ML17320A793
| ML17320A793 | |
| Person / Time | |
|---|---|
| Site: | Cook  |
| Issue date: | 10/03/1982 |
| From: | LIPARULO N J, TSAI S S WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
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| 2693Q:1, NUDOCS 8310140046 | |
| Download: ML17320A793 (24) | |
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FOGINERTINGCRITERIAFORHYOROGEN/AIRMIXTURESS.S.Tsai,andN.J.LiparuloRiskAssessmentTechnologyWestinghouseElectricCorporationPresentedatSecondInternationalWorkshopontheImpactoff{ydrogenonWaterReactorSafetyAlbuquerque,NewMexicoOctober3-7,1982(83i0140046831010lPDRADOCK05000315PPDR2693/:1
FOGINERTIHGCRITERIAFORHYDROGEN/AIRMIXTURESS.S.Tsai,andN.J.LiparuloRiskAssessmentTechnologyMestinghouseElectricCorporationABSTRACTAdistributedignitionsystemhasbeenproposedtoignitehydrogenatlowconcentrationintheicecondensercontainmentduringsevereaccidents.Thepost-accidentcontainmentatmospherecouldbemistyduetofoggenerationfromthebreakflowandcondensationintheicebed.Thusitisimportanttoestablishafoginertingcriterionforeffectiveperformanceo'ftheignitionsystem.Thispaperpresentssuchacriterionthatspecifiesthenecessaryfoggingconditions,i.e.,fogconcentrationanddropsize,forinertingahydrogen/airmixture.Thecriterionshowsthattheminimumfoginertingconcentrationvarieswiththesquareofthevolumemeanfogdropsize.ThepresentfoginertingcriterionisshowntobeingeneralagreementwiththeFactoryMutualtestdata.1.INTRODUCTIONAdistributedignitionsystemhasbeenproposedtoignitehydrogenatlowconcentrationintheicecondensercontainmentduringsevereacci-dents.Thepost-accidentcontainmentatmospheremaybemistybecauseoffoggenerationbythebreakflowandcondensationintheicebed.Thusitisofimportanttoestablishafoginertingcriterionforeffectiveperformanceoftheignitionsystem.ZaloshandBajpai(>)haverecentlyconductedhydrogenflamaabilityteststodeterminetheeffectofwaterfoggingonthehydrogenlowerflammabilitylimit.Inthetests,theminimumfoginertingconcentra-tionsforvariousvolumemeandropsizesandtpdrogenconcentrationsweremeasured.Itwasfoundthatan8volpercenthydrogenmixturecouldbeinertedathighfogconcentration.FogformationmayalsohavebeenresponsibleforfailureofthetwoLawrenceLivermoretests(2)athighsteamconcentrationsto'gnitetpdrogenduringthetestvesselcool-downperiod.Thecapabilityoffogdropletsin.,inhibitingcombustionorquenchingflamesisduetotheirheatabsorbingcapability-highheatofvapor-izationandsmalldropsizes(ontheorderof10p).Duetothesmalldropsizes,fogdropletscouldvaporizerapidly(ontheorderofmili-seconds)withinapropagatingflamefront("Immthick).Ifasub-stantialamountofthesedropletsarepresent,theflamemaybequenched.ThecriticaldropletdiameterforquenchingahydrogenflamehasbeenestimatedbySandiaNationalLaboratories(>).Itwasassumedthatthemeasured"quenchingdistance"forahydrogenflamepropagatingthrougha2693/:I
tubeorbetweenplatescouldbeusedforfogdroplets.Thismodeldoesnotconsiderheattransferandcombustionoccurringbetweentheburnedgasandthesuspendeddroplets.Thispaperpresentsafoginertingcriterionthatspecifiesthenecessaryfoggingconditions,i.e.,fogconcentrationanddropsize,forinertingahydrogen/airmixture.2.FOGINERTINGCRITERIARecenthydrogenburnexperiments(2)conductedatLawrenceLivermoreLaboratoryindicatedthatsubstantialfogformationcouldoccurwhensaturatedsteamisdischargedintoanunheatedvessel.Itappearedthatthisfogpreventedaglowplugigniterfromsuccessfullyignitingthehydrogenmixtureinthevessel.Theabilityoffogininhibitingandquenchingahydrogencombustioncanbeexplainedasfollows.Thefogdropletssuspendedinthehydrogen-air-steammixtureactasaheatsinkthatcouldabsorbalargeamountofcombustionheatbyvaporization,greatlyreducingthepressureandtemperaturerisesresultingfromhydrogencombustion.Ifdropletsaresufficientlysmallsuchthattheycouldvaporizeinsidethethin(1mm)flamefront,theflamemaybequenchedorinhibited.Foraflamespeedof2m/s,thedropresidencetimeisoftheorderof0.5x1Q-3seconds.(3)Insuchashortperiodoftime,thedropletsofinitialradiuslessthanabout4pwillvaporizeentirelyintheflam'efront.Thequenchingofapropagatingflameisalsogovernedbythedistancebetweendroplets.Asthedropletsbecomecloselypacked,thetota1dropletsurfaceareaavailableforenergylossincreases.Acriticalspacingbetweendropletsexistssuchthatalargefractionoftheheatreleasedisabsorbed,Thuspreventing.flamepropagation.Thiscriticalspacingisknownasthe"quenchingdistance",whichisusuallydeterminedbypropagatingflamesintubes.2.1PREVIOUSMORKTheeffectivenessoffogdropletsininhibitingorquenchingaflamedependsonitsquenchingdistance,whichwasdeterminedbyBermanetal.(3)asq=L4V/SjcritwhereVisthegasvolumeandSistheheattransfersurfacearea.Inthesuspendedfogdroplets,thisvolume-to-surfaceratio(i.e.,V/S)isequaltod(1-n)/6q,wheredisthemeandropletdiameterandqisthevolumefractionofthedroplets.Mhenfourtimesthisratioapproachesthequenchingdistance,acriticaldropletdiametercanbeobtainedasndc=Y~q(2)2693':I Usingthequenchingdistancedataforagivenvolumefractionofwaterandgascomposition,thecriticaldropletdiametercanbedeterminedfromEquation(2).Thedropsizeslessthanthecriticaldropsizearecapableofquenchingaflame.2.2PRESENTTHEORYTheprevioustheoriesdonotmodeltheheattransferandcombustionprocessesoccurringbetweentheburned'gasandthesuspendeddroplets.Anewtheoryhasbeendeveloped,whichmodelstheheatlossandcombustionwithinathinflamefront.1+.K!plwhereeiConsiderahydrogen/air/steam/mistdropletsmixtureinwhichaflameispropagating.Theflamemaybedividedintothreezones:heatingzone,reactionzone,andpost-reactionzoneasshowninFigure1.TheunburnedgasattemperatureTumovesinthereactionzonewiththelaminarburningvelocitySu.Iftheunburnedgasdensityispu,thentheconstantmassflowratemisequaltopuSu.TheunburnedgasisheatedtoignitiontemperatureTiandburnedinthereactionzonetoreachtheflametemperatureTf.Thefogdropletswillactasaheatsinkthatreducestheflametemperature.TheproblemhasbeenformulatedandsolvedbyvonKarmani4).Inhisformulation,threeenergyequations,whichincorporatetheheatlossterms,werewrittenforthethreezonesdescribedabove.Thesolutiontotheseequationsyieldsthefollowingrelationship2KG.1-exp(-~p)(Y-Yf)x'ft(3)Ll+1+(4K/)]x1-uC(Ti-T)/qpiuKei(S/C')e.p1,theratioofheatlossrateperunitvolumeto'theheatreleaserateby'chemicalreactionperunitvolumeheatofcombustionCmeanspecificheatmeanheatconductivityreactionrate(massoffuelconsumedper:unittimeperunitvolume)II2693/:I
hydrogenmassfractionintheheatingzone"f=hydrogenmassfractioninthereactionzone>uSuAplotofEq.(3)isshowninFigure2.Itisseenthatforagiven<oi,thereisaminimumvalueof(Yu-Yf1/ei.Belowthisminimumvalue,thereisnosolutionforthee;.Therefore,thisvalueisconsideredastheflamsabilitylimit.Attheflamsabilitylimit,thevalueofKelcanbedeterminedfromFigure2orfromEq.3as(K)ite.=f((Yfei\sAplotof(K)criteiasafunctionof(Yu-Yf)/eiisshowninFigure3.Equation4maybeexpressedas2ufY-YqpuSu(Yu-Yf)fe;d-212i(TiTu)(4)(5)DetailedderivationprocedureforEq.(5)isgiveninAppendixA.UsingthedataonSufromReference(5)wecancalculatetherighthandsideofEq.(5)foragivencompositionandinitialgastemperature.3-VERIFICATIONOFTHEORIESBYEXPERIMENTSExperimentshavebeenconductedatfactoryMutualtostudytheeffectsofwaterfogdensity,dropletdiameter,andtemperatureonthelowerflaranabilitylimitofhydrogen-air-steammixtures(2).Theresultsindicatedthatmostofthefognozzlestestedat20Conlychangedthelimitfrom4.03volumepercentto4.76percent,correspondingtofogconcentrationintherangeof0.028-0.085volumepercent,andvolumemeandropsizerangingfrom45-90microns.Forthe50Ccase,thelowerflammabilitylimitincreasesto7.2percent,correspondingto0.01-0.04volumepercentoffogand20-50micronvolumemeandropsizes.Theresultsdemonstratedthatthefoginertingeffectismorepronouncedatreduceddropsizesandincreasedtemperature.Figures4through6showthecomparisonbetweenthetestdataandthetheoreticalpredictions.Forthiscomp'arison,thepresenttheoryusedthefreestreamtemperaturetocalculatethethermodynamicpropertiesusedinEquation(5).Thisyieldedsomewhathigherfogconcentrationsthanthosecalculatedbyuseofthemeanoftheflameandfreestreamtemperatures.InFigures4and5,thedatasuggestsalinearrelation-shipbetweenthevolumeconcentrationandvolumemeandropsizeonthelog-logplot.Italsosuggeststhattheminimumfoginertingconcentra-tionvariesapproximatelywiththesquareofthevolumemeandropsize.2693Q:I
ThepresenttheoryisingoodagreementwiththeFactoryMutualdataat4.76percentH2;however,itoverpredictstheminimumfoginertingconcentrationat7.2percentH2.Thecauseofthisdiscrepancyisstillunknown.Thediscrepancymaybecausedbytheuncertaintyofthedata.Thefollowingdiscussionsupportsthisview.Thefogdropletsareverysmallandtheyvaporizeveryfastinaflame.Therefore,thefogdropletsbehaveassteamexceptfortheirlargerheatabsorptioncapability.@henthefogdropletsvaporize,theyabsorbtheheatofvaporizationwhichismuchlargerthanthesteamsensibleheat.Typically,theheatofvaporizationofwaterisabout1000Btu/lbandtheaveragespecificheatofsteaminthetemperaturerangeofinterestisabout0.48Btu/lb.Itiswellknownthatahydrogenflamecannotpropagateinsteamhigherthanabout64percentinasteam-airmixture.At7.9percentH2,theadiabaticflametemperatureisabout1240Fandthereforetheincreaseofthesteamsensibleheatisabout540Btu/lb.Consequently,.forthesameamountoffogdropletsandsteam,thefogdropletsheatabsorptioncapabilityisabout1.9timeshigher.Thismeansthatthefogconcentrationwhichisequivalentto22.1percentsteaminasteam-airmixtureiscapableofinerting7.9percentH2.Thisfoginertingvolumetricconcentrationwascalculatedtobe1.61x10-4ft~H20/ft3mixfor7.9percentH2.Toinert7.2'ercentH2,aminimumfogconcentrationof1.56x10-4ft3H20/ft3mix,equivalenttoabout21.3percentsteaminasteam-airmixtureisrequired.Theseestimatesshowthatthepresentpredictionsarereasonableandconservative.TheestimatesareconsistentwithFactoryMutualdataon7.9percentH2butnoton7.2percentH2-Itshouldbenotedthatintheteststhreefogconcentrationmeasuringtechniqueswereused.Thesethreetechniquesgavesubstantiallydif-ferentresults.Thediscrepancyisatleastoneorderofmagnitudedifference.ThefogconcentrationdatapresentedinFigures4through6wereobtainedfromoneofthetechniques.Invie'woftheuncertaintyofthedata,caremustbeexercisedinusingthemforfoginertinganalysispurposes.Theyshouldbeusedinconjunctionwiththepresentfoginertingcriterionintheassessmentoffoginertingpotentialintheicecondenserplants.Someuncertaintyalsoexistsinthepresentfoginertingtheory.Themaximumuncertairityassociatedwiththeunder-predictionoftheheatlossandtemperaturedependenceofthethermo-physicalpropertiesisestimatedtobe+63percent.4.SUMMARYAkDCONCLUSIOkSAfoginertingcriterionhasbeendevelopedtopredicttheminimumfogconcentrationrequiredtoinertagivenhydrogenconcentrationandvolumemeanfogdropsize.ThepresentfoginertingcriterionhasbeenshowntobeingeneralagreementwiththeFactoryMutualtestdata.Thecriterionshowsthattheminimumfoginertingconcentrationvarieswiththesquareofthevolumemeanfogdropsize.2693':I
ACKNOWLEDGMENTSTheauthorswishtoexpresstheirsinceregratitudetoDrs.V.Srinivas,B.Lewis,andB.Karlovitzforassistance,suggestions,andhelpfuldiscussions,toMessrs.D.F.Paddleford,R.8ryan,F.G.Hudson,D.Renfro,andK.Shiuforvaluablecomments.TheyalsowouldliketothankTYA,DukePower,andAEPforprovidingthefinancialsupport.REFERENCES1.R.G.ZaloshandS.N.Bajpai,"MaterFogInertingofHydrogen-AirMixtures,i",EPRIProjectPreliminaryRpt.1932-1,September,1981.2.B.Lowry,"PreliminaryResults:AStudyofHydrogenIgniters,"ENN80-45,LawrenceLivermoreNationalLaboratory,November17,1980.3.M.Berman,etal.,"Analysisoff{ydrogenMitigationforDegradedCoreAccidentsintheSequoyahNuclearPowerPlant,"Sandiadraftreport,December1,1980.4.T.vonKarman,unpublishednotes,1956.5.S.S.Tsai,andN.D.Liparulo,"FlameTemperatureCriteriaTests,"acceptedforpresentationattheSecondInt.WorkshopontheImpactofHydrogenonMaterReactorSafety,Albuquerque,NewMexico,October3-7,1982.2693/:1
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APPEHDIXADERIYATIOHOFEQUATIOH(5)ThisappendixgivesdetailedprocedurestoderiveEq.(5),startingfromEq.(4)<<)cr;t<;=f<<~u-~f)/oj)(4)wheretheratioofheat.lossrateperunitvolumetotheheatreleaseratebychemicalreactionperunitvolume,(K)crit,isdefinedasKcrit=S/Cpw(A-I)andtheratioofsensibleheattoheatofcombustion,ei,isdefinedase;=Cp(Ti-Tu)/q(A-2)ToarriveatEq.(5),itisnecessarytoassumethatalltheheatlossisattributedtoconvectionheattransfertofogdropletsofonlyonedropsize.Underthisassumption,therateofheatlossperunitvolumeperdegree,S,maybee'xpressedasS=nxd2h(A-3)wheren=numberofdropsperunitvolumedvolumemeandropsizeh=heattransfercoefficientItisfurtherassumedthattherelativevelocitybetweenthedropletsandthemixtureflowissosmallthatheattransfercoefficient,h,canbeapproximatedbytheconductionlimit.Infact,itcanbeshownthatforsmalldropsizes,convectionandradiationareunimportantheattransfermechanismsatthedropsurface.Underthisassumption,Eq.(A-3)reducesto12'=~d(A-4)where7=meanheatconductivitynvolumefractionofmistdroplets(-id)n362693Q:I Theofheatgenerationperunitvolume,w,isrelatedtothelaminarburingveloc>ty,Su,andthethicknessofthereactionzone,x,byS(Y-Yf)w=ThethicknessofthereactionzonemaybeapproximatedbypSZ'A-6)CombiningEqs.(A-l),(A-4),(A-5),and(A-6),wehave12',SZ(Yu-Yf)SubstitutingEqs.(A-2)and(A-7)intoEq.(4),wehaveYu-Yfd127(T.-T)(Y-Y)f(u)1u1(5)Q.E.D.2693Q:1,B 4
FIGUREjSCHEMATICREPRESENTATIONOFTEMPERATUREPROFILETHROUGHTHEFLAMEFRONT0'd,a%5gsO3002.<~v~rV+iFIGURE2THEPARAMETERA.pASAFUNCTIONOF(Y-Y~)/e.FORDIFFERENTVALUESOFK9.
0.30.20.i3(Yu-Yfj/0/FIGURE3(K)8.ATTHEFLAMMABILITYLIMITASAFUNCTIONOF(Y-Yf)/8.cr)tiufi 2147310.175x2PlI-10-2P4I-5z0I-LL2IzO0103CJU0tL50SPRACO2163SPRACO1405-0604QSPRAGO2020-17040SPRACO1806-1605NON-FLAMMABLEZONEBERMANETAL.THEORYPRESENTTHEORYFLAMMABLEZONE10410100200VOLUMEMEANDIAMETER(MICRONS)Figure4.ComparisonbetweenTheoriesandFactoryMutuaiFoginertingExperimentson4.76PercentH2 2147.$0SPRACO2163-7604DSPRACO2020-1704QSONICORE035HX0C4FlI-ROI-lEI-RLuORO2C9OVNON-FLAMMABLE'ONEPRESENTTHEORYFLAMMABLEZONE7.2%H2INAIRAT-50C102030405060708090100VOLUMEMEANDIAMETER(MICRONS)Figure5.ComparisonbetweenthePresentTheoryandFactoryMutualFoglnertingExperimentson7.2PercentH2 V
2147.1NON-FLAMMABLEZONEx5X1030xR10-3I-I-zRo5X10.40OU0PRESENTTHEORYFLAMMABLEZONE6FACTORYMUTUALDATAON7.9%H2INAIR10450100VOLUMEMEANDIAMETER(MICRONS)1000Figure6.ComparisonbetweenthePresentTheoryandFactoryMutualFogtnertingExperimentson7.9PercentH213
,~