Predictionoftheobstacleeffectonfilm-boilingheattransferOriginalResearchArticle

Acorrelationhasbeendevelopedtoaccountfortheeffectofobstacles(simulatingthespacingdevicesinbundles)onheattransferindispersed-flowfilmboiling.Thecorrelationisexpressedasamodificationfactortothereferencegeometrywithoutanyobstacles.Thebasicformofthecorrelationisanexponentialdecayfunctionthatresemblesthediminishingeffectonturbulenceenhancement.Thecoefficientsandconstantsinthecorrelationhavebeenoptimizedwithheat-transferdataoflow-pressuresingle-phase(air)flowandhigh-pressuresteam-waterflowatfilm-boilingconditions. Anexperimenthasbeensetuptoobtainvalidationdatawithaheatedtubeof4.1mminsidediameter.HFC-134awasusedastheworkingfluid.Threetypesofobstacleswiththesameblockage-arearatioof37.8%weretested.Theresultsshowedthattheobstaclesexhibitedastrongenhancementeffectonthefilm-boilingheat-transfercoefficientatlocationsdownstreamoftheobstacles.Acomparisonbetweenpredictionsofthecorrelationforthespacing-deviceeffectandexperimentaldatashowedanunderpredictionoftheheat-transferrateatlocationsdownstreamoftheobstacle.Theunderpredictionisduemainlytotherewettingoftheheatedsurfaceatthelocationofanobstacle,beyondwhichthedevelopingfilm-boilingeffectbecomesdominant.Theagreementbetweenpredictionanddataissignificantlyimprovedafteraccountingforthecriticalheatflux(CHF)enhancementanddevelopingfilm-boilingeffectsinthecalculations.