02 part7 second law thermodynamics

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02 part7 second law thermodynamics

02 part7 second law thermodynamics

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  • 1. Second Law of Thermodynamics S.Gunabalan Associate Professor Mechanical Engineering Department Bharathiyar College of Engineering & Technology Karaikal - 609 609. e-Mail : gunabalans@yahoo.com
  • 2. Second law of thermodynamics • The second law of thermodynamics about the direction of heat transfer • Second law can be visualized in terms of the waterfall – You can not send water up with out energy
  • 3. Second law of thermodynamics • Second Law for Heat Engines It is impossible to extract heat QHot from a hot reservoir and use it all to do work W . Some amount of heat QCold must be exhausted to a cold reservoir. Called Kelvin-Planck statement
  • 4. Second law of thermodynamics • Second Law for Refrigerator • It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Called Clausius statement
  • 5. Second law of thermodynamics Second Law: Entropy A measure of the amount of energy which is unavailable to do work. A state variable whose change is defined for a reversible process at T where Q is the heat absorbed A measure of the disorder of a system.
  • 6. First Law efficiency • Efficiency = output energy of device / input energy of device – Irrespective of form of energy – Availability of energy at different temperature
  • 7. Second Law efficiency • Efficiency = • Defined as the available energy for the work
  • 8. Exergy Balance For a Closed System • Exergy balance for a closed system can be developed by combining the energy and entropy balances for a closed system. • Energy Balance ∫ = ∆ + ----------(1)
  • 9. Exergy Balance For a Closed System • ∫ = ∆ + −−−− −(1) • Entropy Balance ∫ / = ∆ --------------(2) • Multiplying the second equation by T0 and subtracting it from the first one yields • ∫ = . ∆
  • 10. Exergy Balance For a Closed System • ∫ = ∆ + −−−− −(1) • ∫ = . ∆ • Subtract ------------------------------------ • ∫ 1 − = ∆ + − . ∆ • E = u + v2/2+gZ
  • 11. Exergy Balance For a Closed System • ∫ 1 − = ∆ + − . ∆ • E = u + v2/2+gZ 1 − = 2 − 1 + ( 2 2 − 1 2 2 ) + 0( 2 − 1) − ( 2 − 1)
  • 12. Reference • http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html • Rajput, R. K. 2010. Engineering thermodynamics. Jones and Bartlett Publishers, Sudbury, Mass. • Nag, P. K. 2002. Basic and applied thermodynamics. Tata McGraw-Hill, New Delhi.