02 part7 second law thermodynamics
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02 part7 second law thermodynamicsTRANSCRIPT
Second Law of Thermodynamics
S.Gunabalan Associate Professor Mechanical Engineering Department Bharathiyar College of Engineering & Technology Karaikal - 609 609. e-Mail : [email protected]
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
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
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
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.
First Law efficiency
• Efficiency = output energy of device / input energy of device – Irrespective of form of energy – Availability of energy at different temperature
Second Law efficiency
• Efficiency = 풂풗풂풊풍풂풃풍풆풆풏풆풓품풚 푬풙풆풓품풚
• 푬풙풆풓품풚 Defined as the available energy for the work
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
∫ 푑푄 = ∆퐸 + 푊 ----------(1)
Exergy Balance For a Closed System
• ∫ 푑푄 = ∆퐸 + 푊−−−− −(1)
• Entropy Balance
∫ 푑푄/푇 = ∆푆 --------------(2)
• Multiplying the second equation by T0 and subtracting it from the first one yields
• ∫ 푑푄 = 푇표.∆푆
Exergy Balance For a Closed System
• ∫ 푑푄 = ∆퐸 + 푊−−−− −(1)
• ∫ 푑푄 = 푇표.∆푆
• Subtract ------------------------------------
• ∫ 1 − 푑푄 = ∆퐸 + 푊 − 푇표.∆푆
• E = u + v2/2+gZ
Exergy Balance For a Closed System
• ∫ 1 − 푑푄 = ∆퐸 + 푊 − 푇표.∆푆
• E = u + v2/2+gZ
1 −
푇표푇 푑푄 = 푢2 − 푢1 + (
푉22 − 푣1
2
2 ) + 푝0(푉2 − 푉1) − 푇표(푆2 − 푆1)
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.