建筑工程及给排水专业中英文对照翻译.docx

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