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Unit12whatdowemeanbytransportphenomena?Transportphenomenaisthecollectivenamegiventothesystematicandintegratedstudyofthreeclassicalareasofengineeringscience:(i)energyorheattransport,(ii)masstransportordiffusion,and(iii)momentumtransportorfluiddynamics.传递现象是工程科学三个典型领域系统性和综合性研究的总称：能量或热量传递，质量传递或扩散，以及动量传递或流体力学。Ofcourse,heatandmasstransportoccurfrequentlyinfluids,andforthisreasonsomeengineeringeducatorsprefertoincludestheseprocessesintheirtreatmentoffluidmechanics.当然，热量和质量传递在流体中经常发生，正因如此一些工程教育家喜欢把这些过程包含在流体力学的范畴内。Sincetransportphenomenaalsoincludesheatconductionanddiffusioninsolids,however,thesubjectisactuallyofwiderscopethanfluidmechanics.由于传递现象也包括固体中的热传导和扩散，因此，传递现象实际上比流体力学的领域更广。Itisalsodistinguishedfromfluidmechanicsinthatthestudyoftransportphenomenamakeuseofthesimilaritiesbetweentheequationsusedtodescribetheprocessesofheat,mass,andmomentumtransport.传递现象的研究充分利用描述传热，传质，动量传递过程的方程间的相似性，这也区别于流体力学。Theseanalogies,astheyareusuallycalled,canoftenberelatedtosimilaritiesinthephysicalmechanismswherebythetransporttakesplace.这些类推(通常被这么叫)常常可以与传递现象发生的物理机制间的相似性关联起来。aconsequence,anunderstandingofoneAstransportprocesscanreadilyleadtoanunderstandingofotherprocesses.因此，一个传递过程的理解能够容易促使其他过程的理解。Moreover,ifthedifferentialequationsandboundaryconditionsarethesame,asolutionneedbeobtainedforonlyoneoftheprocessessincebychangingthenomenclaturethatsolutioncanbeusedtoobtainthesolutionforanyothertransportprocess.而且，如果微分方程和边界条件是一样的，只需获得一个传递过程的解决方案即可，因为通过改变名称就可以用来获得其他任何传递过程的解决方案。Itmustbeemphasized,however,thatwhiletherearesimilaritiesbetweenthetransportprocesses,therearealsoimportantdifferences,especiallybetweenthetransportofmomentum(avector)andthatofheatormass(scalars).必须强调,虽然有相似之处,也有传递过程之间的差异,尤其重要的是运输动量(矢量)和热或质量(标量).Nevertheless,asystematicstudyofthesimilaritiesbetweenthetransportprocessesmakesiteasiertoidentifyandunderstandthedifferencesbetweenthem.然而,系统地研究了相似性传递过程之间的相似性,使它更容易识别和理解它们之间的差别。1．HowWeApproachtheSubject怎么研究传递过程？．Inordertodemonstratetheanalogiesbetweenthetransportprocesses,wewillstudyeachoftheprocessinparallel-insteadofstudyingmomentumtransportfirst,thenenergytransport,andfinallymasstransport.为了找出传递过程间的相似性，我们将同时研究每一种传递过程——取代先研究动量传递，再传热，最后传质的方法。Besidespromotingunderstanding,thereisanotherpedagogicalreasonfornotusingtheserialapproachthatisusedinothertextbooks:ofthethreeprocesses,theconceptsandequationsinvolvedinthestudyofmomentumtransportarethemostdifficultforthebeginnertounderstandandtouse.除了促进理解之外，对于不使用在其他教科书里用到的顺序法还有另一个教学的原因：在三个过程中，包含在动量传递研究中的概念和方程对初学者来说是最难以理解并使用。Becauseitisimpossibletocoverheatandmasstransportthoroughlywithoutpriorknowledgeofmomentumtransport,oneisforcedundertheserialapproachtotakeupthemostdifficultsubject(momentumtransport)first.因为在不具有有关动量传递的知识前提下一个人不可能完全理解传热和传质，在顺序法的情况下他就被迫先研究最难的课程即动量传递。Ontheotherhand,ifthesubjectsarestudiedinparallel,momentumtransportbecomesmoreunderstandablebyreferencetothefamiliarsubjectofheattransport.另一方面，如果课程同时被研究，通过参照有关传热的熟悉课程动量传递就变得更好理解。Furthermore,theparalleltreatmentmakesitpossibletostudythesimplerthephysicalprocessesthatareoccurringratherthanthemathematicalproceduresandrepresentations.而且，平行研究法可以先研究较为简单的概念，再深入到较难和较抽象的概念。我们可以先强调所发生的物理过程而不是数学性步骤和描述。Forexample，wewillstudyone-dimensionaltransportphenomenafirstbecauseequationsinsteadofpartialrequiringvectornotationandwecanoftenuseordinarydifferentialequationsinsteadofpartialdifferentialequations,whicharehardertosolve.例如，我们将先研究一维传递现象，因为它在不要求矢量标注下就可以被解决，并且我们常常可以使用普通的微分方程代替难以解决的偏微分方程。Thisprocedureisalsojustifiedbythefactthatmanyofthepracticalproblemsoftransportphenomenacanbesolvedbyone-dimensionalmodels.加上传递现象的许多实际问题可以通过一维模型解决的这样一个事实，这种处理做法也是合理的。2.WhyShouldEngineersStudyTransportPhenomena?为什么工程师要研究传递现象？Sincethedisciplineoftransportphenomenadealswithcertainlawsofnature,somepeopleclassifyitasabranchofengineering.因为传递现象这个学科牵扯到自然界定则，一些人就把它划分为工程的一个分支。Forthisreasontheengineer,whoisconcernedwiththeeconomicaldesignandoperationofplantsandequipment,quiteproperlyshouldaskhowtransportphenomenawillbeofvalueinpractice.正因如此，对于那些关心工厂和设备设计和操作经济性的工程师而言，十分应该探知在实际中传递现象如何起到价值作用。Therearetwogeneraltypesofanswerstothosequestions.对于那些问题有两种通用型答案。Thefirstrequiresonetorecognizethatheat,mass,andmomentumtransportoccurinmanykindsofengineering,e.g.,heatexchangers,compressors,nuclearandchemicalreactors,humidifiers,aircoolers,driers,fractionaters,andabsorbers.第一种要求大家认识到传热，传质和动量传递发生在许多工程设备中，如热交换器，压缩机，核化反应器，增湿器，空气冷却器，干燥器，分离器和吸收器。Thesetransportprocessesarealsoinvolvedinthehumanbodyaswellasinthecomplexprocesseswherebypollutantsreactanddiffuseintheatmosphere.这些传递过程也发生在人体内以及大气中污染物反应和扩散的一些复杂过程中。Itisimportantthatengineershaveanunderstandingofthephysicallawsgoverningthesetransportprocessesiftheyaretounderstandwhatistakingplaceinengineeringequipmentandtomakewisedecisionswithregardtoitseconomicaloperation.如果工程师要知道工程设备中正在发生什么并要做出能达到经济性操作的决策，对主导这些传递过程的物理定律有一个认识很重要。Thesecondansweristhatengineersneedtobeabletousetheirunderstandingofnaturallawstodesignprocessequipmentinwhichtheseprocessesareoccurring.第二种答案是工程师需要能够运用自然定律的知识设计包含这些过程的工艺设备。Todosotheymustbeabletopredictratesofheat,mass,ormomentumtr