Lecture- 10 Introduction to turbomachinery

1. Fluid Mechanics: Fundamentals of Fluid Mechanics, 7th Edition, Bruce R. Munson. Theodore H. Okiishi. Alric P. Rothmayer John Wiley & Sons, Inc.l, 2013 Lecture- 10Introduction to turbomachinery Lecture slides by Dhafeer M. AL-Shamkhi 2014-2015 Department of Automotive Technical Engineering

4. BasicEnergy Considerations Velocity trianglesare obtainedfrom: Fromangular moment (momentofmomentum) conservation,we canobtain: Thisequationis knownas the Eulerturbomachineequation and since theshaftpowerofa rotatingmachineis givenby

5. TheCentrifugalPump Oneofthe most commonradial-flowturbomachinesis thecentrifugal pump.Thistypeof pump hastwomain components:animpellerattachedtoa rotatingshaft,anda stationarycasing,housing,orvoluteenclosingtheimpeller.The impellerconsistsof a numberofblades (usually curved),also sometimescalledvanes,arrangedinaregularpattern aroundthe shaft. Astheimpellerrotates,fluidis sucked inthroughtheeye ofthecasing andflows radially outward. Forlargecentrifugalpumps,a different designis oftenusedinwhichdiffuser guidevanes surround theimpeller.The diffuser vanes deceleratethe flowas thefluidis directedintothe pump casing.Thistype ofcentrifugalpump is referredtoasadiffuserpump.

6. Impellers aregenerally oftwotypes: (a)Openimpeller,(b)enclosedor shroudedimpeller. Pump impellerscanalso besingleordoublesuction.For thesingle-suctionimpellerthefluid enters throughthe eyeononlyoneside ofthe impeller,whereas forthe double-suction impellerthe fluidentersthe impelleralongitsaxisfrombothsides. Pumpscanbesingleor multistage.For a single-stage pump,onlyone impelleris mounted on theshaft,whereasfor multistage pumps,several impellersare mountedonthe same shaft(stages operate inseries). Centrifugal pumpscomeina variety ofarrangements,butthe basicoperatingprinciple remains thesame.Work is doneonthe fluidbythe rotatingblades, creatingalarge increaseinkineticenergy ofthefluidflowingthroughtheimpeller.This kineticenergy is converted intoanincreaseinpressure as the fluidflowsfromthe impellerintothecasingenclosingthe impeller.

7. TheoreticalConsiderations Fortherotatingshaft,thepowertransferred, is givenby Itfollowsthat the shaftpowerperunitmassofflowingfluidis Forincompressiblepumpflow,theenergy equationgives Where H is thetotalhead

8. Anidealpumpwithoutlosseswouldgive atotal headrise: The actualhead riseha is lessthenhi by hL representingthelossesinthepump. Analternateexpression of hi is given by Often the fluidhasnotangentialcomponentofvelocityorswirl,as itenters theimpeller. sothat (α1 = 90 Vθ1=0)and theexpressionreducesto: Theflowrate,Q,is relatedtotheradialcomponent ofthe absolute velocitythroughtheequation Andthevelocity trianglegives

9. Bladeswithβ2<90⁰are calledbackwardcurved,whereas bladeswithβ2>90⁰ are called forward curved. Pumps arenotusually designedwithforwardcurvedvanessince such pumpstendto suffer unstableflowconditions.Thenormalrange is 20⁰ <β2<25⁰and 15⁰<β2<50⁰. Effect of lossesonthe pumphead– flowrate curve(backwardcurved bladespump) Actual pump performanceis determinedexperimentallythroughtestsonthe pump.From these tests, pumpcharacteristicsare determinedandpresentedaspump performance curves.It is this informationthat is most helpfultothe engineerresponsiblefor incorporatingpumps intoa givenflowsystem.

10. PumpPerformance Characteristics The actualhead rise,deliveredby apump tothefluidcanbedeterminedwithan experimentalarrangement If the differenceinelevationis neglected And thepowergainby thefluidis

11. Inadditiontothe headorpoweraddedtothe fluid,theoverallefficiency, is ofinterest, where The denominatorofthisrelationshiprepresents thetotal powerappliedtothe shaftofthe pump andis oftenreferredto asbrake horsepower(bhp).Thus, hydraulicefficiency mechanicalefficiency volumetricefficiency

12. Typical performancecharacteristicsfora centrifugalpumpofagivensize operatingat a constantimpellerspeed. withthedischargevalveclosed:no flow, relatedefficiency is zero,andthepower suppliedbythepump issimply dissipated asheat. bestefficiencypoint s(BEP) Alsoreferredtoascapacity

13. SystemCharacteristicsandPump Selection Foratypicalflowsystem inwhicha pump isusedis shown,the energy equation applied betweenpoints1and 2indicatesthat with systemequation

14. Toselecta pumpfor a particularapplication,itis necessarytoutilizeboththesystemcurve andthepumpperformancecurve. Theirintersectionrepresentstheoperating point for thesystem.Ideally,we wantthe operatingpointtobenearthe best efficiency point (BEP) for thepump.

15. Pumpscanbearrangedinseriesorinparalleltoprovidefor additionalheador flowcapacity. Whentwopumpsare placedinseries, the resultingpumpperformance curveis obtainedby addingheadsat the sameflowrate. Fortwopumps inparallel,the combinedperformancecurveis obtainedby addingflowratesatthe samehead.

16. Specific Speed Ausefulnondimensionalparametersoftenusedincharacterizingthe operationofpumps is thespecificspeed,definedas: Eachfamilyor class ofpumps hasa particularrange of valuesofspecificspeedassociated withit. ForpumpswithlowQand highha the specificspeedis lowcomparedtoa pumpwithhighQ and lowha.Centrifugalpumpstypicallyare low-capacity,high-headpumps,andtherefore havelowspecific speeds. Theconceptofspecific speed isveryusefulto engineers anddesigners,since iftherequired head, flowrate,andspeedare specified,it is possibletoselect anappropriate(mostefficient) type ofpump fora particularapplication. centrifugal pumpsare radial-flowmachinesthat operate most efficientlyfor applications requiringhighheads atrelatively lowflowrates.This head–flowratecombinationtypically yieldsspecificspeedsthat arelessthanapproximately 1.5.

17. Variationinspecific speedat maximum efficiencywithtype ofpump.

18. Other important dimensionlessparameters Inadditiontothe specificspeed, threeothernondimensionalparameters are oftenusedforengineeringpurposeswhendealingwithpumps. Theheadrisecoefficient Thepowercoefficient Theflowcoefficient

19. Axial-FlowandMixed-FlowPumps Centrifugalpumps areradial-flowmachines that operatemost efficiently for applications requiringhighheadsatrelatively lowflowrates.Formany applications,suchas those associated withdrainageand irrigation,highflowratesat lowheadsarerequiredand centrifugal pumpsare notsuitable. Axial-flowpumps usuallyhave specific speeds inexcess of3.3. Inthis case,axial-flowpumps are commonlyused.This type ofpumpconsistsessentiallyofa propeller confinedwithina cylindricalcasing.Axial-flow pumpsare oftencalledpropeller pumps. Whereas thehead developedbya centrifugalpumpincludesa contributionduetocentrifugal action,thehead developedbyan axial-flowpumpis due primarilytothe tangentialforceexerted bythe rotor bladesonthefluid.The rotor is connectedtoa motor througha shaft, and as itrotates (usuallyat a relatively highspeed)the fluidis sucked inthroughthe inlet. Typically thefluiddischargesthrougha rowoffixedstator (guide)vanes usedtostraighten the flowleaving therotor. Someaxial-flowpumps also have inletguidevanesupstream of the rotor row,andsomearemultistage inwhichpairs (stages)ofrotatingblades(rotor blades) andfixed vanes(stator blades)are arranged inseries. Theconceptsthat were developedforcentrifugalpumps arealso applicabletoaxial-flow pumps.The actualflowcharacteristics,however,are quitedifferent.

20. 375 18 300 15 225 12 150 9 6 3 m 40 80 120 160 200 240 kL/min

21. Forapplicationsrequiringspecificspeedsintermediatetothose forcentrifugalandaxial-flow pumps, mixed-flowpumps havebeendevelopedthat operateefficiently inthespecific speed range1.5<Ns < 3.3 . Asthename implies,theflowina mixed-flowpump hasbotha radialandan axial component.

22. Fans Whenthefluidtobemovedis air,orsomeothergas or vapor,fansare commonlyused. Typesoffans varyfrom the small fanused forcoolingdesktopcomputersto large fans used inmany industrialapplicationssuchas ventilatingoflargebuildings. Fanstypicallyoperate at relatively lowrotationspeeds and arecapableofmovinglargevolumesofgas. Althoughthefluidofinterestis a gas, thechange ingas density throughthe fan does not usuallyexceed7%.Thus, indealingwithfans, the gas density is treatedasaconstant, and the flowanalysisis basedonincompressibleflowconcepts. Turbomachinesusedtoproducelargerchangesingas density andpressure thanpossible withfansare calledcompressors. Asis thecase for pumps, fan designs includecentrifugal(radial-flowfans), as well as mixed-flow andaxial-flow(propeller)fans. Theanalysisoffan performancecloselyfollowsthat previouslydescribedfor pumps. However,fan head-risedataare oftengivenintermsofpressure rise, eitherstaticortotal, rather thanthe moreconventionalheadrisecommonlyusedfor pumps.

23. Scalingrelationshipsfor fansare thesameas thosedevelopedfor pumps, but replacingthe headhawiththepressureheadpa/ρg: where, as before,the subscripts1 and2 refer toany twofansfromthe familyof geometrically similarfans. Theseequations arecalledthefanlawsand canbeusedtoscale performancecharacteristicsbetweenmembers ofafamily ofgeometricallysimilar fans.

24. Turbines Turbines aredevicesthat extract energy froma flowingfluid.The geometry ofturbinesis suchthat the fluidexerts a torque ontherotor inthe directionofitsrotation. The shaft power generatedis availabletodrive generators orotherdevices. Hydraulicturbines :theworkingfluidis water. Gasturbines:the workingfluidis air(+fuel);steam turbines:theworkingfluidis steam . Turbines areclassified intotwobasic types—impulseturbinesandreaction turbines. Forhydraulicimpulseturbines,thepressure dropacross therotor is zero;all ofthepressure dropacross the turbinestage occursinthe nozzlerow.ThePelton wheelis aclassical example ofan impulseturbine.

25. Forreactionturbines,onthe otherhand,therotor is surroundedby a casing(orvolute), whichis completelyfilledwiththe workingfluid.There is botha pressure dropanda fluid relative speedchangeacross therotor. Bothimpulseand reactionturbines canbeanalyzedusingthemoment-of-momentum principles. Ingeneral, impulseturbines arehigh-head,low-flowratedevices,whilereactionturbinesare low-head,high-flowratedevices.

26. ImpulseTurbines Thistype ofturbines ismost efficient whenoperatedwitha large head(for example, a water source fromalake locatedsignificantlyabove theturbinenozzle),whichis converted intoa relativelylarge velocityat the exitofthe nozzle.

27. ReactionTurbines Reactionturbinesare best suitedfor higherflowrateand lowerhead situations suchas are oftenencounteredinhydroelectricpowerplantsassociatedwithadammedriver,for example. Aswithpumps,turbinesare manufacturedinavariety ofconfigurations—radial-flow, mixed-flow,and axial-flow.Typicalradial-and mixed-flowhydraulicturbines arecalledFrancis turbines, namedafter JamesB. Francis,an American engineer.At verylowheadsthe mostefficienttypeofturbineis theaxial-floworpropellerturbine.The Kaplanturbine,namedafter Victor Kaplan,aGerman professor, is an efficientaxial-flowhydraulicturbinewithadjustable blades. TheEulerturbomachineequationandthe correspondingpowerequationfor a centrifugal pump (radial)areequallyvalidfora radial-inflowturbine. foranaxial-flowKaplanturbine,thefluidflows throughthe inletguidevanesandachievesa tangential velocity inavortex(swirl)motionbefore itreaches therotor.Flowacross therotorcontainsa majoraxialcomponent.Both theinletguidevanesandthe turbineblades can beadjustedbychangingtheirsettinganglestoproducethe bestmatchfor thespecific operatingconditions.For example,the operatinghead availablemay change fromseason to seasonand/orthe flowratethroughtherotor may vary.

28. Typical radial-flowFrancisturbine Typical axial-flowKaplanturbine

29. Pumpsandturbines areoftenthought ofasthe“inverse”ofeachother. Pumps addenergy tothefluid;turbines remove energy. Turbine efficiencyis theinverseofpumpefficiency: