热能与动力工程专业英语翻译.pdf

该【热能与动力工程专业英语翻译 】是由【小辰GG】上传分享，文档一共【7】页，该文档可以免费在线阅读，需要了解更多关于【热能与动力工程专业英语翻译 】的内容，可以使用淘豆网的站内搜索功能，选择自己适合的文档，以下文字是截取该文章内的部分文字，如需要获得完整电子版，请下载此文档到您的设备，方便您编辑和打印。Throughtheapplicationofthermodynamicprinciples,modernheatengineshavebeen
developed.
Wearefacingtherealitythatfossilfuelreservesarediminishingandwillbeinsufficientin
theforseeablefuture.
Consequently,tothosewhostudythermodynamics,increasingefficiencyintheuseoffossil
fuelsandthedevelopmentofalternatesourcesofthermalenergyaretherealchallengesto
technologyfortodayandtomorrow.
Thermodynamicsisabranchofsciencewhichdealswithenergy,itsconversionfromone
formtoanother,andthemovementofenergyfromonelocationtoanother.
Thermodynamicsisinvolvedwithenergyexchangesandtheassociatedchangesinthe
propertiesoftheworkingfluidorsubstance.
Althoughthermodynamicsdealswithsystemsinmotion,itdoesnotconcernitselfwiththe
speedatwhichsuchprocessesorenergyexchangesoccur.
Thermodynamics,likeotherphysicalsciences,isbasedonobservationofnature.
Engineeringthermodynamicsconsistsofseveralparts,suchasbasiclaws,thermalproperties
oftheworkingfluids,processandcycleandsoon.
Energyisaprimitive(原始的)(假定)thatitissomethingthatall
matterhas.
Kineticenergyandpotentialenergyaretwoformsofmechanicalenergy.
Achangeofthetotalenergyisequaltotherateofworkdoneonthesystemplustheheat
transfertothesystem.
EnthalpycanbeusedeitherasanextensivepropertyHorasanintensivepropertyh.
Thetwotermsv2/2andgzrepresentskineticenergyandpotentialenergyrespectively.
Althoughthenetheatsuppliedtoathermodynamicsystemisequaltothenetworkdoneby
thesystem,thegrossenergysuppliedtothesystemmustbegreaterthanthenetworkdone
bythesystem.
Notalloftheinputheatisavailableforproducingoutputworkbecausesomeheatmust
alwaysberejectedbythesystem.
Relatedtothesecondlawstatementsaretheconceptsofavailabilityofenergy,entropy,
processreversibilityandthermalefficiency.
Inallreversibleprocessesthereisnochangeintheavailabilityoftheenergyevolvedinthe
process.
Duetothisconceptofavailabilityofenergy,thefollowingstatementscanbemade:Onlya
portionofheatenergymaybeconvertedintowork.
EntropySisanabstractthermodynamicpropertyofasubstancethatcanbeevaluatedonly
bycalculation.
Fromtheaboveexpressiononecanfindthatthevalueofentropyofthesystemwillincrease
whentheheatistransferredintothesystem.
Processesthatreturntotheirinitialstatearecalledcyclicprocesses.
TheCarnotcycleismostefficientcyclepossibleoperatingbetweentwogiventemperature
levels.
IntheidealRankinecycletheefficiencymaybeincreasedbytheuseofareheatersection.
Theprocessofreheatingingeneralraisestheaveragetemperatureatwhichheatissupplied
tothecycle,thusraisingthetheoreticalefficiency.
Afterpartialexpansionthesteamiswithdrawnfromtheturbineandreheatedatconstant
.
Fortheportionoftheheat-additionprocessfromthesubcooledliquidtosaturatedliquid,
theaveragetemperatureismuchbelowthetemperatureofthevaporizationand
superheatingprocess.
Fromtheviewpointofthesecondlaw,thecycleefficiencyisgreatlyreduced.
Ifthisrelativelylow-temperatureheat-additionprocesscouldberaised,theefficiencyofthe
cyclewouldmorenearlyapproachthatoftheCarnotcycle.
Therefrigerationcycleisusedtotransferenergy(heat)fromacoldchamber,whichisata
temperaturelowerthanitssurroundings.
Thebasicrefrigerationcycleconsistsofasequenceofprocessesutilizingaworkingfluid,
calledtherefrigerant,usuallyincontinuouscirculationwithinaclosedsystem.
Therefrigerantreceivesenergyintheevaporator(coldchamber)atatemperaturebelow
thatofthesurroundings,andthenrejectsthisenergyinthecondenser(hotchamber)prior
toreturningtoitsinitialstate.
Intheabsenceoffrictionthesemechanicalenergiesarecompletelyinterchangeable;thatis,
oneunitofpotentialenergycanbeideallyconvertedintooneunitofkineticenergy,and
viceversa.
Itrepresentsenergymodesonthemicroscopiclevel,suchasenergyassociatedwithnuclear
spin,molecularbinding,magnetic-dipolemoment,moleculartranslation,molecularrotation,
molecularvibration,andsoon.
Inastaticfluid,thereisnomotionofonelayeroffluidrelativetoanadjacentlayer,sothere
arenoviscousshearforces.
Aknowledgeoffluidstaticsisnecessaryforthesolutionofmanyfamiliarproblems,suchas
thedeterminationoftotalwaterforceonadam,thecalculationofpressurevariation
throughouttheatmosphere.
Withnorelativemotionbetweenfluidparticles,therearenoshearforcesactingonthe
element,onlynormalforces(duetopressure)andthegravityforce.
Inordertosolveproblemsinfluidflow,itisoftennecessarytodeterminethevariationof
pressurewithvelocityfrompointtopointthroughouttheflowfield.
Asoneknows,astreamlineisacontinuouslinedrawninthedirectionofthevelocityvector
ateachpointintheflow.
Forone-dimensionalflow,theflowpropertiesofwhichdonotvaryinthedirectionnormal
tothestreamline,theconstantintheBernoulliequationisthesameforallstreamlines.
Thetermpviscalledflowwork(energy/mass),thetermv2/2isthekineticenergyperunit
mass;andgzisthepotentialenergyperunitmass.
Therearetwobasictypesofflow,eachpossessingfundamentallydifferentcharacteristics.
Thefirsttypeiscalledlaminarflow,thesecondturbulentflow.
Thetransversemovementofaparticleoffluidfromafaster-movinglayertoa
slower-movinglayerwillhavetheeffectofincreasingthevelocityintheslower-moving
layer.
Theinletlengthrequiredtoattainfullydevelopedflowisdependentonthetypeofflow.
Inananalysisofflowthroughapipe,weareinterestedinthetypeofflow,whetherlaminar
orturbulent,sincetheshearstressandresultantfrictionalforcesactingonthefluidvary
greatlyforthetwotypes.
Anotherwayoflookingatthedifferencebetweenlaminarandturbulentflowsistoconsider
whathappenswhenasmalldisturbanceisintroducedintoaflow.
Thethicknessofthelaminarsublayerdependsonthedegreeofturbulenceofthemain
stream—themoreturbulenttheflow,thethinnerthesublayer.
Weknowthatwhenafluidflowsthroughapipe,thelayeroffluidatthewallhaszero
velocity;layersoffluidatprogressivelygreaterdistancesfromthepipesurfacehavehigher
velocities,withthemaximumvelocityoccurringatthepipecenterline.
However,eventhoughthevelocityfluctuationsaresmall,theyhaveagreateffectonthe
flowcharacteristics.
Furthermore,withthelargenumberofrandomparticlefluctuationspresentinaturbulent
flow,thereisatendencytowardmixingofthefluidandamoreuniformvelocityprofile.
Whensmokeleavesacigarette,ittravelsupwardinitiallyinasmooth,regularpattern;ata
certaindistanceabovethecigarette,however,thesmokebreaksdownintoanirregular
pattern.
Eveninturbulentpipeflow,withthegreatmajorityoftheflowcharacterizedbyrough,
irregularmotions,therewillalwaysbeathinlayerofsmoothlaminarflownearawall,for
theparticlefluctuationsdieoutnearaboundary.
Whenthecentralofcoreregionoftheflowdisappears,theflowistermedfullydeveloped
viscousflow.
Thescienceofheattransferisconcernedwiththeanalysisoftherateofheattransfertaking
.
Heatconductionisthetermappliedtothemechanismofinternalenergyexchangefromone
bodytoanother,orfromonepartofabodytoanother.
Heatconductionisrealizedbytheexchangeofthekineticenergyofthemoleculesbydirect
contactorbythedriftoffreeelectronsinthecaseofheatconductioninmetals.
TheFourierlawmaybeusedtodevelopanequationdescribingthedistributionofthe
temperaturethroughoutaheat-conductingsolid.
Theterm“steadystateconduction”wasdefinedastheconditionwhichprevailswhenthe
temperaturesoffixedpointswithinaheat-conductingbodydonotchangewithtime.
Theterm“one-dimensional”isappliedtoaheatconductionproblemwhenonlyonespace
coordinateisrequiredtodescribethedistributionoftemperaturewithinaheat-conducting
body.
Thesolutionofheatconductionproblemsinvolves,ingeneral,thewritingofthegeneral
heatconductionequationintermsoftheappropriatenumberofarbitraryconstantsand
thentheevaluatingoftheseconstantsbyuseoftheimposedboundaryconditions.
Theelectricalanalogymaybeusedtosolvemorecomplexproblemsinvolvingbothseries
andparallelthermalresistances.
Whenfluidflowsoverasolidbodyorinsideachannelwhiletemperaturesofthefluidand
thesolidsurfacedifferent,heattransferbetweenthefluidandthesolidsurfacetakesplace
asaconsequenceofthemotionoffluidrelativetothesurface.
Themultiplicityofindependentvariablesresultsfromthefactthatconvectiontransferis
determinedbytheboundarylayersthatdevelopsonthesurface.
Thevelocityboundarylayerisdefinedasthethinlayernearthewallinwhichoneassumes
thatviscouseffectsareimportant.
Itshouldbeemphasizedthatathermalboundarylayercanalsobedefinedastheregion
betweenthesurfaceandthepointatwhichthefluidtemperaturehasreachedacertain
percentageofthefluidtemperature.
Thethermalboundarylayerisgenerallynotcoincidentwiththevelocityboundarylayer,
althoughitiscertainlydependentonit.
Numerousanalyticexpressionsareavailableforthepredictionofheattransfercoefficientin
laminartubeflow.
Therearenumerousimportantengineeringapplicationsinwhichheattransferforflowover
bodiessuchasaflatplate,asphere,acirculartube,oratubebundleareneeded.
Thetemperaturevariationwithinthefluidwillgenerateadensitygradientwhich,ina
gravitationalfield,willgiverise,inturn,toaconvectivemotionasaresultofbuoyancy
forces.
Thefluidmotionsetupasaresultofthebuoyancyforce(浮力)iscalledfreeconvection,or
naturalconvection.
Theflowvelocityinfreeconvectionismuchsmallerthanthatencounteredinforced
convection;therefore,heattransferbyfreeconvectionismuchsmallerthanthatbyforced
convection.
Accordingtothedifferentcondensingsituation,condensationcanbedividedintofilmwise
condensationanddropwisecondensation.
Thephenomenonofheattransferinboilingisextremelycomplicatedbecauseofalarge
numberofvariablesinvolvedandverycomplexhydrodynamicdevelopmentsoccurringin
theprocess.
Allbodiescontinuouslyemitenergybecauseoftheirtemperature,andtheenergythus
emittediscalledthermalradiation.
Theradiationenergyemittedbyabodyistransmittedinthespaceintheformof
electromagneticwavesaccordingtoMaxwell’sclassicelectromagneticwavetheoryorinthe
formofdiscretephotonsaccordingtoPlanck’shypothesis(假说).
Theemissionorabsorptionofradiationenergybyabodyisabulkprocess;thatis,radiation
originatingfromtheinteriorofthebodyisemittedthroughthesurface.
Heatexchangersaredevicesthatfacilitateheattransferbetweentwoormorefluidsat
differenttemperatures.
.
,unliketheefficiencyofaheatenginecanbemuchlargerthan
unity.
TheessentialpartsofavaporcompressionsystemareEvaporatorCompressorcondenser,
andExpansionvalve.
Therearethreetypesofvaporcompressor:reciprocating,rotary,centrifugal.
Avaporabsorptionsystemusesheat(thermal)energytoproducerefrigeration.
Inanabsorptionsystem,thecommonlyusedworkingsubstanceisasolutionofrefrigerant
andsolvent.
Thefourimportantfactorsinvolvedinacompleteairconditioninginstallationare:(i)
Temperaturecontrol,(ii)Humiditycontrol,(iii)Airmovementandcirculation,(iv)Air
filtering,cleaningandpurification.
Givesomeapplicationsofrefrigeration
domesticrefrigeration
commercialrefrigeration
industrialrefrigeration
Manufactureandpreservationofmedicines
Preservationofbloodandhumantissues
Productionofrocketfuels
Computerfunctioning
marineandtransportationrefrigeration
Whatisavaporcompressionsystem?
AtypicalVaporCompressionRefrigerationSystem
Components
Evaporator:Heatexchangersforrefrigeranttoabsorbheatfromrefrigeratedspace
Compressor:toraisethetemperatureandpressureofrefrigerantbycompression
Condenser:Heatexchangersforrefrigeranttorejectheattotheenvironment
Receivertank:areservoirtostoretheliquidrefrigerant
Expansionvalve:orRefrigerantflowcontrol,toreducerefrigerantpressure
Cyclediagrams
Whatistheworkingprincipleofavaporabsorptionsystem?
Absorptionrefrigerationcycle
Avaporabsorptionsystemusesheat(thermal)energytoproducerefrigeration.
Inanabsorptionsystem,thecommonlyusedworkingsubstanceisasolutionofrefrigerant
andsolvent,suchasAmmonia/waterandWater/lithiumbromide.
Aabsorptionrefrigerationsystemalsocontainsanevaporatorandcondenserwhichoperate
inexactlysamewayasforvaporcompressorcycle.

refrigerantvaporisabsorbedintothesolventatlowpressure,andthereisanetsurfeitof
heatforth