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Le22 - ME 200 Thermodynamics I Lecture 22 Thermodynamic...

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ME 200: Thermodynamics I Lecture 22: Thermodynamic cycles and the 2 nd Law Reading: Sections 5.5-5.7; SP22, 5.19, 5.20, 5.35 10/18/2010 1 ME200 Therm I Lecture 22, Prof. Mongia Professor Hukam Mongia Office Hours: MWF 9:30 to 10:30 AM in ME 83 Other times email for appointment; Phone: 765-494-5640 Course Website: https://engineering.purdue.edu/ME200/ Course Secretaries: Diana Akers (ME 84) and Marilyn Morrison (ME 100) Grader: Abhro Pal , [email protected]

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10/18/2010 2 This lecture covers application of the 2 nd Law to: Power cycles interacting with two reservoirs and Corollaries Refrigeration and Heat Pump cycles interacting with two reservoirs and Corollaries ME200 Therm I Lecture 22, Prof. Mongia
10/18/2010 3 The 1 st Law for Power Cycles: W = Q H -Q C h = W/ Q H = 1-Q C /Q H Can we let Q C =0? No, because of the Kelvin- Planck’s statement. Therefore, h <1.0 for all power cycles --one of corollaries of the 2 nd Law. Boundary W ME200 Therm I Lecture 22, Prof. Mongia

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10/18/2010 4 In deriving the relationship and h < 1.0, did we: Identify the nature of the substance contained within the system? NO. Specify the exact series of processes making up the cycle? NO. Indicate whether the processes are actual processes or somehow idealized? NO. ME200 Therm I Lecture 22, Prof. Mongia
10/18/2010 5 Two corollaries of the 2 nd Law for Power Cycles (Heat Engines) --- Carnot corollaries 1. The thermal efficiency of an irreversible power cycle is always less than the thermal efficiency of a reversible power cycle when each operates between the same two thermal reservoirs. 2. All reversible power cycles operating between the same two thermal reservoirs have the same thermal efficiency. - Independent of the processes - Independent of the working fluid ME200 Therm I Lecture 22, Prof. Mongia

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6 Low-temperature reservoir Rev. R #3 Irrev. HE #1 Q Cirrev. Q H W irrev Rev. HE #2 Q H Q Crev. W rev High-temperature reservoir Consider an irreversible heat engine, HE #1. 1 Cirrev irrev H Q Q Consider a reversible heat engine, HE #2. 1 Crev rev H Q Q Pr rev irrev ove . 2 ? irrev rev Will it lead toviolat Let us ionof ass the umet nd hat law Always remember : The 2 nd law is stated in terms of a single reservoir for heat engines. Therefore, reduce two-reservoir problem to a single-reservoir problem first. Learn to apply this trick. Proof for Corollary 1
10/18/2010 ME200 Therm I Lecture 22, Prof. Mongia 7 Let us try an approach: Q H1 +Q H2 =0; you should come up with another approach on your own.

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