enclosure-fire-dynamics-c06

6©2000byCRCPressLLCGasTemperaturesinVentilatedEnclosureFiresItisofconsiderableimportancetothefireprotectionengineertobeabletoroughlypredictthehotgastemperatureinafirecompartment.Thisknowledgecanbeusedtoassesswhenhazardousconditionsforhumanswilloccur,whenflashovermayoccur,whenstructuralelementsareindangerofcollapsing,andthethermalfeedbacktofuelsourcesorotherobjects.Anypredictionofthehotgastemperatureinacompartmentfiremustbebasedontheconservationofenergyandmass;thisusuallyleadstoanumberofdifferentialequationsthatcanbesolvednumericallytopredictthetemperature.Bysimplifyingtheenergyandmassbalances,onecanarriveatfairlysimpleanalyticalequationsthatcanpredictthehotgastemperatureforanumberofcompartmentfirescenarios.Thesescenariosusuallyrequirethatthefireiswellventilated,i.e.,thatthereisanopeningtotheoutside.Thischapterreviewsanumberofmethodsthathavebeendevelopedforpredictingtemperaturesinbothpre-andpost-flashoverphasesofwell-ventilatedcompartmentfires.CONTENTS6.1Terminology6.2Introduction6.3ThePre-FlashoverFire6.3.1ASimplifiedEnergyBalance6.3.2ExperimentsandStatisticalCorrelation6.3.3CalculationoftheEffectiveHeatTransferCoefficient6.3.4CalculationalProcedure6.3.5LimitsofApplicability6.3.6PredictingTimetoFlashover6.3.7SomeRelatedExpressionsforSpecialCases6.4ThePost-FlashoverFire6.4.1DefinitionsofSomeTerms6.4.2TheEnergyandMassBalance6.4.3MethodofMagnussonandThelandersson6.4.4OtherRelatedMethodsReferencesProblemsandSuggestedAnswers6.1TERMINOLOGYFireload—Thefireloadforanenclosureisameasureofthetotalenergyreleasedbycombustionofallcombustiblematerialsintheenclosure.ItisassignedthesymbolQandisgivenin[MJ].Fireloaddensity—Thefireloaddensityisthefireloadperunitarea.ThefireloaddensityisassignedthesymbolQ″andisgivenin[MJ/m2].Insomecountries,thefireloadisgivenperunitfloorareaoftheenclosure;inothercountriesthisisgivenintermsofthetotalenclosuresurfacearea,At.©2000byCRCPressLLCTheMQHmethod—Ahand-calculationmethodforcalculatinggastemperaturesinthepre-flashoverstage.Nominaltemperature–timecurves—Thesearestandardtemperature–timecurvestowhichstructuralcomponentscanassumedtobesubjectedduringthepost-flashoverfire.Thebestknownsuchcurveistermedthe“ISO834”curve.Thestandardcurves,definedbydifferentstandardsorganizations,donottakeintoaccountdifferentcompartmentgeometries,open-ings,fuelcontent,andthermalproperties.Theopeningfactor—TheopeningfactorisgivenastheventilationfactordividedbythetotalenclosuresurfaceareaAt.TheopeningfactoristhereforeAo/At.Thisfactorhasbeenfoundtobeveryusefulwhensystemizingcalculatedtemperature–timecurvesforthepost-flashovercase.Thepost-flashoverstage—Whentheobjectiveoffiresafetyengineeringdesignistoensurestructuralstabilityandsafetyoffirefighters,thepost-flashoverstageisofgreatestconcern.Thedesignloadinthiscaseischaracterizedbythetemperature–timecurveassumedforthefullydevelopedfirestage.Thepre-flashoverstage—Thegrowthstageofafire,wheretheemphasisinfiresafetyengineeringdesignisonthesafetyofhumans.Thedesignloadinthiscaseischar-acterizedbyanenergyreleaseratecurve,wherethegrowthphaseofthefireisofmostimportance.Simulatednaturalfireexposure—Referstomethodsforcalculatingorapproximatingtemperature–timecurvesforthepost-flashoverstage,wherethecalculationsarebasedonasolutionoftheenclosuremassandenergybalances.Thesemethodstakeintoaccountdifferentcompartmentgeometries,openings,fuelcontent,andthermalproperties.Totalenclosuresurfacearea—Thetotalsurfaceareaboundingtheenclosure,includingopenings,isgiventhesymbolAt.NotethatintheprevioussectionswehaveusedthetermATtorepresenttheenclosuresurfacearea,notincludingopenings.Theventilationfactor—ThefactorAo(whereAoisopeningareaandHoisopeningheight)hasbeenfoundtobedirectlyproportionaltothemassflowrateofairinthroughanopeningduringthepost-flashoverstage.Thisfactoristermedtheventilationfactor.6.2INTRODUCTIONWhentheenvironmentalconditionsduetoafireinanenclosurearetobeevaluated,estimationofthetemperatureofthegasesisofcentralimportance.Notonlywillthetemperaturehaveadirectimpactonhumansafetyandstructuralsafety,butanestimateoftemperaturealsoisnecessaryforpredictingmassflowratesinandoutthroughopenings,thermalfeedbacktothefuelandothercombustibleobjects,andthermalinfluenceondetectionandsprinklersystems,tonameafewimportantprocesses.Heatposesasignificantphysicaldangertohumans,bothwhentheskinorlungsareindirectcontactwithheatedairandwhenheatisradiatedfromadistance.Buildingregulationsthereforetypicallyspecifyacertainmaximumtemperaturetowhichhumansmaybeexposed,aswellassomemaximumradiativeheatflux.Calculationsoftheload-bearingandseparatingcapacityofstructuralelementsarealsobasedonthetemperaturetowhichtheelementsareexposedincaseoffire.Buildingfireregulationsthuscommonlyrequirethattwomainobjectivesbemet:lifesafetyoftheoccupantsandstructuralstabilityofthebuilding.Twodistinctlydifferentdesignproceduresareappliedineachcase,theformertodowiththepre-flashoverstageofthefire,thelatterwiththepost-flashoverstage.Pre-flashoverstage:Inthecasewheretheobjectiveistofacilitateescapefortheoccupantsthetimeframeisusuallyrelativelyshort(mostoftenlessthan30minutes)andthedesignfireis