Numerical Simulation of Flow Field for a whole cen

VOLUME11,NUMBER2HVAC&RRESEARCHAPRIL2005263NumericalSimulationofFlowFieldforaWholeCentrifugalFanandAnalysisoftheEffectsofBladeInletAngleandImpellerGapZhaoYuSongLi,PhDWenqiHeWeixiongWangDongtaoHuang,PhDZhichiZhuReceivedMarch29,2004;acceptedAugust18,2004Numericalsimulationoftheflowfieldwascarriedoutforawholecentrifugalfantoconsidertheinteractionofthreeparts—inlet,impeller,andscroll.Amodernoptimumdesignmethodforacentrifugalfanhasbeenpresentedusingacomputationalfluiddynamics(CFD)techniquetoreplacetheconventionalmethod.Asamplefanbasedontheoptimumdesignwasmanufacturedandtested.Thenumericalpredictionagreeswellwiththetestdata.Theexcellentperformanceofthesamplefanhasprovedthevalidityofthemethodadopted.Moreover,theeffectsofthebladeinletangleandthegapbetweenimpellerandinletonthefanperformancearequantita-tivelyanalyzed.Particularlytheenergylossineverypassage,includingthegaplossandtheleakageflowrateinthegap,arediscussedindetailinthepaper.Itisfoundthatthebladeinletangleandtheimpellergapplayanimportantroleforfanperformance.INTRODUCTIONTheaerodynamicdesignofacentrifugalfanhaslongfollowedtheconventionalmethod,whichisbasedontheassumptionofone-ortwo-dimensionalidealflowandsomeempiricalcoefficients.Thethreemainparts,i.e.,impeller,scrolltypeofhousing(calledscrollinthefol-lowing),andinletring(calledinletinthefollowing),aredesigned,respectively,andtheirinter-actionsareneglected(Eck1973).Furthermore,thefanperformancecannotbepredictedatthedesignstage.Infact,thereisathree-dimensional,complicated,andhighlyturbulentflowinacentrifugalfan,andtheeffectofinteractionbetweenthemainpartsonfanperformanceisseri-ous,whichcannotbeneglected.Thus,theexistingsimplifieddesignmethodisinsufficienttodescribetherealflowsituation,andtheperformanceofafandesignedbasedonthismethodiscommonlynotverygood.Sometimes,inordertoobtainanexcellentperformance,ittakesalotoftimeandcosttomakeseveralcyclesofdesign-test-redesign.Therefore,theexistingconven-tionaldesignmethodneedstobeimproved.Computationalfluiddynamics(CFD)hasmadegreatprogressincalculatingthethree-dimensionalflowincentrifugalpumpsandfans.Zhangetal.(1996)computedthethree-dimensionalviscousflowinabladepassageofabacksweptcentrifugalimpelleratthedesignpointwiththestandardk-εmodel.Thecomparisonofcomputedandmeasuredvaluesshowedgoodagreement.Thecomputationalsosuccessfullypredictsthejet-wakestructure,thoughslightlydifferentinlocationandsizefromthemeasuredones.Inrecentyearsthek-εmodelhasbeenmorewidelyusedinnumericalsimulationofflowfieldsinturbomachineryZhaoYuandWenqiHeareresearchassistants,SongLiisalecturer,WeixiongWangisagraduatestudent,andDong-taoHuangandZhichiZhuareprofessorsattheFluidAcousticsLab,DepartmentofEngineeringMechanics,TsinghuaUniversity,Beijing,China.264HVAC&RRESEARCHengineering.Itiswellknownthatthek-εmodelisonlyapplicabletoaflowwithsmallseparationandisunabletocapturethedetailsofseparationcharacteristicsandthevortexinalargeseparationregion.Butforgoodaerodynamicaldesignandneardesignoperation,inwhichtheflowissmootherinthechannelandonlysmallseparationoccurs,theoverallpeformanceandtheapproximatevelocityandpressuredistributioncanthenbepredictedusingthek-εmodel.Forexample,thestandardk-εmodelhasbeensuccessfullyappliedinCFDformultibladecentrifugalfans(Seoetal.2003),centrifugalpumps(Gonzalezetal.2002),andcentrifugalcompressors(Zangenehetal.2004).Itisreportedtobecapableofpredictingthecorrecttrendsofvelocityandpressureprofilesandgivesespeciallygoodpredictionofoverallperformance,suchastotalpressurerise,shaftpower,andefficiency.OtherstudiesbyThakuretal.(2002)foracentrifugalblower,Mugglietal.(2002)foramixedflowpump,andMiner(2000)foraxialandmixedflowpumpsalsoprovedthevalidityofthek-εmodelinthepredictionofglobalperformanceortheshapeandmagnitudeofthevelocityandpressureprofiles.Toconsidertheunsteadinesscausedbytheinteractionoftheimpellerandthescroll,the“frozenrotor”modelandthe“mixingplane”modelhavebeendevelopedforquasi-steadysimulation.Theyaremoreadvancedmodelsthansingle-bladepassagesimulation,astheycanreflecttheeffectofthenon-uniformityofflowdistributionindifferentbladepassages.Both“frozenrotor”and“mixingplane”modelscanprovidegoodpredictionforfanperformance,includingstaticpressurerise,power,andefficiency(Thakuretal.2002).Seoetal.(2003)studiedthree-dimensionalturbulentflowinamulti-blade(squirrelcage)centrifugalfanbythe“frozenrotor”method,andafinitevolumemethodwiththek-εturbulencemodelwasemployed.Theresultshowedanagreementwiththeexperiment,especiallyathigherflowratecoefficients.Fullyunsteadysimulationandlargeeddysimulation(LES)hasalsobeenappliedtothesimu-lationofflowincentrifugalpumps.Pedersenetal.(2003)usedLEStosimulatetheflowinacentrifugalpump,andthedetailedflowfieldagreedwellwiththePIVandLDVresults.Gonza-lezetal.(2002)usedunsteadysimulationtostudytheinteractionoftheimpellerandthescrollofacentrifugalpumpandsuccessfullypredictedthefluctuationofpressurewithtime.However,theseadvancedskillsaretooexpensiveandtoocomplicatedtobeemployedinpracticalfandesign.TheapplicationofCFDisdevelopinginthedesignmethodforcentrifugalfansandp