動力傳遞與流動阻力導(dǎo)論演示文稿

動力傳遞與流動阻力導(dǎo)論演示文稿現(xiàn)在是1頁\一共有33頁\編輯于星期五（優(yōu)選）第五節(jié)動力傳遞與流動阻力導(dǎo)論現(xiàn)在是2頁\一共有33頁\編輯于星期五（動量濃度）所以（動量濃度梯度）現(xiàn)在是3頁\一共有33頁\編輯于星期五在渦流動量通量里動量通量=動量擴(kuò)散系數(shù)×動量濃度梯度ε稱為渦流黏度，是湍動程度和管路形狀、位置等的函數(shù)?！玖鲃幼枇Ξa(chǎn)生的機(jī)理】流動阻力產(chǎn)生的原因在于：流體具有黏性，流動時存在內(nèi)摩擦現(xiàn)象，這是流動阻力產(chǎn)生的根本原因（內(nèi)因）；流體與其相接觸的固體壁面之間的作用，促使流體內(nèi)部發(fā)生相對運(yùn)動，提供阻力產(chǎn)生的條件（外因）。因而，流動阻力產(chǎn)生的大小與流體的性質(zhì)、流動類型、流過距離、壁面形狀等有關(guān)?，F(xiàn)在是4頁\一共有33頁\編輯于星期五【VelocityDistributioninPipe

(流體在圓管內(nèi)的速度分布

)】Whetherlayerfloworturbulentflow,thevelocityoffluidmassponitflowinginapipechangeswithdistancebetweenthemasspointandthepipecenterline—velocitydistribution

(速度分布).Generally,thevelocityofmassponitatpipewallisdeemedtobezero.Thenearerthemasspointtothe

pipecenterline,thefasteritmoves.ForLayerFlowThedistributionofvelocityisparabolicintheradialdirection.Thevelocityhasthelargestvalueatthepipecenter.

Meanvelocityishalfthemaximumvelocity,that現(xiàn)在是5頁\一共有33頁\編輯于星期五ForTurbulentFlowmasspointsvigorouslymixandcollideeachother,whichallowvelocitydistributiontobecomeuniformThegreaterthe

valueofRe

,theflatterthecurvetop.Owingtothevigorousmixingandcollisionbetweenmassponits,theflowresistanceinturbulentflowismuchlargerthanthatinlayerflow.

Thevelocitynearthewalldropssuddenly.現(xiàn)在是6頁\一共有33頁\編輯于星期五1.5.1.2管內(nèi)流動阻力的分類直管阻力:

resistancelosscausedbyinternalfrictionwhenfluidflowsthroughastraightpipe,denotedbyhf.gowiththewholeflowprocess,alsocalledon-wayresistancetotalresistance

Sometimes,theresistanceloss

couldbeexpressedaspressuredrop

（壓強(qiáng)降）,

Differingfrom

Δp,Δpf

isnotinvolvedin

energyconversion.Localresistance(局部阻力):

resistancelosscausedby

when

fluidflowsthrough

pipefitting,valves,sectiononsuddenlyshrunkenandexpanded

andotherlocalplaces,denotedby

hj.現(xiàn)在是7頁\一共有33頁\編輯于星期五1.5.1.3計(jì)算直管阻力的通式Whensteady-stateflowingthedrivingforce

=thefrictionalresistancethatTheequationabove

showsthatinternalfrictionchangeslinearlyintheradiusdirectionwhenfluidflowsinapipe.

foranarbitraryfluiddifferentialunitwithlengthLandradiusr,itsforceanalysisthesamedirectiondrivingforce:frictionalresistance:oppositedirection

現(xiàn)在是8頁\一共有33頁\編輯于星期五Whenfluidflowsinahorizontalandequal-diameterstraightpipe,

ΔpisnumericallyequaltotheresistancelossΔpfcausedby

internalfriction(內(nèi)摩擦力).

Atthewall,r=ri=d/2,theequationabovecanbeconvertedto

τs

—

fluidshearingstrengthatthewall

現(xiàn)在是9頁\一共有33頁\編輯于星期五soTheequationaboveis

relationexpressionsbetweentheresistancelossandfrictionstress.τisrelatedtotheflowpattern.kineticenergy

u2/2hasthesameunitas

hf,sohfcanbeexpressedascertain

multiples

of

u2/2.letor現(xiàn)在是10頁\一共有33頁\編輯于星期五generalformulaofcalculationofstraightpiperesistance,calledFanningequation

(范寧公式

)λ—frictioncoefficient,dimensionless,it’srelatedtotheflowpatternandroughnessofpipewall.and1.5.2圓管內(nèi)的穩(wěn)態(tài)層流1.5.2.1速度分布Newton'slawofviscosity

現(xiàn)在是11頁\一共有33頁\編輯于星期五onintegrating穩(wěn)態(tài)流動

velocitydistributioninlayerflow現(xiàn)在是12頁\一共有33頁\編輯于星期五let

r=0anotherformof

velocitydistribution

inlayerflow

foranannularfluiddifferentialunitwiththickness

dr1.5.2.2層流時的摩擦系數(shù)annularsectionarea

現(xiàn)在是13頁\一共有33頁\編輯于星期五totalflowinpipevolumeflowrateofthedifferentialunit

meanvelocityatacross-section

soHagon-Poiseuilleequation

(哈根-泊謖葉方程)現(xiàn)在是14頁\一共有33頁\編輯于星期五soFanningfrictionfactor(范寧摩擦因子)

comparedwithFanningequation

現(xiàn)在是15頁\一共有33頁\編輯于星期五1.5.3圓管內(nèi)的湍流1.5.3.1管內(nèi)粗糙度對摩擦系數(shù)的影響pipewallconditionisrepresentedinroughness.dabsoluteroughness

(絕對粗糙度e),averageheightofwallbulgerelativeroughness(相對粗糙度e/d),ratioofabsoluteroughnesstopipediameterⅠ.Forlayerflowλisindependentofevalueroughness

doesnotaffectflowresistance現(xiàn)在是16頁\一共有33頁\編輯于星期五Ⅱ.Forturbulentflow

laminarbottom(層流內(nèi)層)δb①

e

<δbλisalsoindependentofevaluehydraulicsmooth(水力光滑)②

e

>δbThelargerthevalueofReandthesmallerthevalueofδb,

themoresignificanttheeffectofe

onλ.現(xiàn)在是17頁\一共有33頁\編輯于星期五1.5.3.2湍流時的摩擦系數(shù)

Dimensionalanalysismethod(因次分析法)—serveralphysicalquantities

arecombinedintooneormoredimensionlessgroups

(無因次數(shù)群)bydimensionlesstreatment

(無因次化),thentherelationbetween

dimensionlessgroups

aresetupwithhelpofexperiments.Buckinghamstheorem(白金漢定理)

WhereN=n–m,m—

numbersofthefundamentaldimension

現(xiàn)在是18頁\一共有33頁\編輯于星期五Exampledimensionofvariousphysicalquantitiesfundamentalphysicalquantities—mass,lengthandtime

dimensionoffundamentalphysicalquantities—massM,lengthLandtimeθ

Theequationabovecanbeexpressedasapowerfunction

現(xiàn)在是19頁\一共有33頁\編輯于星期五dimensionequation

(因次方程)現(xiàn)在是20頁\一共有33頁\編輯于星期五ThevaluesofK,b,kandqneedtobedeterminedthroughtheexperiments.

①Blasiusformula(柏拉休斯公式)theappliedrange,

Re=3×103~1×105Ⅰ.Forsmoothpipe(光滑管)現(xiàn)在是21頁\一共有33頁\編輯于星期五②Guyuzhenformula(顧毓珍公式)

Re=3×103~3×106Ⅱ.Forroughpipe(粗糙管)

Colebrookformula(柯爾布魯克公式)

Moodyplot

(摩擦系數(shù)圖—莫迪圖)現(xiàn)在是22頁\一共有33頁\編輯于星期五現(xiàn)在是23頁\一共有33頁\編輯于星期五現(xiàn)在是24頁\一共有33頁\編輯于星期五現(xiàn)在是25頁\一共有33頁\編輯于星期五現(xiàn)在是26頁\一共有33頁\編輯于星期五1.5.3.4非圓形直管的摩擦阻力Replacecircularpipediameter(d)

withequivalentdiameter(de)

例：一套管換熱器，內(nèi)管與外管均為光滑管，尺寸分別為φ30×2.5mm與φ56×3mm。平均溫度為40℃的水以10m3/h的流量流過套管環(huán)隙。求每米管長的壓力降。水在40℃時：ρ

=992kg/m3，μ=65.6×10-5Pa·ssofromMoodyplotλ=0.0196現(xiàn)在是27頁\一共有33頁\編輯于星期五1.5.4邊界層與局部阻力的概念1.5.4.1邊界層(boundaryLayer)邊界層—aflowlayerinwhichvelocitygradientexists

Flowresistanceisdeemedtoconcentratemainlyintheboundarylayer.現(xiàn)在是28頁\一共有33頁\編輯于星期五1.5.4.2局部阻力

(localresistance)Ⅰ.阻力系數(shù)法

(resistancecoefficientmethod)①突然擴(kuò)大現(xiàn)在是29頁\一共有33頁\編輯于星期五②突然縮小inletexit

現(xiàn)在