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Lesson2
FluidFlow
LiQiongCollegeofArchitectureEngineeringNorthChinaInstituteofScienceandtechnologyVersion:2012-2013Content2.1Fluidproperties2.2Basicrelationsoffluid2.3BasicflowprocessLesson2
FluidFlowFlowingfluidsinHVAC&Rsystemscan
transfer
heat,massandmomentum.Thischapterintroducesthebasicsoffluidmechanicsthatare
relatedtoHVACprocess,reviewpertinent
flowprocess.Lesson2
FluidFlow2.1FluidpropertiesFluids
differfrom
solidsintheirreactiontoshearing.Whenplacedunder
shearstress,asoliddeformsonlyafiniteamount,whereasafluiddeformscontinuouslyforaslongastheshearisapplied.Lesson2
FluidFlowBothliquidsandgasesarefluids.Althoughliquidsandgasesdifferstrongly
inthenatureof
molecularactions,theirprimarymechanicaldifferencesareinthedegreeofcompressibility
andliquidformationofafreesurface
(interface)Fluidmotioncanusuallybedescribedby
oneofseveral
simplifiedmodes
ofactionormodels.Thesimplestistheideal-fluidmodel,whichassumesnoresistancetoshearing.(许多简单运行形式或模型通常可以描述流体移动.)Lesson2
FluidFlowMostfluidsinHVACapplicationscanbetreatedas
Newtonian,wheretherateofdeformation
isdirectlyproportionaltotheshearingstress.Turbulencecomplicatesfluidsbehavior,andviscosity
influencesthenatureoftheturbulentflow.Turbulence,whichcomplicatesfluidbehavior,andviscositydoestendtoinfluenceturbulence.Lesson2
FluidFlowFluidpropertiesDensity
(密度),kg/m3Thedensitiesofairandwater
atstandardindoorconditionsof20oCand101.325Pa
(sealevelatmosphericpressure)areair=1.20kg/m3 water=998kg/m3Viscosity(粘性),Ns/m2
Absoluteviscosityordynamicviscosity(绝对粘度,动力粘度)Differentialequation:Lesson2
FluidFlowThevelocitygradientassociatedwithviscousshearforasimplecaseinvolvingflowvelocityinthexdirectionbutofvaryingmagnitudeintheydirectionisillustratedinFigure1B.Lesson2
FluidFlow
Absoluteviscositydependsprimarilyontemperature.Forgases,viscosityincreaseswith
thesquarerootoftheabsolutetemperature.Liquidviscositydecreaseswith
increasingtemperature.Shearingstress(切应力),NKinematicviscosity(运动粘度),m2/sTheratioofabsoluteviscositytodensity.Velocitygradient(速度梯度)Lesson2
FluidFlow2.2
Basicrelationsoffluiddynamics
Thissectionconsidershomogeneous,
constant-property,incompressiblefluids
andintroducesfluiddynamic
considerationsusedinmostanalyses.Continuity(连续性)
ConservationofmatterappliedtofluidflowinaconduitrequiresthatLesson2
FluidFlowBothandvmayvaryoverthecrosssectionAoftheconduit.Ifbothandvareconstantoverthecross-sectionalareanormaltotheflow,thenFortheideal-fluidmodel,flowpatternsaroundbodies(orinconduitsectionchanges)resultfromplacementeffects.Anobstructioninafluidstream,suchasastrutinafloworabumpontheconduitwall,pushestheflowsmoothlyoutoftheway,sothatbehindtheobstruction,theflowbecomesuniformagain.Theeffectoffluidinertia(density)appearsonlyinpressurechanges.Lesson2
FluidFlowPressureVariationAcrossFlowPressurevariationinfluidflowisimportantandcanbeeasilymeasured.Variationacrossstreamlinesinvolvesfluidrotation(vorticity).Thisrelationexplainsthepressuredifferencefoundbetweentheinsideandoutsidewallsofabendandnearotherregionsofconduitsectionchange.Italsostatesthatpressurevariationishydrostaticacrossanyconduitwherestreamlinesareparallel.Lesson2
FluidFlowBernoulliequationandpressurevariation
alongflowAbasictooloffluidflowanalysisistheBernoullirelation,whichinvolvestheprincipleofenergyconservation
alongastreamline.Thefirstlawofthermodynamics
canbeappliedtomechanicalflowenergies(kineticandpotential)andthermalenergies:heatisaformofenergy
and
energyisconserved.Thechangeinenergycontent
Eperunitmassofflowingmaterialis
a
resultfrom
the
workWdoneonthesystemplustheheatQabsorbed:Lesson2
FluidFlow
Fluidenergyiscomposedofkinetic,potential(duetoelevationz),andinternal
(u)energies.Per
unitmassoffluid,theaboveenergychangerelationbetweentwosectionsofthesystemistheexternalworkfromafluidmachinethepressureorflowworkLesson2
FluidFlowRearranging,theenergyequationcanbewrittenasthegeneralizedBernoulliequation:Lesson2
FluidFlowLaminarflow
Forsteady,fullydevelopedlaminarflow
inaparallel-walledconduit,theshearstressvarieslinearlywith
distanceyfromthecenterlineParabolicvelocityprofile(抛物线速度分布)Lesson2
FluidFlowTurbulenceTurbulencecanbequantified
bystatisticalfactors.Thus,thevelocitymostoftenusedinvelocityprofilesisthetemporalaveragevelocity
v,andthestrengthoftheturbulence
ischaracterizedbyroot-mean-square
oftheinstantaneousvariationinvelocityaboutthismean.Theeffectsofturbulencecausethefluidtodiffusemomentum,heat,andmassveryrapidlyacrosstheflow.Lesson2
FluidFlowReynoldsnumberRe,adimensionlessquantity,givestherelativeratioofinertialtoviscousforces;Inflowthroughroundpipesandtubes,thecharacteristiclengthisthediameter.Generally,laminarflowinpipescanbeexpectediftheReynoldsnumber,whichisbasedonthepipediameter,islessthanabout2300;Lesson2
FluidFlowFullyturbulentflow
existswhenRe>10000;Between2300and10000,theflowisintransitionstate.Inothergeometries,differentcriteria
fortheReynoldsnumberexist.Lesson2
FluidFlowHydraulicdiameter(水力直径)wettedperimeterthecross-sectionalareaofthepipeLesson2
FluidFlow2.3
BasicflowprocessWallfriction(壁面摩擦力)Attheboundaryofreal-fluidflow,therelativetangentialvelocity
atthefluidsurfaceiszero;Sometimesinturbulentstudies,velocityatthewallmayappearfinite,implyingafluidslipatthewall.However,thisisnotthecase.Lesson2
FluidFlowZerovelocityleadstoahighshearstressnearthewallboundaryandaslowingdownofadjacentfluidlayers.Avelocityprofilesdevelopsnearawall,withthevelocityincreasing
fromzeroatthewalltoanexteriorvalue
withinafinitelateraldistanceLaminarandturbulentflowdiffersignificantly
intheir
velocityprofiles.Lesson2
FluidFlowTurbulentflowprofilesareflatcomparedto
themorepointedprofilesoflaminarflow.Nearthewall,velocitiesof
theturbulentprofilesmustdroptozeromorerapidlythan
thoseof
thelaminarprofile,sotheshearstressandfrictionaremuchgreaterintheturbulentflowcase.Lesson2
FluidFlowFullydevelopedconduitflow
maybecharacterizedbythe
pipefactor,whichistheratioofaverage
tomaximumvelocity.BoundarylayerInmostflows,thefrictionofaboundingwallonthefluidflowisevidencedbya
boundarylayerTheboundarylayeristheregioncloseto
thewallwher
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