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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Lesson from First Law
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Second Law of Thermodynamics
The first law of thermodynamics requires that
energy be conserved during a process, but place
no restriction on the direction of a process.
Q
Q
Soda 4 ºC
Satisfying the first law does not guarantee that
a process will actually occur.
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Second Law of Thermodynamics
The inadequacy of the first law to identify
whether a process can take place is remedied by
introducing the second law of thermodynamics.
A process will not occur unless it satisfies both
the first and second laws of thermodynamics.
The second law asserts that
1. Processes occur in a certain direction.
2. Energy has quality as well as quantity.
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Second Law of Thermodynamics
Thermal Energy Reservoir
A hypothetical body with a relatively large
thermal energy capacity that can supply or absorb
finite amount of energy as heat without
undergoing any change in temperature.
Source
A reservoir that supplies energy in the form of
heat
Sink
A reservoir that absorbs energy in the form of
heat
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Second Law of Thermodynamics
Heat Engines
Devices that are used to convert heat to work.
Characteristics of Heat Engines
1. They receive heat from a high-temperature
source.
2. They convert part of this heat to work.
3. They reject the remaining waste heat to a
low- temperature sink.
4. They operate on a cycle.
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Second Law of Thermodynamics
High-temperature Reservoir at TH
QH
W
W QH QL
HE
QL
Low-temperature Reservoir at TL
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Second Law of Thermodynamics
Performance
Thermal Efficiency
lt 100
Automobile Engine 20 Diesel Engine
30 Gas Turbine 30 Steam Power
Plant 40
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Example 1
Heat is transferred to a heat engine from a
furnace at a rate of 80 MW. If the rate of waste
heat rejection to a nearby river is 50 MW,
determine the net power output and the thermal
efficiency.
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Second Law of Thermodynamics
Kelvin-Planck Statement
It is impossible for any device that operates on
a cycle to receive heat from a single reservoir
and produce an equivalent amount of work.
No heat engine can have a thermal efficiency of
100
The impossibility of having 100 efficiency heat
engine is not due to friction or other
dissipative effects.
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Second Law of Thermodynamics
Refrigerators/Heat Pumps
Devices that are used to transfer heat from
low- temperature medium to high-temperature one.
Like heat engines, they are cyclic devices.
Refrigerators and heat pumps operate on the
same cycle but differ in their objectives.
Refrigerators maintain the refrigerated space
at a low temperature.
Heat pumps maintain the heated space at a
high temperature.
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Second Law of Thermodynamics
High-temperature Reservoir at TH
QH
QL QH - W
W
Ref
QL
Objective
Low-temperature Reservoir at TL
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Second Law of Thermodynamics
Objective
High-temperature Reservoir at TH
QH
QH W QL
W
HP
QL
Low-temperature Reservoir at TL
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Second Law of Thermodynamics
Performance
Coefficient of Performance (COP)
Refrigerators
gt 1
Heat Pumps
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Second Law of Thermodynamics
Energy Efficient Rating (EER)
The amount of heat removed from the cooled space
in BTUs for 1 Watt-hour of electricity
consumed.
Air-conditioners
1 Wh 3.412 BTU
EER 3.412 COPR
Most air conditioners have an EER between 8 and
12.
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Example 2
The food compartment of a refrigerator is
maintained at 4 ºC by removing heat from it at a
rate of 360 kJ/min. If the required power input
is 2 kW, determine the COP and the rate of
heat discharged.
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Second Law of Thermodynamics
Clausius Statement
It is impossible to construct a device that
operates on a cycle and produce no effect other
than the transfer of heat from a low-temperature
body to a high-temperature body.
Equivalence of the two statements
A violation of one statement leads to the
violation of the other statement.
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Equivalence of the Two Statements
High-temperature Reservoir at TH
Net QOUT QL
QH QL
QH
Ref
HE
HE Ref
W QH
QL
Net QIN QL
Low-temperature Reservoir at TL
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Equivalence of the Two Statements
High-temperature Reservoir at TH
Net QIN QH - QL
QH
QL
HE Ref
HE
Ref
W QH QL
QL
QL
Net W QH - QL
Low-temperature Reservoir at TL