OnthemodellingofbubblyflowinverticalpipesOriginalResearchArticle

ToqualifyCFDcodesfortwo-phaseflows,theyhavetobeequippedwithconstitutivemodelsstandingfortheinteractionbetweenthegaseousandtheliquidphases.Incaseofbubblyflowthisparticularlyconcernstheforcesactingonthebubblesandbubblecoalescenceandbreak-up.Applyingatwofluidapproach,besidesthedragforcesdescribingthemomentumexchangeinflowdirection,thenon-dragforcesactingperpendiculartotheflowdirectionplayanimportantroleforthedevelopmentoftheflowstructure.Gas–liquidflowinverticalpipesisaverygoodobjectforstudyingthecorrespondingphenomena.Here,thebubblesmoveunderclearboundaryconditions,resultinginashearfieldofnearlyconstantstructurewherethebubblesriseforacomparativelylongtime.Theevolutionoftheflowwithinthepipedependsonaverycomplexinteractionbetweenbubbleforcesandbubblecoalescenceandbreak-up,e.g.thelift-force,whichstronglyinfluencestheradialdistributionofthebubbles,changesitssigndependingonthebubblediameter.Theconsequenceistheradialseparationofsmallandlargebubbles.Neglectingthisphenomenon,modelsarenotabletodescribethecorrectflowstructure.Extensiveexperimentsmeasuringtheradialgasvolumefractiondistribution,thebubblesizedistributionandtheradialresidenceofbubblesdependentontheirsizeweredeterminedfordifferentdistancesfromthegasinjection.BasingontheseexperimentstheapplicabilityandthelimitsforthesimulationofbubbleflowwithcurrentCFD-codesaredemonstrated,usingthesimulationofverticalpipeflowwithCFX-4asanexample.Usingasimplifiedmodelfocusingparticularlyontheradialphenomenadescribedabove,parametricstudieswereconducted.Theygiveanindicationfornecessaryimprovementsofthecodes.FinallyapossiblewayfortheimprovementoftheCFD-codesisshown.