建筑工程及给排水专业中英文对照翻译.docx
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建筑工程及给排水专业中英文对照翻译
LaminarandTurbulentFlow
Observationshowsthattwoentirelydifferenttypesoffluidflowexist.Thiswasdemon-stratedbyOsborneReynoldsin1883throughanexperimentinwhichwaterwasdischargedfromatankthroughaglasstube.Therateofflowcouldbecontrolledbyavalveattheoutlet,andafinefilamentofdyeinjectedattheentrancetothetube.Atlowvelocities,itwasfoundthatthedyefilamentremainedintactthroughoutthelengthofthetube,showingthattheparticlesofwatermovedinparallellines.Thistypeofflowisknownaslaminar,viscousorstreamline,theparticlesoffluidmovinginanorderlymannerandretainingthesamerelativepositionsinsuccessivecross-sections.
Asthevelocityinthetubewasincreasedbyopeningtheoutletvalve,apointwaseventuallyreachedatwhichthedyefilamentatfirstbegantooscillateandthenbrokeupsothatthecolourwasdiffusedoverthewholecross-section,showingthattheparticlesoffluidnolongermovedinanorderlymannerbutoccupieddifferentrelativepositioninsuccessivecross-sections.Thistypeofflowisknownasturbulentandischaracterizedbycontinuoussmallfluctuationsinthemagnitudeanddirectionofthevelocityofthefluidparticles,whichareaccompaniedbycorrespondingsmallfluctuationsofpressure.
Whenthemotionofafluidparticleinastreamisdisturbed,itsinertiawilltendtocarryitoninthenewdirection,buttheviscousforcesduetothesurroundingfluidwilltendtomakeitconformtothemotionoftherestofthestream.Inviscousflow,theviscousshearstressesaresufficienttoeliminatetheeffectsofanydeviation,butinturbulentflowtheyareinadequate.Thecriterionwhichdetermineswhetherflowwillbeviscousofturbulentisthereforetheratiooftheinertialforcetotheviscousforceactingontheparticle.
Theratio
Thus,thecriterionwhichdetermineswhetherflowisviscousorturbulentisthequantityρvl/μ,knownastheReynoldsnumber.Itisaratioofforcesand,therefore,apurenumberandmayalsobewrittenasul/vwhereisthekinematicviscosity(v=μ/ρ).
ExperimentscarriedoutwithanumberofdifferentfluidsinstraightpipesofdifferentdiametershaveestablishedthatiftheReynoldsnumberiscalculatedbymaking1equaltothepipediameterandusingthemeanvelocityv,then,belowacriticalvalueofρvd/μ=2000,flowwillnormallybelaminar(viscous),anytendencytoturbulencebeingdampedoutbyviscousfriction.ThisvalueoftheReynoldsnumberappliesonlytoflowinpipes,butcriticalvaluesoftheReynoldsnumbercanbeestablishedforothertypesofflow,choosingasuitablecharacteristiclengthsuchasthechordofanaerofoilinplaceofthepipediameter.Foragivenfluidflowinginapipeofagivendiameter,therewillbeacriticalvelocityofflowcorrespondingtothecriticalvalueoftheReynoldsnumber,belowwhichflowwillbeviscous.
Inpipes,atvaluesoftheReynoldsnumber>2000,flowwillnotnecessarilybeturbulent.LaminarflowhasbeenmaintaineduptoRe=50,000,butconditionsareunstableandanydisturbancewillcausereversiontonormalturbulentflow.Instraightpipesofconstantdiameter,flowcanbeassumedtobeturbulentiftheReynoldsnumberexceeds4000.
PipeNetworks
Anextensionofcompoundpipesinparallelisacasefrequentlyencounteredinmunicipaldistributionsystem,inwhichthepipesareinterconnectedsothattheflowtoagivenoutletmaycomebyseveraldifferentpaths.Indeed,itisfrequentlyimpossibletotellbyinspectionwhichwaytheflowtravels.Nevertheless,theflowinanynetworks,howevercomplicated,mustsatisfythebasicrelationsofcontinuityandenergyasfollows:
1.Theflowintoanyjunctionmustequaltheflowoutofit.
2.Theflowineachpipemustsatisfythepipe-frictionlawsforflowinasinglepipe.
3.Thealgebraicsumoftheheadlossesaroundanyclosedcircuitmustbezero.
Pipenetworksaregenerallytoocomplicatedtosolveanalytically,aswaspossibleinthesimplercasesofparallelpipes.Apracticalprocedureisthemethodofsuccessiveapproximations,introducedbyCross.Itconsistsofthefollowingelements,inorder:
1.Bycarefulinspectionassumethemostreasonabledistributionofflowsthatsatisfiescondition1.
2.Writecondition2foreachpipeintheform
hL=KQn(7.5)
whereKisaconstantforeachpipe.Forexample,thestandardpipe-frictionequationwouldyieldK=1/C2andn=2forconstantf.Minorlosseswithinanycircuitmaybeincluded,butminorlossesatthejunctionpointsareneglected.
3.Toinvestigatecondition3,computethealgebraicsumoftheheadlossesaroundeachelementarycircuit.∑hL=∑KQn.Considerlossesfromclockwiseflowsaspositive,counterclockwisenegative.Onlybygoodluckwilltheseaddtozeroonthefirsttrial.
4.Adjusttheflowineachcircuitbyacorrection,ΔQ,tobalancetheheadinthatcircuitandgive∑KQn=0.TheheartofthismethodliesinthedeterminationofΔQ.Foranypipewemaywrite
Q=Q0+ΔQ
whereQisthecorrectdischargeandQ0istheassumeddischarge.Then,foracircuit
(7.6)
ItmustbeemphasizedagainthatthenumeratorofEq.(7.6)istobesummedalgebraically,withdueaccountofsign,whilethedenominatorissummedarithmetically.ThenegativesigninEq.(7.6)indicatesthatwhenthereisanexcessofheadlossaroundaloopintheclockwisedirection,theΔQmustbesubtractedfromclockwiseQ0’sandaddedtocounterclockwiseones.Thereverseistrueifthereisadeficiencyofheadlossaroundaloopintheclockwisedirection.
5.Aftereachcircuitisgivenafirstcorrection,thelosseswillstillnotbalancebecauseoftheinteractionofonecircuituponanother(pipeswhicharecommontotwocircuitsreceivetwoindependentcorrections,oneforeachcircuit).Theprocedureisrepeated,arrivingatasecondcorrection,andsoon,untilthecorrectionsbecomenegligible.
EitherformofEq.(7.6)maybeusedtofindΔQ.AsvaluesofKappearinbothnumeratoranddenominatorofthefirstform,valuesproportionaltotheactualKmaybeusedtofindthedistribution.Thesecondformwillbefoundmostconvenientforusewithpipe-frictiondiagramsforwaterpipes.
Anattractivefeatureoftheapproximationmethodisthaterrorsincomputationhavethesameeffectaserrorsinjudgmentandwilleventuallybecorrectedbytheprocess.
Thepipe-networksproblemlendsitselfwelltosolutionbyuseofadigitalcomputer.Programmingtakestimeandcare,butoncesetup,thereisgreatflexibilityandmanyman-hoursoflaborcanbesaved.
TheFutureofPlasticPipeatHigherPressures
ParticipantsinanAGAmeetingpanelonplasticpipediscussedthepossibilityofusingpolyethylenegaspipeathigherpressures.Topicsincludedthedesignequation,includingworkbeingdonebyISOonanupdatedversion,andtheevaluationofrapidcrackpropagationinaPEpiperesin.Thisisofcriticalimportancebecauseaspipeisusedathigherpressureandinlargerdiameters,thepossibilityofRCPincreases.
Severalyearsago,AGA’sPlasticPipeDesignEquationTaskGroupreviewedthedesignequationtodetermineifhigheroperatingpressurescouldbeusedinplasticpipingsystems.Membersfelttheperformanceofourpiperesinswasnottrulyreflectedbythedesignequation.Itwasgenerallyacceptedthatthelong-termpropertiesofmodernresinsfarsurpassedthoseofolderresins.Majorconsiderationswerenewequationsbeingdevelopedandselectionofanappropriatedesignfactor.
Improvedpipeperformance
Manyutilitiesmonitoredtheperformanceofplasticpiperesins.Herearesomeofthelong-termtestsusedandthekindsofperformancechangetheyhaveshownfortypicalgaspiperesins.
Elevatedtemperaturebursttest
TheyusedtestsliketheElevatedTemperatureBurstTest,inwhichthelong-termperformanceofthepipeischeckedbymeasuringthetimerequiredforformationofbrittlecracksinthepipewallunderhightemperaturesandpressures(often80degreesCandaround4to5-MPahoopstress).AtConsumersGasweexpectedearlyresinstolastatleast170hrs.at80degreesCandahoopstressof3MPa.Extrapolationshowedthatresinspassingtheselimitsshouldhavealifeexpectancyofmorethan50yrs.Qualitycontroltestingonshipmentsofpipemadefromtheseresinssometimesresultedinproductrejectionforfailuretomeetthiscriterion.
Atthesametemperature,today’sresinslastthousandsofhoursathoopstressesof4.6MPa.Testsperformedonpipemadefromnewresinshavebeenterminatedwithnofailureattimesexceeding5,700hrs.Theseresultswereperformedonsamplesthatweresqueezedoffbeforetesting.Suchstresseswereneverappliedinearlytesting.Whenextrapolatedtooperatingconditions,thisdifferenceintestperformanceisequivalenttoanincreaseinlifetimeofhundreds(andinsomecaseseventhousands)ofyears.
Environmentalstresscrackresistancetest
SomecompaniesalsousedtheEnvironmentalStressCrackResistancetestwhichmeasuredbrittlecrackformationinpipesbutwhichusedstresscrackingagentstoshortentesttimes.
Thistesthasalsoshowndramaticimprovementinresistancebrittlefailure.Forexample,atmycompanyatesttimeofmorethan20hrs.at50degreesCwasrequiredonourearlyresins.Today’sresinslastwellabove1,000hrs.withnofailure.
Notchtests
Notchtests,whicharequicklyrun,measurebrittlecrackformationinnotchedpipeormoldedcouponsamples.Thisisimportantforthenewerresinssincesomeotherteststofailurecantakeverylongtimes.Notchtestresultsshowthatwhileearlyresinslastedfortesttimesrangingbetween1,000to10,000min.,currentresinsusuallylast