大学热动专业英语1-2章翻译

SpecializedEnglishforThermalEnergy&PowerEngineeringCOURSEOUTLINETextbook:热能与动力工程专业英语〔SpecializedEnglishforThermalEnergy&PowerEngineering〕(3th)阎维平，中国电力出版社〔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.第一章热科学介绍1.1工程热力学根底热力学是一门研究能量储存、转换及传递的科学。能量以内能〔与温度有关〕、动能〔由物体运动引起〕、势能〔由高度引起〕和化学能〔与化学组成相关〕的形式储存。不同形式的能量可以相互转化，而且能量在边界上可以以热和功的形式进行传递。Inthermodynamics,wewillderiveequationsthatrelatethetransformationandtransfersofenergytopropertiessuchastemperature,pressure,anddensity.Substancesandtheirproperties,thus,becomeveryimportantinthermodynamics.Manyofourequationswillbebasedonexperimentalobservationthathavebeenorganizedintomathematicalstatementsorlaws;thefirstandsecondlawsofthermodynamicsarethemostwidelyused.在热力学中，我们将推导有关能量转化和传递与物性参数，如温度、压强及密度等关系间的方程。因此，在热力学中，物质及其性质变得非常重要。许多热力学方程都是建立在实验观察的根底之上，而且这些实验观察的结果已被整理成数学表达式或定律的形式。其中，热力学第一定律和第二定律应用最为广泛。1.1.1ThermodynamicsystemandcontrolvolumeAthermodynamicsystemisafixedquantityofmattercontainedwithinsomeenclosure.Thesurfaceisusuallyanobviousone(likethatsurroundingthegasinthecylinder).However,itmaybeanimaginedboundary(likethedeformingboundaryofacertainamountofmassasitflowsthroughapump).Allmatterandspaceexternaltoasystemiscollectivelycalleditssurroundings.Thermodynamicsisconcernedwiththeinteractionsofasystemanditssurroundings热力系统和控制体热力系统是一包围在某一封闭边界内的具有固定质量物质的系统。系统边界通常是比拟明显的〔如气缸内气体的固定边界〕。然而，系统边界也可以是假想的〔如一定质量的流体流经泵时不断变形的边界〕。系统之外的所有物质和空间统称外界或环境。热力学主要研究系统与外界或系统与系统之间的相互作用。系统通过在边界上进行能量传递，从而与外界进行相互作用，但在边界上没有质量交换。当系统与外界间没有能量交换时，这样的系统称为孤立系统。Inmanycases,ananalysisissimplifiedifattentionisfocusedonaparticularvolumeinspaceintowhich,orfromwhich,asubstanceflows.Suchavolumeisacontrolvolume.Apump,aturbine,andaninflatingordeflatingballoonareexamplesofcontrolvolumes.Thesurfacethatcompletelysurroundsthecontrolvolumeiscalledacontrolsurface.Thus,wemustchoose,inaparticularproblem,whetherasystemistobeconsideredorwhetheracontrolvolumeismoreuseful.Ifthereismassfluxacrossaboundary,thenacontrolvolumeisrequired;otherwise,asystemisidentified.在许多情况下，当我们只关心空间中有物质流进或流出的某个特定体积时，分析可以得到简化。这样的特定体积称为控制体。例如泵、透平、充气或放气的气球都是控制体的例子。包含控制体的外表称为控制外表。因此，对于具体的问题，我们必须确定是选取系统作为研究对象有利还是选取控制体作为研究对象有利。如果边界上有质量交换，那么选取控制体有利；反之，那么应选取系统作为研究对象。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、过程和循环对于某一参考系统，假设系统内各点温度完全相同。当物质内部各点的特性参数均相同且不随时间变化时，那么称系统处于热力学平衡状态。当系统边界某局部的温度突然上升时，那么系统内的温度将自发地重新分布，直至处处相同。当系统从一个平衡状态转变为另一个平衡状态时，系统所经历的一系列由中间状态组成的变化历程称为过程。假设从一个状态到达另一个状态的过程中，始终无限小地偏离平衡态，那么称该过程为准静态过程，可以把其中任一个中间状态看作为平衡状态。准静态过程可近似视为许多过程的叠加结果，而不会显著减小其精确性，例如气体在内燃机内的压缩和膨胀过程。如果系统经历一系列不平衡状态〔如燃烧〕，从一个平衡状态转变为另一个平衡状态，那么其过程为非平衡过程。Whenasysteminagiveninitialstateexperiencesaseriesofprocessesandreturnstotheinitialstate,thesystemundergoesacycle.Attheendofthecycle,thepropertiesofthesystemhavethesamevaluestheyhadatthebeginning.Theprefixiso-isattachedtothenameofanypropertythatremainsunchangedinaprocess.Atisothermalprocessisoneinwhichthetemperatureisheldconstant;inanisobaricprocess,thepressureremainsconstant;anisometricprocessisaconstant-volumeprocess.当系统从一给定的初始状态出发，经历一系列中间过程又回到其初始状态，那么称系统经历了一个循环。循环结束时，系统中的各参数又与初始参数相同。在任一特性参数名称前加上前缀iso-，表示该参数在整个过程保持不变。等温〔isothermal〕过程中温度保持不变；等压〔isobaric〕过程中压强恒定；等容〔isometric〕过程中体积保持不变。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纯物质的气-液相平衡如图1-1(a)所示，由活塞和气缸组成的装置中装有1kg水。假定活塞和其上的重物使气缸内压强维持在，初始温度20℃。当有热量开始传递给水时，缸内水温迅速上升，而比容略有增加，气缸内压强保持恒定不变。当水温到达℃时，如假设再增加传热量，水将发生相变，如图1-1(b)所示。也就是说，一局部水开始气化变为蒸汽，在此相变过程中，温度和压强始终保持不变，但比容却有大幅度的增加。当最后一滴液体被气化时，进一步的加热将使蒸汽温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).在给定压强下发生气化的温度称为饱和温度，压强称为给定温度下的饱和压强。因此，℃水的饱和压强是，水的饱和温度为℃。如果某一工质为液态并处于其饱和温度和饱和压强下，那么称该液体为饱和液体。如果液体温度低于当前压强下的饱和温度，那么称该液体为过冷液体〔说明液体的当前温度低于给定压强下的饱和温度〕或压缩液体〔说明液体的当前压强大于给定温度下的饱和压强〕。Whenasubstanceexistsaspartliquidandpartvaporatthesaturationtemperature,itsqualityisdefinedastheratioofthemassofvaportothetotalmass.Thus,inFig.1-1(b),ifthemassofthevaporis0.2kgandthemassoftheliquidis0.8kg,thequalityis0.2or20%.Qualityhasmeaningonlywhenthesubstanceisinasaturatedstate.Ifasubstanceexistsasvaporatthesaturationtemperature,itiscalledsaturatedvapor.(Sometimesthetermdrysaturatedvaporisusedtoemphasizethatthequalityis100%).Whenthevaporisatatemperaturegreaterthanthesaturationtemperature,itissaidtoexistassuperheatedvapor.Thepressureandtemperatureofsuperheatedvaporareindependentproperties,sincethetemperaturemayincreasewhilethepressureremainsconstant.假设某一工质在饱和温度下以液、气共存的形式存在，那么称蒸汽质量与总质量之比为干度。因此，如图1-1(b)所示，假设蒸汽质量为，液体质量为，那么其干度为或20%。干度只有在饱和状态下才有意义。假设某一工质处于饱和温度下并以蒸汽形态存在，那么称该蒸汽为饱和蒸汽〔有时称为干饱和蒸汽，意在强调其干度为100%〕。当蒸汽温度高于其饱和温度时，那么称之为过热蒸汽。过热蒸汽的压强和温度是彼此独立的，因为温度上升时，压强可能保持不变。Letusplotonthetemperature-volumediagramofFig.1-2theconstant-pressurelinethatrepresentsthestatesthroughwhichthewaterpassesasitisheatedfromtheinitialstateof0.1MPaand20C.LetstateArepresenttheinitialstate,BC),andlineABtheprocessinwhichtheliquidisheatedfromtheinitialtemperaturetothesaturationtemperature.PointCisthesaturated-vaporstate,andlineBCistheconstant-temperatureprocessinwhichthechangeofphasefromliquidtovaporoccurs.LineCDrepresentstheprocessinwhichthesteamissuperheatedatconstantpressure.Temperatureandvolumebothincreaseduringthisprocess.在图1-2所示的温度-比容图上作等压线，表示水由初压、初温20℃被加热的过程。点A代表初始状态，点B为饱和液态〔℃〕，线AB表示液体由初始温度被加热至饱和温度所经历的过程。点C表示饱和蒸汽状态，线BC表示等温过程，即液体气化转变为蒸汽的过程。线CD表示在等压条件下蒸汽被加热至过热的过程，在此过程中，温度和比容均增大。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热力学第一定律通常把热力学第一定律称为能量守恒定律。在根底物理课程中，能量守恒定律侧重动能、势能的变化以及和功之间的相互关系。更为常见的能量守恒形式还包括传热效应和内能的变化。当然，也包括其它形式的能，如静电能、磁场能、应变能和外表能。历史上，用热力学第一定律来描述循环过程：净传热量等于循环过程中对系统所做的净功。1.1.5ThesecondlawofthermodynamicsThesecondlawofthermodynamicscanbestatedinavarietyofways.Herewepresenttwo:theClausiusstatementandtheKelvin-Planckstatement.ClausiusStatementItisimpossibletoconstructadevicethatoperatesinacycleandwhosesoleeffectisthetransferofheatfromacoolerbodytoahotterbody.Thisstatementrelatestoarefrigerator(oraheatpump).Itstatesthatitisimpossibletoconstructarefrigeratorthattransfersenergyfromacoolerbodytoahotterbodywithouttheinputofwork;thisviolationisshowninFig.1-3a.1.1.5热力学第二定律热力学第二定律有多种表述形式。在此列举两种：克劳修斯表述和凯尔文-普朗克表述。克劳修斯表述：制造一台唯一功能是把热量从低温物体传给高温物体的循环设备是不可能的。以冰箱〔或热泵〕为例，不可能制造一台不用输入功就能把热量从低温物体传给高温物体的冰箱，如图1-3(a)所示。Kelvin-PlanckStatementItisimpossibletoconstructadevicethatoperatesinacycleandproducesnoothereffectthantheproductionofworkandthetransferofheatfromasinglebody.Inotherwords,itisimpossibletoconstructaheatenginethatextractsenergyfromareservoir,doeswork,anddoesnottransferheattoalow-temperaturereservoir.Thisrulesoutanyheatenginethatis100percentefficient,liketheoneshowninFig.1-3b.凯尔文-普朗克表述：制造一台从单一热源吸热和做功的循环设备是不可能的。换句话说，制造这样一台从某一热源吸热并对外做功，而没有与低温热源进行换热的热机是不可能的。因此，该表述说明了不存在工作效率为100%的热机，如图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卡诺循环卡诺机是低温热源和高温热源间运行效率最高的热机。卡诺机是一个理想热机，利用多个可逆过程组成一循环过程，该循环称为卡诺循环。卡诺机非常有用，因为它的运行效率为任何实际热机最大可能的效率。因此，如果一台实际热机的效率要远低于同样条件下的卡诺机效率，那么有可能对该热机进行一些改良以提高其效率。理想的卡诺循环包括四个可逆过程，如图1-4所示：1→2等温膨胀；2→3绝热可逆膨胀；3→4等温压缩；4→1可逆绝热压缩。卡诺循环的效率为:〔1-1〕注意，提高TH〔提高吸热温度〕或降低TL〔降低放热温度〕均可使循环效率提高。1.1.7TheRankinecycleThefirstclassofpowercyclesthatweconsiderarethoseutilizedbytheelectricpowergeneratingindustry,namely,powercyclesthatoperateinsuchawaythattheworkingfluidchangesphasefromaliquidtoavapor.Thesimplestvapor-powercycleiscalledtheRankinecycle,shownschematicallyinFig.1-5a.Amajorfeatureofsuchacycleisthatthepumprequiresverylittleworktodeliverhigh-pressurewatertotheboiler.Apossibledisadvantageisthattheexpansionprocessintheturbineusuallyentersthequalityregion,resultingintheformationofliquiddropletsthatmaydamagetheturbineblades.1.1.7朗肯循环我们所关心的第一类动力循环为电力生产工业所采用的，也就是说，动力循环按这样的方式运行：工质发生相变，由液态变为气态。最简单的蒸汽-动力循环是朗肯循环，如图1-5(a)所示。朗肯循环的一个主要特征是泵消耗很少的功就能把高压水送入锅炉。其可能的缺点为工质在汽机内膨胀做功后，通常进入湿蒸汽区，形成可能损害汽轮机叶片的液滴。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.朗肯循环是一个理想循环，其忽略了四个过程中的摩擦损失。这些损失通常很小，在初始分析时可完全忽略。朗肯循环由四个理想过程组成，其T-s图如图1-5(b)所示：1→2为泵内等熵压缩过程；2→3为炉内定压吸热过程；3→4为汽轮机内等熵膨胀做功过程；4→1为凝汽器内定压放热过程。泵用于提高饱和液体的压强。事实上，状态1和状态2几乎完全一样，因为由2点开始的较高压强下的吸热过程线非常接近饱和曲线，图中仅为了解释说明的需要分别标出。锅炉〔也称蒸汽发生器〕和凝汽器均为换热器，它们既不需要功也不产生功。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.如果忽略动能和势能的变化，输出的净功等于T-s图曲线下面的面积，即图1-5(b)中1-2-3-4-1所包围的面积，由用热力学第一定律可证明。循环过程中工质的吸热量对应面积a-2-3-b-a。因此，朗肯循环的热效率可表示为〔1-2〕即，热效率等于输出能量除以输入能量〔所购能量〕。显然，通过增大分子或减小分母均可以提高热效率。这可以通过增大泵出口压强p2，提高锅炉出口温度T3，或降低汽机出口压强p4来实现。1.1.8TheReheatcycleItisapparentthatwhenoperatinginaRankinecyclewithahighboilerpressureoralowcondenserpressureitisdifficulttopreventliquiddropletsfromforminginthelow-pressureportionoftheturbine.Sincemostmetalcannotwithstandtemperaturesaboveabout600C,thereheatcycleisoftenusedtopreventliquid-dropletformation:thesteampassingthroughtheturbineisreheatedatsomeintermediatepressure,therebyraisingthetemperaturetostate5intheT-sdiagramofFig.1-6.Thesteamthenpassesthroughthelow-pressuresectionoftheturbineandentersthecondenseratstate6.Thiscontrolsorcompletelyeliminatesthemoistureproblemintheturbine.Often,theturbineisseparatedintoahigh-pressureturbineandalow-pressureturbine.1.1.8再热循环对于一个处于高锅炉压强和低凝汽器压强条件下的朗肯循环，显然，很难阻止液滴在汽轮机低压局部的形成。由于大多数金属不能承受600℃以上的高温，因此，通常采用再热循环来防止液滴的形成。再热过程如下：经过汽轮机的局部蒸汽在某中间压强下被再热，从而提高蒸汽温度，直至到达状态5，如图1-6所示。然后这局部蒸汽进入汽轮机低压缸，而后进入凝汽器〔状态6〕。再热循环方式可以控制或者完全消除汽轮机中的湿蒸汽问题，因此，通常汽轮机分成高压缸和低压缸两局部。Thereheatcycledoesnotsignificantlyinfluencethethermalefficiencyofthecycle,butitdoesresultinasignificantadditionalworkoutput,representedinthefigurebyarea4-5-6-4-4ofFig.1-6.Thereheatcycledemandsasignificantinvestmentinadditionalequipment,andtheuseofsuchequipmentmustbeeconomicallyjustifiedbytheincreasedworkoutput.Ifreheatisnotusedtoavoiddropletformation,thecondenserpressuremustbequitehigh,resultingrelativelylowcycleefficiency.Inthatsense,reheatsignificantlyincreasescycleefficiencywhencomparedtothecyclewithnoheatbutwiththehighercondenserpressure.虽然再热循环不会显著影响循环热效率，但带来了显著的额外的输出功，如图1-6中的面积4-5-6-4-4所示。当然，再热循环需要一笔可观的投资来购置额外的设备，这些设备的使用效果必须通过与多增加的输出功进行经济性分析来判定。如果不采用再热循环来防止液滴的形成，那么凝汽器出口压强必须相当地高，因而导致循环热效率较低。在这种意义上，与无再热循环且高凝汽器出口压强的循环相比，再热可以显著提高循环效率。1.2FundamentalofFluidMechanicsFluidmotionsmanifestthemselvesinmanydifferentways.Somecanbedescribedveryeasily,whileothersrequireathoroughunderstandingofphysicallaws.Inengineeringapplications,itisimportanttodescribethefluidmotionsassimplyascanbejustified.Thisusuallydependsontherequiredaccuracy.Often,accuraciesof±10%areacceptable,althoughinsomeapplicationshigheraccuracieshavetobeachieved.Thegeneralequationsofmotionareverydifficulttosolve;consequently,itistheengineer'sresponsibilitytoknowwhichsimplifyingassumptionscanbemade.This,ofcourse,requiresexperienceand,moreimportant,anunderstandingofthephysicsinvolved.1.2流体力学根底流体运动表现出多种不同的运动形式。有些可以简单描述，而其它的那么需要完全理解其内在的物理规律。在工程应用中，尽量简单地描述流体运动是非常重要的。简化程度通常取决于对精确度的要求，通常可以接受±10%左右的误差，而有些工程应用那么要求较高的精度。描述运动的一般性方程通常很难求解，因此，工程师有责任了解可以进行哪些简化的假设。当然，这需要丰富的经验，更重要的是要深刻理解流动所涉及的物理内涵。Somecommonassumptionsusedtosimplifyaflowsituationarerelatedtofluidproperties.Forexample,undercertainconditions,theviscositycanaffecttheflowsignificantly;inothers,viscouseffectscanbeneglectedgreatlysimplifyingtheequationswithoutsignificantlyalteringthepredictions.Itiswellknownthatthecompressibilityofagasinmotionshouldbetakenintoaccountifthevelocitiesareveryhigh.Butcompressibilityeffectsdonothavetobetakenintoaccounttopredictwindforcesonbuildingsortopredictanyotherphysicalquantitythatisadirecteffectofwind.Afterourstudyoffluidmotions,theappropriateassumptionsusedshouldbecomemoreobvious.Hereweintroducesomeimportantgeneralapproachesusedtoanalyzefluidmechanicsproblemsandgiveabriefoverviewofdifferenttypesofflow.一些常见的用来简化流动状态的假设是与流体性质有关系的。例如，黏性在某些条件下对流体有显著的影响；而在其它条件下，忽略黏性效应的影响可以大大地简化方程，但并不会显著改变计算结果。众所周知，气体速度很高时必须考虑其压缩性，但在预测风力对建筑物的影响程度，或者预测受风力直接影响的其它物理量时，可以不计空气的压缩性。学完流体运动学之后，可以更明显地看出采用了哪些恰当的假设。这里，将介绍一些重要的用来分析流体力学问题的一般性方法，并简要介绍不同类型的流动。1.2.1LagrangianandEuleriandescriptionsofmotionInthedescriptionofaflowfield,itisconvenienttothinkofindividualparticleseachofwhichisconsideredtobeasmallmassoffluid,consistingofalargenumberofmolecules,thatoccupiesasmallvolumethatmoveswiththeflow.Ifthefluidisincompressible,thevolumedoesnotchangeinmagnitudebutmaydeform.Ifthefluidiscompressible,asthevolumedeforms,italsochangesitsmagnitude.Inbothcasestheparticlesareconsideredtomovethroughaflowfieldasanentity.1.2.1拉格朗日运动描述和欧拉运动描述描述流场时，将着眼点放在流体质点上是非常方便的。每个质点都包含了微小质量的流体，它由大量分子组成。质点占据很小的体积，并随流体流动而移动。对不可压缩流体，其体积大小不变，但可能发生形变。对可压缩流体，不但体积发生形变，而且大小也将改变。在上述两种情况下，均将所有质点看作一个整体在流场中运动。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.质点力学主要研究单个质点，质点运动是时间的函数。任一质点的位移、速度和加速度可表示为s(x0,y0,z0,t)，V(x0,y0,z0,t)，a(x0,y0,z0,t)，其它相关参量也可计算。坐标(x0,y0,z0)表示质点的起始位置，也是每个质点的名字。这就是拉格朗日运动描述，以约瑟夫Ｌ拉格朗日的名字命名，该描述方法通常用于质点动力学分析。拉格朗日法跟踪多个质点的运动过程并考虑质点间的相互作用。然而，由于实际流体包含质点数目巨大，因而采用拉格朗日法研究流体流动那么非常困难。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.与分别跟踪每个流体质点不同的另一种方法是将着眼点放在空间点上，然后观察质点经过每个空间点时的质点速度，由此可以得到质点流经各空间点时的速度变化率，即V/x，V/y，V/z；还可以判断某一点上的速度是否随时间变化，即计算V/t。这种描述方法称为欧拉运动描述，以莱昂哈德欧拉的名字命名。在欧拉法中，速度等流动参数是空间和时间的函数。在直角笛卡儿坐标系中，速度表示为V=V(x,y,z,t)。我们所研究的流动区域称为流场。1.2.2PathlinesandstreamlinesTwodifferentlineshelpusindescribingaflowfield.Apathlineisthelocusofpointstraversedbyagivenparticleasittravelsinafieldofflow;thepathlineprovidesuswitha"history"oftheparticle'slocations.Aphotographofapathlinewouldrequireatimeexposureofanilluminatedparticle.Astreamlineisalineintheflowpossessingthefollowingproperty:thevelocityvectorofeachparticleoccupyingapointonthestreamlineistangenttothestreamline,thatis,Vdr=0.SinceVanddrareinthesamedirection;recallthatthecrossproductoftwovectorsinthesamedirectioniszero.Aphotographofastreamlinecannotbemadedirectly.Forageneralunsteadyflowthestreamlinescanbeinferredfromphotographsofshortpathlinesofalargenumberofparticles.1.2.2迹线和流线可采用两种不同的流动线来帮助我们描述流场。迹线是某一给定质点在流场中运动时所经过的不同空间点形成的轨迹，它记录了质点的“历史〞位置。一定曝光时间下可以拍得发亮粒子的运动迹线。流线是流场中具有这样特性的线：任一质点在流线上某点处的速度矢量与该流线相切，即Vdr=0。这是因为V和dr具有相同的方向，而具有相同方向的两个矢量的叉乘积等于零。同迹线相比，流线不能直接由相机拍摄获得。对于一般的非定常流动，根据大量质点的短迹线相片可以推断出流线的形状。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)].1.2.3一维、二维和三维流动一般来说，欧拉运动描述中的速度矢量取决于三个空间变量和时间变量，即V=V(x,y,z,t)。这样的流动称为三维流动，因为速度矢量依赖于三个空间坐标。三维流动的求解非常困难，并且也超出了序言的范围。即使假设流动为定常的〔如，V=V(x,y,z)〕，该流动仍为三维流动。三维流动常常可以近似成二维流动。例如，对于一个很宽的大坝，受坝两端条件的影响，水流经大坝时的流动为三维流动；但远离坝端的中间局部的流动可看作是二维的。一般来说，二维流动是指其速度矢量只取决于两个空间坐标的流动。平面流动即是如此，速度矢量只依赖于x，y两个空间坐标，而与z坐标无关〔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.一维流动的速度矢量只依赖于一个空间坐标。这类流动常发生在长直管内和平行平板间。管内流动的速度只随到管轴的距离变化，即u=u(r)。平行平板间的速度也只与y坐标有关，即u=u(y)。即使流动为非定常流动，如启动时的情形，u=u(y,t)，但该流动仍是一维的。对于完全开展的流动，其速度轮廓线并不随流动方向上的空间坐标而改变。这要求研究区域要远离入口处或几何形状突然改变的区域。有许多流体力学方面的工程问题，其流场可以简化为均匀流动：速度和其它流体特性参数在整个区域内均为常数。这种简化只对速度在整个区域内均保持不变时才成立，而且这种情况非常普遍。例如：管内的高速流动和溪水的流动。平均速度可能从一个断面到另一个断面有所不同，而流动条件仅取决于流动方向上的空间变量。1.2.4Newtonianfluidandnon-NewtonianfluidANewtonianfluid