Numerical Study of Fluid Flow and Heat Transfer

AnbarJournalforEngineeringScienceS88AJES-2012,Vol.5,No.1NumericalStudyofFluidFlowandHeatTransferoveraBankofOval-TubesHeatExchangerwithVortexGeneratorsAbdulmajeedA.RamadhanDepartmentofMechanicalEngineering-CollegeofEngineeringUniversityofAnbarReceivedon:16/8/2011Acceptedon:3/4/2012ABSTRACTThepresentworkrepresentsatwo-dimensionalnumericalinvestigationofforcedlaminarflowheattransferovera3-rowsoval-tubebankinstaggeredarrangementwithrectangularlongitudinalvortexgenerators(LVGs)placedbehindeachtube.TheeffectsofReynoldsnumber(from250to1500),thepositions(3inx-axisand2iny-axis)andanglesofattack(30oand45o)ofrectangularVGsareexamined.ThestudyfocusesontheInfluenceofthedifferentparametersofVGsonheattransferandfluidflowcharacteristicsofthreerowsoval-tubebanks.ThecharacteristicsofaverageNunumberandskinfrictioncoefficientarestudiednumericallybytheaidofthecomputationalfluiddynamics(CFD)commercialcodeofFLUENT6.3.TheresultsshowedincreasingintheheattransferandskinfrictioncoefficientwiththeincreasingofRenumberanddecreasingtherelativedistanceofpositionsofLVGs.IthasbeenobservedthattheoverallNuavnumberofthreeoval-tubesincreasesby10–20.4%andby10.4–27.7%withanglesof30oand45orespectively,withincreasingintheoverallaverageofskinfrictioncoefficientofthreeoval-tubesreachedto53%and72%withtwoanglesusedrespectively,incomparisonwiththecasewithoutVGs.Keywords:Heattransferenhancement,oval-tubebanks,vortexgenerators.1.INTRODUCTIONThecompactheatexchangeriswidelyusedinmanyfieldssuchasautomobile,airconditioning,powersystem,chemicalengineering,electronicchipcoolingandaerospace,etc.Themainsubjecttodesignthecompactheatexchangerishowtoenhancetheheattransfersothatitsintegralperformancemaybeimprovedtomeetthedemandofhighefficiency(energysaving)andlowcostwiththevolumeassmallaspossibleandtheweightaslightaspossible.Manystudieshavebeencarriedoutandmanymethodshavebeenappliedtotheheattransferenhancementinthecompactheatexchangersince1960s.Incompactheatexchangers,thethermalresistanceoftheair-sideisgenerallydominantandmayaccountfor80%ormoreofthetotalthermalresistance.Asweknow,howtoreducethethermalresistanceisthekeyfortheheattransferenhancement.Onefrequentlyusedmethodforheattransferenhancementemployssurfacesthatareinterruptedperiodicallyalongthestreamwisedirection.Typically,thesesurfacesareintheformofwavy,louver,slit,oroffsetstripfins.Despitethefactthatinterruptedsurfacescansignificantlyimprovetheheattransferperformance,theassociatedpenaltyofpressuredropisalsotremendous.Anothercommonmethodistoapplyvortexgenerators(VGs),suchasribs,obstacles,wingsandwinglets.Vortexgeneratorsusuallyareincorporatedintoasurfacebymeansofembossing,AnbarJournalforEngineeringScienceS89AJES-2012,Vol.5,No.1stamping,punching,orattachmentprocess.Theygeneratelongitudinalvortices,whichswirltheprimaryflowandincreasethemixingofdownstreamregions.Inaddition,thevortexgeneratordeterminesthesecondaryflowpattern.Thus,heattransferenhancementisassociatedwiththesecondaryflowwithrelativelylowpenaltyofpressuredrop.ThefirstliteraturereportingtheenhancementofheattransferofusingsurfaceprotrusionvortexgeneratorsisbyEdwardsandAlker[1].TheynotedamaximumincreaseinthelocalNusseltnumberof40%.Russelletal.[2]presentedthefirststudyontheair-sideheattransferenhancementusingvortexgeneratorsfortheheatexchanger.ThenumericalstudiesbyBiswasetal.[3]andJahromietal.[4]showedthattheheattransferinthewakeregioncanbeenhancedsignificantlyinthepresenceofwinglettypelongitudinalvortexgeneratorsbehindthetubes.Extensivestudieshavebeendoneonheattransfercharacteristicsandflowstructureforheatexchangerswithlongitudinalvortexgenerators(LVGs).Inrecentyears,theapplicationofvortexgeneratorsincompactheatexchangershasreceivedmoreandmoreattention.AnexperimentalstudywasconductedbyToriietal.[5]toobtainheattransferandpressurelossinafin-and-tubeheatexchangerwithin-lineorstaggeredtubebankswithdeltawingletvortexgeneratorsofvariousconfigurations.Thewingletswereplacedinaspecialorientationtoaugmentheattransferandreduceformdrag.Theyshowedthatincaseofstaggeredtubebanks,theheattransferwasaugmentedby30–10%,andthecorrespondingpressurelosswasreducedby55–34%fortheReynoldsnumberrangingfrom350to2100.GentryandJacobi[6]experimentallyexploredtheheattransferenhancementbydelta-wing-generatedtipvorticesinflat-plateanddevelopingchannelflows.Theyreportedthatonthecompletechannelsurfacethelargestspatiallyaveragedheattransferenhancementwas55%accompaniedbya100%increaseinthepressuredroprelativetothesamechannelflowwithnodelta-wingvortexgenerator.Leuetal.[7]numericallyandexperimentallystudiedtheheattransferandflowintheplate-finandtubeheatexchangerswithinclinedblockshapevortexgeneratorsmountedbehindthetubes.Theypointedoutthattheproposedheattransferenhancementtechniqueisabletogeneratelongitudinalvorticesandtoimprovetheheattransferperformanceinthewakeregions.SommersandJacobi[8]experimentallyinvestigatedtheair-sideheattransferenhancementofarefrigeratorevaporatorusingvortexgeneration.Theynotedthatforair-sideReynoldsnumbersbetween500and1300,theair-sidethermalresistancewasreducedby35–42%wh