ME200 - Lecture 34

ME200 - Lecture 34 - 11/16/2010 1 Ideal Rankine Cycle ME...

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Unformatted text preview: 11/16/2010 1 Ideal Rankine Cycle ME 200 Thermodynamics I Purdue University , Dr. Tim Pourpoint timothee@purdue.edu Lecture 33 November 17 th , 2010 Last Lecture: Heat transfer and work during a reversible, steady process Outline Difference between Pdv (related to closed system) and vdP (related to open system) work Strategies to minimize either work output in multi stage turbines (heating the working substance between successive stages) or work input in multi stage compressors (cooling the working substance between successive stages) Today: ME 200 ME 200 2 Ideal Rankine Cycle Next week: Exam #3 this evening from 8 to 9pm in WTHR 200 Covers lectures 1 to 32, emphasis on 2 nd law Solve SP32 BEFORE the exam , there will be an isentropic efficiency problem 11/16/2010 2 Vapor Power Cycle Goal: To generate electricity from heat input ME 200 ME 200 3 Heat Input Sources: Fossil fuels (oil, coal, natural gas) Nuclear fission Geothermal Continue Vapor Power Cycle Solar radiation Garbage/Trash Cycle Working Fluid: Water/Steam ME 200 ME 200 4 Environmental Issues: Air pollution Thermal pollution Waste products Safety 11/16/2010 3 Question: What is the upper limit of vapor power cycle performance? Answer: Continue Vapor Power Cycle 1 2: adiabatic, reversible compression 2 3: isothermal, reversible heat input 3 4: adiabatic, reversible expansion 4 1: isothermal, reversible heat rejection 2 3 T Q H = T S 23 W net net H L 3 H H 23 Note : W Q Q Q T d s T S T H ME 200 ME 200 5 1 4 s Q L = T S 14 H H 23 2 1 L L 4 1 4 Q T d s T S T L Continue Carnot Power Cycle System Schematic: hot reservoir @ T H T H = constant H Q T L = constant L Q in W out W reversible, isothermal reversible, isothermal reversible, adiabatic ME 200 ME 200 6 net H L L L 4 1 L th,C H H H H 23 H Thermal Efficiency : W Q Q Q T S T 1 1 1 Q Q Q T S T cold reservoir @ T L 11/16/2010 4 Example 36.1: Given: T H = 450 o F and T L = 212 o F Find: Thermal efficiency of Carnot Cycle Solution: th,C = 0.26 Continue Vapor Power Cycle th,C Question: How to implement the Carnot Power Cycle with a real working fluid? ME 200 ME 200 7 T L /T H 1 Answer: Put the cycle in the two phase dome Carnot power cycle T 2 3 T H s 1 4 s 1 =s 2 s 3 =s 4 H T L ME 200 ME 200 8 However, in real life we have to deal with deviations from Carnot: finite temperature difference across the heat exchanger difficult to compress 2 phase mixture non isentropic compression and expansion Introduce Rankine power cycle 11/16/2010 5 Ideal Rankine Power Cycle System Schematic and T s diagram: ME 200 ME 200 9 Define ideal Rankine power cycle: Neglect the finite temperature difference of heat exchanger...
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ME200 - Lecture 34 - 11/16/2010 1 Ideal Rankine Cycle ME...

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