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Experimentalresearchonnaturalconvectiveheattransferofwaternearitsdensitymaximuminahorizontalannulus

You-RongLi ,Xiao-FengYuan,Yu-PengHu,Jing-WenTang

KeyLaboratoryofLow-GradeEnergyUtilizationTechnologiesandSystemsofMinistryofEducation,CollegeofPowerEngineering,ChongqingUniversity,Chongqing400044,China

articleinfoabstract

Inthispaper,theheattransfercharacteristicsofthenaturalconvectionofwaternearitsdensitymaxi-muminahorizontalannulushadbeenexperimentallyinvestigated.Theannuluswasformedwithtwocoppercylindricalwalls,whichwereconcentricoreccentric.Thetemperatureoftheoutercylinderwaskeptataround0°Candthatoftheinnercylinderwasrangedfrom0to12°C.Inordertoenhanceheattransfer,three-dimensionalexternal nswereappliedontheinnerwall.Theeffectsofthegapwidthandtheeccentricityofannulusandthe ngeometricparametersonheattransferwerestudied.TheresultsshowthatthetwoturningpointsoftheaverageNusseltnumberontheinnerwallappearatthetemperaturedifferencescloseto4°Cand8°C,respectively.Withtheincreaseoftemperaturediffer-ence,theaverageNusseltnumber rstlyincreases,andthendecreases, nallyincreasesagain.Themin-imumvalueoftheaverageNusseltnumberlocatesnearthetemperaturedifference8°C.Additionally,itincreaseswiththeincreaseofthegapwidthofannulusandtheeccentricity.Furthermore,whenthree-dimensionalexternal nswereapplied,theaverageNusseltnumberincreaseswiththeincreaseofthe naxialpitch,whereasitdecreaseswiththeincreaseofthe nwidth.Intherangeofexperimentalvalues,thein uencesofthe nheightandcircularpitchareslight.Finally,theexperimentalempiricalcorrela-tionsofheattransferontheinnerwallareobtained.

Ó2012ElsevierInc.Allrightsreserved.

Articlehistory:

Received7April2012

Receivedinrevisedform5July2012Accepted15August2012

Availableonline15September2012Keywords:

NaturalconvectionHeattransfer

DensitymaximumHorizontalannulus

Three-dimensionalexternal n

1.Introduction

Naturalconvectiveheattransferbetweentwoconcentricoreccentrichorizontalcylindricalwallsareencounteredinsomeimportantprocessinengineering,suchasthecoolingofelectronicequipments,solarcollectorspaces,theiceformingandmeltinginthecool-thermalenergystoragesystem,andcrystalgrowth.KuhenandGoldstein[1,2]conductedancomprehensiveexperimentalinvestigationonthenaturalconvectiveheattransferofairincon-centricandeccentrichorizontalannulifortheRayleigh(Ra)num-bersfrom2.2Â102to7.7Â107.Itwasreportedthattheeccentricitysubstantiallyalteredthelocalheattransferabilityonbothcylinders,buttheoverallheattransfercoef cientwaschan-gedbylessthan10%.Later,someexperimentalandnumericalinvestigationshadbeencarriedoutfornaturalconvectioninahor-izontalannuluswithdifferentinclinationanglemeasuredfromthehorizontallevel,byTakataetal.[3],Hamadetal.[4,5]andNada[6].Intheseworks,itwasconcludedthatthein uencesoftheannulusgapandtheRayleighnumberonnaturalconvectionheattransferweremoresigni cantthanthatoftheinclinationangle.TheheattransferrateincreasedwiththeincreaseoftheannulusCorrespondingauthor.Tel.:+862365112284;fax:+862365102473.

E-mailaddress:liyourong@http://doc.guandang.net(Y.-R.Li).

0894-1777/$-seefrontmatterÓ2012ElsevierInc.Allrightsreserved.http://doc.guandang.net/10.1016/j.exptherm usci.2012.08.016

gapwidthandtheRayleighnumberandthedecreaseoftheincli-nationangleoftheannulus.Furthermore,manynumericalsimula-tionsandlinearstabilityanalysishadbeenalsoperformedfornaturalconvectiveheattransferinconcentricandeccentrichori-zontalannulus.Inpreviousworks,thedensityofthe uidwascon-sideredasalinearfunctionoftemperature,andthein uencesofthegeometryparameters[7–9]andtheRayleighnumber,thePra-ndtlnumber[10–12],eccentricity[13,14]werestudied,andthetransitionandmultiplicityof ows[15–18]werealsoexhibited.Uptonow,variousnumericalsimulationsonthenaturalcon-vectionofcoldwaternearitsdensitymaximuminconcentricandeccentrichorizontalannulihadbeenperformed.Nguyenetal.[19],Vasseuretal.[20],RaghavaraoandSanyasiraju[21]ob-tainedtheheattransfercharacteristics,thevelocitypro les,thelo-calandaverageNusselt(Nu)numbersofthesteadynaturalconvectionofcoldwaterbetweentwohorizontalconcentriccylin-derswithconstantsurfacetemperaturesfordifferentRayleighnumber,radiusratioanddensityinversionparameter.Itwasalsofoundthatthe owpatternwasgreatlyin uencedbythedensityinversionparameterofwater.TheworkofRaghavaraoandSan-yasiraju[22]onthesteady-statenaturalconvectionofwaterinaneccentricannulusshowedthatthe owwassymmetricaboutverticalaxisforverticaleccentricity.Theheattransferratedependedonthepositionofthemaximumdensitypointinthe

Y.-R.Lietal./ExperimentalThermalandFluidScience44(2013)544–549545

NomenclatureABcpdDeghHlNuPPrqqmQRaT

heattransfersurfacearea,m2 nwidth,m

speci cheatatconstantpressure,J/(kgK)

eccentricdistancebetweenthecenterofinnercylinderandoutercylinder,m

cylindricalwalldiameter,meccentricity,e=2d/(DoÀDi)accelerationofgravity,m/s2

averageheattransfercoef cient,W/(m2K) nheight,m

thegapwidthofannulus,l=(DoÀDi)/2,maverageNusseltnumber,–pitchof n,m

Prandtlnumber,Pr=m/a,–

exponentindensity-temperatureEq.(1)massrateofcirculating uid,kg/sheattransferrate,W

Rayleighnumberbasedonthegapwidth,gcDTql3/(ma)temperatures,°C

Greeksymbolsathermaldiffusivity,m2/sccoef cientindensity-temperatureEq.(1),(°C)Àqldynamicviscous,kg/(ms)Hmdensityinversionparameter,Hm=(TmÀTo)/(TiÀTo)qdensity,kg/m3kthermalconductivity,W/(mK)Subscriptsaaxialccirculariinnerwalloouterwallmdensityinversionpointininletoutoutletfthecirculating uidintheinnertubeaveaverage

annulus.LiandYuanetal.[23,24]revealedthecomplicated ow

patternsandtheheattransfercharacteristicsofthenaturalconvec-tionofwaternearitsdensitymaximumfromsteady-statetooscil-latory owbetweenhorizontalcylinderswithdifferentradiusratio,densityinversionparameterandeccentricitybynumericalsimulation.Ontheotherhand,Sekietal.[25]experimentallyinvestigatedtheeffectofdensi …… 此处隐藏：21532字，全部文档内容请下载后查看。喜欢就下载吧 ……