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PHY_10_7_V2_P_Adiabatic process.pptx
Adiabatic processes.
Adiabatic processes
Learning objectives
Two processes are key to designing an efficient heat engine:
isothermal processes
adiabatic processes
Two important processes
PV diagrams help us analyze these processes.
Two processes are key to designing an efficient heat engine:
isothermal processes
adiabatic processes
Two important processes
An isothermal process is one in which temperature is kept constant.
Isothermal processes
An isothermal process is one in which temperature is kept constant.
In isothermal processes, pressure and volume are inversely related:
If pressure doubles, volume is halved, and vice-versa.
Isothermal processes
While not quite achievable in real engines, isothermal heat transfer has the highest possible theoretical efficiency.
Isothermal processes
Adiabatic processes
An adiabatic process is one in which no heat is exchanged.
Adiabatic processes
An adiabatic process is one in which no heat is exchanged.
Examples of adiabatic processes:
A process that takes place in a perfectly insulated container.
An adiabatic process is one in which no heat is exchanged.
Examples of adiabatic processes:
A process that takes place in a perfectly insulated container.
A process that happens so quickly that there is not enough time for heat exchange.
Adiabatic processes
PV diagram curves for adiabatic processes differ from isothermal curves.
For adiabatic curves:
Adiabatic processes
where:
Why are adiabatic processes important?
Adiabatic processes are efficient.
No heat is exchanged, so adiabatic processes are reversible (ΔS = 0).
Reversibility is the “zero friction” limit of efficiency.
A refrigerator causes heat to flow “backwards”—from cold to hot.
How is this possible?
Doesn’t this violate the law of entropy?
How does a refrigerator work?
If you put a refrigerator in a closed room, and left the refrigerator door open, would the room warm up or cool down?
Let’s see.
What would happen?
A refrigerator causes heat to flow from cold to hot by using a reversed thermodynamic cycle.
How does a refrigerator work?
A refrigerator causes heat to flow from cold to hot by using a reversed thermodynamic cycle.
In a reversed cycle, work is done on the working fluid, instead of by the working fluid.
How does a refrigerator work?
When a fluid is compressed, its temperature increases even if no heat is exchanged.
The underlying principle
Compression
temperature increases
The underlying principle
When a fluid is compressed, its temperature increases even if no heat is exchanged.
When a fluid freely expands, its temperature decreases even if no heat is exchanged.
This is what allows refrigeration to work.
Expansion
temperature decreases
Reflection:
What has been learned
What remained unclear
What is necessary to work on
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