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ME200 - Lecture 210

# ME200 - Lecture 210 - Thermodynamics,Reversibleand ME200...

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10/16/2010 1 Introduction to the Second Law of Thermodynamics, Reversible and Irreversible Processes ME 200 Thermodynamics I L 21 Purdue University , Dr. Tim Pourpoint – [email protected] Lecture 21 October 15 th , 2010 • Applications of First Law of Thermodynamics – Nozzles – Diffusers – Turbines – Compressors Th ttli D i Last Lecture – Throttling Devices – Mixing Chambers – Heat Exchangers TODAY: Second Law of Thermodynamics ME 200 2

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10/16/2010 2 This Lecture • Second Law of Thermodynamics – Direction of Thermodynamic Processes – Clausius Statement of the Second Law – Thermal Energy Reservoir Thermal Energy Reservoir – Kelvin – Planck Statement – Entropy Statement – Reversible and Irreversible Processes ME 200 3 • First law of thermodynamics – states conservation of energy principle when transforming one form of energy into other – considers only change in energy (or quantity) in such Why Second Law? considers only change in energy (or quantity) in such transformations • Second law of thermodynamics – establishes direction/possibility of processes – provides means for measuring the quality of energy ME 200 4 provides means for measuring the quality of energy – determines theoretical limits on the performance of engineering devices
10/16/2010 3 Second Law: Direction (a) Spontaneous heat transfer (b) Spontaneous expansion (c) Falling mass Can the reverse process occur spontaneously without any change in ME 200 5 surroundings? T = 300 K Consider hot iron block dropped into a tank of room temperature water: State 1 Second Law: Direction T = 800 K T = 280 K T = 320 K State 2a State 2b ME 200 6 T = 900 K T = 700 K

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10/16/2010 4 T = 300 K Both states 2a and 2b are allowed by the First Law. State 1 The Second Law eliminates State 2a as impossible due to Second Law: Direction T = 800 K T = 280 K T = 320 K State 2a State 2b required increase in entropy. ME 200 7 T = 900 K T = 700 K Consider rigid tank of high pressure, high temperature air in large sealed room. The tank has a valve that is closed initially. Second Law: Quality of Energy ME 200 8
10/16/2010 5 The valve is opened and the high temperature, high pressure gas escapes to the room. The whole system now reaches equilibrium at temperature and pressure slightly above room Second Law: Quality of Energy values.

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