自由紊流流动和换课件.ppt

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1、主要内容8.1自由剪切层流动8.2热射流8.3热尾流远离固体壁面的自由紊流的动量传递和换热特性研究内容：自由紊流自由剪切层流动热射流热尾流自然对流热尾流强制对流热尾流8.1自由剪切层流动层流剪切层过渡紊流剪切层周围环境虚拟原点两个自由剪切流解决问题：剪切层厚度变化速度分布温度分布及通过y＝0平面的热流边界层性质假设数学模型模型求解8.2热射流剪切层区域热射流区域虚拟原点时间平均界面两个自由剪切流层会合的流动为热射流。二维热射流热射流区域的流动具有边界层的性质轴对称圆形射流虚拟原点时间平均界面环状剪

2、切流圆形射流区剪切流区域射流区域数学模型8.3热尾流自然对流热尾流大尺寸涡环境热源虚拟原点时间平均界面集中热源上方的自然对流热尾流及时间平均的漏斗形流场浮升力数学模型8.1PhysicalConsiderationsFreeconvection:BuoyanceforcesForcedconvection:FluidmotionForcedforcesfluiddensitygradientBodyforceBuoyanceforcesTemperaturegradientGravitationa

3、lforceUnstablefluidcirculationStable(a)(b)(a)Thedensitydecreasesinthedirectionofthegravitationalforces.Thisconditionisunstable.Heattransferoccursfromthebottomtothetopsurfacebyfreeconvection.(b)Thedensitynolongerdecreasesinthedirectionofthegravitationa

4、lforces.Thisconditionisnowstable.Heattransferoccursfromthebottomtothetopsurfacebyconduction.Notethatthepresenceofafluiddensitygradientinagravitationalfielddoesnotensuretheexistenceoffreeconvectioncurrents.8.2TheGoverningEquationsQuiescentfluidgFigure8

5、.1boundarylayerdevelopmentonaheatedverticalplateAssumptions:1.Steady-state.2.Two-dimensional.3.Constantproperty.4.Incompressible.Equations:Inthequiescentregionoutsidetheboundarylayer.u=v=0BuoyancyforceThrvolumetricthermalexpansioncoefficientStronglyco

6、upledForaperfectgas,9.3SimilarityConsiderationNondimensionalizingGrashofnumberGrLTheGrashofnumberindicatestheratioofthebuoyancyforcetotheviscousforceactingonthefluid.ConclusionItisimportanttonotethatsuchcorrelationsarepertinentonlywhenForced-andfree-c

7、onvectioneffectsarecomparable.EffectsoffreeandforcedconvectionmustbeconsideredEffectsoffreeconvectionmaybeneglectedEffectsofforcedconvectionmaybeneglected8.4LaminarFreeConvectiononaVerticalSurfaceGoverningequationsBoundaryconditionsMathematicalmodel8.

8、5TheEffectsofTurbulenceThermalinstabilityTurbulenceflowQuiescentfluidgFigure8.2Freeconvectionboundarylayertransitiononaverticalplate.LaminarTurbulentTransitionRayleighnumberTransitioninafreeconvectionboundarylayerdependsontherelativemagnitudeo