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Cengel ch07

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Cengel ch07

  1. 1. CHAPTER 7Exergy: A Measureof Work Potential
  2. 2. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-1A system that is in equilibriumwith its environment is said tobe at the dead state.7-1
  3. 3. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-3The immediatesurroundings of ahot potato aresimply thetemperaturegradient zone of theair next to thepotato.7-2
  4. 4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-8As a closed system expands, somework needs to be done to push theatmospheric air out of the way(Wsurr).7-3
  5. 5. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-10The differencebetween reversiblework and actual usefulwork is theirreversibility.7-4
  6. 6. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-15Two heat enginesthat have thesame thermalefficiency, butdifferentmaximumthermalefficiencies.7-5
  7. 7. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-16Second-law efficiencyis a measure of theperformance of adevice relative to itsperformance underreversible conditions.7-6
  8. 8. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-20The exergy of aspecified mass at aspecified state is theuseful work thatcan be produced asthe mass undergoesa reversible processto the state of theenvironment.7-7
  9. 9. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-21The exergy of a coldmedium is also apositive quantitysince work can beproduced bytransferring heat toit.7-8
  10. 10. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-22The exergy of flow energy isthe useful work that wouldbe delivered by animaginary piston in the flowsection.7-9
  11. 11. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-23The energy and exergycontents of (a) a fixedmass and (b) a fluidstream.7-10
  12. 12. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-26The Carnot efficiencyη c = 1 - T0 /T representsthe fraction of theenergy of a heat sourceat temperature T thatcan be converted towork in anenvironment attemperature T0 .7-11
  13. 13. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-27The transfer anddestruction ofexergy during a heattransfer processthrough a finitetemperaturedifference.7-12
  14. 14. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-29Mass containsenergy, entropy, andexergy, and thusmass flow into orout of a system isaccompanied byenergy, entropy, andexergy transfer.7-13
  15. 15. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-30The isolated systemconsidered in thedevelopment of the decreaseof exergy principle.7-14
  16. 16. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-32Mechanisms ofexergy transfer fora general system.7-15
  17. 17. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-33Exergy balance for a closedsystem when the directionof heat transfer is taken tobe to the system and thedirection of work from thesystem.7-16
  18. 18. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-34Exergy destroyedoutside systemboundaries can beaccounted for bywriting an exergybalance on theextended system thatincludes the system andits immediatesurroundings.7-17
  19. 19. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-42Exergy is transferred into orout of a control volume bymass as well as heat and worktransfer.7-18
  20. 20. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-49The irreversibility associated with astudent studying and watching a movieon television, each for two hours.7-19
  21. 21. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.FIGURE 7-50A poeticexpression ofexergy andexergydestruction.7-20

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