Synopsis : ExperimentandnumericalsimulationofRHcirculationflowwascarriedoutonwatermodel.Heliumgaswasinjectedintowaterinup-leg andflowratewasestimatedatthebottomofdown-leg.Volumeoffluid(VOF)simulationwasalsoperformedbycompressibleInterFoam solverofOpenFOAM,opensourceCFDtoolbox.Threepairsofdiameterofup/down-leg,100mm/100mm,140mm/70mmand170mm/30 mmwereestimatedandthecirculationflowratewascomparedtoOnoetal.'sestimatedformula.Theflowratein100mm/100mmand140

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... .'sformula,butitwashalfin170mm/30mm.Simulatedflowrateofallpairswere almostconsistentwithmeasurement.Thevisualizationofexperimentandsimulationshowedthatdownflowthroughup-legoccurredincase of170mm/30mm,anditwasthoughttocauselowerflowratethroughdown-legthanOnoetal.'srelation.Itwasimportanttousesufficiently smallmeshforup-legtogetflowratecorrectlybecaseofthenecessityofresolutingtheshapeofbubble.Finallythesimulationwithscaled upmeshandmoltensteelpropertieswascarriedout.Thevolumeofinjectedgasbecamelargeraccordingtorisingintheup-legbecauseof hydrostaticpressuredistributionandthebehaviorofgas-liquidinterfaceinvesselwassimulatedqualitatively.