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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 12 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 772 Work Work is entropyfree, 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, nonquasiequilibrium 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 773 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 unitmass 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 774 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 775...
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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.
 Fall '08
 Hassan

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