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quiz_04_solution - EML 5104 Classical Thermodynamics,...

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Unformatted text preview: EML 5104 Classical Thermodynamics, Spring 2010 Quiz 4, 5 Feb 2010 Name (Print): ___________________________________ UF ­ID: ___________________________________ Please sign the honor pledge: "On my honor, I have neither given nor received unauthorized aid in doing this assignment." _________________________________ Signature Only mark one of the multiple choice answers Q1: Exergy and the exergy equation are a consequence of (1.5 P) a) the first law of thermodynamics and the ideal gas law combined b) the second law of thermodynamics and the Guy ­Stodola theorem c) Newton’s second law of motion d) the first and the second law of thermodynamics combined Q2: Specific methalpy h0 is a shorthand notation for enthalpy plus macroscopic forms of energy. Neglecting non ­mechanical forms of energy it is given by (1.5 P): a) h + v2/2 + gz b) e + P v c) u + P v + v2/2 + gz d) All of the above Q3: Mark the definition of non ­flow availability (1.5 P) a) E  ­ T0S+P0 V b) U + m V2/2+m g z c) n R T d) R m ln(p2/p1) Q4: Mark the definition of specific flow ­availability (note: there is a difference between b) and c)) (1.5 P) a) u + pv b) h0  ­ T0s c) h0  ­ T0s d) none of the above Q5: Available (useful) work EW is (1.5 P): a) b) c) d) (E ) (E ) W rev − T0 Sgen − Wlost W rev dV W − P0 dt All of the above Q6: Give the mathematical relation called Guy ­Stodola theorem, also called lost work theorem. It relates loss of available work and entropy generation. Explain each term of the equation (2 P). Wlost = T0 Sgen Turn page! EML 5104 Classical Thermodynamics, Spring 2010 Q6: Below the mathematical law governing available work for open, non ­steady systems is given. Express the terms marked with curly braces in terms of heat, work, pressure, temperature, entropy, methalpy, etc. Use appropriate symbols for intensive and extensive properties (2.5). dV W − P0 dt Ew d =− A dt E + P0V − T0 S + ∑ i =1 N ⎛ T0 ⎞ ⎜ 1− T ⎟ Qi ⎝ ⎠ i EQ , i + ∑m in h − T0 S 0 b − ∑m out h − T0 S 0 b − T0 Sgen Q7: Qualitatively, draw the curves for the 1st law efficiency of a heat engine, the COP of a refrigeration process, and the COP of a heat pump (4 P) See Fig. 3.11 in Bejan’s book Q8: Heat flow across a finite temperature difference is an important source of irreversibilities. In the sketch below, steady state heat flow across a layer of material is indicated. Start with the equation given in Q6. Remove all irrelevant terms. Assume Q1 , T1, and T0 are known. Knowing that no work is done, give an expression for the entropy generation rate (4 P). Turn page! EML 5104 Classical Thermodynamics, Spring 2010 dV W − P0 dt N Ew N d =− A + ∑ EQ , i + ∑ m b − ∑ m b − T0 Sgen dt E + P0V − T0 S i =1 0 0 in out h − T0 S h − T0 S ⎛ T0 ⎞ ⎜ 1− T ⎟ Qi ⎝ ⎠ i ⎛ T⎞ 0 = ∑ ⎜ 1 − 0 ⎟ Qi −T0 Sgen Ti ⎠ i =1 ⎝ Sgen ⎛ T0 ⎞ ⎜1 − T ⎟ Q ⎝ i⎠ = T0 Points: _______/ 20 P Turn page! ...
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