动力传递与流动阻力导论演示文稿

动力传递与流动阻力导论演示文稿当前1页，总共33页。（优选）第五节动力传递与流动阻力导论当前2页，总共33页。（动量浓度）所以（动量浓度梯度）当前3页，总共33页。在涡流动量通量里动量通量=动量扩散系数×动量浓度梯度ε称为涡流黏度，是湍动程度和管路形状、位置等的函数。【流动阻力产生的机理】流动阻力产生的原因在于：流体具有黏性，流动时存在内摩擦现象，这是流动阻力产生的根本原因（内因）；流体与其相接触的固体壁面之间的作用，促使流体内部发生相对运动，提供阻力产生的条件（外因）。因而，流动阻力产生的大小与流体的性质、流动类型、流过距离、壁面形状等有关。当前4页，总共33页。【VelocityDistributioninPipe

(流体在圆管内的速度分布

)】Whetherlayerfloworturbulentflow,thevelocityoffluidmassponitflowinginapipechangeswithdistancebetweenthemasspointandthepipecenterline—velocitydistribution

(速度分布).Generally,thevelocityofmassponitatpipewallisdeemedtobezero.Thenearerthemasspointtothe

pipecenterline,thefasteritmoves.ForLayerFlowThedistributionofvelocityisparabolicintheradialdirection.Thevelocityhasthelargestvalueatthepipecenter.

Meanvelocityishalfthemaximumvelocity,that当前5页，总共33页。ForTurbulentFlowmasspointsvigorouslymixandcollideeachother,whichallowvelocitydistributiontobecomeuniformThegreaterthe

valueofRe

,theflatterthecurvetop.Owingtothevigorousmixingandcollisionbetweenmassponits,theflowresistanceinturbulentflowismuchlargerthanthatinlayerflow.

Thevelocitynearthewalldropssuddenly.当前6页，总共33页。1.5.1.2管内流动阻力的分类直管阻力:

resistancelosscausedbyinternalfrictionwhenfluidflowsthroughastraightpipe,denotedbyhf.gowiththewholeflowprocess,alsocalledon-wayresistancetotalresistance

Sometimes,theresistanceloss

couldbeexpressedaspressuredrop

（压强降）,

Differingfrom

Δp,Δpf

isnotinvolvedin

energyconversion.Localresistance(局部阻力):

resistancelosscausedby

when

fluidflowsthrough

pipefitting,valves,sectiononsuddenlyshrunkenandexpanded

andotherlocalplaces,denotedby

hj.当前7页，总共33页。1.5.1.3计算直管阻力的通式Whensteady-stateflowingthedrivingforce

=thefrictionalresistancethatTheequationabove

showsthatinternalfrictionchangeslinearlyintheradiusdirectionwhenfluidflowsinapipe.

foranarbitraryfluiddifferentialunitwithlengthLandradiusr,itsforceanalysisthesamedirectiondrivingforce:frictionalresistance:oppositedirection

当前8页，总共33页。Whenfluidflowsinahorizontalandequal-diameterstraightpipe,

ΔpisnumericallyequaltotheresistancelossΔpfcausedby

internalfriction(内摩擦力).

Atthewall,r=ri=d/2,theequationabovecanbeconvertedto

τs

—

fluidshearingstrengthatthewall

当前9页，总共33页。soTheequationaboveis

relationexpressionsbetweentheresistancelossandfrictionstress.τisrelatedtotheflowpattern.kineticenergy

u2/2hasthesameunitas

hf,sohfcanbeexpressedascertain

multiples

of

u2/2.letor当前10页，总共33页。generalformulaofcalculationofstraightpiperesistance,calledFanningequation

(范宁公式

)λ—frictioncoefficient,dimensionless,it’srelatedtotheflowpatternandroughnessofpipewall.and1.5.2圆管内的稳态层流1.5.2.1速度分布Newton'slawofviscosity

当前11页，总共33页。onintegrating稳态流动

velocitydistributioninlayerflow当前12页，总共33页。let

r=0anotherformof

velocitydistribution

inlayerflow

foranannularfluiddifferentialunitwiththickness

dr1.5.2.2层流时的摩擦系数annularsectionarea

当前13页，总共33页。totalflowinpipevolumeflowrateofthedifferentialunit

meanvelocityatacross-section

soHagon-Poiseuilleequation

(哈根-泊谡叶方程)当前14页，总共33页。soFanningfrictionfactor(范宁摩擦因子)

comparedwithFanningequation

当前15页，总共33页。1.5.3圆管内的湍流1.5.3.1管内粗糙度对摩擦系数的影响pipewallconditionisrepresentedinroughness.dabsoluteroughness

(绝对粗糙度e),averageheightofwallbulgerelativeroughness(相对粗糙度e/d),ratioofabsoluteroughnesstopipediameterⅠ.Forlayerflowλisindependentofevalueroughness

doesnotaffectflowresistance当前16页，总共33页。Ⅱ.Forturbulentflow

laminarbottom(层流内层)δb①

e

<δbλisalsoindependentofevaluehydraulicsmooth(水力光滑)②

e

>δbThelargerthevalueofReandthesmallerthevalueofδb,

themoresignificanttheeffectofe

onλ.当前17页，总共33页。1.5.3.2湍流时的摩擦系数

Dimensionalanalysismethod(因次分析法)—serveralphysicalquantities

arecombinedintooneormoredimensionlessgroups

(无因次数群)bydimensionlesstreatment

(无因次化),thentherelationbetween

dimensionlessgroups

aresetupwithhelpofexperiments.Buckinghamstheorem(白金汉定理)

WhereN=n–m,m—

numbersofthefundamentaldimension

当前18页，总共33页。Exampledimensionofvariousphysicalquantitiesfundamentalphysicalquantities—mass,lengthandtime

dimensionoffundamentalphysicalquantities—massM,lengthLandtimeθ

Theequationabovecanbeexpressedasapowerfunction

当前19页，总共33页。dimensionequation

(因次方程)当前20页，总共33页。ThevaluesofK,b,kandqneedtobedeterminedthroughtheexperiments.

①Blasiusformula(柏拉休斯公式)theappliedrange,

Re=3×103~1×105Ⅰ.Forsmoothpipe(光滑管)当前21页，总共33页。②Guyuzhenformula(顾毓珍公式)

Re=3×103~3×106Ⅱ.Forroughpipe(粗糙管)

Colebrookformula(柯尔布鲁克公式)

Moodyplot

(摩擦系数图—莫迪图)当前22页，总共33页。当前23页，总共33页。当前24页，总共33页。当前25页，总共33页。当前26页，总共33页。1.5.3.4非圆形直管的摩擦阻力Replacecircularpipediameter(d)

withequivalentdiameter(de)

例：一套管换热器，内管与外管均为光滑管，尺寸分别为φ30×2.5mm与φ56×3mm。平均温度为40℃的水以10m3/h的流量流过套管环隙。求每米管长的压力降。水在40℃时：ρ

=992kg/m3，μ=65.6×10-5Pa·ssofromMoodyplotλ=0.0196当前27页，总共33页。1.5.4边界层与局部阻力的概念1.5.4.1边界层(boundaryLayer)边界层—aflowlayerinwhichvelocitygradientexists

Flowresistanceisdeemedtoconcentratemainlyintheboundarylayer.当前28页，总共33页。1.5.4.2局部阻力

(localresistance)Ⅰ.阻力系数法

(resistancecoefficientmethod)①突然扩大当前29页，总共33页。②突然缩小inletexi