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本文档共33页；当前第1页；编辑于星期六\0点1分（优选）第五节动力传递与流动阻力导论本文档共33页；当前第2页；编辑于星期六\0点1分（动量浓度）所以（动量浓度梯度）本文档共33页；当前第3页；编辑于星期六\0点1分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。本文档共33页；当前第4页；编辑于星期六\0点1分【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本文档共33页；当前第5页；编辑于星期六\0点1分ForTurbulentFlowmasspointsvigorouslymixandcollideeachother,whichallowvelocitydistributiontobecomeuniformThegreaterthe

valueofRe

,theflatterthecurvetop.Owingtothevigorousmixingandcollisionbetweenmassponits,theflowresistanceinturbulentflowismuchlargerthanthatinlayerflow.

Thevelocitynearthewalldropssuddenly.本文档共33页；当前第6页；编辑于星期六\0点1分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.本文档共33页；当前第7页；编辑于星期六\0点1分1.5.1.3计算直管阻力的通式Whensteady-stateflowingthedrivingforce

=thefrictionalresistancethatTheequationabove

showsthatinternalfrictionchangeslinearlyintheradiusdirectionwhenfluidflowsinapipe.

foranarbitraryfluiddifferentialunitwithlengthLandradiusr,itsforceanalysisthesamedirectiondrivingforce:frictionalresistance:oppositedirection

本文档共33页；当前第8页；编辑于星期六\0点1分Whenfluidflowsinahorizontalandequal-diameterstraightpipe,

ΔpisnumericallyequaltotheresistancelossΔpfcausedby

internalfriction(内摩擦力).

Atthewall,r=ri=d/2,theequationabovecanbeconvertedto

τs

—

fluidshearingstrengthatthewall

本文档共33页；当前第9页；编辑于星期六\0点1分soTheequationaboveis

relationexpressionsbetweentheresistancelossandfrictionstress.τisrelatedtotheflowpattern.kineticenergy

u2/2hasthesameunitas

hf,sohfcanbeexpressedascertain

multiples

of

u2/2.letor本文档共33页；当前第10页；编辑于星期六\0点1分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

本文档共33页；当前第11页；编辑于星期六\0点1分onintegrating稳态流动

velocitydistributioninlayerflow本文档共33页；当前第12页；编辑于星期六\0点1分let

r=0anotherformof

velocitydistribution

inlayerflow

foranannularfluiddifferentialunitwiththickness

dr1.5.2.2层流时的摩擦系数annularsectionarea

本文档共33页；当前第13页；编辑于星期六\0点1分totalflowinpipevolumeflowrateofthedifferentialunit

meanvelocityatacross-section

soHagon-Poiseuilleequation

(哈根-泊谡叶方程)本文档共33页；当前第14页；编辑于星期六\0点1分soFanningfrictionfactor(范宁摩擦因子)

comparedwithFanningequation

本文档共33页；当前第15页；编辑于星期六\0点1分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本文档共33页；当前第16页；编辑于星期六\0点1分Ⅱ.Forturbulentflow

laminarbottom(层流内层)δb①

e

<δbλisalsoindependentofevaluehydraulicsmooth(水力光滑)②

e

>δbThelargerthevalueofReandthesmallerthevalueofδb,

themoresignificanttheeffectofe

onλ.本文档共33页；当前第17页；编辑于星期六\0点1分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

本文档共33页；当前第18页；编辑于星期六\0点1分Exampledimensionofvariousphysicalquantitiesfundamentalphysicalquantities—mass,lengthandtime

dimensionoffundamentalphysicalquantities—massM,lengthLandtimeθ

Theequationabovecanbeexpressedasapowerfunction

本文档共33页；当前第19页；编辑于星期六\0点1分dimensionequation

(因次方程)本文档共33页；当前第20页；编辑于星期六\0点1分ThevaluesofK,b,kandqneedtobedeterminedthroughtheexperiments.

①Blasiusformula(柏拉休斯公式)theappliedrange,

Re=3×103~1×105Ⅰ.Forsmoothpipe(光滑管)本文档共33页；当前第21页；编辑于星期六\0点1分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)本文档共33页；当前第22页；编辑于星期六\0点1分本文档共33页；当前第23页；编辑于星期六\0点1分本文档共33页；当前第24页；编辑于星期六\0点1分本文档共33页；当前第25页；编辑于星期六\0点1分本文档共33页；当前第26页；编辑于星期六\0点1分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本文档共33页；当前第27页；编辑于星期六\0点1分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.本文档共33页；当前第28页；编辑于星期六\0点1分1.5.4.2局部阻力

(localresistance)Ⅰ.阻力系数法

(resistancecoefficientmethod)①突然扩大本文档共33页；当前第29页；编辑于星期六\0点1分②突然缩小inletexit

本文档共33