Download Thermodynamics Chapter 4

4
CHAPTER
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Thermodynamics
The First Law of
Thermodynamics:
Control Volumes
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4-1
Velocity Profiles for Flow in a Pipe
(fig. 4-6)
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4-2
Volume Flow Rate
Volume flow rate is the volume of fluid flowing through
a cross section per unit of time
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Thermodynamics
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4-3
Mass Flow, Heat, and Work
Affect Energy Content
The energy content of a control volume can be changed by mass flow as well
as heat and work interactions
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4-4
Control Volume May Involve
Boundary, Electrical, and Shaft Work
(Fig. 4-9)
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4-5
Schematic for Flow Work
(Fig. 4-10)
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Thermodynamics
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4-6
During Steady Flow Process, Volume
Flow Rates are not Necessarily Conserved
(Fig. 4-19)
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4-7
A Water Heater Under Steady
Operation
.
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(Fig. 4-21)
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Thermodynamics
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4-8
Steady-Flow Devices Operate
Steadily for Long Periods
(Fig. 4-25)
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4-9
Nozzle and Diffuser Shapes Cause
Large Changes in Fluid Velocities
Nozzles and Diffusers are shaped so that they cause large
changes in fluid velocities and thus kinetic energies
(Fig. 4-27)
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4-10
Schematic for Example 4-2
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4-11
Schematic for Example 4-4
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Thermodynamics
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4-12
Throttling Valve Devices Cause
Large Pressure Drops in Fluid
(Fig.4-32)
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4-13
Ideal Gas Temperature Does Not
Change During a Throttling
The temperature of an ideal gas does not change during
a throttling(h =constant) process since h = h (T)
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4-14
T-Elbow Serves as Mixing Chamber
for Hot and Cold Water Steams
The T-ebow of an ordinary shower serves as the mixing chamber
for hot- and cold-water streams.
(Fig. 4-35)
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Thermodynamics
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4-15
Heat Transfer Via Heat Exchanger
Depends on System Selection
The heat transfer associated with a heat exchanger may be zero or nonzero
depending on how the system is selected
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Thermodynamics
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4-16
Schematic for Example 4-9
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4-17
Rigid Tank Charging From a Supply
Line is an Unsteady-Flow Process
Charging of a rigid tank from a supply line is an unsteady-flow process
since it involves changes within the control volume
(Fig. 4-47)
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4-18
Temperature of Steam Rises Entering Tank,
Flow Energy Converts to Internal Energy
The Temperature of Steam rises from 300 to 456°C as it enters a tank as a
result of flow energy being converted to internal energy
(Fig. 4-54)
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4-19
Enthalpy of a Saturated Vapor
at a Given Pressure
In a pressure cooker, the enthalpy of the existing steam is Hg@P
(enthalpy of the saturated vapor at the given pressure)
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Chapter Summary
Thermodynamics
• A control volume differs from a closed system in
that it involves mass transfer. Mass carries energy
with it, and thus the mass and energy content of a
system change when mass enters or leaves.
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4-21
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Chapter Summary
• The mass and energy balances for any system
undergoing any process can be expressed as
Thermodynamics
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4-22
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Chapter Summary
Thermodynamics
• The mass and energy balances for any system
undergoing any process can be expressed in the
rate form as
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4-23
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Thermodynamics
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Chapter Summary
• Mass flow through a cross section per unit time is
.
called the mass flow rate and is denoted m. It
is expressed as
where

= density, kg/m3 (= 1/v)
= average fluid velocity normal to A, m/s
A
= cross-sectional area, m2
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4-24
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Chapter Summary
Thermodynamics
• The fluid volume flowing through a cross section
.
per unit time is called the volume flow rate V. It is
given by
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4-25
Chapter Summary
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Thermodynamics
• The mass and volume flow rates are related by
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4-26
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Chapter Summary
Thermodynamics
• Thermodynamic processes involving control
volumes can be considered in two groups: steadyflow processes and unsteady-flow processes.
During a steady-flow process, the fluid flows
through the control volume steadily, experiencing
no change with time at a fixed position. The mass
and energy content of the control volume remain
constant during a steady-flow process.
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4-27
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Thermodynamics
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Chapter Summary
• Taking heat transfer to the system and work done
by the system to be positive quantities, the
conservation of mass and energy equations for
steady-flow processes are expressed as
for each exit
for each inlet
where the subscript i stands for inlet and e for
exit. These are the most general forms of the
equations for steady-flow processes.
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Chapter Summary
• For single-stream (one-inlet--one-exit) systems
such as nozzles, diffusers, turbines, compressors,
and pumps, the steady flow equations simplify to
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Thermodynamics
In the above relations, subscripts 1 and 2 denote
the inlet and exit states, respectively.
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4-29
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Chapter Summary
Thermodynamics
• During a uniform-flow process, the state of the
control volume may change with time, but it may
do so uniformly. Also, the fluid properties at the
inlets and the exits are assumed to remain
constant during the entire process. The
conservation of energy equation for a uniformflow process reduces to
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4-30
Çengel
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Chapter Summary
Thermodynamics
• When the kinetic and potential energy changes
associated with the control volume and the fluid
streams are negligible, the conservation of energy
equation for a uniform-flow process simplifies to
Third Edition
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