Chapter_7_2

A engel m a boles 7 72 work work is entropy free and

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Unformatted text preview: ction of heat transfer since the absolute temperature T is always a positive quantity. When the temperature is not constant, the entropy transfer for process 1-2 can be determined by integration (or by summation if appropriate) as Sheat = z 2 1 δQ Qk ≅∑ T Tk Chapter 7 - 71 Student Study Guide for 5th edition of Thermodynamics by Y. A. Çengel & M. A. Boles 7-72 Work Work is entropy-free, and no entropy is transferred by work. Energy is transferred by both work and heat, whereas entropy is transferred only by heat and mass. Entropy transfer by work : Mass flow S work = 0 Mass contains entropy as well as energy, and the entropy and energy contents of a system are proportional to the mass. When a mass in the amount m enters or leaves a system, entropy in the amount of ms enters or leaves, where s is the specific entropy of the mass. Entropy transfer by mass: Smass = ms Entropy Generation, Sgen Irreversibilities such as friction, mixing, chemical reactions, heat transfer through a finite temperature difference, unrestrained expansion, non-quasiequilibrium expansion, or compression always cause the entropy of a system to increase, and entropy generation is a measure of the entropy created by such effects during a process. Chapter 7 - 72 Student Study Guide for 5th edition of Thermodynamics by Y. A. Çengel & M. A. Boles 7-73 For a reversible process, the entropy generation is zero and the entropy change of a system is equal to the entropy transfer. The entropy transfer by heat is zero for an adiabatic system and the entropy transfer by mass is zero for a closed system. The entropy balance for any system undergoing any process can be expressed in the general form as Sin − Sout + S gen = ∆Ssystem Net entropy transfer by heat and mass Entropy generation Change in entropy 3 ( kJ / K ) The entropy balance for any system undergoing any process can be expressed in the general rate form, as Sin − Sout Rate of net entropy transfer by heat and mass + S gen Rate of entropy generation 3 = ∆S system Rate of change of entropy ( kW / K ) where the rates of entropy transfer by heat transferred at a rate of mass flowing at a rate of m are Sheat = Q / T and Smass = ms . Q and The entropy balance can also be expressed on a unit-mass basis as ( sin − sout ) + sgen = ∆ssystem ( kJ / kg ⋅ K ) The term Sgen is the entropy generation within the system boundary only, and not the entropy generation that may occur outside the system boundary during the process as a result of external irreversibilities. Sgen = 0 for the internally reversible process, but not necessarily zero for the totally reversible process. The total entropy generated during any process is Chapter 7 - 73 Student Study Guide for 5th edition of Thermodynamics by Y. A. Çengel & M. A. Boles 7-74 obtained by applying the entropy balance to an Isolated System that contains the system itself and its immediate surroundings. ∆ Closed Systems Taking the positive direction of heat transfer to the system to be positive, the general entropy balance for the closed system is Qk ∑ T + S gen = ∆Ssystem = S2 − S1 k For an adiabatic process (Q = 0), this reduces to ( kJ / K ) Adiabatic closed system: S gen = ∆Sadiabatic system Chapter 7 - 74 Student Study Guide for 5th edition of Thermodynamics by Y. A. Çengel & M. A. Boles 7-75...
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This note was uploaded on 09/17/2009 for the course MAE 301 taught by Professor Hassan during the Fall '08 term at N.C. State.

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