enclosure-fire-dynamics-c03

3©2000byCRCPressLLCEnergyReleaseRatesTherateatwhichenergyisreleasedinacompartmentfirecontrolstoaconsiderableextenttheenvironmentalconsequencesofthefire,suchastheplumeflows,thehotgastemperatures,andtherateofdescentofthehotgaslayer.Thefollowingchaptersinthistextbookoutlineanumberofcalculationalprocedurestoenableanestimationoftheenvironmentalconsequencesofafire.Mostoftheseproceduresrequiresomeknowledgeoftheenergyreleaserate.Thischapteroutlinesthemethodscommonlyusedtoestimatetheenergyreleaserateproducedbyafire.CONTENTS3.1Terminology3.2Introduction3.3FactorsControllingEnergyReleaseRatesinEnclosureFires3.3.1BurningRateandEnergyReleaseRate3.3.2EnclosureEffects3.4EnergyReleaseRatesBasedonFreeBurnMeasurements3.4.1MeasurementTechniquesandParametersMeasured3.4.2PoolFires3.4.3VariousProducts3.4.4Thet-squaredFire3.5TheDesignFire3.5.1Background3.5.2TheGrowthPhase3.5.3TheSteadyPhase3.5.4TheDecayPhase3.5.5AMoreComplexDesignFire3.5.6EnergyReleaseRatesUsedinThisBookReferencesProblemsandSuggestedAnswers3.1TERMINOLOGYBurningrateormasslossrate—Themassrateofsolidorliquidfuelvaporizedandburned.Itisexpressedasmassflowperunittime,typicallyinkg/sorg/sandisheredenotedas.Itcanalsobeexpressedasmassfluxormassburningrateperunitarea,typicallyinkg/(m2s).Inthiscaseitisdenotedas″.Adistinctionshouldbemadebetweenburningrateandmasslossrate(fuelsupplied),sinceallofthefuelsuppliedmaynotbeburned.Forburningobjectswithunlimitedairsupply,thetermsaresynonymous.Combustionefficiency—Theratiobetweentheeffectiveheatofcombustionandthecompleteheatofcombustionistermedthecombustionefficiencyandisdenotedχ.Energyreleaserateorheatreleaserate—Whenanobjectburnsitreleasesacertainamountofenergyperunittime,usuallygiveninkW(=kJ/s)anddenoted.Formostmaterialstheenergyreleaseratechangeswithtime.Thisisalsooftentermedheatreleaserate˙m˙m˙Q©2000byCRCPressLLC(sometimesshortenedtoHRR).Thetermenergyreleaserateis,however,moreappropriate,becauseheatisstrictlyspeakingenergytransportedduetoatemperaturedifference.Butbecauseofthegeneralusageoftheexpressions,thesetermswillbetakenassynonymous.Heatofcombustion—Ameasureofhowmuchenergyisreleasedwhenaunitmassofmaterialcombusts,typicallygiveninkJ/kgorkJ/g.Itisimportanttodistinguishbetweenthecompleteheatofcombustion,denoted∆Hc,andtheeffectiveheatofcombustion,denoted∆Heff.Theformerisameasureoftheenergyreleasedwhenthecombustioniscomplete,leavingnoresidualfuelandreleasingallofthechemicalenergyofthematerial.Theeffectiveheatofcombustionismoreappropriateforfires,wheresomeresidueisleftandthecombustionisnotnecessarilycomplete.Thisisalsosometimestermedthechemicalheatofcombustion.Heatofgasification—Ameasureofhowmuchenergyisneededtogasifysomeunitmassofthefuel.ItistypicallygiveninkJ/kgandisdenoted∆Hg.3.2INTRODUCTIONEnergyreleaserate(oftentermedheatreleaserateorHRR)ismeasuredinwatts,kilowatts,ormegawatts.Table3.1givessomecharacteristicvaluesofenergyreleasedbyvariousburningfuelpackagesandheatoutputfromvarioussources.Firedevelopmentisgenerallycharacterizedintermsofenergyreleaseratevs.time.Onceanengineerhasarrivedattheenergyreleaseratevs.timerelationshipforacertainscenario,itistermedthedesignfire.Table3.1indicatesthatformanydesignpurposesthedesignfireenergyoutputcouldbeintherange100kWto50MW.Therearebasicallytwoapproachesavailablewhendeterminingthedesignfireforagivenscenario.Oneisbasedonknowledgeoftheamountandtypeofcombustiblematerialsinthecompartmentoffireorigin.Theotherisbasedonknowledgeofthetypeofoccupancy,whereverylittleisknownaboutthedetailsofthefireload.Inthefirstcase,anobjectisassumedtoigniteandstarttoburn.Theresultingenergyreleaseratevs.timecaninmanycasesbeestimatedusingdatafrompreviousexperimentswhereenergyreleaseratehasbeenmeasured.Suchresultshavebeensummarizedinanumberofpublications.1,2,3However,inmanydesignsituationsthereisverylittleinformationavailableonthecombustiblecontentoftheroomoffireorigin.Inthiscase,knowledgeofthetypeofoccupancy,anyavailablestatistics,andengineeringjudgmentmustbeusedtoarriveatadesignfire.InSection3.3,wediscussthefactorscontrollingenergyreleaseratesinenclosurefires.Section3.4providesexamplesofhowexperimentaldatacanbeusedtoestimatethisforvariousfuelsandproducts.Finally,Section3.5discusseshowthisinformationcanbeusedtoarriveatadesignfireforacertainscenario.TABLE3.1RoughMeasureofEnergyReleasedorGeneratedfromVariousSourcesAburningcigarette5WAtypicallightbulb60WAhumanbeingatnormalexertion100WAburningwastepaperbasket100kWAburning1m2poolofgasoline2.5MWBurningwoodpallets,stackedtotheheightof3m7MWBurningpolystyrenejars,incartons,2m2,4.9mhigh30–40MWOutputfromatypicalreactorataNuclearPowerPlant500–1000MW©2000byCRCPressLLC3.3FACTORSCONTROLLINGENERGYRELEASERATESINENCLOSUREFIRESTherateatwhichenergyisreleasedinafiredependsmainlyonthetype,quantity,andorientationoffuelandontheeffectsthatanenclosuremayhaveontheenergyreleaserate.Theenergyreleaseratewillvarywithtime.Figure3.1showsaschematicgraphoftheenergyreleaseratevs.timemeasuredwhenwoodpalletstacksofdifferentheightsburn.Suchmeasure-mentsareoftentermed“freeburn”tests,indicatingthattheitemsareburningwithoutanyeffe