文獻名稱(中文)-化工工業(yè)離心泵

PAGE1文獻名稱（外文）CENTRIFUGALPUMPSINTHECHEMICALINDUSTRYAbstract:Acentrifugalpumpconvertstheinputpowertokineticenergyintheliquidbyacceleratingtheliquidbyarevolvingdevice-animpeller.Themostcommontypeisthevolutepump.Fluidentersthepumpthroughtheeyeoftheimpellerwhichrotatesathighspeed.Thefluidisacceleratedradiallyoutwardfromthepumpchasing.Avacuumiscreatedattheimpellerseyethatcontinuouslydrawsmorefluidintothepump.Thisarticlestressesonaseriesofcentrifugalpumps，F(xiàn)romabriefintroductiontotheprinciples.Keywords:centrifugalpump,Introduction,Workingprinciple,Cavitation，MechanismofCavitation，SolutionandRemedies1.IntroductionPump,deviceusedtoraise,transfer,orcompressliquidsandgases.Fourgeneralclassesofpumpsforliquidsaredescribedbelow.Inallofthem,stepsaretakentopreventcavitation(theformationofavacuum),whichwouldreducetheflowanddamagethestructureofthepump.Pumpsusedforgasesandvaporsareusuallyknownascompressors.Thestudyoffluidsinmotioniscalledfluiddynamics.Waterpump,deviceformovingwaterfromonelocationtoanother,usingtubesorothermachinery.Waterpumpsoperateunderpressuresrangingfromafractionofapoundtomorethan10,000poundspersquareinch.Everydayexamplesofwaterpumpsrangefromsmallelectricpumpsthatcirculateandaeratewaterinaquariumsandfountainstosumppumpsthatremovewaterfrombeneaththefoundationsofhomes.Onetypeofmodernpumpsusedtomovewateristhecentrifugalpump.Earlyversionofthecentrifugalpump,thescrewpump,consistsofacorkscrew-shapedmechanisminapipethat,whenrotated,pullswaterupward.Screwpumpsareoftenusedinwaste-watertreatmentplantsbecausetheycanmovelargeamountsofwaterwithoutbecomingcloggedwithdebris.IntheancientMiddleEasttheneedforirrigationoffarmlandwasastronginducementtodevelopawaterpump.Earlypumpsinthisregionweresimpledevicesforliftingbucketsofwaterfromasourcetoacontaineroratrench.GreekmathematicianandinventorArchimedesisthoughttohavedevisedthefirstscrewpumpinthethirdcenturyBC.LaterGreekinventorCtesibiusdevelopthefirstliftpump.Duringthelate17thandearly18thCenturiesAD,BritishengineerThomasSavery,FrenchphysicistDenisPapin,AndBritishblacksmithandinventorThomasNewcomencontributedtothedevelopmentofawaterpumpthatusedsteamtopowerthepump’piston.Thesteam-poweredwaterpump’sfirstwideusewasinpumpingwateroutofmines.Modern-dayexamplesofcentrifugalpumpsarethoseusedattheGrandCouleeDamontheColumbiaRiver.Thispumpsystemhasthepotentialtoirrigateoveronemillionacresofland.Alsoknownasrotarypumps,centrifugalpumpshavearotatingimpeller,alsoknownasablade,thatisimmersedintheliquid.Liquidentersthepumpneartheaxisoftheimpeller,andtherotatingimpellersweepstheliquidouttowardtheendsoftheimpellerbladesathighpressure.Theimpelleralsogivestheliquidarelativelyhighvelocitythatcanbeconvertedintopressureinastationarypartofthepump,knownasthediffuser.Inhigh-pressurepumps,anumberofimpellermaybeusedinseries,andthediffusersfollowingeachimpellermaycontainguidevanestograduallyreducetheliquidvelocity.Forlower-pressurepumps,thediffuserisgenerallyaspiralpassage,knownasavolute,withitscross-sectionalareaincreasinggraduallytoreducethevelocityefficiently.Theimpellermustbeprimedbeforeitcanbeginoperation,thatis,theimpellermustbesurroundedbyliquidwhenthepumpisstarted.Thiscanbedonebyplacingacheckvalveinthesuctionline,whichholdstheliquidinthepumpwhentheimpellerisnotrotating.Ifthisvalveleaks,thepumpmayneedtobeprimedbytheintroductionofliquidfromanoutsidesourcesuchasthedischargereservoir.Acentrifugalpumpgenerallyhasavalveinthedischargelinetocontroltheflowandpressure.Forlowflowsandhighpressures,theactionoftheimpellerislargelyradial.Forhigherflowsandlowerdischargepressure,thedirectionoftheflowwithinthepumpismorenearlyparalleltotheaxisoftheshaft,andthepumpissaidtohaveanaxialflow.Theimpellerinthiscaseactsasapropeller.Thetransitionfromonesetoffloeconditionstotheotherisgradual,andforintermediatecondition,thedeviceiscalledamixed-flowpump.2.TheCentrifugalPumpThecentrifugalpumpisbyfarthemostwidelyusedtypeinthechemicalandpetroleumindustries.Itwillpumpliquidswithverywiderangingpropertiesandsuspensionswithahighsolidscontentincluding,forexample,cementslurries,andmaybeconstructedfromaverywiderangofcorrosionresistantmaterials.Thewholepumpcasingmaybeconstructedfromplasticsuchaspolypropyleneoritmaybefittedwithacorrosion-resistantlining.Becauseitoperatesathighspeed,itmaybedirectlycoupledtoanelectricmotoranditwillgiveahighflowrateforitssize.Inthistypeofpump,thefluidisfedtothecentreofarotatingimpellerandisthrownoutwardbycentrifugalaction.Asaresultofthehighspeedofrotationtheliquidacquiresahighkineticenergyandthepressuredifferencebetweenthesuctionanddeliverysidesarisesfromtheconversionofkineticenergyintopressureenergy.Theimpellerconsistsofaseriesofcurvedvanessoshapedthattheflowwithinthepumpisassmoothaspossible.Thegreaterthenumberofvanesontheimpeller,thegreateristhecontroloverthedirectionoftheliquidandhencethesmallerarethelossesduetoturbulenceandcirculationbetweenthevanes.Intheopenimpeller,thevanesarefixedtoacentralhub,whereasintheclosedtypethevanesareheldbetweentwosupportingplatesandleakageacrosstheimpellerisreduced.Aswillbeseenlater,theangleofthetipsofthebladesverylargelydeterminestheoperatingcharacteristicsofthepump.Theliquidentersthecasingofthepump，normallyinanaxialdirection，andispickedupbythevanesoftheimpeller．Inthesimpletypeofcentrifugalpump，theliquiddischargesintoavolute，achamberofgraduallyincreasingcross—sectionwithatangentialoutlet．AvolutetypeofpumpisshowninFig.(a)．Intheturbinepump[-Fig．(b)]theliquidflowsfromthemovingvanesoftheimpellerthroughaseriesoffixedvanesformingadiffusionring．Thisgivesamoregradualchangeindirectiontothefluidandmoreefficientconversionofkineticenergyintopressureenergythanisobtainedwiththevolutetype．Theangleoftheleadingedgeofthefixedvanesshouldbesuchthatthefluidisreceivedwithoutshock．Theliquidsflowsalongthesurfaceoftheimpellervanewithacertainvelocitywhilstthetipofthevaneismovingrelativetothecasingofthepump．Thedirectionofmotionoftheliquidrelativetothepumpcasing--andtherequiredangleofthefixedvanes—isfoundbycompoundingthesetwovelocities．InFig．c，c.isthevelocityoftheliquidrelativetothevaneandisthetangentialvelocityofthetipofthevane；compoundingthesetwovelocitiesgivestheresultantvelocityoftheliquid．Itisapparent，therefore，thattherequiredvaneangleinthediffuserisdependentonthethroughput，thespeedofrotation，andtheangleoftheimpellerblades．Thepumpwillthereforeoperateatmaximumefficiencyonlyoveranarrowrangeofconditions．VirtualheadofacentrifugalpumpThemaximumpressureisdevelopedwhenthewholeoftheexcesskineticenergyofthefluidisconvertedintopressureenergy.Asindicatedbelow．theheadisproportionaltothesquareoftheradiusandtothespeed，andisoftheorderof60mforasingle—stagecentrifugalpump；forhigherpressures，multistagepumpsmustbeused．Considertheliquidwhichisrotatingatadistanceofbetweenrandr+drfromthecentreofthepump(Fig．d)．dThemassofthiselementoffluiddmisgivenby2πrdrdρ，whereρisthedensityofthefluidand6isthewidthoftheelementoffluid。Ifthefluidistravelingwithavelocityuandatanangleθtothetangentialdirection．Theangularmomentumofthismassoffluid=dM(urcosθ)Thetorqueactingonthefluiddτisequaltotherateofchangeofangularmomentumwithtime，asitgoesthroughthepumpDτ=dMα/αt(urcosθ)=2πrbρdrα/αt(urcosθ)Thevolumetricrateofflowofliquidthroughthepump：Q=2πrbα/αtDr=Qρd(urcosθ)Thetotaltorqueactingontheliquidinthepumpisthereforeobtainedintegratingdτbetweenthelimitsdenotedbysuffix1andsuffix2，wheresuffix1referstotheconditionsattheinlettothepumpandsuffix2referstotheconditionatthedischarge．Thus，τ=Qρ(cos-cos)TheadvantagesanddisadvantagesofthecentrifugalpumpThemainadvantagesare：(1)Itissimpleinconstructionandcan，therefore，bemadeinawiderangeofmaterials(2)Thereisacompleteabsenceofvalves．(3)Itoperatesathighspeed(upto100Hz)and，therefore，canbecoupleddirectlytoanelectricmotor.Ingeneral，thehigherthespeedthesmallerthepumpandmotorforagivenduty．(4)Itgivesasteadydelivery．(5)Maintenancecostsarelowerthanforanyothertypeofpump．(6)Nodamageisdonetothepumpifthedeliverylinebecomesblocked，provideditisnotruninthisconditionforaprolongedperiod．(7)Itismuchsmallerthanotherpumpsofequalcapacity．Itcan，therefore，bemadeintoasealedunitwiththedrivingmotorandimmersedinthesuctiontank．(8)Liquidscontaininghighproportionsofsuspendedsolidsarereadilyhandled．Themaindisadvantagesare：(1)Thesingle—stagepumpwillnotdevelopahighpressure．Multistagepumpswilldevelopgreaterheadsbattheyareverymuchmoreexpensiveandcannotreadilybemadeincorrosion—resistantmaterialbecauseoftheirgreatercomplexity．Itisgenerallybettertouseveryhighspeedsinordertoreducethenumberofstagesrequired．(2)Itoperatesatahighefficiencyoveronlyalimitedrangeofconditions;thisappliesespeciallytoturbinepumps．(3)Itisnotusuallyself-priming.(4)Ifanon-returnvalveisnotincorporatedinthedeliveryorsuctionline,theliquidwillrunbackintothesuctiontankassoonasthepumpstops．(5)Veryviscousliquidscannothehandledefficiently．3.Cavitationincentrifugalpump（1）Theterm‘cavitation’comesfromtheLatinwordcavus,whichmeansahollowspaceoracavity.Webster’sDictionarydefinestheword‘cavitation’astherapidformationandcollapseofcavitiesinaflowingliquidinregionsofverylowpressure.Inanydiscussiononcentrifugalpumpsvarioustermslikevaporpockets,gaspockets,holes,bubbles,etc.areusedinplaceofthetermcavities.Theseareoneandthesamethingandneednotbeconfused.Thetermbubbleshallbeusedhereafterinthediscussion.Inthecontextofcentrifugalpumps,thetermcavitationimpliesadynamicprocessofformationofbubblesinsidetheliquid,theirgrowthandsubsequentcollapseastheliquidflowsthroughthepump.1.Vaporbubblesareformedduetothevaporisationofaprocessliquidthatisbeingpumped.ThecavitationconditioninducedbyformationandcollapseofvaporbubblesiscommonlyreferredtoasVaporousCavitation.2.Gasbubblesareformedduetothepresenceofdissolvedgasesintheliquidthatisbeingpumped(generallyairbutmaybeanygasinthesystem).ThecavitationconditioninducedbytheformationandcollapseofgasbubblesiscommonlyreferredtoasGaseousCavitation.Totalpressure：Thesumofstaticpressureanddynamicpressureisdefinedasthetotalpressure.Itisameasureoftotalenergyofthemovingfluidstream.i.e.bothpotentialandkineticenergy.Vaporpressureisthepressurerequiredtokeepaliquidinaliquidstate.Ifthepressureappliedtothesurfaceoftheliquidisnotenoughtokeepthemoleculesprettyclosetogether,themoleculeswillbefreetoseparateandroamaroundasagasorvapor.Thevaporpressureisdependentuponthetemperatureoftheliquid.Higherthetemperature,higherwillbethevaporpressure.（3）CavitationDamage：Cavitationcandestroypumpsandvalves,andcavitationcausesalossofefficiencyinpumpsimmediately,andalsoacontinuouslyincreasinglossofefficiencyastheequipmentdegradesduetoerosionofthepumpcomponentsbycavitation.ThereforeItisimportanttounderstandthephenomenasufficientlytopredictandthereforereducecavitationanddamagefromcavitation,andalsotodiagnoseandfindpracticalsolutionstocavitationproblems。1）CavitationEnhancedChemicalErosionPumpsoperatingundercavitationconditionsbecomemorevulnerabletocorrosionandchemicalattack.Metalscommonlydevelopanoxidelayerorpassivatedlayerwhichprotectsthemetalfromfurthercorrosion.Cavitationcanremovethisoxideorpassivelayeronacontinuousbasisandexposeunprotectedmetaltofurtheroxidation.Thetwoprocesses(cavitation&oxidation)thenworktogethertorapidlyremovemetalfromthepumpcasingandimpeller.Stainlesssteelsarenotinvulnerabletothisprocess.2）MaterialsSelectionThereisnometal,plastic,oranyothermaterialknowntoman,thatcanwithstandthehighlevelsofenergyreleasedbycavitationintheformsofheatandpressure.Inpracticehowever,materialscanbeselectedthatresultinlongerlifeandcustomervalueintheirabilitytowithstandcavitationenergies,sothatattentiontopumpconstructionmaterialsisvaluableandproductive.Wherecavitationisnotaproblemornotpredictedtobeaproblem,commonmaterialssuchascastironandbronzearesuitableforpumpconstruction.Therearemillionsofcastironandbronzepumpsthatworkfinefor20yearsormorewithoutanyproblemeventhoughmanyofthosepumpsexperiencesomecavitation.（4）MechanismofCavitation：ThephenomenonofcavitationisastepwiseprocessasshowninFigure(below).StepOne,Formationofbubblesinsidetheliquidbeingpumped.Thebubblesforminsidetheliquidwhenitvaporisesi.e.phasechangefromliquidtovapor.Buthowdoesvaporizationoftheliquidoccurduringapumpingoperation?Vaporizationofanyliquidinsideaclosedcontainercanoccurifeitherpressureontheliquidsurfacedecreasessuchthatitbecomesequaltoorlessthantheliquidvaporpressureattheoperatingtemperature,orthetemperatureoftheliquidrises,CollapseofaVaporBubbleraisingthevaporpressuresuchthatitbecomesequaltoorgreaterthantheoperatingpressureattheliquidsurface.Forexample,ifwateratroomtemperature(about77°F)iskeptinaclosedcontainerandthesystempressureisreducedtoitsvaporpressure(about0.52psia),thewaterquicklychangestoavapor.Also,iftheoperatingpressureistoremainconstantatabout0.52psiaandthetemperatureisallowedtoriseabove77°F,thenthewaterquicklychangestoavapor.CollapseofaVaporBubbleJustlikeinaclosedcontainer,vaporizationoftheliquidcanoccurincentrifugalpumpswhenthelocalstaticpressurereducesbelowthatofthevaporpressureoftheliquidatthepumpingtemperature.StepThree,Collapseofbubbles，Asthevaporbubblesmovealongtheimpellervanes,thepressurearoundthebubblesbeginstoincreaseuntilapointisreachedwherethepressureontheoutsideofthebubbleisgreaterthanthepressureinsidethebubble.Thebubblecollapses.Theprocessisnotanexplosionbutratheranimplosion(inwardbursting).Hundredsofbubblescollapseatapproximatelythesamepointoneachimpellervane.Bubblescollapsenon-symmetricallysuchthatthesurroundingliquidrushestofillthevoidformingaliquidmicrojet.Themicrojetsubsequentlyrupturesthebubblewithsuchforcethatahammeringactionoccurs.Bubblecollapsepressuresgreaterthan1GPa(145x106psi)havebeenreported.Thehighlylocalizedhammeringeffectcanpitthepumpimpeller.Thepittingeffectisillustratedschematicallyinthisthefigure.Afterthebubblecollapses,ashockwaveemanatesoutwardfromthepointofcollapse.Thisshockwaveiswhatweactuallyhearandwhatwecall"cavitation".Theimplosionofbubblesandemanationofshockwaves(redcolor).Innutshell,themechanismofcavitationisallaboutformation,growthandcollapseofbubblesinsidetheliquidbeingpumped.Buthowcantheknowledgeofmechanismofcavitationcanreallyhelpintroubleshootingacavitationproblem.Theconceptofmechanismcanhelpinidentifyingthetypeofbubblesandthecauseoftheirformationandcollapse.(5)SolutionandRemedies：Forvaporizationproblems(cavitation)(1.Tocurevaporizationproblemsyoumusteitherincreasethesuctionhead,lowerthefluidtemperature,ordecreasetheN.P.S.H.Required.Weshalllookateachpossibility:1).3)reducetheN.P.S.H.Required? 1.Removedebrisfromsuctionline.2.Movepumpclosertosourcetank/sump3.Increasesuctionlinediameter.4.Decreasesuctionliftrequirement5.InstalllargerpumprunningslowerwhichwilldecreasetheNetPositiveSuctionHeadRequiredbythepump(NPSHR).6.Increasedischargepressure.7.FullyopenSuctionlinevalve.(3.Fordischargecavitation:1.Removedebrisfromdischargeline.2.Decreasedischargelinelength3.Increasedischargelinediameter.4.Decreasedischargestaticheadrequirement.5.Installlargerpump,whichwillmaintaintherequiredflowwithoutdischargecavitating.6.Fullyopendischargelinevalve.(4.ForRecirculationcavitation:1.Designingthepumpforlowersuction-specificspeedsandlimitingtherangeofoperationtoflowcapacitiesabovethepointofrecirculation.2.Raisingthesuctionhead.Selectedfrom:1.J.M.Coucson,J.F.Richardson,ChemicalEngineering,Butterworth-HeinemannLtd.,19952.Delgosha,O.C.,Patella,R.F.,Reboud,J.L.:ExperimentalandNumericalStudiesinaCentrifugalPumpwithTwo-DimensionalCurvedBladesinCavitatingCondition.JournalofFluidsEngineering,vol.125,pp.970―978,(2003).3.Zhang,J.F.,Yuan,S.Q.,Fu,Y.D.:NumericalForecastoftheInfluenceofSplitterBladesontheFlowFieldandCharacteristicsofaCentrifugalpump,ChineseJournalofChemicalEngineering,vol.45,pp.131-137,(2009).4.P.D.Lyapkov,TrudyVNII,No.5,Gostoptekhizdat,Moscow(1959).5.Zhang,J.F.,Yuan,S.Q.,Fu,Y.D.:NumericalForecastoftheInfluenceofSplitterBladesontheFlowFieldandCharacteristicsofaCentrifugalpump,ChineseJournalofChemicalEngineering,vol.45,pp.131-137,(2009).文獻名稱（中文）化工工業(yè)離心泵摘要：離心泵是通過葉輪的旋轉把液體的內能轉換成動能的一種旋轉裝置。液體由吸入口進入蝸殼，通過高速旋轉的葉輪，液體呈放射狀加速從泵中向外輸出，這時葉輪附近留出一個真空，不斷吸引更多的流體進入泵的葉輪附近，這樣由葉輪的旋轉來完成液體的進出。這篇文章主要講述了關于離心泵的發(fā)展史，離心泵工作原理的分析，汽蝕的基本原理和預防汽蝕的措施等的一系列問題。從而幫助我們加深對離心泵的理解。關鍵詞：離心泵介紹工作原理汽蝕汽蝕原理預防措施1.介紹泵的提出，最先是用于轉移或壓縮液體和氣體的設備。在所有泵中，我們一步步采取措施來防止氣蝕，氣蝕將減少流量并且破壞泵的結構。用來處理氣體和蒸汽的泵稱為氣體壓縮機，研究流體的運動的科學稱為流體力學。水泵是用管子連接的機械把水從一個地方傳到另一個地方。水泵的操作壓力從一磅到一萬磅每平方英尺。日常生活中，泵是很多見的，有用于在魚池和噴泉使水循環(huán)和向水中充氣的電泵，還有用于從住宅處把水引走的污水泵。離心泵的早期形式螺桿泵，是通過一個管子連接一根螺桿組成的，它是利用螺桿的旋轉把水提升上去。螺旋泵經(jīng)常用在污水處理廠中，因為它們可以運輸大量的水，而不會因為碎片而堵塞。在遠古的中東，因為對農(nóng)場進行灌溉的需求，所以有一種強大的動力去推進水泵的進程。在這些區(qū)域里，早期的泵是為了將水一桶一桶的從水源或河渠中提升到容器中。古希臘的發(fā)明家和數(shù)學家阿基米德被認為是公元前3世紀首先提出螺旋泵的發(fā)明家。之后，古希臘發(fā)明家發(fā)明了第一個提水泵。在十七世紀末和十八世紀初，英國的工程師ThomasSavory，法國的物理學家DenisPa]pin，和英國的鐵匠和發(fā)明家TomasNewcomen,它們發(fā)明了用蒸汽驅動活塞的水泵。蒸汽驅動的水泵首先廣泛的被應用是在從煤礦往外輸水過程中?，F(xiàn)在離心泵使用的例子，是來自于哥倫比亞河上使用的大古利水壩。這個泵有超過灌溉一百萬英畝的土地能力。離心泵被認為是旋轉泵，它有一個旋轉地葉輪，葉輪上有葉片，葉片是侵入液體中的。液體也是由葉輪軸向進入泵，并且旋轉的葉輪將液體甩向葉片根部。同時葉輪也給液體一個較高的速度，這個速度通過泵的一個固定部件轉化成壓力。我們一般稱為擴壓器。在高壓泵里，很多葉輪可以被系列選用，并且在一個葉輪后有一個擴壓器，也可能含有導輪，可以逐漸的降低液體的速度。對于低壓泵來說，擴壓泵一般就是一個螺旋形的通道，成為蝸殼，作用原理是攔截面逐漸增加可以有效降低流體的速度。在泵工作前，葉輪必須被灌注，也就是在泵啟動時，葉輪必須被液體包圍。也可以通過在吸入線上放另一個截止閥來實現(xiàn)，截止閥在泵停止工作時是液體保留在泵內。如果截止閥泄露了，泵可以通過閥的入口，從外面的水源比如說蓄水池來取水灌注。一般離心泵在排水線的地方也有一個閥控制流體和壓力。對于小流量和高壓力來說，葉輪作用很大部分是放射狀的。對于高速流體和低壓排水壓力，泵中流體的方向可以近似于與軸的軸向平行，這時泵有一個軸流。這時葉輪就近似于螺旋推進器。從一種流動的狀態(tài)轉換到另一種流動的狀態(tài)是漸進的，對于中間狀態(tài)，設備可稱為混流泵。2.離心泵離心泵是化工和石油工業(yè)中應用最廣泛的一種泵。它能輸送性能非常廣泛的液體和固體含量高的懸浮液，像泥泥漿，可以用多種抗腐蝕材料建造。泵的整個外殼可用像聚丙烯這樣的塑料來建造，或者用腐蝕襯里加工。由于它的高速運轉，可將其直接耦合到電動機上，由電動機的規(guī)格大小決定流量高低。在這樣的泵中，液體被吸入到旋轉葉輪的中心，通過離心作用向外流動。由于高速旋轉，液體在吸入口和因動能轉化為壓能的出口側獲得較高的動能和壓力差。葉輪由一系列弧形葉片組成，因此能使液體的流動盡可能平穩(wěn)。葉輪中葉片越多，則液體的流動方向越好控制，那么液體循環(huán)流動時因波動引起的損失就越少。在開式葉輪中，葉片被固定在中心輪轂上，而在閉式中葉片則是用兩塊鋼板支撐以減少漏液。由此可以看出，在很大程度上，葉片末端的角度決定了泵的工作特性。流體通常在軸向上通過葉片的上升進入泵殼。在這種簡單類型的離心泵中，液體由切向方向隨著橫截面逐步流到蝸殼中。圖（a）所示為旋渦型泵。圖（b）中，在渦輪泵中的液體隨移動的葉輪在一系列固定葉片中形成擴散環(huán)。這種旋渦能逐漸改變流體的流動方向，并有效地將動能轉化成壓能。固定葉片前緣處的流體應該沒有受到?jīng)_擊。沿著葉輪葉片，液體的流動具有一定速度，同時，葉片末端相對于泵體有移動。液體的運動方向相對于泵殼——和固定葉片所需的角度一樣——是兩個速度的合成方向。在圖c中，c.是液體相對于葉片的速度，是葉片上某點的切向速度；將這兩個速度合成即可得到液體的速度。因此，很明顯，在擴散環(huán)中所需要的葉輪角由葉輪的產(chǎn)量、旋轉速度和葉片的角度決定。所以，泵在很嚴格的條件下才能有最大的運行效能。 2.1離心泵的有效壓頭 當流體所剩余的動能全部轉化為壓能時，壓力最大。如下文所述，有效壓頭和半徑的平方以及速度成正比，壓力更高時，必須使用多級泵?？紤]到液體在離泵中心r到r+dr的距離內旋轉，如圖dd.所示。這一部分流體的質量為dM=2πrdrdρ，其中ρ是流體的密度，b是這部分流體的寬度。 如果流體在與切向方向成θ角上以速度u流動，則這部分質量流體的角動量為=dM(urcosθ) 流體通過泵所產(chǎn)生的扭轉力等于角動量對時間的改變量dτ=dM(urcosθ)=2πrbρdr(urcosθ) 液體的體積流速為：Q=2πrbDr=Qρd(urcosθ) 因此，液體在泵中受到總的扭轉力由dτ在小標1和2之間積分而得，下標1引用的是泵入口處的條件，小標2是出口時的條件。于是有：τ=Qρ（u2r2cosθ2–u1r1cosθ1） 2.2離心泵的優(yōu)缺點主要優(yōu)點有：（1）制造簡單，可用多種材料加工。（2）無閥門。（3）高速運轉（高達100赫茲），因此可直接耦合到電動機上。一般地，速度越大，泵和電動機的效率越小。（4）能平穩(wěn)傳送。（5）維修費用比其他類型的泵少。（6）輸送堵塞時，只要不是長時間運作，泵就不會被損壞。（7）與其它泵相比，體積較小。因此，它可以利用電動機做成密封裝置沉浸在吸收罐中。（8）能容易輸送含有高比例懸浮固體的液體。主要缺點有：（1）單級泵不能提高壓力。而多級泵能提高壓頭，但價格昂貴而且由于它們的復雜性不能用抗腐蝕的材料加工建造。通常用較高的速度來減少所需要的級數(shù)。（2）只有在有限條件下才能以最高效能運作：尤其是渦輪泵。（3）它不能自動注水。（4）在輸送和吸收管道中，如果沒有止回閥，液體就會在泵停止瞬間倒流到吸入槽內。（5）不能有效處理粘性液體。3.離心泵中的汽蝕（1）“汽蝕”一詞來源于拉丁語高弓足，這意味著一個中空的空間或空腔。韋氏詞典定義的字是在一個非常低的壓力區(qū)域流動的液體腔內迅速形成和崩潰的“腔”。在離心泵中的任何地方像蒸氣泡沫，氣體泡沫，氣體破洞，氣泡等各種條件長期作用都會造成汽蝕。這是一個各種結果同時作用的事情，不能簡單地看待。汽蝕的形成討論如下。在離心泵的蝸殼中，汽蝕意味著一個氣泡內的液體，他們的形成，成長和隨后通過泵的液體流動崩潰所經(jīng)歷的動態(tài)過程。一般來說，液體內氣泡的形成有兩種類型：蒸汽氣泡或氣態(tài)空泡。1.由于一個進程正在進行的液體汽化而引起的泡沫的形成。蒸汽氣泡的形成和崩潰引起的汽蝕條件通常被稱為霧狀氣蝕。2.泡沫形成的過程中，由于正在往泵中輸送的液體中溶解入氣體（一般空氣的存在，但可能是系統(tǒng)中的任何氣體），由這些氣體的形成和崩潰引起的汽蝕條件通常被稱為氣態(tài)空泡。度作者：J.M.Coucson,J.F.Richardson出版日期（期刊號）：ChemicalEngineering,1995出版單位：Butterworth-HeinemannLtd參考文獻[1]關醒凡.泵的理論與設計.機械工業(yè)出版社，1987.[2]高殿榮.工程流體力學.機械工業(yè)出版社，2000.[3]韓占忠.Fluent流體工程仿真計算實例與應用.北京理工出版社，2004.指導教師意見：指導教師簽字：年月日系(教研室)意見：主任簽字：年月日注：此表單獨作為一頁?；贑8051F單片機直流電動機反饋控制系統(tǒng)的設計與研究基于單片機的嵌入式Web服務器的研究MOTOROLA單片機MC68HC（8）05PV8/A內嵌EEPROM的工藝和制程方法及對良率的影響研究基于模糊控制的電阻釬焊單片機溫度控制系統(tǒng)的研制基于MCS-51系列單片機的通用控制模塊的研究基于單片機實現(xiàn)的供暖系統(tǒng)最佳啟停自校正（STR）調節(jié)器單片機控制的二級倒立擺系統(tǒng)的研究基于增強型51系列單片機的TCP/IP協(xié)議棧的實現(xiàn)基于單片機的蓄電池自動監(jiān)測系統(tǒng)基于32位嵌入式單片機系統(tǒng)的圖像采集與處理技術的研究基于單片機的作物營養(yǎng)診斷專家系統(tǒng)的研究基于單片機的交流伺服電機運動控制系統(tǒng)研究與開發(fā)基于單片機的泵管內壁硬度測試儀的研制基于單片機的自動找平控制系統(tǒng)研究基于C8051F040單片機的嵌入式系統(tǒng)開發(fā)基于單片機的液壓動力系統(tǒng)狀態(tài)監(jiān)測儀開發(fā)模糊Smith智能控制方法的研究及其單片機實現(xiàn)一種基于單片機的軸快流CO〈,2〉激光器的手持控制面板的研制基于雙單片機沖床數(shù)控系統(tǒng)的研究基于CYGNAL單片機的在線間歇式濁度儀的研制基于單片機的噴油泵試驗臺控制器的研制基于單片機的軟起動器的研究和設計基于單片機控制的高速快走絲電火花線切割機床短循環(huán)走絲方式研究基于單片機的機電產(chǎn)品控制系統(tǒng)開發(fā)基于PIC單片機的智能手機充電器基于單片機的實時內核設計及其應用研究基于單片機的遠程抄表系統(tǒng)的設計與研究基于單片機的煙氣二氧化硫濃度檢測儀的研制基于微型光譜儀的單片機系統(tǒng)單片機系統(tǒng)軟件構件開發(fā)的技術研究基于單片機的液體點滴速度自動檢測儀的研制基于單片機系統(tǒng)的多功能溫度測量儀的研制基于PIC單片機的電能采集終端的設計和應用基于單片機的光纖光柵解調儀的研制氣壓式線性摩擦焊機單片機控制系統(tǒng)的研制基于單片機的數(shù)字磁通門傳感器基于單片機的旋轉變壓器-數(shù)字轉換器的研究基于單片機的光纖Bragg光柵解調系統(tǒng)的研究單片機控制的便攜式多功能乳腺治療儀的研制基于C8051F020單片機的多生理信號檢測儀基于單片機的電機運動控制系統(tǒng)設計Pico專用單片機核的可測性設計研究基于MCS-51單片機的熱量計基于雙單片機的智能遙測微型氣象站MCS-51單片機構建機器人的實踐研究基于單片機的輪軌力檢測基于單片機的GPS定位儀的研究與實現(xiàn)基于單片機的電液伺服控制系統(tǒng)用于單片機系統(tǒng)的MMC卡文件系統(tǒng)研制基于單片機的時控和計數(shù)系統(tǒng)性能優(yōu)化的研究基于單片機和CPLD的粗光柵位移測量系統(tǒng)研究單片機控制的后備式方波UPS提升高職學生單片機應用能力的探究基于單片機控制的自動低頻減載裝置研究基于單片機控制的水下焊接電源的研究基于單片機的多通道數(shù)據(jù)采集系統(tǒng)基于uPSD3234單片機的氚表面污染測量儀的研制基于單片機的紅外測油儀的研究96系列單片機仿真器研究與設計基于單片機的單晶金剛石刀具刃磨設備的數(shù)控改造基于單片機的溫度智能控制系統(tǒng)的設計與實現(xiàn)基于MSP430單片機的電梯門機控制器的研制基于單片機的氣體測漏儀的研究基于三菱M16C/6N系列單片機的CAN/USB協(xié)議轉換器基于單片機和DSP的變壓器油色譜在線監(jiān)測技術研究基于單片機的膛壁溫度報警系統(tǒng)設計基于AVR單片機的低壓無功補償控制器的設計基于單片機船舶電力推進電機監(jiān)測系統(tǒng)基于單片機網(wǎng)絡的振動信號的采集系統(tǒng)基于單片機的大容量數(shù)據(jù)存儲技術