Thermo Chapter 5 - Second Law of Thermodynamicsmazlan/?download=Thermo Chapter 5...1 CHAPTER...

CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER 55555555

FACULTY OF MECHANICAL ENGINEERINGUNIVERSITI TEKNOLOGI MALAYSIASKUDAI, JOHO, MALAYSIA THERMODYNAMICS SECOND LAW OF THERMOSECOND LAW OF THERMOSECOND LAW OF THERMOSECOND LAW OF THERMOSECOND LAW OF THERMOSECOND LAW OF THERMOSECOND LAW OF THERMOSECOND LAW OF THERMO

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CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER 55555555Reversible process = “a process that can be reversed withoutleaving any trace on the surroundings”. That is, both system andthe surroundings are returned to their initial states at the end ofthe reverse process.

Irreversible process= processesthat are not reversible !Irreversible process= processesthat are not reversible !

In the real world, there is no reversible process. We can onlyapproximate actual processes as reversible processes, but theycan never be achieved.

•When processesare reversible, work-producing devices (car

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•When processesare reversible, work-producing devices (carengines, gas/steam turbines) deliverthe most work.

When processes are reversible, work-consuming devices(compressors, fans, and pumps) consumethe least work.

CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER 55555555Irreversibilities = factors that cause a process to be irreversible.

The factors include friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical electric resistance, inelastic deformation of solids, and chemical reactions. A reversible processinvolves noneof these !

•Friction –

When two bodies in contact are forced to move relative to each

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When two bodies in contact are forced to move relative to each other (e.g. a piston in a cylinder), some work is converted to heat due to friction. To restore the initial state, all this heat must be converted back to work – which violates the second law !

•Unrestrained expansion of a gas –

Considera gasseparatedfrom a vacuum by a membraneConsidera gasseparatedfrom a vacuum by a membraneinside a tank. When the membrane is ruptured, the gasfills the whole tank. To restore the system to its originalstate, the gas is compressed to its initial volume, whiletransferring heat from the gas until it reaches its initialtemperature. The heat transferred from the gas is equalto the work done on the gas by the surroundings. To

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restore the surroundings requires converting this heatcompletely to work – a clear violation of the second law !

•Heat transfer through a finite temperature difference –

Consider a cold can of soft drink left in a warm room.Heat is transferred from the room air to the cooler softHeat is transferred from the room air to the cooler softdrink, raising its temperature. The soft drink can berestored to its initial temperature by refrigeration, whichrequires some work input. The refrigeration systemrejects excess heat (equal in magnitude to work supplied)to the room air. To restore the air to its initial state, thisexcessheat must be completely converted to back work –

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excessheat must be completely converted to back work –also a violation of the second law !

Internally and Externally Reversible Processes

•A process in internally reversible if no irreversibilities occurwithin the boundariesof a systemduring a process. A quasi-within the boundariesof a systemduring a process. A quasi-equilibrium process in an internally reversible process

•A process is externally reversible if no irreversibilities occuroutside the system boundaries during a process.

•A processis totally reversible(or reversible),if it involves no

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•A processis totally reversible(or reversible),if it involves noirreversibilities within the system or its surroundings.

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CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER 55555555The Carnot Cycle

A Carnot cycle is a totally reversible cycle, proposed by SadiCarnot in 1824. The theoretical heatengine operating on theCarnot cycle is called the Carnot heat engine. The Carnot cycleis composed of four reversible processes – 2 isothermal and 2adiabatic processes.adiabatic processes.

The Carnot cycle is the most efficient cycle (maximumefficiency) operating between two specified temperature limits.By reversing the processes in the Carnot cycle, the reversedCarnot cycle is obtained. The reversed Carnot cycle is anexampleof a theoretical refrigeration cycle with the maximum

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exampleof a theoretical refrigeration cycle with the maximumCOP.The Carnot and reversed Carnot cycles can be executed eitherin a closedor a steady-flowsystem.

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Thermo Chapter 5 - Second Law of Thermodynamicsmazlan/?download=Thermo Chapter 5...1 CHAPTER...

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