大學(xué)熱動專業(yè)英語1-2章翻譯

SpecializedEnglishforThermalEnergy&PowerEngineeringCOURSEOUTLINETextbook:熱能與動力工程專業(yè)英語〔SpecializedEnglishforThermalEnergy&PowerEngineering〕(3th)閻維平，中國電力出版社〔第三版〕COURSEOUTLINECourseGoals:TounderstandthebasiccharacteristicsofSpecializedEnglish.Torecognizesometechnicalwordsinthermalenergyandpowerengineering.Toknowhowtowritetheabstractforapaperorathesis(P155).Grading:Exercisesintheclass20% Finalexam80%ContentsChapter1IntroductiontoThermalSciences1.1Fundamentalofengineeringthermodynamics1.2Fundamentaloffluidmechanics1.3FundamentalofheattransferChapter2Boiler2.1Introduction2.2Developmentofutilityboiler2.3Fuelandcombustion2.4Pulverizingsystem2.5Systemarrangementandkeycomponents2.6On-loadcleaningofboilers2.7EnergybalanceChapter1IntroductiontoThermalSciences1.1FundamentalofengineeringthermodynamicsThermodynamicsisascienceinwhichthestorage,transformation,andtransferofenergyarestudied.Energyisstoredasinternalenergy(associatedwithtemperature),kineticenergy(duetomotion),potentialenergy(duetoelevation)andchemicalenergy(duetochemicalcomposition);itistransformedfromoneoftheseformstoanother;anditistransferredacrossaboundaryaseitherheatorwork.第一章熱科學(xué)介紹1.1工程熱力學(xué)根底熱力學(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,wewillderiveequationsthatrelatethetransformationandtransfersofenergytopropertiessuchastemperature,pressure,anddensity.Substancesandtheirproperties,thus,becomeveryimportantinthermodynamics.Manyofourequationswillbebasedonexperimentalobservationthathavebeenorganizedintomathematicalstatementsorlaws;thefirstandsecondlawsofthermodynamicsarethemostwidelyused.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)觀察的根底之上，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。1.1.1ThermodynamicsystemandcontrolvolumeAthermodynamicsystemisafixedquantityofmattercontainedwithinsomeenclosure.Thesurfaceisusuallyanobviousone(likethatsurroundingthegasinthecylinder).However,itmaybeanimaginedboundary(likethedeformingboundaryofacertainamountofmassasitflowsthroughapump).Allmatterandspaceexternaltoasystemiscollectivelycalleditssurroundings.Thermodynamicsisconcernedwiththeinteractionsofasystemanditssurroundings熱力系統(tǒng)和控制體熱力系統(tǒng)是一包圍在某一封閉邊界內(nèi)的具有固定質(zhì)量物質(zhì)的系統(tǒng)。系統(tǒng)邊界通常是比擬明顯的〔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〕。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,andaninflatingordeflatingballoonareexamplesofcontrolvolumes.Thesurfacethatcompletelysurroundsthecontrolvolumeiscalledacontrolsurface.Thus,wemustchoose,inaparticularproblem,whetherasystemistobeconsideredorwhetheracontrolvolumeismoreuseful.Ifthereismassfluxacrossaboundary,thenacontrolvolumeisrequired;otherwise,asystemisidentified.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。包含控制體的外表稱為控制外表。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，那么選取控制體有利；反之，那么應(yīng)選取系統(tǒng)作為研究對象。1.1.2Equilibrium,processandcycleWhenthetemperatureofasystemisreferredto,itisassumedthatallpointsofthesystemhavethesame,oressentiallythesame,temperature.Whenthepropertiesareconstantfrompointtopointandwhenthereisnotendencyforchargewithtime,aconditionofthermodynamicequilibriumexists.Ifthetemperature,say,issuddenlyincreasedatsomepartofthesystemboundary,spontaneousredistributionisassumedtooccuruntilallpartsofthesystemareatthesametemperature.Whenasystemchangesfromoneequilibriumstatetoanother,thepathofsuccessivestatesthroughwhichthesystempassesiscalledaprocess.If,inthepassingfromonestatestothenext,thedeviationfromequilibriumisinfinitesimal,aquasi-equilibriumprocessoccurs,andeachstateintheprocessmaybeidealizedasanequilibriumstate.Quasi-equilibriumprocessescanapproximatemanyprocesses,suchasthecompressionandexpansionofgasesinaninternalcombustionengine,withnosignificantlossofaccuracy.Ifthesystemgoesfromoneequilibriumstatetoanotherthroughaseriesofnonequilibriumstates(asincombustion),anonequilibriumprocessoccurs.平衡、過程和循環(huán)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，那么稱系統(tǒng)處于熱力學(xué)平衡狀態(tài)。當(dāng)系統(tǒng)邊界某局部的溫度突然上升時，那么系統(tǒng)內(nèi)的溫度將自發(fā)地重新分布，直至處處相同。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。假設(shè)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)，那么稱該過程為準(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)，那么其過程為非平衡過程。Whenasysteminagiveninitialstateexperiencesaseriesofprocessesandreturnstotheinitialstate,thesystemundergoesacycle.Attheendofthecycle,thepropertiesofthesystemhavethesamevaluestheyhadatthebeginning.Theprefixiso-isattachedtothenameofanypropertythatremainsunchangedinaprocess.Atisothermalprocessisoneinwhichthetemperatureisheldconstant;inanisobaricprocess,thepressureremainsconstant;anisometricprocessisaconstant-volumeprocess.當(dāng)系統(tǒng)從一給定的初始狀態(tài)出發(fā)，經(jīng)歷一系列中間過程又回到其初始狀態(tài)，那么稱系統(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。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-liquidphaseequilibriuminapuresubstanceConsiderasasystem1kgofwatercontainedinthepiston/cylinderarrangementshowninFig.1-1(a).Supposethatthepistonandweightmaintainapressureof0.1MPainthecylinderandthattheinitialtemperatureis20℃.Asheatistransferredtothewater,thetemperatureincreasesappreciably,thespecificvolumeincreasesslightly,andthepressureremainsconstant.Whenthetemperaturereaches99.6℃,additionalheattransferresultsinachangeofphase,asindicatedinFig.1-1(b).Thatis,someoftheliquidbecomesvapor,andduringthisprocessboththetemperatureandpressureremainconstant,butthespecificvolumeincreasesconsiderably.Whenthelastdropofliquidhasvaporized,furthertransferofheatresultsinanincreaseinbothtemperatureandspecificvolumeofthevapor,asshowninFig.1-1(c).1.1.3純物質(zhì)的氣-液相平衡如圖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，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)水溫到達(dá)℃時，如假設(shè)再增加傳熱量，水將發(fā)生相變，如圖1-1(b)所示。也就是說，一局部水開始?xì)饣優(yōu)檎羝诖讼嘧冞^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。Thetermsaturationtemperaturedesignatesthetemperatureatwhichvaporizationtakeplaceatagivenpressure.Thepressureiscalledthesaturationpressureforthegiventemperature.Thus,forwaterat99.6℃thesaturationpressureis0.1MPa,andforwaterat0.1MPathesaturationtemperatureis99.6℃.Ifasubstanceexistsatthesaturationtemperatureandpressure,itiscalledsaturatedliquid.Ifthetemperatureoftheliquidislowerthanthesaturationtemperaturefortheexistingpressure,itiscalledeitherasubcooledliquid(implyingthatthetemperatureislowerthanthesaturationtemperatureforthegivenpressure)oracompressedliquid(implyingthatthepressureisgreaterthanthesaturationpressureforthegiventemperature).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，℃水的飽和壓強(qiáng)是，水的飽和溫度為℃。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，那么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，那么稱該液體為過冷液體〔說明液體的當(dāng)前溫度低于給定壓強(qiáng)下的飽和溫度〕或壓縮液體〔說明液體的當(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),ifthemassofthevaporis0.2kgandthemassoftheliquidis0.8kg,thequalityis0.2or20%.Qualityhasmeaningonlywhenthesubstanceisinasaturatedstate.Ifasubstanceexistsasvaporatthesaturationtemperature,itiscalledsaturatedvapor.(Sometimesthetermdrysaturatedvaporisusedtoemphasizethatthequalityis100%).Whenthevaporisatatemperaturegreaterthanthesaturationtemperature,itissaidtoexistassuperheatedvapor.Thepressureandtemperatureofsuperheatedvaporareindependentproperties,sincethetemperaturemayincreasewhilethepressureremainsconstant.假設(shè)某一工質(zhì)在飽和溫度下以液、氣共存的形式存在，那么稱蒸汽質(zhì)量與總質(zhì)量之比為干度。因此，如圖1-1(b)所示，假設(shè)蒸汽質(zhì)量為，液體質(zhì)量為，那么其干度為或20%。干度只有在飽和狀態(tài)下才有意義。假設(shè)某一工質(zhì)處于飽和溫度下并以蒸汽形態(tài)存在，那么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，那么稱之為過熱蒸汽。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-volumediagramofFig.1-2theconstant-pressurelinethatrepresentsthestatesthroughwhichthewaterpassesasitisheatedfromtheinitialstateof0.1MPaand20C.LetstateArepresenttheinitialstate,BC),andlineABtheprocessinwhichtheliquidisheatedfromtheinitialtemperaturetothesaturationtemperature.PointCisthesaturated-vaporstate,andlineBCistheconstant-temperatureprocessinwhichthechangeofphasefromliquidtovaporoccurs.LineCDrepresentstheprocessinwhichthesteamissuperheatedatconstantpressure.Temperatureandvolumebothincreaseduringthisprocess.在圖1-2所示的溫度-比容圖上作等壓線，表示水由初壓、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)〔℃〕，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。Inasimilarmanner,aconstantpressureof10MPaisrepresentedbylineIJKLC.Atapressureof22.09MPa,representedbylineMNO,wefind,however,thatthereisnoconstant-temperaturevaporizationprocess.Instead,pointNisapointofinflectionwithazeroslope.Thispointiscalledthecriticalpoint.Atthecriticalpointthesaturated-liquidandsaturated-vaporstatesareidentical.Thetemperature,pressure,andspecificvolumeatthecriticalpointarecalledthecriticaltemperature,criticalpressure,andcriticalvolume.Thecritical-pointdataforsomesubstancesaregiveninTable1-1.Fig.1-2Temperature-volumediagram.1.1.4ThefirstlawofthermodynamicsThefirstlawofthermodynamicsiscommonlycalledthelawofconservationofenergy.Inelementaryphysicscourses,thestudyofconservationofenergyemphasizeschangesinkineticandpotentialenergyandtheirrelationshiptowork.Amoregeneralformofconservationofenergyincludestheeffectsofheattransferandinternalenergychanges.Otherformsofenergycouldalsobeincluded,suchaselectrostatic,magnetic,strain,andsurfaceenergy.Historically,thefirstlawofthermodynamicswasstatedforacycle:thenetheattransferisequaltothenetworkdoneforasystemundergoingacycle.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。在根底物理課程中，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)變能和外表能。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.ClausiusStatementItisimpossibletoconstructadevicethatoperatesinacycleandwhosesoleeffectisthetransferofheatfromacoolerbodytoahotterbody.Thisstatementrelatestoarefrigerator(oraheatpump).Itstatesthatitisimpossibletoconstructarefrigeratorthattransfersenergyfromacoolerbodytoahotterbodywithouttheinputofwork;thisviolationisshowninFig.1-3a.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?？藙谛匏贡硎觯褐圃煲慌_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。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。Kelvin-PlanckStatementItisimpossibletoconstructadevicethatoperatesinacycleandproducesnoothereffectthantheproductionofworkandthetransferofheatfromasinglebody.Inotherwords,itisimpossibletoconstructaheatenginethatextractsenergyfromareservoir,doeswork,anddoesnottransferheattoalow-temperaturereservoir.Thisrulesoutanyheatenginethatis100percentefficient,liketheoneshowninFig.1-3b.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。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。1.1.6TheCarnotcycleTheheatenginethatoperatesthemostefficientlybetweenahigh-temperaturereservoirandalow-temperaturereservoiristheCarnotengine.Thisisanidealenginethatusesreversibleprocessestoformitscycleofoperation;suchacycleisCarnotcycle.TheCarnotengineisveryuseful,sinceitsefficiencyestablishesthemaximumpossibleefficiencyofanyrealengine.IftheefficiencyofarealengineissignificantlylowerthantheefficiencyofaCarnotenginebetweenthesamelimits,thenadditionalimprovementsmaybepossible.TheidealCarnotcycleinFig.1-4iscomposedoffourreversibleprocesses:1→2:Isothermalexpansion;2→3:Adiabaticreversibleexpansion;3→4:Isothermalcompression;4→1:Adiabaticreversiblecompression.TheefficiencyofaCarnotcycleis〔1-1〕NotethattheefficiencyisincreasedbyraisingthetemperatureTHwhichheatisaddedorbyloweringthetemperatureTLatwhichheatisrejected.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)?？ㄖZ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，那么有可能對該熱機(jī)進(jìn)行一些改良以提高其效率。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:〔1-1〕注意，提高TH〔提高吸熱溫度〕或降低TL〔降低放熱溫度〕均可使循環(huán)效率提高。1.1.7TheRankinecycleThefirstclassofpowercyclesthatweconsiderarethoseutilizedbytheelectricpowergeneratingindustry,namely,powercyclesthatoperateinsuchawaythattheworkingfluidchangesphasefromaliquidtoavapor.Thesimplestvapor-powercycleiscalledtheRankinecycle,shownschematicallyinFig.1-5a.Amajorfeatureofsuchacycleisthatthepumprequiresverylittleworktodeliverhigh-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消耗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。TheRankinecycleisanidealizedcycleinwhichfrictionlossesineachofthefourcomponentsareneglected.Thelossesusuallyarequitesmallandwillbeneglectedcompletelyininitialanalysis.TheRankinecycleiscomposedofthefouridealprocessesshownontheT-sdiagraminFig.1-5b:1→2:Isentropiccompressioninapump;2→3:Constant-pressureheatadditioninaboiler;3→4:Isentropicexpansioninaturbine;4→1:Constant-pressureheatrejectioninacondenser.Thepumpisusedtoincreasethepressureofthesaturatedliquid.Actually,states1and2areessentiallythesame,sincethehigh-pressurelinesareextremelyclosetothesaturationcurve;theyareshownseparatedforillustrationonly.Theboiler(alsocalledasteamgenerator)andthecondenserareheatexchangersthatneitherrequirenorproduceanywork.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。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);thisistruesincethefirstlawrequiresthatWnet=Qnet.Theheattransfertotheworkingsubstanceisrepresentedbyareaa-2-3-b-a.Thus,thethermalefficiencyηoftheRankinecycleis〔1-2〕thatis,thedesiredoutputdividedbytheenergyinput(thepurchasedenergy).Obviously,thethermalefficiencycanbeimprovedbyincreasingthenumeratororbydecreasingthedenominator.Thiscanbedonebyincreasingthepumpoutletpressurep2,increasingtheboileroutlettemperatureT3,ordecreasingtheturbineoutletpressurep4.如果忽略動能和勢能的變化，輸出的凈功等于T-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。循環(huán)過程中工質(zhì)的吸熱量對應(yīng)面積a-2-3-b-a。因此，朗肯循環(huán)的熱效率可表示為〔1-2〕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)p2，提高鍋爐出口溫度T3，或降低汽機(jī)出口壓強(qiáng)p4來實(shí)現(xiàn)。1.1.8TheReheatcycleItisapparentthatwhenoperatinginaRankinecyclewithahighboilerpressureoralowcondenserpressureitisdifficulttopreventliquiddropletsfromforminginthelow-pressureportionoftheturbine.Sincemostmetalcannotwithstandtemperaturesaboveabout600C,thereheatcycleisoftenusedtopreventliquid-dropletformation:thesteampassingthroughtheturbineisreheatedatsomeintermediatepressure,therebyraisingthetemperaturetostate5intheT-sdiagramofFig.1-6.Thesteamthenpassesthroughthelow-pressuresectionoftheturbineandentersthecondenseratstate6.Thiscontrolsorcompletelyeliminatesthemoistureproblemintheturbine.Often,theturbineisseparatedintoahigh-pressureturbineandalow-pressureturbine.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)低壓局部的形成。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)的局部蒸汽在某中間壓強(qiáng)下被再熱，從而提高蒸汽溫度，直至到達(dá)狀態(tài)5，如圖1-6所示。然后這局部蒸汽進(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局部。Thereheatcycledoesnotsignificantlyinfluencethethermalefficiencyofthecycle,butitdoesresultinasignificantadditionalworkoutput,representedinthefigurebyarea4-5-6-4-4ofFig.1-6.Thereheatcycledemandsasignificantinvestmentinadditionalequipment,andtheuseofsuchequipmentmustbeeconomicallyjustifiedbytheincreasedworkoutput.Ifreheatisnotusedtoavoiddropletformation,thecondenserpressuremustbequitehigh,resultingrelativelylowcycleefficiency.Inthatsense,reheatsignificantlyincreasescycleefficiencywhencomparedtothecyclewithnoheatbutwiththehighercondenserpressure.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)來防止液滴的形成，那么凝汽器出口壓強(qiáng)必須相當(dāng)?shù)馗?，因而?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。1.2FundamentalofFluidMechanicsFluidmotionsmanifestthemselvesinmanydifferentways.Somecanbedescribedveryeasily,whileothersrequireathoroughunderstandingofphysicallaws.Inengineeringapplications,itisimportanttodescribethefluidmotionsassimplyascanbejustified.Thisusuallydependsontherequiredaccuracy.Often,accuraciesof±10%areacceptable,althoughinsomeapplicationshigheraccuracieshavetobeachieved.Thegeneralequationsofmotionareverydifficulttosolve;consequently,itistheengineer'sresponsibilitytoknowwhichsimplifyingassumptionscanbemade.This,ofcourse,requiresexperienceand,moreimportant,anunderstandingofthephysicsinvolved.1.2流體力學(xué)根底流體運(yùn)動表現(xiàn)出多種不同的運(yùn)動形式。有些可以簡單描述，而其它的那么需要完全理解其內(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)用那么要求較高的精度。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.Itiswellknownthatthecompressibilityofagasinmotionshouldbetakenintoaccountifthevelocitiesareveryhigh.Butcompressibilityeffectsdonothavetobetakenintoaccounttopredictwindforcesonbuildingsortopredictanyotherphysicalquantitythatisadirecteffectofwind.Afterourstudyoffluidmotions,theappropriateassumptionsusedshouldbecomemoreobvious.Hereweintroducesomeimportantgeneralapproachesusedtoanalyzefluidmechanicsproblemsandgiveabriefoverviewofdifferenttypesofflow.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(jì)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(jì)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,itisconvenienttothinkofindividualparticleseachofwhichisconsideredtobeasmallmassoffluid,consistingofalargenumberofmolecules,thatoccupiesasmallvolumethatmoveswiththeflow.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,velocity,andaccelerationofeachparticlearelistedass(x0,y0,z0,t),V(x0,y0,z0,t),anda(x0,y0,z0,t)andquantitiesofinterestcanbecalculated.Thepoint(x0,y0,z0)locatesthestartingpoint--thename--ofeachparticle.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(x0,y0,z0,t)，V(x0,y0,z0,t)，a(x0,y0,z0,t)，其它相關(guān)參量也可計(jì)算。坐標(biāo)(x0,y0,z0)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那么非常困難。Analternativetofollowingeachfluidparticleseparatelyistoidentifypointsinspaceandthenobservethevelocityofparticlespassingeachpoint;wecanobservetherateofchangeofvelocityastheparticlespasseachpoint,thatis,V/x,V/yandV/zandwecanobserveifthevelocityischangingwithtimeateachparticularpoint,thatis,V/t.InthisEuleriandescription,namedafterLeonhardEuler,ofmotion,theflowproperties,suchasvelocity,arefunctionsofbothspaceandtime.Inrectangular,CartesiancoordinatesthevelocityexpressedasV=V(x,y,z,t).Theregionofflowthatisbeingconsiderediscalledaflowfield.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(jì)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.Apathlineisthelocusofpointstraversedbyagivenparticleasittravelsinafieldofflow;thepathlineprovidesuswitha"history"oftheparticle'slocations.Aphotographofapathlinewouldrequireatimeexposureofanilluminatedparticle.Astreamlineisalineintheflowpossessingthefollowingproperty:thevelocityvectorofeachparticleoccupyingapointonthestreamlineistangenttothestreamline,thatis,Vdr=0.SinceVanddrareinthesamedirection;recallthatthecrossproductoftwovectorsinthesamedirectioniszero.Aphotographofastreamlinecannotbemadedirectly.Forageneralunsteadyflowthestreamlinescanbeinferredfromphotographsofshortpathlinesofalargenumberofparticles.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。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(x,y,z,t).Suchaflowisathree-dimensionalflow,becausethevelocityvectordependsonthreespacecoordinates.Thesolutionstoproblemsinsuchaflowareverydifficultandarebeyondthescopeofanintroductorycourse.Eveniftheflowcouldbeassumedtobesteady[i.e.,V=V(x,y,z)],itwouldremainathree-dimensionalflow.Oftenathree-dimensionalflowcanbeapproximatedasatwo-dimensionalflow.Forexample,theflowoverawidedamisthree-dimensionalbecauseoftheendconditions,buttheflowinthecentralportionawayfromtheendscanbetreatedastwo-dimensional.Ingeneral,atwo-dimensionalflowisaflowinwhichthevelocityvectordependsononlytwospacevariables.Anexampleisaplaneflow,inwhichthevelocityvectordependsontwospatialcoordinates,xandy,butnotz[i.e.,V=V(x,y)].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。三維流動常?？梢越瞥啥S流動。例如，對于一個很寬的大壩，受壩兩端條件的影響，水流經(jīng)大壩時的流動為三維流動；但遠(yuǎn)離壩端的中間局部的流動可看作是二維的。一般來說，二維流動是指其速度矢量只取決于兩個空間坐標(biāo)的流動。平面流動即是如此，速度矢量只依賴于x，y兩個空間坐標(biāo)，而與z坐標(biāo)無關(guān)〔如，V=V(x,y)〕。Aone-dimensionalflowisaflowinwhichthevelocityvectordependsononlyonespacevariable.Suchflowsoccurinlong,straightpipesorbetweenparallelplates.Thevelocityinthepipevariesonlywithri.e.,u=u(r).Thevelocitybetweenparallelplatesvariesonlywiththecoordinateyi.e.,u=u(y).Eveniftheflowisunsteadysothatu=u(y,t),aswouldbethesituationduringstartup,theflowisone-dimensional.Asfordevelopedflows,thevelocityprofilesdonotvarywithrespecttothespacecoordinateinthedirectionofflow.Thisdemandsthattheregionofinterestbeasubstantialdistancefromanentranceorasuddenchangeingeometry.Therearemanyengineeringproblemsinfluidmechanicsinwhichaflowfieldissimplifiedtoauniformflow:thevelocity,andotherfluidproperties,areconstantoverthearea.Thissimplificationismadewhenthevelocityisessentiallyconstantoverthearea,arathercommonoccurrence.Examplesofsuchflowsarerelativelyhighspeedflowinapipesection,andflowinastream.Theaveragevelocitymaychangefromonesectiontoanother;theflowconditionsdependonlyonthespacevariableintheflowdirection.一維流動的速度矢量只依賴于一個空間坐標(biāo)。這類流動常發(fā)生在長直管內(nèi)和平行平板間。管內(nèi)流動的速度只隨到管軸的距離變化，即u=u(r)。平行平板間的速度也只與y坐標(biāo)有關(guān)，即u=u(y)。即使流動為非定常流動，如啟動時的情形，u=u(y,t)，但該流動仍是一維的。對于完全開展的流動，其速度輪廓線并不隨流動方向上的空間坐標(biāo)而改變。這要求研究區(qū)域要遠(yuǎn)離入口處或幾何形狀突然改變的區(qū)域。有許多流體力學(xué)方面的工程問題，其流場可以簡化為均勻流動：速度和其它流體特性參數(shù)在整個區(qū)域內(nèi)均為常數(shù)。這種簡化只對速度在整個區(qū)域內(nèi)均保持不變時才成立，而且這種情況非常普遍。例如：管內(nèi)的高速流動和溪水的流動。平均速度可能從一個斷面到另一個斷面有所不同，而流動條件僅取決于流動方向上的空間變量。1.2.4Newtonianfluidandnon-NewtonianfluidANewtonianfluid