TurbulentheatmixingofaheavyliquidmetalflowintheMEGAPIEtargetgeometry—TheheatedjetexperimentOriginalResearchArticle

TheMEGAPIEtargetinstalledatthePaul–ScherrerInstituteisanexampleofaspallationtargetusingeutecticliquidlead–bismuth(Pb45Bi55)bothascoolantandneutronsource.Anadequatecoolingofthetargetrequiresaconditioningoftheflow,whichisrealizedbyamainflowtransportedinanannulargapdownwards,u-turnedatahemisphericalshellintoacylindricalrisertube.Inordertoavoidastagnationpointclosetothelowestpartoftheshellajetflowissuperimposedtothemainflow,whichisdirectedtowardstothestagnationpointandflowstangentiallyalongtheshell. TheheatedjetexperimentconductedintheTHEADESloopoftheKALLAlaboratoryisnearly1:1representationofthelowerpartoftheMEGAPIEtarget.Itisaimedtostudythecoolingcapabilityofthisspecificgeometryindependenceontheflowrateratio(Qmain/Qjet)ofthemainflow(Qmain)tothejetflow(Qjet).Here,aheatedjetisinjectedintoacoldmainflowatMEGAPIErelevantflowrateratios.Theliquidmetalexperimentisaccompaniedbyawaterexperimentinalmostthesamegeometrytostudythemomentumfieldaswellasathree-dimensionalturbulentnumericalfluiddynamicsimulation(CFD).BesidesadetailedstudyoftheenvisagednominaloperationoftheMEGAPIEtargetwithQmain/Qjet=15deviationsfromthismodeareinvestigatedintherangefrom7.5≤Qmain/Qjet≤20inordertogiveanestimateonthesafeoperationalthresholdofthetarget. Theexperimentshowsthat,theflowpatternestablishinginthisspecificdesignandtheturbulenceintensitydistributionessentiallydependsontheflowrateratio(Qmain/Qjet).AllQmain/Qjet-ratiosinvestigatedexhibitanunstabletimedependentbehavior.TheMEGAPIEdesignishighlysensitiveagainstchangesofthisratio. Mainlythreecompletelydifferentflowpatternswereidentified.Asufficientcoolingofthelowertargetshell,however,isonlyensuredifQmain/Qjet≤12.5.Inthiscasethejetflowcoversthewholelowershell.AlthoughforQmain/Qjet≤12.5theflowismoreunstablecomparedtotheotherpatternsmostofthefluctuationsclosetothecenterlineareinthehighfrequencyrange(>1Hz),sothattheywillnotleadtoseveretemperaturefluctuationsinthelowershellmaterial.Inthiscasethethermalmixingoccursonlargescalesandisexcellent. ForflowrateratiosQmain/Qjet>12.5complexflowpatternsconsistingofseveralfluidstreaksandvorticeswereidentified.SinceinthesecasesthejetflowdoesnotfullycoverthelowershellanadequatecoolingoftheMEGAPIEtargetcannotbeguaranteedandthustemperaturesmayappearexceedingmaterialacceptablelimits. Allconductedexperimentsshowahighsensitivitytoasymmetriesevenfarupstream.Acomparisonofthenumericalsimulation,whichassumedasymmetricflow,withtheexperimentaldatawasduetotheexperimentallyfoundasymmetryonlypartiallypossible.