advanced engineering thermodynamics chapter1.pdf

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1、1TheFirstLawofThermodynamics1.1ELEMENTSOFTHERMODYNAMICSTERMINOLOGYThefirststepinthethermodynamicanalysisofanythingmustbethedefinitionoftheentitythatisbeingsubjectedtoanalysis.Werefertothisentity(col-lectionofmatter,regioninspace)asasystem.Althoughtheneedforexe-cutingthi

2、sfirstmoveandfordoingitonceandunambiguouslyisobvious,thetemptationisgreattoassumethesystemdefinitionitselfobviousandomitit.Thisisasourceofconfusion,ifnotoutrighterror.Paradoxicaldifferencesbetweentworesultsclaimedbytwoexpertswhoattack‘‘thesameproblem’’areoftenexplainedb

3、ytherealizationthatthetwoproblemsolversweremen-tallyaddressingdifferentsystems.Aswewillseeverysoon,theprecisedef-initionofthesystemiscriticalwhendeterminingthelocationandmagnitudeofthermodynamicirreversibility.Thetendencytoforgettodefinethesystemisduemainlytotradition.

4、Inconsiderablyolderdisciplinessuchassolid-bodymechanics,thesystemisindeedobvious,asthemeresketchingofabodyfocusestheattentionofbothproblemsolverandcritic.Influidmechanicsandheattransfer,thesystemisagainunderstoodoncetheboundaryconditionsnecessaryforsolvingtheNavier–Sto

5、kesequationsarespecified.However,eveninfluidmechanicsandheattransfer,theunambiguousdefinitionofthesystemisamustifthemethodisorderofmagnitudeorCOPYRIGHTEDMATERIALscaleanalysis[1]—thatis,iftheanalystreplacestheNavier–Stokesequationswithapproximatealgebraicstatementsthatcan

6、-notbesubjectedtoboundaryconditions.Todefinethesystemmeansalsotoidentifysharplythesystem’senviron-ment,orsurroundings.Theenvironmentistheportionofmatterorregioninspacethatresidesoutsidethesystemselectedforanalysis.Whatdifferentiatesbetweenthesystemanditsenvironmentisth

7、esurfacecalledboundary.Oneveryimportantdefiningfeaturethatsometimesfallspreytothesameforcesoftraditionisthattheboundaryisasurface,notanothersystem(notethatthethicknessofasurfaceiszero;therefore,theboundarycanneithercontain12THEFIRSTLAWOFTHERMODYNAMICSmatternorfillavolum

8、einspace).Saidanotherway,thevalueofapropertythatismeasuredatapointonthesurfacecalledboundarymustbesharedbyboththesystemandth