ENGR 2213 Thermodynamics

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ENGR 2213 Thermodynamics

• F. C. Lai
• School of Aerospace and Mechanical
• Engineering
• University of Oklahoma

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First Law of Thermodynamics
Energy can be neither created or destroyed it
can only change forms.
Conservation of Energy Principle
Closed Systems
Change in the total energy of the system
Net energy transferred in - by heat
Net energy transferred out by work
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First Law of Thermodynamics
E total energy includes kinetic energy,
potential energy and other forms of energy
All other forms of energy are lumped together
as the internal energy U.
Internal energy U is an extensive
property. Specific internal energy u U/m
is an intensive
property
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Internal Energy
? Sensible Energy
It is associated with the kinetic energies of
the molecules
- Translational kinetic energy
- Rotational kinetic energy
- Vibrational kinetic energy
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Internal Energy
? Latent Energy
It is associated with the phase change of a
system. It is the energy used to overcome the
intermolecular forces.
? Chemical (Bond) Energy
It is associated with the atomic bonds of a
molecule
? Nuclear Energy
It is associated with the bonds within the
nucleus of an atom.
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Internal Energy
For saturated mixtures
For compressed liquid
based on the given temperature
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Example 1
A piston-cylinder assembly contains 8 kg of
superheated water vapor at 500 kPa and 300 ºC.
Steam is now cooled at constant pressure until
70 of it, by mass, condenses. Find the work
associated with this process.
A piston-cylinder assembly contains 8 kg of
superheated water vapor at 500 kPa and 300 ºC.
Steam is now cooled at constant pressure until
70 of it, by mass, condenses. Find the work
associated with this process.
p
1
2
W
v
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Example 2
An insulated rigid tank contains saturated water
vapor at 100 ºC. The water is rapidly stirred
until the pressure becomes 200 kPa. Find the
water temperature at the end of the process and
work associated with this process.
An insulated rigid tank contains saturated water
vapor at 100 ºC. The water is rapidly stirred
until the pressure becomes 200 kPa. Find the
water temperature at the end of the process and
work associated with this process.
p
2
1
v
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Example 3
Steam in a radiator (V 20 L) is at P1 300 kPa
and T1 250 ºC. If the valves are closed now,
determine the amount of heat that will transfer
to the room when the pressure drops to 100 kPa.
Steam in a radiator (V 20 L) is at P1 300 kPa
and T1 250 ºC. If the valves are closed now,
determine the amount of heat that will transfer
to the room when the pressure drops to 100 kPa.
p
1
Q
2
v
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Example 4
Water contained in a piston-cylinder
assembly undergoes two processes in series.
Initially, water is at 1 MPa and 400 ºC. 1 ? 2
water is cooled as it is compressed at
constant pressure to saturated vapor at p2
1 MPa. 2 ? 3 water is cooled at
constant volume to 150 ºC. Find heat and work
involved in these processes.
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Example 4 (continued)
p
1
2
3
W12
v
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Example 5
A water mixture with an initial quality of 25 is
contained in a piston-cylinder assembly. The
mass of the piston is 40 kg, and its diameter is
10 cm. The atmospheric pressure is 100 kPa. As
the water is heated, the pressure inside the
cylinder remains constant until the piston hits
the stops. Heat transfer to the water continues
until the pressure becomes 300 kPa. Neglect the
friction between the piston and cylinder wall.
Determine the total amount of heat transfer.
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Example 5 (continued)
p
3
2
1
4 cm
W12
1 cm
v
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Example 6
Two rigid tanks are connected by a valve. Tank A
contains 0.2 m3 of water at 400 kPa and 80
quality. Tank B contains 0.5 m3 of water at 200
kPa and 250 ºC. The valve is now opened, and
the two tanks eventually come to the same state.
Determine the pressure and the amount of heat
transfer when the system reaches thermal
equilibrium with the surroundings at 25 ºC.
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Example 6 (continued)
Tank B
Tank A
pB1 200 kPa TB1 250 ºC
pA1 400 kPa xA1 0.8
Q