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
6-1 introduction to the second lawE.g: we use electric resistor to heat the room. Is itpossible to transfer some heat to an equivalent amountof electric energy to be generated in the resistor ?Heat=0W·Transferring heat to a paddlewheel will not cause itto rotate.Heat
6-1 introduction to the second law • E.g: we use electric resistor to heat the room. Is it possible to transfer some heat to an equivalent amount of electric energy to be generated in the resistor ? 6 •Transferring heat to a paddle wheel will not cause it to rotate
6-1 introduction to the second lawSatisfying thefirstlawdoes not ensurethattheprocesscan actually occur. We need to introduce the secondlawofthermodynamicsAprocessmustsatisfybothIstlaw2nd lawPROCESSthefirstandsecondlawsofthermodynamicstoproceed.A process cannot occur unless it satisfies both laws
6-1 introduction to the second law • Satisfying the first law does not ensure that the process can actually occur. We need to introduce the second law of thermodynamics. • A process cannot occur unless it satisfies both laws. 7 A process must satisfy both the first and second laws of thermodynamics to proceed
6-1 introduction to the second law The second law of thermodynamics:- identify the direction of a process.assertthatenergyhasnotonlyquantity(数量）butalsoquality(品质）- provide the necessary means to determine the quality as well asthe degradation of energy during a process-be usedto determinethetheoretical limitsforperformanceofcommonlyusedengineeringsystems(heatenginesandrefrigerators).-Twostatementswill be discussedlater
6-1 introduction to the second law • The second law of thermodynamics: – identify the direction of a process. – assert that energy has not only quantity(数量) but also quality(品质). – provide the necessary means to determine the quality as well as the degradation of energy during a process – be used to determine the theoretical limits for performance of commonly used engineering systems (heat engines and refrigerators) . – Two statements will be discussed later. 8
6-2 thermal energy reservoirsThermal energy reservoir: supply or absorb finiteamounts of heat without undergoing any changein temperature: has big thermal energy capacity(mass*specific heat, mCp)Heat source(热源): a reservoir that suppliesenergy in the form of heat. Heat sink(热沉): a reservoir that absorbs energyin the form of heat
6-2 thermal energy reservoirs • Thermal energy reservoir: supply or absorb finite amounts of heat without undergoing any change in temperature: has big thermal energy capacity (mass*specific heat, mCP) • Heat source(热源): a reservoir that supplies energy in the form of heat. • Heat sink(热沉): a reservoir that absorbs energy in the form of heat. 9
6-3 heat engines (热机) Work can be converted to heat directly and completely,but converting heat to work requires the use of somespecial devices. These devices are called heat engines (热机)Receive heat (Qμ) from a high temperature source (solar energyfurnace)Convert part of these heat to work (W) (usually rotating shaft)- Reject the remaining waste heat (Q) to a low-temperature sink (theatmosphere,rivers,etc.)Operateonacycle Usually involves a fluid, called working fluid(工质)Broadly heat engine is often include work producing devices that do notoperate in a thermodynamic cycle. (like gas turbines, car engines, whichoperate in a mechanical cycle not in a thermodynamics (working fluiddoesnotundergoacompetecycle))10
6-3 heat engines (热机) • Work can be converted to heat directly and completely, but converting heat to work requires the use of some special devices. These devices are called heat engines ( 热机). – Receive heat (QH) from a high temperature source (solar energy, furnace) – Convert part of these heat to work (W) (usually rotating shaft) – Reject the remaining waste heat (QL ) to a low-temperature sink (the atmosphere, rivers, etc.) – Operate on a cycle. – Usually involves a fluid, called working fluid(工质). – Broadly heat engine is often include work producing devices that do not operate in a thermodynamic cycle. (like gas turbines, car engines, which operate in a mechanical cycle not in a thermodynamics (working fluid does not undergo a compete cycle)) 10