Theoreticalmodelingofdropwisecondensationheattransferincludinglengtheffectatlowheatflux

Abstract

응축열전달은원자력발전소의피동격납건물냉각계통(PCCS)에서주요한역할을한다.PCCS의신뢰성을평가하기위하여격납건물조건하에서의응축열전달을정확하게평가하는것은매우중요하다.원자로의격납건물은공기로채워져있으며,비응축성기체는응축열전달에상당히부정적인영향을미치기에,기존대부분의연구들은비응축성기체의영향을분석하는것을주목표로하였다.근래에는혁신적인설계를동반한소형모듈형원자로(SMR)의등장으로,원자력학계의주관심영역은SMR에집중되었다.그중에서도진공을유지하는격납건물개념은수소위험성제거,정상상태단열,사고상황응축열전달증진의장점을가지고있는혁신적인설계로평가된다.본설계에서는비응축성기체의영향은더이상학계의주관심사가아니며,응축열전달에영향을미칠수있는다른요소들을파악하는것이앞으로의SMR개발의주관심사가될것으로보인다. 본저자는적응축(DWC)현상이비응축성기체가없는환경에서응축열전달에큰영향을미치는요소로판단하였다.비응축성기체를배제한실험을수행하여,Rivulet으로명명되는특정한현상이수직상향표면에서적응축이발현될시나타나는것을확인하였다.표면최상단에서멀어질수록감소하는열유속경향을확인하였으며,이는rivulet의효과로판단된다.Rivulet효과는기존적응축연구자들에게서고려되지않은현상이기에,본연구에서는이를해석하기위한새로운열전달모델을개발하였다.기존적응축열전달모델을기반으로rivulet에의한열전달과그영향을추가하여모델을완성하였다.Rivulet영향은적응축중에나타나는액적의최대직경을감소시켜열전달을증진시키는효과와,표면을덮고있는rivulet에의해열전달을감소시키는효과가동시에적용되며,그결과로길이가긴수직표면일수록하단에서더낮은열유속이나타난다.개발된모델을실험결과보다약간높게예측하였지만,길이에따라열유속이감소하는경향은기존모델들과달리성공적으로예측하였다. |Condensationheattransferplaysakeyroleinthepassivecontainmentcoolingsystem(PCCS)ofnuclearpowerplants(NPPs).ToevaluatethereliabilityofthePCCS,thecondensationheattransferincontainmentmustbeunderstood.Theeffectofthenoncondensablegas(NCG)hasbeenamajorissueincondensationheattransferbecauseoftheair-filledcontainmentinconventionalNPPsandtheirremarkabledegradationeffect.Nowadays,innovativesmallmodularreactor(SMR)designshaveattractedconsiderableresearchinterest,andevacuatedcontainmenthasemergedasaleadingtechniquethathaslessNCG.TheNCGeffectisnotabasicissueinSMR;therefore,findingeffectivefactorsforcondensationheattransferwithanevacuatedcontainmentenvironmentwillattractmuchattentioninthefuture. Dropwisecondensation(DWC)canbeacriticalfactortoevaluateheattransfer,becausethecondensationmodecannotbenegligibleunderalowNCGconcentration.ExperimentswereperformedunderpuresteamconditionstounderstandtheeffectofDWContheheattransfer.Adistinctivedropletbehaviorwasobserved,withthecoalescenceofdropletsformingarivuletflowonaverticalsurfaceduringDWC.Inaddition,resultsshowthattheDWCheatfluxdecreasesastheaxialpositionincreases,asdoestheeffectofrivuletflow,suchasheattransferthroughitself.TherivuletflowcanbeobservedonlyonlongverticalsurfacessuchasPCCSheatexchangertubes;thishasnotbeenaddressedinDWCresearch.Therefore,anewheattransfermodelisdevelopedtoevaluatetheheattransferperformanceofPCCSinSMRcontainment.ThetheoreticalapproachconsistsofthepreviousDWCheattransfermodelthatisbasedonthepopulationdensityandtherivuletheattransfermodeledinthisthesis.Itincorporatesadecreasedmaximumdropletradiusandincreasedrivuletcoveredareaasthelengthincreasesfromthetopofthesurface.Thedevelopedmodelshowsgoodagreementwiththeexperimentaldata,andtherefore,itcanexplainthereducedtrendofheatfluxwiththeaxialpositionunderPCCSconditionsinSMRcontainment.