Thermodynamics

470 | ThermodynamicsExergytransferby work:X work e W W surrW1for boundary work21for other forms of work2whereX # heat 11 T 0 >T2Q #Exergytransfer X mass mcby mass:The exergy of an isolated system during a process alwaysdecreases or, in the limiting case of a reversible process,remains constant. This is known as the decrease of exergyprinciple and is expressed as¢X isolated 1X 2 X 1 2 isolated 0Exergy balance for any system undergoing any process canbe expressed asGeneral: X in X out X destroyed ¢X systemX # work W # usefulX # mass m # cFor a reversible process, the exergy destruction term X destroyeddrops out. Taking the positive direction of heat transfer to be tothe system and the positive direction of work transfer to befrom the system, the general exergy balance relations can beexpressed more explicitly asa a 1 T 0T kb Q k 3W P 0 1V 2 V 1 24 ainmc aoutmc X destroyed X 2 X 1Net exergy transfer Exergy Changeby heat, work, and mass destruction in exergyX # in X # outGeneral, X # destroyed dX system >dtrate form: Rate of net exergy transfer Rate of exergy Rate of changeby heat, work, and mass destruction in exergyGeneral,unit-mass basis:⎫ ⎪⎬⎪⎭⎫⎪⎬⎪⎭⎫⎪⎬⎪⎭⎫ ⎪⎬⎪⎭⎫ ⎪⎬⎪⎭⎫⎪⎬⎪⎭1x in x out 2 x destroyed ¢x systema a 1 T 0b Q # k a W # P dV CV0 bT k dt ainm # c aoutm # c X # destroyed dX CVdtREFERENCES AND SUGGESTED READINGS1. J. E. Ahern. The Exergy Method of Energy SystemsAnalysis. New York: John Wiley & Sons, 1980.2. A. Bejan. Advanced Engineering Thermodynamics. 2nded. New York: Wiley Interscience, 1997.3. A. Bejan. Entropy Generation through Heat and FluidFlow. New York: John Wiley & Sons, 1982.4. Y. A. Çengel. “A Unified and Intuitive Approach toTeaching Thermodynamics.” ASME InternationalCongress and Exposition, Atlanta, Georgia, November17–22, 1996.5. M. S. Moran and H. N. Shapiro. Fundamentals ofEngineering Thermodynamics. 3rd ed. New York: JohnWiley & Sons, 1996.6. K. Wark and D. E. Richards. Thermodynamics. 6th ed.New York: McGraw-Hill, 1999.PROBLEMS*Exergy, Irreversibility, Reversible Work,and Second-Law Efficiency8–1C How does reversible work differ from useful work?8–2C Under what conditions does the reversible work equalirreversibility for a process?8–3C What final state will maximize the work output of adevice?8–4C Is the exergy of a system different in differentenvironments?8–5C How does useful work differ from actual work? Forwhat kind of systems are these two identical?*Problems designated by a “C” are concept questions, and studentsare encouraged to answer them all. Problems designated by an “E”are in English units, and the SI users can ignore them. Problemswith a CD-EES icon are solved using EES, and complete solutionstogether with parametric studies are included on the enclosed DVD.Problems with a computer-EES icon are comprehensive in nature,and are intended to be solved with a computer, preferably using theEES software that accompanies this text.