ch 6b 2nd law
TRANSCRIPT
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6.4 .Reversible and Irreversible Processes
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Reversible Processes
The second law of thermodynamics state that no heat engine can have an efficiency of 100%.
Then one may ask, what is the highest efficiency that a heat engine can possibly have.
Before we answer this question, we need to define an idealized process first, which is called the reversible process.
The processes discussed earlier occurred in a certain direction. They can not reverse themselves irreversible processes.
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Reversible Processes A reversible process is defined as a process that
can be reversed without leaving any trace on either system or surroundings.
This is possible if the net of heat and net work exchange between the system and the surrounding is zero for the combined process (original and reverse).
Quasi-equilibrium expansion or compression of a gas
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Reversible processes actually do not occur in nature.
They are simply idealization of actual processes.
Reversible processes can never be achieved.
You may be wondering, then, why we are bothering with such fictitious processes:
1. Easy to analyze
2. Serve as idealized model
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Engineers are interested in reversible processes because:
when Reversible processes are approximated instead of the Actual ones
1. Work-producing devices such as car engine and gas or steam turbine deliver the most work, and
2. Work-consuming devices such as compressors, fan, and pumps consume the least work.
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Reversible processes can be viewed as theoretical limits for the corresponding not reversible ones.
We may never be able to have a reversible process, but we may certainly approach it.
The more closely we approximate a reversible process, the more work delivered by a work-producing device or the less work required by a work-consuming device.
Processes that are not reversible are called Irreversible processes.
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Reversible processes
Ideal processes
Irreversible processes
Actual processes
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6.5. Carnot Cycle
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Cycles that are composed of reversible processes will give the maximum net work and consumes the minimum work.
One of these cycles is the
Carnot Cycle. Named for French engineer Nicolas Sadi
Carnot (1769-1832) It is composed of four processes as
follows:
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Process 1-2: A reversible isothermal expansion
The gas is allowed to expand isothermally by receiving heat ( QH) from a hot reservoir.
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Process 2-3: A reversible adiabatic expansion
The cylinder now is insulated and the gas is allowed to expand adiabatically and thus doing work on the surrounding.The gas temperature decreases from TH to TL.
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Process 3-4: A reversible isothermal compression
The insulation is removed and the gas is compressed isothermally by rejecting heat (QL) to a cold reservoir.
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Process 4-1: A reversible adiabatic compression
The cylinder is insulated again and the gas is compressed adiabatically to state 1, raising its temperature from TL to TH
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Net work done by Carnot cycle is the area enclosed by all process
The Carnot cycle is the most efficient cycle operation between two specified temperatures limits.
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Carnot cycle can be executed in many different ways
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Reversed Carnot Cycle
Process 2-3: The gas expands isothermally at TL while receiving QL from the cold reservoir.Process 3-4: The gas is compressed adiabatically raising its temperature to TH.Process 4-1: The gas is compressed isothermally by rejecting QH to the hot reservoir.
Process 1-2: The gas expands adiabatically (throttling valve) reducing its temp from TH to TL.
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Reversed Carnot Cycle
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Carnot principles1. No heat engine is more
efficient than a reversible one operating between the same two reservoirs.
2. The thermal efficiencies of all reversible heat engines operating between the same two reservoirs are the same.
Low temperature reservoir at TL
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The Thermodynamic Temperature Scale
A temperature scale that is independent of the properties of the substances that are used to measure temperature is called a thermodynamic temperature scale.
That is the Kelvin scale, and the temperatures on this scale are called absolute temperatures.
L
H
revL
H
T
T
Q
Q
cycles reversible ForThe second Carnot principle state that the thermal efficiencies of all reversible heat engines operating between the same two reservoirs are the same.
hth, rev = f (TH,TL)
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Efficiency of a Carnot Engine For a reversible cycle the amount of heat
transferred is proportional to the temperature of the reservoir.
H
Lrev Q
Q1
H
L
T
T1
Only true for the reversible case
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COP of a Reversible Heat Pump and a Reversible Refrigerator
HLrevHP QQ
COP
1
1,
HL TT1
1
1
1,
LH
revR QQCOP
1
1
LH TT
Only true for the reversible case
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How do Reversible Carnot Heat Engine compare with real engines?
engine heat impossible
engine heat reversible
engine heat leirreversib
rev,th
rev,th
rev,th
th
thermal th
engine heat impossible
engine heat reversible
engine heat leirreversib
rev,th
rev,th
rev,th
th
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orrefrigerat impossible COP
orrefrigerat reversible COP
orrefrigerat leirreversib COP
COP
rev,R
rev,R
rev,R
R
1
1
L
HR
COP
1
1
L
Hrev,R
TT
,COP
COP of Carnot Refrigerator
How do Carnot Refrigerator compare with real Refrigerator?
COP of Refrigerator
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COP of Carnot Heat PumpCOP of real Heat Pump
H
LHP
COP
1
1
H
Lrev,HP
TT
COP
1
1
Pump Heat impossible COP
Pump Heat reversible COP
Pump Heat leirreversib COP
COP
rev,HP
rev,HP
rev,HP
HP
How do Carnot Heat Pump compare with real one?
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How to increase the efficiency of a real heat engine?
H
Lrev,th
H
Lth
T
T
Q
Q
1
1
1- Increase TH but you are limited with melting temperature of the engine material.
2- Decrease TL but you are limited with your environment.
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6.6 .Equivalence of Ideal-Gas and Thermodynamic
Temperature Scales
Lecture Notes
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Example (5-8): Heating a House by a Carnot Heat Pump
A heat pump is to be used to heat a house during the winter, as shown in the figure at right. The house is to be maintained at 21oC at all times. The house is estimated to be losing heat at a rate of 135,000 kJ/h when the outside temperature drops to -5oC. Determine the minimum power required to drive this heat pump.
Sol:
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Example (1)
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Example (2)