ME200 - Lecture 23

ME200 - Lecture 23 - Temperaturescales ME200...

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10/18/2010 1 More on Thermodynamic Cycles Temperature scales ME 200 Thermodynamics I L 23 Purdue University , Dr. Tim Pourpoint – [email protected] Lecture 23 October 20 th , 2010 Up: Temperature distribution on the sun Right: Space Shuttle Main Engine with combustion temperature of ~3600 K Lecture Outline Last Lecture: More on the entropy statement of 2 nd law Heat Engines R f i d h Refrigerators and heat pumps Perpetual motion machines Carnot cycle This Lecture: Carnot principles ME 200 2 Thermodynamic temperature scale Carnot heat engine Carnot refrigerator and heat pump
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10/18/2010 2 • 2 nd law limits operation of cyclic devices as expressed by KP and Clausius statements – Heat engine cannot operate by exchanging heat with a single reservoir Carnot Principles – Refrigerator cannot operate without net work input from external source • Carnot principles – The efficiency of an irreversible heat engine (or COP of an irreversible refrigerator/heat pump) is always less than the efficiency of a reversible one operating between the same two ME 200 3 reservoirs – The efficiencies of all reversible heat engines (or COP of all refrigerators/heat pumps) operating between the same two reservoirs are the same irrespective of the working fluid Carnot principles illustrated High Temperature Reservoir at T H 1 Irrev. HE 2 Rev. HE 3 Rev. HE ME 200 4 Low Temperature Reservoir at T L th,1 < th,2 th,2 = th,3
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10/18/2010 3 Proof of first Carnot principle High Temperature Reservoir at T H Q H Q H Rev. HE (or R) Irrev. HE Q L, irrev < Q L,rev Q L,rev W rev Combined HE + R Q L, irrev Q L,rev W irrev W irrev W rev ME 200 5 Low Temperature Reservoir at T L Low Temperature Reservoir at T L A reversible and an irreversible heat engine operating between the same two reservoirs (the reversible heat engine is then reversed to run as a refrigerator) The equivalent combined system Second Carnot principle illustrated High Temperature Reservoir at T H =1000 K A reversible HE, th,A Another reversible HE, th,B ME 200 6 Low Temperature Reservoir at T L =300 K th,A = th,B = 70%
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10/18/2010 4 • Independent of properties of substances used to measure temperature • Recall 2 nd Carnot principle says all reversible heat engines (RHE ) h ffi i i b Thermodynamic temperature scale (RHEs) have same efficiency operating between same two reservoirs • So efficiency of RHEs is independent of working fluid, it properties, details of cycle and engine • Since energy reservoirs are characterized by their
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