
4-6
Mixing Chambers
•
•
•
•
Usually no work or heat transfer
Usually, ke and pe are neglected
m1 + m2 = mout
m1h1 + m2h2 = mouthout
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FIGURE 4-42
The heat transfer associated with a
heat exchanger may be zero or
nonzero depending on how the
system is selected.
4-7
Kind of like a mixing chamber, but no mixing.