Chap5 SummaryConservationofmassprinciple:thenetmasstransfertoorfroma controlvolumeduringatimeintervalisequaltotheConservationofmassnetchangeinthetotal masswithinthecontrol volumedmcymoutAmcmindtoutinFlow energy:Totalenergyofaflowingfluidof1kg2(kJ/kg)Waow=PV=h+ke+pe=hgzEnergyofflowingfluid2V2Rate of energy transportE=mo=gzIm2V=大mMassbalanceIncompressibleOUDUinSingle streamPVA=PVA2nmSteadyflowProcess/systemV=V,-VA=VA2IncompressibleSinglestreamIFor△KE=0,△PE=0q-W=h2-hTurbines,compressorsNozzles,diffusersHeatexchangersSteadyflowdevicesPipeand ductflowThrottling valvesMixingchambers?
Chap5 Summary 1 Conservation of mass principle: the net mass transfer to or from a control volume during a time interval is equal to the net change in the total mass within the control volume. Energy of flowing fluid Nozzles, diffusers Steady flow Process/system Steady flow devices Conservation of mass Mass balance Flow energy: Total energy of a flowing fluid of 1kg Enthalpy is associated with the energy pushing the fluid into or out of CV Single stream Incompressible Incompressible Single stream Energy balance for general steady flow systems For For single stream △KE=0, △PE=0 Turbines, compressors Throttling valves Mixing chambers Heat exchangers Pipe and duct flow Rate of energy transport
Chap6 Summary-1Whyweneed2ndLaw?Allprocessessatisfy1stLawSatisfying1stdoesnotensuretheprocesscanactuallyoccurIntroductionto2ndLawAprocesshasdirectionEnergyhasqualityandquantityHeat SinkHeat SourceHeatengineThermalenergyReservoirWact.ouOReceive heat Qfromahightemperature sourceMthQHWConvertpartQHtowork WnetoutnetoutQHeatEnginesQReject wasteheat QLto a lowtemperature sinkih2ndlaw,Kelin-PlanckStatement:It isimpossibleforanydevicethatoperatesonacycletoreceiveheatfromasinglereservoirandproduceanetamountofwork.Noheatenginecanhaven=100%Refrigerators/heatpump:ThedevicesdriveheatQtransferfromT,toTHW.RefrigeratorTheworkinputtotherefrigerator/heatpumpnet,inwants QL0HeatQabsorbedfromrefrigeratedspaceTHeatpumpQHHeat Qrejected tohightemperature THwants QHRefrigerator,HeatPumpDesired outputuDesired outputQAirCOPCOPHCOPW.WaeLinRequired inputRequired inputConditioner2nd law,Clausius Statement:Heatdoesnot,of its own volition,transferfromacoldmediumtoawarmerone.（热不能自发地、不付代价地从低温物体传到高温物体）
Chap6 Summary-1 2 Why we need 2nd Law? All processes satisfy 1st Law; Satisfying 1st does not ensure the process can actually occur Heat Engines Refrigerator, Heat Pump Introduction to 2nd Law Refrigerators/heat pump: The devices drive heat Q transfer from TL to TH, Thermal energy Reservoir Receive heat QH from a high temperature source The work input to the refrigerator/heat pump Heat QL absorbed from refrigerated space TL A process has direction Energy has quality and quantity Heat Source Heat Sink Convert part QH to work Wnet,out Reject waste heat QL to a low temperature sink Heat engine 2nd law, Kelvin-Planck Statement: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. No heat engine can have η=100% Heat QH rejected to high temperature TH Refrigerator wants QL Heat pump wants QH COP 2nd law, Clausius Statement: Heat does not, of its own volition, transfer from a cold medium to a warmer one. (热不能自发地、不付代价地从低温物体传到高温 物体) Air Conditioner
Chapter6The second law of thermodynamics
Chapter 6 The second law of thermodynamics
6-1 introduction to the second law In chap.4 and chap.5, the first law of thermo-dynamics to processes of-closedsystems- opensystems (controlvolumes).Noprocessisknowntohavetakenplaceinviolation of the first law of thermodynamics: A process must satisfy the first law to occur
6-1 introduction to the second law • In chap.4 and chap.5, the first law of thermodynamics to processes of – closed systems – open systems (control volumes). • No process is known to have taken place in violation of the first law of thermodynamics. • A process must satisfy the first law to occur. 4
6-1 introduction to the secondlaw.However,satisfyingthefirst law alonedoes not ensure that the process willactuallytakeplaceE.g:acupofhotwaterisgetting colderinacoolerroomHowabout,thereverseprocess(will itget hotterina coolerroom?)
6-1 introduction to the second law • However, satisfying the first law alone does not ensure that the process will actually take place. – E.g: a cup of hot water is getting colder in a cooler room. – How about, the reverse process (will it get hotter in a cooler room?) 5 ? Q Q