Chapter4

TheFirstLawofThermodynamics:

ControlVolumes

4-1-1ConservationofMassprinciple4-1ThermodynamicanalysisofControlVolume4-1-2ConservationofEnergyprincipleforControlVolume

ThenetenergychangeinenergyofCVTotalenergycrossingboundary+TotalenergyofmassenteringCV=-TotalenergyofmassleavingCV

4-1-3FlowWork

4-1-4TotalEnergyofaFlowingFluidOnaunit-massbasis,theenergystoredinfluid:Consideringtheflowwork,wehaveThetotalenergyofaflowingfluid(denotedθ):

4-1-5EnthalpyInternalenergyandflowworkusuallytransferredatthesametimeinflowingfluid,sowedefineProfessorJ.Kestinproposedin1966thatthetermθbecalledmethalpy(frommetaenthalpy,whichmeansbeyondenthalpy)

4-2TheSteady-FlowProcess4-2-1DefinitionofSteady-FlowProcessAprocessduringwhichafluidflowsthroughacontrolvolumesteadily4-2-2CharacteristicsofSteady-FlowProcessNopropertiesinthecontrolvolumechangewithtime2.Nopropertieschangesattheboundariesofthecontrolvolumewithtime

3.Theheatandworkinteractionsbetweenasteadyflowsystemanditssurroundingsdonotchangewithtime4-2-3EnergyEquationofSteady-FlowProcessFromtheabovediscussionwecanconcludethat:2.ThenetchangeinenergyofCVisZero

EnergyEquationofSteady-FlowTotalenergycrossingboundaryperunittime=TotalenergyofmassleavingCVperunittime-TotalenergyofmassenteringCVperunittime

Usingsubscript1andsubscript2fordenotinginletandexitstatesDividingtheequationbyyields

Work

3-3SomeSteady-FlowEngineeringdevices3-3-1NozzlesandDiffusersq=Δh+ΔV2/2+gΔz+wq=0Δz=0w=0

3-3-2SteamTurbinefromq=Δh+ΔV2/2+gΔz+wsifq=0;Δc=0;Δz=0thenws=-Δh=h1-h2

3-3-3ThrottlingValvesFromq=Δh+ΔV2/2+gΔz+wsasq=0;ws=0;z=0;ΔV2/2=0thenΔh=0Thatish1=h2

3-3-4MixingChambersF