熱能與動力工程專業(yè)英語

......Chapter1IntroductiontoThermal.Chapter1IntroductiontoThermalScience第一章熱科學(xué)基礎(chǔ)Acousticflowmeter聲波流量計Corrugatedfin波狀散熱片Adiabatic[]絕熱的Crossproduct矢量積Aerodynamics空氣動力學(xué)Denominator分母Affiliation聯(lián)系Developedflow充分發(fā)展流Airfoil機(jī)翼，螺旋槳Diffusion擴(kuò)散Alternative替代燃料Dopplereffect多普勒效應(yīng)Anemometer風(fēng)速計Double-pipeheatexchanger套管式換熱器Angularspeed角速度Drysaturatedvapor干飽和蒸汽Areadensity表面密度Electrode電極Baffle擋板Electrolyte電解，電解液Bifurcation分形Electrostatic靜電的Blackbody黑體Emissivity發(fā)射率Blade漿葉，葉片Equilibrium平衡Boiler鍋爐Fluidmechanics流體力學(xué)Boundarylayer邊界層Forcedconvection強(qiáng)制對流CarnotCycle卡諾循環(huán)Freeconvection自然對流Cartesiancoordinates笛卡爾坐標(biāo)系Frictionloss摩擦損失CelsiusDegree攝氏度Glassceramic微晶玻璃，玻璃陶瓷Compactheatexchanger緊湊式換熱器Heatengine熱機(jī)Composition成分，合成物Heatpump熱泵Compressedliquid壓縮液體Hydrofoil水翼Compressibility可壓縮性，壓縮率Hypersonicspeed高超音速Condensation凝結(jié)Infinitesimal無窮小的Condenser冷凝器Inflating/deflating充氣/壓縮Conduction導(dǎo)熱Internalcombustionengine內(nèi)燃機(jī)Controlvolume控制體Isentropic等熵的Convection對流 Isobaric等壓的Coriolis-accelarationflowmeter科Isolatedsystem孤立體系的氏加速流量計Isometric等容的Rough-walltube粗糙管Isothermal等溫的Saturation飽和Kinematicviscosity運(yùn)動黏度Shearstress剪切力、切應(yīng)力Laminar層流Shell-and-tubeheatexchanger管殼式換熱器Manuscript手稿，原稿Specificvolume比容Moisture濕度，水分Steady穩(wěn)態(tài)的，定常的Molecule(化學(xué))分子Stiflingengine斯特林機(jī)Moltenpolymer熔融聚合物Strainrate變形速度，應(yīng)變率Muti-disciplinary多學(xué)科的Streamline流線NewtonianFluid牛頓流體Strut支撐，支柱Nominaltemperaturegradient法向Subcooledliquid過冷液體溫度梯度Numerator（數(shù)學(xué)）分子 Superheatedvapor過熱蒸汽Parallelflow平行流動，并流 Surrounding環(huán)境，外界Pathline跡線 Thermalconductivity熱傳導(dǎo)率Phasechange相變 Thermalefficiency熱效率Planeflow平面流，二元流 Thermodynamics熱力學(xué)PlateandflameheatexchangerTorsional扭力的，扭轉(zhuǎn)的板式換熱器Polymersolution膠漿Trailingedge機(jī)翼后緣、尾緣Proof校樣Transmitter傳送裝置、發(fā)送器Propeller螺旋槳，推進(jìn)器Turbinemeter渦輪流量計Pump泵Turbulent湍流的Qulity干度Ultrosonic超聲波的Qusi-equilibrium準(zhǔn)平衡、準(zhǔn)靜態(tài)Uniformflow均勻劉Radiation輻射Vacuum真空RankinCycle朗肯循環(huán)Viewfactor角系數(shù)RegenerativeheatexchangerViscous黏性的蓄熱/再生式換熱器Reservoir水庫，蓄水池Cortexshedding漩渦脫落Reversible可逆的Waterfaucet水龍頭，水嘴Rotameter轉(zhuǎn)子流量計BiBiotnumber比澳數(shù)NPSH汽蝕余量CFD計算流體力學(xué)NTU傳熱單元數(shù)CHF臨界熱流量Nu努謝爾特數(shù)COP制冷系數(shù)PE勢能Eu歐拉數(shù)Pr普朗特數(shù)Fo富立葉數(shù)Ra瑞利數(shù)Fr弗勞德數(shù)Re雷諾數(shù)Gr格拉曉夫數(shù)Sc施密特數(shù)KE動能St斯坦頓數(shù)，斯特勞哈數(shù)LMTD對數(shù)平均溫差We韋伯?dāng)?shù)1.1FundamentalofEngineeringThermodynamics1.1工程熱力學(xué)基礎(chǔ)Thermodynamicsisascienceinwhichthestorage,transformationandtransferofenergyarestudied.Energyisstoredasinternalenergy(associatedwithtemperature),kineticenergy(dutomotion),potentialenergy(duetoelevation)andchemicalenergy(duetochemicalcomposition);itistransformedfromoneoftheseformstoanother;anditistransferredacrossaboundaryaseitherheatorwork.熱力學(xué)是一門研究能量儲存、轉(zhuǎn)換及傳遞的科學(xué)。能量以內(nèi)能（與溫度有關(guān)）、動能（由物體運(yùn)動引起）、勢能（由高度引起）和化學(xué)能（與化學(xué)組成相關(guān)）的形式儲存。不同形式的能量可以相互轉(zhuǎn)化，而且能量在邊界上可以以熱和功的形式進(jìn)行傳遞。Inthermodynamics,wewillderiveequationsthatrelatethetransformationsandtransfersofenergytopropertiessuchastemperature,pressureanddensity.Substancesandtheirproperties,thus,becomeveryimportantinthermodynamics.Manyofourequationswillbebasedonexperimentalobservationsthathavebeenorganizedintomathematicalstatementsorlaws,thefirstandsecondlawsofthermodynamicsaremostwidelyused.在熱力學(xué)中，我們將推導(dǎo)有關(guān)能量轉(zhuǎn)化和傳遞與物性參數(shù)，如溫度、壓強(qiáng)及密度等關(guān)系間的方程。因此，在熱力學(xué)中，物質(zhì)及其性質(zhì)變得非常重要。許多熱力學(xué)方程都是建立在實(shí)驗(yàn)觀察的基礎(chǔ)之上，而且這些實(shí)驗(yàn)觀察的結(jié)果已被整理成數(shù)學(xué)表達(dá)式或定律的形式。其中，熱力學(xué)第一定律和第二定律應(yīng)用最為廣泛。1.1.1Thermodynamicsystemandcontrolvolume1.1.1熱力系統(tǒng)和控制體Athermodynamicsystemisafixedquantityofmattercontainedwithinsomeenclosure.Thesurfaceisusuallyanobviousone(likethatsurroundingthegasinthecylinder).However,itmaybeanimaginedboundary(likethedeformingboundaryofacertainamountofmassasitflowsthroughapump).熱力系統(tǒng)是一包圍在某一封閉邊界內(nèi)的具有固定質(zhì)量的物質(zhì)。系統(tǒng)邊界通常是比較明顯的（如氣缸內(nèi)氣體的固定邊界）。然而，系統(tǒng)邊界也可以是假想的（如一定質(zhì)量的流體流經(jīng)泵時不斷變形的邊界）。Allmatterandspaceexternaltoasystemiscollectivelycalleditssurroundings.Thermodynamicsisconcernedwiththeinteractionofasystemanditssurroundings,oronesysteminteractingwithanother.Asysteminteractswithitssurroundingsbytransferringenergyacrossitsboundary.Nomaterialcrossestheboundaryofasystem.Ifthesystemdoesnotexchangeenergywiththesurroundings,itisanisolatedsystem.系統(tǒng)之外的所有物質(zhì)和空間統(tǒng)稱外界或環(huán)境。熱力學(xué)主要研究系統(tǒng)與外界或系統(tǒng)與系統(tǒng)之間的相互作用。系統(tǒng)通過在邊界上進(jìn)行能量傳遞，從而與外界進(jìn)行相互作用，但在邊界上沒有質(zhì)量交換。當(dāng)系統(tǒng)與外界間沒有能量交換時，這樣的系統(tǒng)稱為孤立系統(tǒng)。Inmanycases,ananalysisissimplifiedifattentionisfocusedonaparticularvolumeinspaceintowhich,orfromwhich,asubstanceflows.Suchavolumeisacontrolvolume.Apump,aturbine,andaninflatingordeflatingballoonareexamplesofcontrolvolume.Thesurfacethatcompletelysurroundsthecontrolvolumeiscalledacontrolsurface.在許多情況下，當(dāng)我們只關(guān)心空間中有物質(zhì)流進(jìn)或流出的某個特定體積時，分析可以得到簡化。這樣的特定體積稱為控制體。例如泵、透平、充氣或放氣的氣球都是控制體的例子。包含控制體的表面稱為控制表面。Thus,wemustchoose,inaparticularproblem,whetherasystemistobeconsideredorwhetheracontrolvolumeismoreuseful.Ifthereismassfluxacrossaboundary,thenacontrolvolumeisrequired;otherwise,asystemisidentified.因此，對于具體的問題，我們必須確定是選取系統(tǒng)作為研究對象有利還是選取控制體作為研究對象有利。如果邊界上有質(zhì)量交換，則選取控制體有利；反之，則應(yīng)選取系統(tǒng)作為研究對象。1.1.2Equilibrium,processandcycle平衡、過程和循環(huán)Whenthetemperatureofasystemisreferredto,itisassumedthatallpointsofthesystemhavethesame,oressentiallythesametemperature.Whenthepropertiesareconstantfrompointtopointandwhenthereisnotendencyforchangewithtime,aconditionofthermodynamicequilibriumexists.Ifthetemperature,say,issuddenlyincreasedatsomepartofthesystemboundary,spontaneousredistributionisassumedtooccuruntilallpartsofthe教材1頁systemareatthesametemperature.對于某一參考系統(tǒng)，假設(shè)系統(tǒng)內(nèi)各點(diǎn)溫度完全相同。當(dāng)物質(zhì)內(nèi)部各點(diǎn)的特性參數(shù)均相同且不隨時間變化時，則稱系統(tǒng)處于熱力學(xué)平衡狀態(tài)。當(dāng)系統(tǒng)邊界某部分的溫度突然上升時，則系統(tǒng)內(nèi)的溫度將自發(fā)地重新分布，直至處處相同。Whenasystemchangesfromoneequilibriumstatetoanother,thepathofsuccessivesatesthroughwhichthesystempassesiscalledprocess.If,inthepassingonestatetothenext,thedeviationfromequilibriumisinfinitesimal,aquasi-equilibriumprocessoccurs,andeachstateintheprocessmaybeidealizedasanequilibriumstate.Quasi-equilibriumprocessescanapproximatemanyprocesses,suchasthecompressionandexpansionofgasesinaninternalcombustionengine,withnosignificantlossofaccuracy.Ifthesystemgoesfromoneequilibriumstatetoanotherthroughaseriesofnon-equilibriumstates(asincombustion),anon-equilibriumprocessoccurs.當(dāng)系統(tǒng)從一個平衡狀態(tài)轉(zhuǎn)變?yōu)榱硪粋€平衡狀態(tài)時，系統(tǒng)所經(jīng)歷的一系列由中間狀態(tài)組成的變化歷程稱為過程。若從一個狀態(tài)到達(dá)另一個狀態(tài)的過程中，始終無限小地偏離平衡態(tài)，則稱該過程為準(zhǔn)靜態(tài)過程，可以把其中任一個中間狀態(tài)看作為平衡狀態(tài)。準(zhǔn)靜態(tài)過程可近似視為許多過程的疊加結(jié)果，而不會顯著減小其精確性，例如氣體在內(nèi)燃機(jī)內(nèi)的壓縮和膨脹過程。如果系統(tǒng)經(jīng)歷一系列不平衡狀態(tài)（如燃燒），從一個平衡狀態(tài)轉(zhuǎn)變?yōu)榱硪粋€平衡狀態(tài)，則其過程為非平衡過程。Whenasysteminagiveninitialstateexperiencesaseriesofprocessandreturnstotheinitialstate,thesystemgoesacycle.Attheendofthecycle,thepropertiesofthesystemhavethesamevaluestheyhadatthebeginning.當(dāng)系統(tǒng)從一個給定的初始狀態(tài)出發(fā)，經(jīng)歷一系列中間過程又回到其初始狀態(tài)，則稱系統(tǒng)經(jīng)歷了一個循環(huán)。循環(huán)結(jié)束時，系統(tǒng)中的各參數(shù)又與初始參數(shù)相同。Theprefixiso-isattachedtothenamesofanypropertythatremainunchangedinaprocess.Anisothermalprocessisoneinwhichthetemperatureisheldconstant;inaniso-baricprocess,thepressureremainsconstant;anisometricprocessisaconstant-volumeprocess.在任一特性參數(shù)名稱前加上前綴iso-，表示該參數(shù)在整個過程保持不變。等溫（isothermal）過程中溫度保持不變；等壓（isobaric）過程中壓強(qiáng)恒定；等容（isometric）過程中體積保持不變。1.1.3Vapor-liquidphaseequilibriuminpuresubstance純物質(zhì)的氣-液相平衡Considerasasystem1kgofwatercontainedinthepistonorcylinderarrangementshowninFig.1-1(a).Supposethepistonandweightmaintainapressureof0.1MPainthecylinderandthattheinitialtemperatureis20℃.Asheatistransferredtothewater,thetemperatureincreaseappreciably,thespecificvolumeincreaseslightly,andthepressureremainsconstant.Whenthetemperaturereaches99.6℃,additionalheattransferresultsinachangeofphase,asindicatedinFig.1-1(b).Thatis,someoftheliquidbecomesvapor,andduringthisprocessboththetemperatureandpressureremainconstant,butthespecificvolumeincreasesconsiderably.Whenthelastdropofliquidhasvaporized,furthertransferofheatresultsinanincreaseinbothtemperatureandspecificvolumeofthevapor,asshowninFig.1-1(c).如圖1-1(a)所示，由活塞和氣缸組成的裝置中裝有1kg水。假定活塞和其上的重物使氣缸內(nèi)壓強(qiáng)維持在0.1Mpa，初始溫度20℃。當(dāng)有熱量開始傳遞給水時，缸內(nèi)水溫迅速上升，而比容略有增加，氣缸內(nèi)壓強(qiáng)保持恒定不變。當(dāng)水溫達(dá)到99.6℃時，如若再增加傳熱量，水將發(fā)生相變，如圖1-1(b)所示。也就是說，一部分水開始?xì)饣優(yōu)檎羝?，在此相變過程中，溫度和壓強(qiáng)始終保持不變，但比容卻有大幅度的增加。當(dāng)最后一滴液體被氣化時，進(jìn)一步的加熱將使蒸汽溫度和比容均有所增加，如同1-1(c)所示。圖1-1液體在常壓下的蒸發(fā)過程Thetermsaturationtemperaturedesignatesthetemperatureatwhichvaporizationtakesplaceatagivenpressure.Thispressureiscalledthesaturationpressureforthegiventemperature.Thus,forwaterat99.6℃,thesaturationpressureis0.1MPa,andforwaterat0.1MPathesaturationtemperatureis99.6℃.在給定壓強(qiáng)下發(fā)生氣化的溫度稱為飽和溫度，壓強(qiáng)稱為給定溫度下的飽和壓強(qiáng)。因此，99.6℃水的飽和壓強(qiáng)是0.1MPa，0.1MPa水的飽和溫度為99.6℃。Ifasubstanceexistsasliquidatthesaturationtemperature,itiscalledsaturatedliquid.Ifthetemperatureoftheliquidislowerthanthesaturationtemperaturefortheexistingpressure,itiscalledeitherasubcooledliquid(implyingthatthetemperatureislower教材2頁thanthesaturationtemperatureforthegivenpressure)oracompressedliquid(implyingthatthepressureisgreaterthanthesaturationpressureforthegiventemperature).如果某一工質(zhì)為液態(tài)并處于其飽和溫度和飽和壓強(qiáng)下，則稱該液體為飽和液體。如果液體溫度低于當(dāng)前壓強(qiáng)下的飽和溫度，則稱該液體為過冷液體（表明液體的當(dāng)前溫度低于給定壓強(qiáng)下的飽和溫度）或壓縮液體（表明液體的當(dāng)前壓強(qiáng)大于給定溫度下的飽和壓強(qiáng)）。Whenasubstanceexistsaspartliquidandpartvaporatthesaturationtemperature,itsqualityisdefinedastheratioofthemassofvaportothetotalmass.Thus,inFig.1-1(b),ifthemassofvaporis0.2kgandthemassofliquidis0.8kg,thequalityis0.2or20%.Qualityhasmeaningonlywhenthesubstanceisinasaturatedstate.若某一工質(zhì)在飽和溫度下以液、氣共存的形式存在，則稱蒸汽質(zhì)量與總質(zhì)量之比為干度。因此，如圖1-1(b)所示，若蒸汽質(zhì)量為0.2kg，液體質(zhì)量為0.8kg，則其干度為0.2或20%。干度只有在飽和狀態(tài)下才有意義。Ifasubstanceexistsasvaporatthesaturationtemperature,itiscalledsaturationvapor(Sometimesthetermdrysaturationvaporisusedtoemphasizethatthequalityis100%).Whenthevaporisatatemperaturegreaterthanthesaturationtemperature,itissaidtoexistassuperheatedvapor.Thepressureandtemperatureofsuperheatedvaporareindependentproperties,sincethetemperaturemayincreasewhilethepressureremainsconstant.若某一工質(zhì)處于飽和溫度下并以蒸汽形態(tài)存在，則稱該蒸汽為飽和蒸汽（有時稱為干飽和蒸汽，意在強(qiáng)調(diào)其干度為100%）。當(dāng)蒸汽溫度高于其飽和溫度時，則稱之為過熱蒸汽。過熱蒸汽的壓強(qiáng)和溫度是彼此獨(dú)立的，因?yàn)闇囟壬仙龝r，壓強(qiáng)可能保持不變。Letusplotonthetemperature-valuediagramofFig.1-2theconstant-pressurelinethatrepresentsthestatesthroughwhichthewaterpassesasitisheatedfromtheinitialstateof0.1MPaand20℃.LetstateArepresenttheinitialstate,Bthesaturated-liquidstate(99.6℃),andlineABtheprocessinwhichtheliquidisheatedfromtheinitialtemperaturetothesaturationtemperature.PointCisthe.....saturated-vaporstate,andlineBCistheconstant-temperatureprocess.saturated-vaporstate,andlineBCistheconstant-temperatureprocessinwhichthechangeofphasefromliquidtovaporoccurs.LineCDrepresents[]theprocessinwhichthesteamissuperheatedatconstantpressure.Temperatureandvolumebothincreaseduringthisprocess.在圖1-2所示的溫度-比容圖上作等壓線，表示水由初壓MPa0.1、初溫20℃被加熱的過程。點(diǎn)A代表初始狀態(tài)，點(diǎn)B為飽和液態(tài)（99.6℃），線AB表示液體由初始溫度被加熱至飽和溫度所經(jīng)歷的過程。點(diǎn)C表示飽和蒸汽狀態(tài)，線BC表示等溫過程，即液體氣化轉(zhuǎn)變?yōu)檎羝倪^程。線CD表示在等壓條件下蒸汽被加熱至過熱的過程，在此過程中，溫度和比容均增大。圖1-2溫度-比容曲線表1-1一些物質(zhì)的臨界參數(shù)Inasimilarname,aconstantpressureof10MPaisrepresentedbylineIJKL,forwhichthesaturationtemperatureis311.1℃.Atapressureof22.09MPa,representedbylineMNO,wefind,however,thatthereisnoconstant-temperaturevaporizationprocess.Instead,pointNisapointofinflectionwithazeroslope.Thispointiscalledthecriticalpoint.Atthecriticalpointthesaturated-liquidandsaturated-vaporstatesareidentical.Thetemperature,pressureandspecificvolumeatcriticalpointarecalledthecriticaltemperature,criticalpressureandcriticalvolume.Thecritical-pointdataforsomesubstancesaregiveninTable1-1.類似地，線IJKL表示壓強(qiáng)為10MPa下的等壓線，相應(yīng)的飽和溫度為311.1℃。但是，在壓強(qiáng)為22.09MPa條件下（線MNO），不存在等溫蒸發(fā)過程。相反，點(diǎn)N是個轉(zhuǎn)折點(diǎn)，在該點(diǎn)上，切線斜率為零，通常把N點(diǎn)稱為臨界點(diǎn)。在臨界點(diǎn)處，飽和液體和飽和氣體的狀態(tài)都是相同的。臨界點(diǎn)下的溫度、壓強(qiáng)和比容分別稱為臨界溫度、臨界壓強(qiáng)和臨界比容。一些工質(zhì)的臨界點(diǎn)數(shù)據(jù)如表1-1所示。1.1.4ThefirstlawofthermodynamicsThefirstlawofthethermodynamicsiscommonlycalledthelawofconservationofenergy.教材3頁.Inelementaryphysicscourses,thestudyofconservationofenergy.Inelementaryphysicscourses,thestudyofconservationofenergyemphasizeschangesinkineticandpotentialenergyandtheirrelationshiptowork.Amoregeneralformofconservationofenergyincludestheeffectsofheattransferandinternalenergychanges.Otherformsofenergycouldalsobeincluded,suchaselectrostatic,magnetic,strainandsurfaceenergy.1.1.4熱力學(xué)第一定律通常把熱力學(xué)第一定律稱為能量守恒定律。在基礎(chǔ)物理課程中，能量守恒定律側(cè)重動能、勢能的變化以及和功之間的相互關(guān)系。更為常見的能量守恒形式還包括傳熱效應(yīng)和內(nèi)能的變化。當(dāng)然，也包括其它形式的能，如靜電能、磁場能、應(yīng)變能和表面能。Historically,[]thefirstlawofthermodynamicswasstatedforacycle:thenetheattransferisequaltothenetworkdoneforasystemundergoingacycle.歷史上，用熱力學(xué)第一定律來描述循環(huán)過程：凈傳熱量等于循環(huán)過程中對系統(tǒng)所做的凈功。1.1.5ThesecondlawofthermodynamicsThesecondlawofthermodynamicscanbestatedinavarietyofways.Herewepresenttwo:theClausiusstatementandtheKelvin-Planckstatement.1.1.5熱力學(xué)第二定律熱力學(xué)第二定律有多種表述形式。在此列舉兩種：克勞修斯表述和凱爾文-普朗克表述。ClausiusstatementItisimpossibletoconstructadevicethatoperatesinacycleandwhosesoleeffectistransferofheatfromacoolerbodytoahotterbody.克勞修斯表述：制造一臺唯一功能是把熱量從低溫物體傳給高溫物體的循環(huán)設(shè)備是不可能的。圖1-3第二定律的違背Thisstatementrelatestoarefrigerator(oraheatpump).Itstatesthatitisimpossibletoconstructarefrigeratorthattransfersenergyfromacoolerbodytoahotterbodywithouttheinputofwork;thisviolationisshowninFig.1-3(a).以冰箱（或熱泵）為例，不可能制造一臺不用輸入功就能把熱量從低溫物體傳給高溫物體的冰箱，如圖1-3(a)所示。Itisimpossibletoconstructadevicethatoperatesinacycleandproducesnoothereffectthantheproductionofworkandthetransferofheatfromasinglebody.凱爾文-普朗克表述：制造一臺從單一熱源吸熱和做功的循環(huán)設(shè)備是不可能的。Inotherwords,itisimpossibletoconstructaheatenginethatextractsenergyfromreservoir,doeswork,anddoesnottransferheattoalow-temperaturereservoir.Thisrulesoutanyheatenginethatis100percentefficient,liketheoneshowninFig.1-3(b).換句話說，制造這樣一臺從某一熱源吸熱并對外做功，而沒有與低溫?zé)嵩催M(jìn)行換熱的熱機(jī)是不可能的。因此，該表述說明了不存在工作效率為100%的熱機(jī)，如圖1-3(b)所示。1.1.6TheCarnotCycleTheheatenginethatoperatesmostefficientlybetweenahigh-temperaturereservoirandalow-temperaturereservoiristheCarnotengine.Thisisanidealenginethatusesreversibleprocesstoformitscycleofoperation;suchacycleisCarnotcycle.TheCarnotengineisveryuseful,sinceitsefficiencyestablishesthemaximumpossibleefficiencyofanyrealengine.IftheefficiencyofarealissignificantlylowerthantheefficiencyofCarnotenginebetweenthesametemperaturelimits,thenadditionalimprovementsmaybepossible.1.1.6卡諾循環(huán)卡諾機(jī)是低溫?zé)嵩春透邷責(zé)嵩撮g運(yùn)行效率最高的熱機(jī)。卡諾機(jī)是一個理想熱機(jī)，利用多個可逆過程組成一循環(huán)過程，該循環(huán)稱為卡諾循環(huán)?？ㄖZ機(jī)非常有用，因?yàn)樗倪\(yùn)行效率為任何實(shí)際熱機(jī)最大可能的效率。因此，如果一臺實(shí)際熱機(jī)的效率要遠(yuǎn)低于同樣條件下的卡諾機(jī)效率，則有可能對該熱機(jī)進(jìn)行一些改進(jìn)以提高其效率。......圖1-4卡諾循環(huán)TheidealCarnotcycleinFig.1-4iscomposedoffourreversibleprocesses:12:Isothermalexpansion:23；Adiabaticreversibleexpansion:34；Isothermalcompression：41；Adiabaticreversiblecompression.TheefficiencyofaCarnotcycleis教材4頁T1LTHNotethattheefficiencyisincreasedbyraisingthetemperatureTHatwhichheatisaddedorbydecreasingthetemperatureTatwhichheatLisrejected.理想的卡諾循環(huán)包括四個可逆過程，如圖1-4所示：1→2等溫膨脹；2→3絕熱可逆膨脹；3→4等溫壓縮；4→1可逆絕熱壓縮。卡諾循環(huán)的效率為T1L(1-1)TH注意，提高T（提高吸熱溫度）或降低T（降低放熱溫度）均可使循環(huán)效率提高。 H L1.1.7TheRankinecycleThefirstclassofpowercyclesthatweconsiderarethoseutilizedbytheelectricpowergeneratingindustry,namely,powercyclesthatoperatesinsuchawaythattheworkingfluidchangesphasesfromaliquidtovapor.Thesimplestvapor-powercycleiscalledtheRankinecycle,shownschematicallyinFig.1-5(a).Amajorfeatureofsuchcycleisthatthepumprequiresverylittleworktodeliverhigh-pressurewatertotheboiler.Apossibledisadvantageisthattheexpansionprocessintheturbineusuallyentersthequalityregion,resultingintheformationofliquiddropletsthatmaydamagetheturbineblades.1.1.7朗肯循環(huán)我們所關(guān)心的第一類動力循環(huán)為電力生產(chǎn)工業(yè)所采用的，也就是說，動力循環(huán)按這樣的方式運(yùn)行：工質(zhì)發(fā)生相變，由液態(tài)變?yōu)闅鈶B(tài)。最簡單的蒸汽-動力循環(huán)是朗肯循環(huán)，如圖1-5(a)所示。朗肯循環(huán)的一個主要特征是泵耗費(fèi)很少的功就能把高壓水送入鍋爐。其可能的缺點(diǎn)為工質(zhì)在汽機(jī)內(nèi)膨脹做功后，通常進(jìn)入濕蒸汽區(qū)，形成可能損害汽輪機(jī)葉片的液滴。圖1-5朗肯循環(huán)TheRankinecycleisanidealizedcycleinwhichfrictionlossesineachofthefourcomponentsareneglected.Thelossesusuallyarequitesmallandwillbeneglectedcompletelyininitialanalysis.TheRankinecycleiscomposedofthefouridealprocessesshownontheT-sdiagraminFig.1-5(b):12:Isentropiccompressioninapump；23:Constant-pressureheatadditioninaboiler;34:Isentropicexpansioninaturbine;41:Constantpressureheatrejectioninacondenser.朗肯循環(huán)是一個理想循環(huán)，其忽略了四個過程中的摩擦損失。這些損失通常很小，在初始分析時可完全忽略。朗肯循環(huán)由四個理想過程組成，其T-s圖如圖1-5(b)所示：1→2為泵內(nèi)等熵壓縮過程；2→3為爐內(nèi)定壓吸熱過程；3→4為汽輪機(jī)內(nèi)等熵膨脹做功過程；4→1為凝汽器內(nèi)定壓放熱過程。Thepumpisusedtoincreasethepressureofthesaturatedliquid.Actually,states1and2areessentiallythesame,sincethehigh-pressurelinesareextremelyclosetothesaturationcurve;theyareshownseparatedforillustrationonly.Theboiler(alsocalledasteamgenerator)andthecondenserareheatexchangersthatneitherrequirenorproduceanywork.泵用于提高飽和液體的壓強(qiáng)。事實(shí)上，狀態(tài)1和狀態(tài)2幾乎完全一樣，因?yàn)橛?點(diǎn)開始的較高壓強(qiáng)下的吸熱過程線非常接近飽和曲線，圖中僅為了解釋說明的需要分別標(biāo)出。鍋爐（也稱蒸汽發(fā)生器）和凝汽器均為換熱器，它們既不需要功也不產(chǎn)生功。Ifweneglectkineticenergyandpotentialenergychanges,thenetworkoutputistheareaundertheT-sdiagram,representedbyarea1-2-3-4-1ofFig.1-5(b);thisistruesincethefirstlawrequiresthatW=Q.Theheattransfertotheworkingsubstanceisrepresentedbyareaout neta-2-3-b-a.Thus,thethermalefficiencyηoftheRankinecycleisarea12341areaa23baThatis,thedesiredoutputdividedbytheenergyinput(thepurchasedenergy).Obviously,教材5頁thethermalefficiencycanbeimprovedbyincreasingthenumeratororbydecreasingthenominator.Thiscanbedonebyincreasingthepumpoutletpressurep,increasingtheboileroutlettemperatureT,ordecreasing 2 3theturbineoutletpressurep.如果忽略動能和勢能的變化，輸?shù)膬艄Φ扔赥-s圖曲線下面的面積，即圖1-5(b)中1-2-3-4-1所包圍的面積，由用熱力學(xué)第一定律可證明WQ。循環(huán)過程中工質(zhì)的吸熱量 net net對應(yīng)面積a-2-3-b-a。因此，朗肯循環(huán)的熱效率可表示為面積12341(1-2)面積a23ba即，熱效率等于輸出能量除以輸入能量（所購能量）。顯然，通過增大分子或減小分母均可以提高熱效率。這可以通過增大泵出口壓強(qiáng)p，提高鍋爐出口溫度T，或降低汽機(jī)出口壓 2 3強(qiáng)p來實(shí)現(xiàn)。41.1.8TheReheatcycleItisapparentthatwhenoperatinginaRankinecyclewithahighboilerpressureoralowcondenserpressureitisdifficulttopreventliquiddropletsfromforminginthelow-pressureportionoftheturbine.Sincemostmetalcannotwithstandtemperaturesaboveabout600℃,thereheatcycleisoftenusedtopreventliquid-dropletformation:thesteampassingthroughtheturbineisreheatatsomeintermediatepressure,therebyraisingthetemperaturetostate5intheT-sdiagramofFig.1-6.Thesteamthenpassesthroughthelow-pressuresectionoftheturbineandentersthecondenseratstate6.Thiscontrolsorcompletelyeliminatesthemoistureproblemintheturbine.Thereheatcycledosenotsignificantlyinfluencesthethermalefficiencyofthecycle,butitdoesresultinasignificantadditionalworkoutput,representedinthefigurebyarea4-5-6-4’-4ofFig.1-6.Thereheatcycledemandsasignificantinvestmentinadditionalequipment,andtheuseofsuchequipmentmustbeeconomicallyjustifiedbytheincreasedworkoutput.Ifreheatisnotusedtoavoiddropletformation,thecondenserpressuremustbequitehigh,resultingrelativelylowcycleefficiency.Inthatsense,reheatsignificantlyincreasecycleefficiencywhencomparedtocyclewithnoreheatbutwiththehighercondenserpressure.1.1.8再熱循環(huán)對于一個處于高鍋爐壓強(qiáng)和低凝汽器壓強(qiáng)條件下的朗肯循環(huán)，顯然，很難阻止液滴在汽輪機(jī)低壓部分的形成。由于大多數(shù)金屬不能承受600℃以上的高溫，因此，通常采用再熱循環(huán)來防止液滴的形成。再熱過程如下：經(jīng)過汽輪機(jī)的部分蒸汽在某中間壓強(qiáng)下被再熱，從而提高蒸汽溫度，直至達(dá)到狀態(tài)5，如圖1-6所示。然后這部分蒸汽進(jìn)入汽輪機(jī)低壓缸，而后進(jìn)入凝汽器（狀態(tài)6）。再熱循環(huán)方式可以控制或者完全消除汽輪機(jī)中的濕蒸汽問題，因此，通常汽輪機(jī)分成高壓缸和低壓缸兩部分。雖然再熱循環(huán)不會顯著影響循環(huán)熱效率，但帶來了顯著的額外的輸出功，如圖1-6中的面積4-5-6-4-4所示。當(dāng)然，再熱循環(huán)需要一筆可觀的投資來購置額外的設(shè)備，這些設(shè)備的使用效果必須通過與多增加的輸出功進(jìn)行經(jīng)濟(jì)性分析來判定。如果不采用再熱循環(huán)來避免液滴的形成，則凝汽器出口壓強(qiáng)必須相當(dāng)?shù)馗撸蚨鴮?dǎo)致循環(huán)熱效率較低。在這種意義上，與無再熱循環(huán)且高凝汽器出口壓強(qiáng)的循環(huán)相比，再熱可以顯著提高循環(huán)效率。圖1-6再熱循環(huán)1.2FundamentalofFluidMechanicsFluidmotionsmanifestthemselvesinmanydifferentways.Somecanbedescribedveryeasily,whileothersrequireathoroughunderstandingofphysicallaws.Inengineeringapplications,itisimportanttodescribethefluidmotionsassimplyascanbejustified.Thisusuallydependsontherequiredaccuracy.Often,accuraciesof±10%areacceptable,althoughinsomeapplicationshigheraccuracieshavetobeachieved.Thegeneralequationsofmotionareverydifficulttosolve;consequently,itistheengineer’sresponsibilitytoknowwhichsimplifyingassumptionscanbemade.This,ofcourse,requiresexperienceand,moreimportant,教材6頁aunderstandingofthephysicsinvolved.1.2流體力學(xué)基礎(chǔ)流體運(yùn)動表現(xiàn)出多種不同的運(yùn)動形式。有些可以簡單描述，而其它的則需要完全理解其內(nèi)在的物理規(guī)律。在工程應(yīng)用中，盡量簡單地描述流體運(yùn)動是非常重要的。簡化程度通常取決于對精確度的要求，通?？梢越邮堋?0%左右的誤差，而有些工程應(yīng)用則要求較高的精度。描述運(yùn)動的一般性方程通常很難求解，因此，工程師有責(zé)任了解可以進(jìn)行哪些簡化的假設(shè)。當(dāng)然，這需要豐富的經(jīng)驗(yàn)，更重要的是要深刻理解流動所涉及的物理內(nèi)涵。Somecommonassumptionsusedtosimplifyaflowsituationarerelatedtofluidproperties.Forexample,undercertainconditions,theviscositycanaffecttheflowsignificantly;inothers,viscouseffectscanbeneglectedgreatlysimplifyingtheequationswithoutsignificantlyalteringthepredictions.Itiswellknownthatthecompressibilityeffectsdonothavetobetakenintoaccounttopredictwindforcesonbuildingsortopredictanyotherphysicalquantitythatisadirecteffectofwind.Afterourstudyoffluidmotions,theappropriateassumptionsusedshouldbecomemoreobvious.Hereweintroducesomeimportantgeneralapproachesusedtoanalyzefluidmechanicsandgiveabriefoverviewofdifferenttypesofflow.一些常見的用來簡化流動狀態(tài)的假設(shè)是與流體性質(zhì)有關(guān)系的。例如，黏性在某些條件下對流體有顯著的影響；而在其它條件下，忽略黏性效應(yīng)的影響可以大大地簡化方程，但并不會顯著改變計算結(jié)果。眾所周知，氣體速度很高時必須考慮其壓縮性，但在預(yù)測風(fēng)力對建筑物的影響程度，或者預(yù)測受風(fēng)力直接影響的其它物理量時，可以不計空氣的壓縮性。學(xué)完流體運(yùn)動學(xué)之后，可以更明顯地看出采用了哪些恰當(dāng)?shù)募僭O(shè)。這里，將介紹一些重要的用來分析流體力學(xué)問題的一般性方法，并簡要介紹不同類型的流動。1.2.1LagrangianandEulerianDescriptionsofMotionInthedescriptionofaflowfield,itisconvenienttothinkofindividualparticlesofwhichisconsideredtobeasmallmassoffluid,consistingofalargenumberofmoleculesthatoccupiesasmallvolumethatmoveswiththeflow.Ifthefluidisincompressible,thevolumedoesnotchangeinmagnitudebutmaydeform.Ifthefluidiscompressible,asthevolumedeforms,italsochangesitsmagnitude.Inbothcasestheparticlesareconsideredtomovethroughaflowfieldasanentity.1.2.1拉格朗日運(yùn)動描述和歐拉運(yùn)動描述描述流場時，將著眼點(diǎn)放在流體質(zhì)點(diǎn)上是非常方便的。每個質(zhì)點(diǎn)都包含了微小質(zhì)量的流體，它由大量分子組成。質(zhì)點(diǎn)占據(jù)很小的體積，并隨流體流動而移動。對不可壓縮流體，其體積大小不變，但可能發(fā)生形變。對可壓縮流體，不但體積發(fā)生形變，而且大小也將改變。在上述兩種情況下，均將所有質(zhì)點(diǎn)看作一個整體在流場中運(yùn)動。Inthestudyofparticlemechanics,whereattentionisfocusedonindividualparticles,motionisobservedasafunctionoftime.Theposition,velocityandaccelerationofeachparticlearelistedass(x,y,z,t),V(x,y,z,t)anda(x,y,z,t)andquantitiesofinterestcan 0 0 0 0 0 0 0 0 0becalculated.Thepoint(x,y,z)locatesthestartingpointthename-of 0 0 0eachparticle.ThisistheLagrangiandescription,namedafterJosephL.Lagrange,ofmotionthatisusedinacourseondynamics.IntheLagrangiandescriptionmanyparticlescanbefollowedandtheirinfluenceononeanothernoted.Thisbecomes,however,adifficulttaskasthenumberofparticlesbecomesextremelylarge,asinafluidflow.質(zhì)點(diǎn)力學(xué)主要研究單個質(zhì)點(diǎn)，質(zhì)點(diǎn)運(yùn)動是時間的函數(shù)。任一質(zhì)點(diǎn)的位移、速度和加速度可表示為s(x,y,z,t)，V(x,y,z,t)，a(x,y,z,t)，其它相關(guān)參量也可計 0 0 0 0 0 0 0 0 0算。坐標(biāo)(x,y,z)表示質(zhì)點(diǎn)的起始位置，也是每個質(zhì)點(diǎn)的名字。這就是拉格朗日運(yùn)動描 0 0 0述，以約瑟夫Ｌ拉格朗日的名字命名，該描述方法通常用于質(zhì)點(diǎn)動力學(xué)分析。拉格朗日法跟蹤多個質(zhì)點(diǎn)的運(yùn)動過程并考慮質(zhì)點(diǎn)間的相互作用。然而，由于實(shí)際流體包含質(zhì)點(diǎn)數(shù)目巨大，因而采用拉格朗日法研究流體流動則非常困難。Analternativetofollowingeachfluidparticleseparatelyistoidentifypointsinspaceandthenobservethevelocityofparticlespassingeachpoint;wecanobservetherateofchangeofvelocityastheparticlespasseachpoint,thatis,V/x，V/y，V/zandwecanobserveifthevelocityischangingwithtimeateachparticularpoint,thatis,V/t.InthisEuleriandescription,namedafterLeonhardEuler,ofmotion,theflowproperties,suchasvelocity,arefunctionsofbothspaceandtime.Inrectangular,CartesiancoordinatesthevelocitiesexpressedasV=V(x,y,z,t).Theregionofflowthatisconsiderediscalledaflowfield.與分別跟蹤每個流體質(zhì)點(diǎn)不同的另一種方法是將著眼點(diǎn)放在空間點(diǎn)上，然后觀察質(zhì)點(diǎn)經(jīng)過每個空間點(diǎn)時的質(zhì)點(diǎn)速度，由此可以得到質(zhì)點(diǎn)流經(jīng)各空間點(diǎn)時的速度變化率，即V/x，V/y，V/z；還可以判斷某一點(diǎn)上的速度是否隨時間變化，即計算V/t。這種描述方法稱為歐拉運(yùn)動描述，以萊昂哈德歐拉的名字命名。在歐拉法中，速度等流動參數(shù)是空間和時間的函數(shù)。在直角笛卡兒坐標(biāo)系中，速度表示為V=V(x,y,z,t)。我們所研究的流動區(qū)域稱為流場。1.2.2PathlinesandstreamlinesTwodifferentlineshelpusindescribingaflowfield.Apathlineisthefocusofpointstraversedbyagivenparticleasittravelsinafieldofflow;thepathlineprovidesuswitha“history”oftheparticle’slocations.Aphotographofapathlinewouldrequiredatimeexposureofanilluminatedparticle.教材7頁1.2.2跡線和流線可采用兩種不同的流動線來幫助我們描述流場。跡線是某一給定質(zhì)點(diǎn)在流場中運(yùn)動時所經(jīng)過的不同空間點(diǎn)形成的軌跡，它記錄了質(zhì)點(diǎn)的“歷史”位置。一定曝光時間下可以拍得發(fā)亮粒子的運(yùn)動跡線。Astreamlineisalineintheflowpossessingthefollowingproperty:thevelocityvectorofeachparticleoccupyingapointonthestreamlineistangenttothestreamline,thatis,V×dr=0.SinceVanddrareinthesamedirection;recallthatthecrossproductoftwovectorsinthesamedirectioniszero.Aphotographofastreamlinescannotbemadedirectly.Forageneralunsteadyflowthestreamlinescanbeinferredfromphotographsofshortpathlinesofalargenumberofparticles.流線是流場中具有這樣特性的線：任一質(zhì)點(diǎn)在流線上某點(diǎn)處的速度矢量與該流線相切，即Vdr=0。這是因?yàn)閂和dr具有相同的方向，而具有相同方向的兩個矢量的叉乘積等于零。同跡線相比，流線不能直接由相機(jī)拍攝獲得。對于一般的非定常流動，根據(jù)大量質(zhì)點(diǎn)的短跡線相片可以推斷出流線的形狀。1.2.3One-,two-,andthree-dimensionalflowsIntheEuleriandescriptionofmotionthevelocityvector,ingeneral,dependsonthreespacevariablesandtime,thatis,V=V(z,y,z,t).Suchaflowisathree-dimensionalflow,becausethevelocityvectordependsonthreespacecoordinates.Thesolutionstoproblemsinsuchaflowareverydifficultandarebeyondthespaceofanintroductorycourse.Eveniftheflowcouldbeassuredtobesteady[i.e,V=V(x,y,z)],itwouldremainathree-dimensionalflow.1.2.3一維、二維和三維流動一般來說，歐拉運(yùn)動描述中的速度矢量取決于三個空間變量和時間變量，即V=V(x,y,z,t)。這樣的流動稱為三維流動，因?yàn)樗俣仁噶恳蕾囉谌齻€空間坐標(biāo)。三維流動的求解非常困難，并且也超出了序言的范圍。即使假設(shè)流動為定常的（如，V=V(x,y,z)），該流動仍為三維流動。Oftenathreedimensionalfl