BubblerisecharacteristicsafterthedeparturefromanucleationsiteinverticalupflowboilingofsubcooledwaterOriginalResearchArticle

Risecharacteristicsofvaporbubblesafterthedeparturefromanucleationsiteinforcedconvectivesubcooledflowboilingwerestudiedvisuallyusingtwosynchronizedhighspeedvideocameras.Thetestsectionwasatransparentglasstubeof20mmininsidediameter,filtratedanddeionizedtapwaterwasusedasaworkingfluid,andtheflowdirectionadoptedwasverticalupward.Theoutersurfaceoftestsectiontubewaselectricallyheatedtogeneratevaporbubblesinsideofthetube.Inthepresentexperiments,themassfluxandliquidsubcoolingwerevariedwithin94–1435kg/m2sand2.2–10K,respectively.Sincetheobservationswereperformedatlowheatfluxestoavoidthesignificantincreaseinthenumberofactivenucleationsites,theobtainedbubbleimageswereclearenoughtocarryoutthedetailedimageanalysisfortherisecharacteristicsofindividualbubbles.Thefollowingthreedifferentbubblerisepathswereobservedafterthedeparturefromnucleationsites:somebubblesslidupwardtheverticalwallforlongdistance,whileotherbubblesweredetachedfromthewallafterslidingforseveralmillimetersandthenmigratedtowardthebulkliquid;afterthemigration,someofthedetachedbubbleswerecollapsedinsubcooledliquidbutothersremainedclosetothewallandwerereattachedtothewall.Theresultsofdetailedimageanalysessuggestedthatthevariationinbubbleshapefromflattenedtomoreroundedwasofprimaryimportancefortheoccurrenceofbubbledetachmentfromthewall.