Stage 1:IgnitionRecall (Lecture:Ignition)::Twotypesoffire:flamingandsmouldering(usuallyflameignition).Flamingignition:auto and piloted ignitionmatch ignites.Ignitionlimit:ignitiontemperature&minimumheating10°Ignition(delay)time:1000Ignition1pcs(Tig - Ta)(thin fuel)X(s).aumdeqe100oAutoignition1(thick fuel)kpcX! Piloted ignitionOq10NoignitionS:fuelthickness:kpc:thermalinertia,102030405060701qe:externalheatflux; Tig:ignitiontemperatureIncident radiant heatflux (kW/m2)11FireScienceDrXinyan Huang
Fire Science 11 Dr Xinyan Huang Stage 1: Ignition Recall (Lecture: Ignition): • Two types of fire: flaming and smouldering (usually flame ignition) • Flaming ignition: auto and piloted ignition • Ignition limit: ignition temperature & minimum heating • Ignition (delay) time: 𝑡𝑖𝑔 ≈ 𝜌𝑐𝛿 𝑇𝑖𝑔 − 𝑇𝑎 𝑞ሶ𝑒 ′′ ∝ 1 𝑞ሶ𝑒 ′′ (thin fuel) 𝑘𝜌𝑐 𝑇𝑖𝑔 − 𝑇∞ 𝑞ሶ𝑒 ′′ 2 ∝ 1 𝑞ሶ𝑒 ′′2 (thick fuel) 𝛿: fuel thickness; 𝑘𝜌𝑐: thermal inertia; 𝑞ሶ𝑒 ′′: external heat flux; 𝑇𝑖𝑔: ignition temperature No ignition Ignition
IgnitionStage:SmokeandHeatDetectionHRR不VStage1IgnitionHeatingpyrolysisSmoulderingSmokedetectorIgnitionFlashoverBurnoutTime(flaming)Ignitionstageforearlywarning.Alarmfireservicedepartment85dBSound.StartevacuationActivatefireprotectionsystem(sprinklersetc.Heat detector12FireScienceDrXinyanHuang
Fire Science 12 Dr Xinyan Huang Ignition Stage: Smoke and Heat Detection Smoke detector Heat detector Ignition stage for early warning • Alarm fire service department • Start evacuation • Activate fire protection system (sprinklers etc.) HRR Ignition (flaming) Flashover Burnout Time Stage 1 Ignition Heating, pyrolysis, Smouldering
CompartmentFireStage2:Growth(FireSpread)IIGrowthStage(1)Roomtemperature is stillow.L(q) + qe)8f(2)Firestartstospreadpocp(Tpy-To)RadiationFireplume00feedback&ceiling jet450400FlashoverFuel-controlfire:oxygensupplyissufficient(n1),soHRRis350controlledbythefuelrelease._s ww)Peak temperature300eunHHR =mrAH.250Ceiling jetPlumecenterline口peaseee200Ceilingjet:Therisinghotgasflowspreads in150ashallowlayerbeneaththeceilingsurface100Aresultofcompartmentation50Triggerfiredetectors,andsprinklersFireExtemalradiantflux(Wcm13FireScienceDrXinyan Huang
Fire Science 13 Dr Xinyan Huang Compartment Fire Stage 2: Growth (Fire Spread) II Growth Stage (1) Room temperature is still low. (2) Fire starts to spread Fuel-control fire: oxygen supply is sufficient (𝜼 → 𝟏), so HRR is controlled by the fuel release. Ceiling jet: The rising hot gas flow spreads in a shallow layer beneath the ceiling surface. ❖ A result of compartmentation ❖ Trigger fire detectors, and sprinklers 𝑽𝒇 = 𝒒ሶ𝒇 ′′ + 𝒒ሶ𝒆 ′′ 𝜹𝒇 𝝆𝜹𝒄𝒑(𝑻𝒑𝒚 − 𝑻𝟎) Flashover 𝑯𝑯𝑹 = 𝒎ሶ 𝑭𝜼𝜟𝑯𝒄
GrowthStage:SuppressionSystemNofiresuppressionFire(Flashover)HRR不Stage3FullydevelopedStage1Stage 2Ventilation-IgnitionGrowthcontrolledfireHeatingWell-ventilatedfireWithfireSmoulderingsuppressionsystemignitionFlashoverTimeFire ServicesActivefireprotectionsystem(sprinkler,watermist,gas,foam,etc.)iseffectiveintheGrowthStage*Noneedtoextinguishthefire,butonlyneedtodelayflashoverandsavetimeforevacuationand fireservices14FireScienceDrXinyan Huang
Fire Science 14 Dr Xinyan Huang Growth Stage: Suppression System Time Fire HRR With fire suppression system ❖ Active fire protection system (sprinkler, water mist, gas, foam, etc.) is effective in the Growth Stage ❖ No need to extinguish the fire, but only need to delay flashover and save time for evacuation and fire services Fire Services
CompartmentFire Stage 3:Fully-DevelopedFully-developedorPost-flashoyerfire:-Highhotsmokelayertemperature(600~800°C)o SmokelayerdescendstothegroundEverything is burning:nomoreinternalfire spread-Flamespreadout-seekmoreoxygeno Spilledflameandfacadefire (verticalfirespread)oSpreadtoothercompartmentsAllfiresuppressionsystemsbecomeincapableChanceofevacuationislow (manycasualties)FirefightingbecomeslesseffectiveBurningintensityanddurationaffectstructurestabilityTimbercharred,andsteelpermanentlyweakened15FireScienceDrXinyanHuang
Fire Science 15 Dr Xinyan Huang Compartment Fire Stage 3: Fully-Developed Fully-developed or Post-flashover fire: ▪ High hot smoke layer temperature (600 ~ 800 ℃) o Smoke layer descends to the ground ▪ Everything is burning: no more internal fire spread ▪ Flame spread out – seek more oxygen o Spilled flame and façade fire (vertical fire spread) o Spread to other compartments All fire suppression systems become incapable ▪ Chance of evacuation is low (many casualties) ▪ Firefighting becomes less effective ▪ Burning intensity and duration affect structure stability o Timber charred, and steel permanently weakened