Hydro-thermal-mechanicalanalysisofthermalfatigueinamixingteeOriginalResearchArticle

ThispapercoversworkcarriedoutbytheCEAtostudythemechanismsleadingtocrackingofpipingasaresultofthermalloadinginflowmixingzones.Themaingoaloftheworkistoanalyse,bycalculation,thethermalloadingcausedbyturbulentmixinginteesandtounderstandthemechanismofinitiationandpropagationofcracksinsuchcomponents.ThisworkissupportedbyIRSN.Thisthermalfatiguephenomenonisstillnotfullyunderstood.Oneofthemainobstaclestoitsunderstandingresidesinthemulti-domainnatureoftheloadingandassociateddamage,involvingthreecomplementaryscientificdisciplines:thermal-hydraulicfield,thermo-mechanicalfieldandmaterialsscience. ThispaperdescribestheapproachadoptedbytheCEAtoestablishnaturalmechanisms(turbulence,pulsingandinstability)whichmightbethecauseofanysubstantialthermo-mechanicalloadinginthepiping. Althoughturbulencemaybethecauseofthethermalstripping(presenceofhigh-frequencythermalfluctuationsontheinnersurfaceofthecomponent),itcannotaloneexplainthepropagationofdeepcracks.Themainreasonisthe“high-passfilter”effectofconvection.Thewallcannotbesubjectedtoconvection-relatedthermalfluctuationsandfrequencieslessthantheinverseoftheturbulencetransittime.Astraightforwardfrequency-basedanalysisoftheloading,carriedoutasafirststage,madeitpossibletoestablishthelimitsoftheloadingcreatedbythesehigh-frequencyevents. However,turbulencecangiverisetoflowinstability(suchaspulsing)oflowerfrequency.Butthiscannotexplaineverything.Thegeometryupstreamofthetee,particularlythesequenceofstraightsectionsandbendscan,incertaincases,dampthepulsesorgreatlyamplifythem.Theuseofsuitablethermal-hydraulicmodellingisdiscussedinthesecondpartofthisarticle. Thefinalresultofthethermo-hydro-mechanicallink-uponapplicationtothecomplex3DgeometryoftheCivauxunit1case(whichincludesamixingtee,bendsandstraightsections)enabledtheobservationsmadeinthisplantcasetobehighlightedandcorrelated.Oneoftheoriginalitiesofthisstudyistocarryouttheoverallanalysis(thermal-hydraulicandthermo-mechanical)withasinglecomputercode,theCAST3McodedevelopedbytheCEA.