動力傳遞與流動阻力導論演示文稿

動力傳遞與流動阻力導論演示文稿當前第1頁\共有33頁\編于星期三\0點（優(yōu)選）第五節(jié)動力傳遞與流動阻力導論當前第2頁\共有33頁\編于星期三\0點（動量濃度）所以（動量濃度梯度）當前第3頁\共有33頁\編于星期三\0點在渦流動量通量里動量通量=動量擴散系數(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，流動時存在內摩擦現(xiàn)象，這是流動阻力產(chǎn)生的根本原因（內因）；流體與其相接觸的固體壁面之間的作用，促使流體內部發(fā)生相對運動，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、流動類型、流過距離、壁面形狀等有關。當前第4頁\共有33頁\編于星期三\0點【VelocityDistributioninPipe

(流體在圓管內的速度分布

)】Whetherlayerfloworturbulentflow,thevelocityoffluidmassponitflowinginapipechangeswithdistancebetweenthemasspointandthepipecenterline—velocitydistribution

(速度分布).Generally,thevelocityofmassponitatpipewallisdeemedtobezero.Thenearerthemasspointtothe

pipecenterline,thefasteritmoves.ForLayerFlowThedistributionofvelocityisparabolicintheradialdirection.Thevelocityhasthelargestvalueatthepipecenter.

Meanvelocityishalfthemaximumvelocity,that當前第5頁\共有33頁\編于星期三\0點ForTurbulentFlowmasspointsvigorouslymixandcollideeachother,whichallowvelocitydistributiontobecomeuniformThegreaterthe

valueofRe

,theflatterthecurvetop.Owingtothevigorousmixingandcollisionbetweenmassponits,theflowresistanceinturbulentflowismuchlargerthanthatinlayerflow.

Thevelocitynearthewalldropssuddenly.當前第6頁\共有33頁\編于星期三\0點1.5.1.2管內流動阻力的分類直管阻力:

resistancelosscausedbyinternalfrictionwhenfluidflowsthroughastraightpipe,denotedbyhf.gowiththewholeflowprocess,alsocalledon-wayresistancetotalresistance

Sometimes,theresistanceloss

couldbeexpressedaspressuredrop

（壓強降）,

Differingfrom

Δp,Δpf

isnotinvolvedin

energyconversion.Localresistance(局部阻力):

resistancelosscausedby

when

fluidflowsthrough

pipefitting,valves,sectiononsuddenlyshrunkenandexpanded

andotherlocalplaces,denotedby

hj.當前第7頁\共有33頁\編于星期三\0點1.5.1.3計算直管阻力的通式Whensteady-stateflowingthedrivingforce

=thefrictionalresistancethatTheequationabove

showsthatinternalfrictionchangeslinearlyintheradiusdirectionwhenfluidflowsinapipe.

foranarbitraryfluiddifferentialunitwithlengthLandradiusr,itsforceanalysisthesamedirectiondrivingforce:frictionalresistance:oppositedirection

當前第8頁\共有33頁\編于星期三\0點Whenfluidflowsinahorizontalandequal-diameterstraightpipe,

ΔpisnumericallyequaltotheresistancelossΔpfcausedby

internalfriction(內摩擦力).

Atthewall,r=ri=d/2,theequationabovecanbeconvertedto

τs

—

fluidshearingstrengthatthewall

當前第9頁\共有33頁\編于星期三\0點soTheequationaboveis

relationexpressionsbetweentheresistancelossandfrictionstress.τisrelatedtotheflowpattern.kineticenergy

u2/2hasthesameunitas

hf,sohfcanbeexpressedascertain

multiples

of

u2/2.letor當前第10頁\共有33頁\編于星期三\0點generalformulaofcalculationofstraightpiperesistance,calledFanningequation

(范寧公式

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

當前第11頁\共有33頁\編于星期三\0點onintegrating穩(wěn)態(tài)流動

velocitydistributioninlayerflow當前第12頁\共有33頁\編于星期三\0點let

r=0anotherformof

velocitydistribution

inlayerflow

foranannularfluiddifferentialunitwiththickness

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

當前第13頁\共有33頁\編于星期三\0點totalflowinpipevolumeflowrateofthedifferentialunit

meanvelocityatacross-section

soHagon-Poiseuilleequation

(哈根-泊謖葉方程)當前第14頁\共有33頁\編于星期三\0點soFanningfrictionfactor(范寧摩擦因子)

comparedwithFanningequation

當前第15頁\共有33頁\編于星期三\0點1.5.3圓管內的湍流1.5.3.1管內粗糙度對摩擦系數(shù)的影響pipewallconditionisrepresentedinroughness.dabsoluteroughness

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

doesnotaffectflowresistance當前第16頁\共有33頁\編于星期三\0點Ⅱ.Forturbulentflow

laminarbottom(層流內層)δb①

e

<δbλisalsoindependentofevaluehydraulicsmooth(水力光滑)②

e

>δbThelargerthevalueofReandthesmallerthevalueofδb,

themoresignificanttheeffectofe

onλ.當前第17頁\共有33頁\編于星期三\0點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

當前第18頁\共有33頁\編于星期三\0點Exampledimensionofvariousphysicalquantitiesfundamentalphysicalquantities—mass,lengthandtime

dimensionoffundamentalphysicalquantities—massM,lengthLandtimeθ

Theequationabovecanbeexpressedasapowerfunction

當前第19頁\共有33頁\編于星期三\0點dimensionequation

(因次方程)當前第20頁\共有33頁\編于星期三\0點ThevaluesofK,b,kandqneedtobedeterminedthroughtheexperiments.

①Blasiusformula(柏拉休斯公式)theappliedrange,

Re=3×103~1×105Ⅰ.Forsmoothpipe(光滑管)當前第21頁\共有33頁\編于星期三\0點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)當前第22頁\共有33頁\編于星期三\0點當前第23頁\共有33頁\編于星期三\0點當前第24頁\共有33頁\編于星期三\0點當前第25頁\共有33頁\編于星期三\0點當前第26頁\共有33頁\編于星期三\0點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當前第27頁\共有33頁\編于星期三\0點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.當前第28頁\共有33頁\編于星期三\0點1.5.4.2局部阻力

(localresistance)Ⅰ.阻力系數(shù)法

(resistancecoefficientmethod)①突然擴大當前第29頁\共有33頁\編于星期三\0點②突然縮小inletexit

當前第