enclosure-fire-dynamics-c02

2©2000byCRCPressLLCAQualitativeDescriptionofEnclosureFiresThischapterprovidesanoverviewandaqualitativedescriptionofthedevelopmentofafireinanenclosureanddiscussestheresultingenvironmentalconditionsthatarise.Itlaysgroundworkforthemorecalculation-intensivechaptersthatfollow.Weintroduceanddefineanumberoftermsnecessaryforoursubsequenttreatmentofthesubjectofenclosurefires.Thechapterbeginswithageneraldescriptionoftheprocessofcombustionandadescriptionofatypicaldevelopmentofafireinanenclosure.Thefiredevelopmentiscommonlydividedintodifferentstages;theseareidentifiedanddiscussed.Finally,wediscussanumberoffactorsthatinfluenceenclosurefiredevelopment.CONTENTS2.1Terminology2.2Introduction2.2.1GeneralDescriptionoftheProcessofCombustion2.2.2GeneralDescriptionofFireGrowthinanEnclosure2.3StagesinEnclosureFireDevelopment2.3.1FireDevelopmentinTermsofEnclosureTemperatures2.3.2FireDevelopmentinTermsofFlowthroughOpenings2.3.3OtherCommonTermsDescribingEnclosureFireStages2.4FactorsInfluencingFireDevelopmentinanEnclosure2.5SummaryReferences2.1TERMINOLOGYBackdraft—Limitedventilationduringanenclosurefirecanleadtotheproductionoflargeamountsofunburntgases.Whenanopeningissuddenlyintroduced,theinflowingairmaymixwiththese,creatingacombustiblemixtureofgasesinsomepartoftheenclosure.Anyignitionsources,suchasaglowingember,canignitethisflammablemixture,resultinginanextremelyrapidburningofthegases.Expansionduetotheheatcreatedbythecombustionwillexpeltheburninggasesoutthroughtheopeningandcauseafireballoutsidetheenclosure.Thephenomenoncanbeextremelyhazardous.Flashover—Thetransitionfromthefiregrowthperiodtothefullydevelopedstageintheenclosurefiredevelopment.TheformaldefinitionfromtheInternationalStandardsOrga-nizationis“therapidtransitiontoastateoftotalsurfaceinvolvementinafireofcombus-tiblematerialwithinanenclosure.”Infiresafetyengineeringthewordisusedtoindicatethedemarcationpointbetweentwostagesofacompartmentfire,i.e.,pre-flashoverandpost-flashover.Fuel-controlledfire—Afterignitionandduringtheinitialfiregrowthstage,thefireissaidtobefuel-controlled,sinceintheinitialstagesthereissufficientoxygenavailablefor©2000byCRCPressLLCcombustionandthegrowthofthefireentirelydependsonthecharacteristicsofthefuelanditsgeometry.Thefirecanalsobefuel-controlledinlaterstages.Fullydevelopedfire—Synonymouswiththepost-flashoverfire,whichlastsfromtheoccurrenceofflashoverthroughthedecaystagetoextinction.Duringmostofthisperiodthefireisventilation-controlled;atsomepointduringthedecayperioditbecomesfuel-controlledagain.Post-flashoverfire—Whentheobjectiveoffiresafetyengineeringdesignistoensurestructuralstabilityandsafetyoffirefighters,thepost-flashoverfireisofgreatestconcern.Thedesignloadinthiscaseischaracterizedbythetemperature–timecurveassumedforthefullydevelopedfirestage.Pre-flashoverfire—Thegrowthstageofafire,wheretheemphasisinfiresafetyengineeringdesignisonthesafetyofhumans.Thedesignloadisinthiscasecharacterizedbyanenergyreleaseratecurve,wherethegrowthphaseofthefireisofmostimportance.Ventilation-controlledfire—Asthefiregrowsitmaybecomeventilation-controlled,whenthereisnotenoughoxygenavailabletocombustmostofthepyrolyzingfuel.Theenergyreleaserateofthefireisthendeterminedbytheamountofoxygenthatenterstheenclosureopenings.2.2INTRODUCTIONFireisaphysicalandchemicalphenomenonthatisstronglyinteractivebynature.Theinteractionsbetweentheflame,itsfuel,andthesurroundingscanbestronglynonlinear,andquantitativeestimationoftheprocessesinvolvedisoftencomplex.Theprocessesofinterestinanenclosurefiremainlyinvolvemassfluxesandheatfluxestoandfromthefuelandthesurroundings.Figure2.1showsaschematicoftheseinteractions,indicatingthecomplexityofthemassandheattransferprocessesoccurringinanenclosurefire.Inordertointroducethemostdominantoftheseprocesses,thissectionincludesageneralqualitativedescriptionofthechemicalandphysicalphenomenaassociatedwithfire.Thediscussionisdividedintotwoparts:ageneralandsummarydiscussionontheprocessofcombustion,andaqualitativedescriptionofthedevelopmentofafireinanenclosureandtheeffectsithasontheenclosureenvironment.2.2.1GENERALDESCRIPTIONOFTHEPROCESSOFCOMBUSTIONThestudyofcombustionisacomplexsubject;itincludesanumberofdisciplinessuchasfluidmechanics,heatandmasstransport,andchemicalkinetics.Infiresafetyengineering,acourseonfirefundamentalsdealsextensivelywiththesubjectofcombustion.Inourintroductorydiscussiononcombustionwewilluseacandleasourfuel;thediscussionisthereforelimitedtoalaminar,steadyflameonasolidsubstrate.Thestudyofaburningcandle,however,isveryillustrativeofthenaturalprocessesinwhichweareinterested.The19thcenturyscientistMichaelFaradaygavelectureson“TheChemicalHistoryofaCandle”attheso-calledChristmasLecturesattheRoyalInstitutioninLondon.2Heclaimedthattherewasnobetterwayonecouldintroducethestudyofnaturalphilosophythanbyconsideringthephysicalphenomenonofacandle.ConsiderFigure2.2,whichshowsaburningcandleandthetemperaturedistributionthroughtheflame.Anignitionsource,amatchforexample,heatsupthewickandstartsmeltingthesolidwax.Thewaxinthewickvaporizes,andthegasesmove,bytheprocessofdiffusion,outintoaregionwhereoxygenisfound.Thegasesareox