河南科技大学热能与动力工程专业英语考试

2.6NaturalConvection自然对流Heattransferinvolvingmotioninafluidcausedbythedifferenceindensityandtheactionofgravityiscallednaturalorfreeconvection.Heattransfercoefficientsfornaturalconvectionaregenerallymuchlowerthanforforcedconvection,anditisthereforeimportantnottoignoreradiationincalculatingthetotalheatlossorgain.Radianttransfermaybeofthesameorderofmagnitudeasnaturalconvection,evenatroomtemperatures,sincewalltemperaturesinaroomcanaffecthumancomfort.由于密度差和重力作用引起流体运动而产生的热传递称为自然对流或自由对流。自然对流的传热系数一般远低于强迫对流，因此在计算总的吸热量或放热量时，主要的一点就是不要忽略辐射热。辐射热传递与自然对流可以有相同的数量级，甚至在室温下也如此，因为室内墙体温度影响人体的舒适感。Naturalconvectionisimportantinavarietyofheatingandrefrigerationequipment:(1)gravitycoilsusedinhighhumiditycoldstorageroomsandinroof-mountedrefrigerantcondensers,(2)theevaporatorandcondenserofhouseholdrefrigerators,(3)baseboardradiatorsandconvectorsforspaceheatingand(4)coolingpanelsforairconditioning.Naturalconvectionisalsoinvolvedinheatlossorgaintoequipmentcasingsandinterconnectingductsandpipes.在各种供热和制冷设备中自然对流是非常重要的：（1）在高湿度冷藏室内及室内安装的制冷剂冷凝器内使用的重力盘管；（2）家用冰箱的蒸发器和冷凝器；（3）空间采暖用的踢脚板散热器和对流器；（4）空调用辐射对流护壁板。设备外壳及其连接风道和管道的放热量和吸热量中也包含自然对流。Considerheattransferbynaturalconvectionbetweenacoldfluidandahotsurface.Thefluidinimmediatecontactwiththesurfaceisheatedbyconduction,becomeslighterandrisesbecauseofthedifferenceindensityoftheadjacentfluid.Themotionisresistedbytheviscosityofthefluid.Theheattransferisinfluencedby:(1)gravitationalforceduetothermalexpansion,(2)viscousdragand(3)thermaldiffusion.Itmaybeexpectedtodependonthegravitationalaccelerationg,thecoefficientofthermalexpansionβ,thekinematicviscosityυ(=μ/ρ),andthethermaldiffusivityα=(k/ρcp).Thesevariablescanbeexpressedintermsofdimensionlessnumbers:theNusseltnumber,Nu,isafunctionoftheproductofthePrandtlnumber,Pr,andGrashofnumber,Gr,which,whencombined,dependonthefluidproperties,thetemperaturedifferencebetweenthesurfaceandthefluid,△t,andthecharacteristiclengthofthesurface,L.Theconstantcandexponentndependonthephysicalconfigurationandnatureofflow.现在我们来考虑冷流体和热表面间由自然对流引起的热传递。与表面直接接触的流体由于导热而被加热、变轻，并且由于与相邻流体的密度差而上升。这种运动由于流体粘性而受到阻碍。热传递受下列因素影响：（1）因热膨胀而引起的重力作用；（2）粘性阻滞；（3）热扩散。这种热传递被认为取决于重力加速度g、热膨胀系数β、运动粘滞系数υ(=μ/ρ)，以及导热系数α=(k/ρcp)。这些变量可以用无因次数的形式给出：努谢尔特数Nu，它是普朗特数Pr和格拉晓夫数Gr乘积的函数，Pr与Gr的乘积，取决于流体特性、表面与流体间的温差△t和表面特征长度L。常数c和指数n取决于物体外形和流动性质。Theentireprocessofnaturalconvectioncannotberepresentedbyasinglevalueofexponentn,butcanbedividedintothreeregions:(1)turbulentnaturalconvectionforwhichnequals0.33,(2)laminarnaturalconvection,forwhichnequals0.25and(3)aregionthathas(Gr·Pr)lessthanforlaminarnaturalconvection,forwhichtheexponentngraduallydiminishesfrom0.25tolowervalues.Notethat,forwires,the(Gr·Pr)islikelytobeverysmall,sothattheexponentnis0.1.自然对流的全部过程不能由单一的指数n表示，但可以分成三个区域：（1）紊流自然对流，n等于0.33；（2）层流自然对流，n等于0.25；（3）（Gr·Pr）小于层流自然对流的区域，指数n从0.25逐渐减小到更低的值。注意，对金属线，（Gr·Pr）可能会很小，所以n是0.1。Tocalculatethenaturalconvectionheattransfercoefficient,determine(Gr·Pr)tofindwhethertheboundarylayerislaminarofturbulent,thenapplytheappropriateequation.Thecorrectcharacteristiclengthindicatedmustbeused.Sincetheexponentnis0.33foraturbulentboundarylayer,thecharacteristiclengthcancelsout,andtheheattransfercoefficientisindependentofthecharacteristiclength.Turbulenceoccurswhenlengthortemperaturedifferenceislarge.Sincethelengthofapipeisgenerallygreaterthanitsdiameter,theheattransfercoefficientforverticalpipesislargerthanforhorizontalpipes.为了计算自然对流传热系数，先算出（Gr·Pr），确定边界是层流还是紊流，然后应用合适的方程。必须使用已指出的正确的特征长度。由于紊流边界层的指数n是0.33，特征长度消去，传热系数与特征长度无关。当长度或温差大时就产生紊流。由于管道的长度一般大于它的直径，竖管的传热系数大于横管的传热系数。Convectionfromhorizontalplatesfacingdownwardwhenheated(orupwardwhencooled)isaspecialcase.Sincethehotairisabovethecolderair,thereisnotheoreticalreasonforconvection.Someconvectioniscaused,however,bysecondaryinfluencessuchastemperaturedifferencesontheedgesoftheplate.Asanapproximation,acoefficientofsomewhatlessthanhalfofthecoefficientforaheatedhorizontalplatefacingupwardcanbeused.热面朝下（冷面朝上）的水平板对流是个特例。由于热空气在较冷空气之上，理论上不会产生对流。然而，一些次要影响，诸如平板边缘的温差，会引起一些对流。作为一种近似，可以使用这样一个（传热）系数，该系数比热面朝上的水平板的系数的一半稍低。Sinceairisoftentheheattransportfluid,simplifiedequationsforairaregiven.Otherinformationonnaturalconvectionisavailableinthegeneralheattransferreferences.由于空气经常是传热流体，空气的简化方程已经给出。其他有关自然对流的资料在一般有关传热的参考书中可以得到。Observeddifferencesinthecomparisonsofrecentexperimentalandnumericalresultswithexistingcorrelationsfornaturalconvectiveheattransfercoefficientsindicatethatcautionshouldbetakenwhenapplyingcoefficientsfor(isolated)verticalplatesrecommendedbyASHRAEforsituationswithverticalsurfacesinenclosedspaces(buildings).Improvedcorrelationsforcalculatingnaturalconvectiveheattransferfromverticalsurfacesinroomsundercertaintemperatureboundaryconditionshavebeendeveloped.将最近的试验和数据结果与现有的自然对流传热系数关系式相比较，观察到的差别表明，在封闭空间（建筑物）内的竖直表面使用ASHRAE推荐的（独立的）竖直平板传热系数时应该小心。计算房间内一定温度边界条件下竖直表面自然对流传热的改进关系式已建立起来。Naturalconvectioncanaffecttheheattransfercoefficientinthepresenceofweakforcedconvection.Astheforcedconvectioneffect,i..e.,theReynoldsnumber,increases,the“mixedconvection”(superimposedforced-on-freeconvection)giveswaytothepureforcedconv