第十一章-气体射流

1FluidMechanics23Chapter11Gasdiffusion§11–1Introduction§11–2Characterofsubmergingturbulentdiffusionininfinitespace§11–3kineticanalysisofdiffusionincircularsection§11–4planediffusion§11–5Temperatureorthicknessdifferenceofdiffusion§11–6IntroductionofotherdiffusioninprojectandequipmentExerciseofChapter114第十一章气体射流§11–1引言§11–2无限空间淹没紊流射流的特征§11–3圆断面射流的运动分析§11–4平面射流§11–5温差或浓差射流§11–6工程设备中常见的其他射流简介第十一章习题5Flowbygasdiffusingfromorifice,pipemouthorslotcalledgassubmergeddiffusionflow.Whenorificevelocityishigh,flowisinturbulentstate,calledturbulentdiffusion.Mostdiffusionappliedinengineeringisturbulentdiffusionofgas.Diffusiondescribeflowvelocity,temperatureandthicknessaftercomingout.Diffusetoinfinitespace,flowisnotrestrictedbysolidwallcalledfreediffusion.Contrarily,calledrestricteddiffusion.Chapter11Diffusionofgas§11-1Introduction6气体自孔口、管嘴或条缝向外喷射所形成的流动，称为气体淹没射流。当出口速度较大，流动呈紊流状态，叫做紊流射流。工程上所应用的射流，多为气体紊流射流。射流讨论的是出流后的流速场、温度场和浓度场。射流到无限大空间中，流动不受固体边壁的限制，为无限空间射流，又称自由射流。反之，为有限空间射流，又称受限射流。第十一章气体射流§11-1引言7§11-2CharacteristicofturbulentdiffusionininfinitespaceTakecircularsectiondiffusionasexampletalkaboutdiffusionmotion.AirflowdiffusedfromcircularsectionmuzzlewithradiiR.Velocityofoutletsectionisconsidereduniformdistributing,allareu0andisturbulentflow.TakeaxisofdiffusionMxasaxisxOwingtodiffusionisturbulentflowtype,transversepulseofdiffusionmakeexchangebetweenmassandmomentumamongdiffusionandmedium.Drivingmediummoving,massflux,transversesectionareaalongxincreasing,comeintoconeformflowdiffusedtoaround,asinfig.11—1CAMDF。8§11-2无限空间淹没紊流射流的特征现以无限空间中圆断面紊流射流为例，讨论射流运动。气流自半径为R的圆断面喷嘴喷出。出口断面上的速度认为均匀分布，皆为u0值，且流动为紊流。取射流轴线Mx为x轴。由于射流为紊流型，紊流的横向脉动造成射流与周围介质之间不断发生质量、动量交换，带动周围介质流动，使射流的质量流量、射流的横断面积沿x方向不断增加，形成了向周围扩散的锥体状流动场，如图11—1所示的锥体CAMDF。9thestartingsegmentthemainbodysegmentthecoretheboundarylayerxs0s0xoMADBCEFFig.11—1DiffusionstructureStructureandcharacteristicofturbulentdiffusion10起始段主体段核心边界层xs0s0xoMADBCEF图11—1射流结构紊流射流的结构及特性。111.InitialandmainsegmentoftransitionsectionDiffusionvelocityisuniformwhenjustejected.Flowalongx,diffusionbringaroundmediumconstantly,makingboundarylarger,andmainvelocityofdiffusionreducegradually,partwithvelocityu0（asin11—6AoDcone）calledcoreofdiffusion,otherpartwithvelocitylowerthanu0calledboundarylayer.Boundarylayerdisperseconstantlyfromoutlettoaroundalongdiffusionline,drivingaroundmediumintoboundarylayer,andexpandingtodiffusioncentertillcertaindistance,boundaryexpendingtoaxesofdiffusion,coreregiondisappeared,onlyvelocityinaxesisu0.Thissectionasinfig.11—1BoE,calledtransitionsection.Taketransitionsectionasdividingline,outletsectiontilltransitionsectioncalledinitialsegmentofdiffusion.Sincetransitionsectioncalledmainsegmentofdiffusion.12一、过流断面（又称转折断面）起始段及主体段刚喷出的射流速度仍然是均匀的。沿x方向流动，射流不断代入周围介质，不仅使边界扩张，而且使射流主体的速度逐渐降低，速度为u0的部分（如图其11—6AoD锥体）称为射流核心，其余部分速度小于u0称为边界层。射流边界层从出口开始沿射程不断地向外扩散，带动周围介质进入边界层，同时向射流中心扩展，至某一距离处，边界层扩展到射流轴心线，核心区域消失，只有轴心上速度为u0。射流这一断面为图11—1上的BoE,称为过渡断面或转折断面。以过渡断面分界，出口断面至过渡断面称为射流起始段。过渡断面以后称为射流主体段。132.CoefficientofturbulentflowandgeometricalcharacterOuterboundarylayerofdiffusionisabeeline,asinfig.11—1ABandDE.AB、DEprolongtomuzzlemeetingatpointM,thispointcalledculmination.Halfofcalledpoleangle,aswellascalleddiffusingangle。BoisradiiRofcirculardiffusionsection（orhalfbreadthofboundarylayerinplanediffusionyb）.Itispositiveratiowithdistancefromculmination,Bo＝Kx。oMisxdistancecalculatedfromculmination.Observingfromfigure,Bo/oM=tan,soAMD1)(11tan—KxKxwhereK—experimentconstant;—formcoefficientofmuzzle,circularmuzzle,＝3.4；—coefficientturbulentflow,determinedbyexperiment14二、紊流系数及几何特征射流外边界层是一条直线，如图11—1上的AB及DE线。AB、DE延至喷嘴内交于M点，此点称为极点，的一半称为极角,又称扩散角。Bo为圆断面射流截面的半径R（或平面射流边界层的半宽度yb）。它和从极点起点算的距离成正比，即Bo＝Kx。oM是从极点起算的x距离。由图看出，Bo/oM=tan,故AMD1)(11tan—KxKx式中K—试验常数；—喷口形状系数，圆形喷嘴，＝3.4；—紊流系数，由实验决定，是表示射流流动结构的特征系数。15Coefficientofturbulentflowrelatetoturbulentflowintensityinoutletsection,Morehighasintensity,morehighasvaluea,whichmakediffusionexpendingangleaincrease.Drivingaroundmediummore,velocityalongdiffusionlinereducingmorefast.arelatestotheuniformofvelocitydistributinginoutletsection.Practicalvalueofturbulentflowcoefficientandexpendingangleondifferentformmuzzle.asintable11—1.Coefficientofturbulentflowtable11—1WindmachineofaxisflowwithmetalgriddingwellshrinkplanemuzzlenarrowgapinplanewallportraitgapofwindwaywithgrindroundoutletkindsofmuzzlemuzzlewithshrinkoutletcylindricalpipeRight-anglepipewithleadingwindplank2kindsofmuzzle2071.0066.008.0076.020.012.00127022500002900368034400155.0118.0108.024.002410132032904780000Fromformula（11—1—1）,aisconformed,outerboundarylineofdiffusionboundarylayer,diffusionexpendingforwardalongcertainexpendinganglea,thisisitsgeometricalcharacter.Applyingthischaracter,varietyruleofdiffusionradiicanbeobtainedalongdiffusionwayincircularsection.16紊流系数与出口断面上紊流强度有关，紊流强度越大，a值也大，使射流扩散角a增大，被带动的周围介质增多，射流速度沿程下降加速。a还与射流出口断面上速度分布的均匀性有关。各种不同形状喷嘴的紊流系数和扩散角的实测值列表11—1。紊流系数表11—1喷嘴种类带有收缩口的喷嘴圆柱形管带有导风板的轴流式通风机带导流板的直角弯管2喷嘴种类2071.0066.008.0076.020.012.00127022500002900368034400带金属网格的轴流风机收缩极好的平面喷口平面壁上锐缘狭缝具有导叶且加工磨圆