SHANDONGUNIVERSITY6-6 reversible and irreversible processesReversible processes do not occur in nature.: They are idealizations of actual processes.All theprocesses occurringinnatureare irreversibleThe possibility of finding a perfect mate is no higher thanthepossibility offindingaperfect(reversible)process.But we need reversible processes:- Easyto analyze:a serious of equilibrium states during a process Act as idealized models (theoretical limits) to be compared byactualprocesses
SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • Reversible processes do not occur in nature. • They are idealizations of actual processes. • All the processes occurring in nature are irreversible. • The possibility of finding a perfect mate is no higher than the possibility of finding a perfect (reversible) process. • But we need reversible processes: – Easy to analyze: a serious of equilibrium states during a process – Act as idealized models (theoretical limits) to be compared by actual processes. 6
SHANDONGUNIVERSITYExpansionCompressionExpansionCompressionPressuredistributionWaterWaterWaterWater(b)Fast(irreversible)process(a)Slow(reversible)processReversible processes deliver themost and consume theleast work
SHANDONG UNIVERSITY 7 Reversible processes deliver the most and consume the least work
SHANDONG UNIVERSITY6-6 reversible and irreversible processesIrreversibilities: The factors that cause a process to beirreversible are called irreversibilities, including=Friction(摩擦)Fnction(e)Fast co20cGASHeunrestrainedexpansion（自由膨胎(b)Fastespansion7OLP50kP(c) Umestnined expansioeW-heattransferacrossafinitetemperaturedifference（温差传热）DuiMehcuimaferfeesmixing of two fluids, electric resistance, inelastic deformation ofsolids,andchemicalreactions
SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • Irreversibilities: The factors that cause a process to be irreversible are called irreversibilities, including – Friction(摩擦) – unrestrained expansion（自由膨胀） – heat transfer across a finite temperature difference （温差传热） – mixing of two fluids, electric resistance, inelastic deformation of solids, and chemical reactions. 8
SHANDONGUNIVERSITYInternallyandExternallyReversibleProcessesInternallyreversibleprocess:Ifnoirreversibilitiesoccurwithintheboundariesofthesystemduringtheprocess.Externallyreversible:IfnoirreversibilitiesoccuroutsidethesystemboundariesTotallyreversibleprocess:ItinvolvesnoirreversibilitieswithinthesystemoritssurroundingsAtotallyreversibleprocessinvolvesnoheattransferthroughafinitetemperaturedifference,nononquasi-equilibriumchanges,andnofriction or otherdissipativeeffects
SHANDONG UNIVERSITY 9 Internally and Externally Reversible Processes • Internally reversible process: If no irreversibilities occur within the boundaries of the system during the process. • Externally reversible: If no irreversibilities occur outside the system boundaries. • Totally reversible process: It involves no irreversibilities within the system or its surroundings. • A totally reversible process involves no heat transfer through a finite temperature difference, no nonquasi-equilibrium changes, and no friction or other dissipative effects
SHANDONGUNIVERSITY6-7 The Carnot Cycle（卡诺循环)Carnot cycle(卡诺循环)is the bestknownreversible cycleComposedoffourreversibleprocesses:Twoisothermalprocesses.Two adiabaticprocessesFirstproposed in1824bySadiCarnotCarnotcyclecanbeexecutedeitherinaclosedorasteadyflow system.Thetheoretical heatengineoperates on aCarnotcycleisCarnotheatengine（卡诺热机）10
SHANDONG UNIVERSITY 6-7 The Carnot Cycle (卡诺循环) • Carnot cycle(卡诺循环) is the best known reversible cycle. – Composed of four reversible processes: • Two isothermal processes • Two adiabatic processes – First proposed in 1824 by Sadi Carnot – Carnot cycle can be executed either in a closed or a steady flow system. – The theoretical heat engine operates on a Carnot cycle is Carnot heat engine（卡诺热机）. 10