DepartmentofTHEHONGKONGBuildingEnvironmentandEnergyEngineeringPOLYTECHNICUNIVERSITY建第璟境及能源工程學系香港理工大學Lecture 5: Ignition and FlammabilityXinyan Huang, Ph.D.AssistantProfessorDept.of Building Environment and Energy EngineeringResearchCentreforFireSafetyEngineering,PolyUxy.huang@polyu.edu.hkFireScience1DrXinyan Huang
Fire Science 1 Dr Xinyan Huang Xinyan Huang, Ph.D. Assistant Professor Dept. of Building Environment and Energy Engineering Research Centre for Fire Safety Engineering, PolyU xy.huang@polyu.edu.hk Lecture 5: Ignition and Flammability
What is ignition?0:07Ignitionisthestartoffire(scientifically).Ignition is the appearance of a flame (generally)https://www.youtube.com/watch?y=jTbUPOsGQT8FireScience2DrXinyan Huang
Fire Science 2 Dr Xinyan Huang What is ignition? • Ignition is the start of fire (scientifically) • Ignition is the appearance of a flame (generally) https://www.youtube.com/watch?v=jTbUP0sGQT8
Whatisexplosion?ExplosionisasuddenincreaseofvolumePhysicalprocess:bubble,balloonChemicalprocess:explosives,ignitionNuclearprocessIgnitionisathermalexplosion,sustainedbyheatreleasefromchemicalchainreactions.Heatrelease→thermalexpansionChainreactions→acceleratedprocessDeflagration(fire/flame)Detonation(super-sonicexplosion)FireScience3DrXinyan Huang
Fire Science 3 Dr Xinyan Huang What is explosion? Explosion is a sudden increase of volume: • Physical process: bubble, balloon • Chemical process: explosives, ignition • Nuclear process Ignition is a thermal explosion, sustained by heat release from chemical chain reactions. • Heat release → thermal expansion • Chain reactions → accelerated process o Deflagration (fire/flame) o Detonation (super-sonic explosion)
Explosion vs.IgnitionH202H2+02COLLFCFire ScienceDrXinyan Huang
Fire Science 4 Dr Xinyan Huang Explosion vs. Ignition
Explosion:fromIgnitionto Denotation1e36.01.357ns0.305.44.80.25]anaa884320.20ul.0.150.100.050.60.0Q0.40.60.88.00.21.0(cm)xSimulationbyElaineOranandHuahuaXiao(UniversityofMaryland)5FireScienceDrXinyan Huang
Fire Science 5 Dr Xinyan Huang Explosion: from Ignition to Denotation Simulation by Elaine Oran and Huahua Xiao (University of Maryland)
DepartmentofTHEHONGKONGBuilding Environmentand Energy EngineeringPOLYTECHNICUNIVERSITY建筑璟境及能源工程學系香港理工大學Outline1.Flammability of GasandIgnition2.lgnitionProcessesPilotedvs.Autoignition. Solid fuelLiquidfuel3.Self-heating ignition6Fire ScienceDrXinyanHuang
Fire Science 6 Dr Xinyan Huang Outline 1. Flammability of Gas and Ignition 2. Ignition Processes • Piloted vs. Auto ignition • Solid fuel • Liquid fuel 3. Self-heating ignition
RequirementsofIgnitionCanwealwaysigniteaflameifwehaveOXYGENoxygen,fuelandheat?HEATFIRERIODICVIDEOSFUELLeft:.02Right:HzFireScience7DrXinyan Huang
Fire Science 7 Dr Xinyan Huang Can we always ignite a flame if we have oxygen, fuel and heat? Left: O2 Right: H2 Requirements of Ignition
ReviewofPremixedFlameThespeedofpremixedflameorlaminar burning velocity:Fuel/airCombustion8αmixtureproductsS.= 0.1~1m/s28tchemFuelGaseousfuelXevolumefraction(%)FuelAirPureairPurefuel1100%0%LowerflammabilityHigherflammabilitylimit (LFL)limit(HFL)Fire Science8DrXinyanHuang
Fire Science 8 Dr Xinyan Huang Review of Premixed Flame Fuel/air mixture Combustion products SL The speed of premixed flame or laminar burning velocity: 𝑺𝑳 ≈ 𝜹 𝒕𝒄𝒉𝒆𝒎 ≈ 𝜶 𝜹 = 𝟎. 𝟏~𝟏 m/s 0% 100% φ Pure air Pure fuel Lower flammability limit (LFL) Higher flammability limit (HFL) + Fuel Fuel Air Gaseous fuel volume fraction (%) XF =
Review:ConceptofStoichiometricStoichiometric is the ideal combustionprocesswherebothfueland oxygenareburned completely.CH4+2(O2+3.76N2)→CO2+2H20+7.52N21XcH4,st = 1 + 2 ×(1 + 3.76)=0.095=9.5%Iftheoxygen (orfuel)is notcompletelyburnt,the combustion isfuel-lean (orfuel-rich)Then, we can define an equivalence ratio (Φ)asForexample:FuelRichFuel-rich(Φ>1,(F/A)CombustionΦ = 1,StoichiometricdFuelair(F/A)stproducts +mixture(Φ<1,Fuel-leanunburnt fuel9FireScienceDrXinyan Huang
Fire Science 9 Dr Xinyan Huang Stoichiometric is the ideal combustion process where both fuel and oxygen are burned completely. CH4 + 2 𝑂2 + 3.76𝑁2 → CO2 + 2𝐻2𝑂 + 7.52𝑁2 X𝐶𝐻4,𝑠𝑡 = 1 1 + 2 × 1 + 3.76 = 0.095 = 9.5% If the oxygen (or fuel) is not completely burnt, the combustion is fuel-lean (or fuel-rich). Then, we can define an equivalence ratio (𝜙) as 𝜙 = 𝐹Τ𝐴 𝐹Τ𝐴 𝑠𝑡 ϕ > 1, Fuel−rich ϕ = 1, Stoichiometric ϕ < 1, Fuel−lean Review: Concept of Stoichiometric Fuel/air mixture Combustion products + unburnt fuel SL For example: Fuel Rich
Review:AdiabaticFlameTemperature (Tf.ad)Tf.ad is the temperature of gas product from a combustion process without heat loss.mpAH,=Ecp mp(Tr.ad -To)= mo,AHo2Fuel +AirCombustionProduct(stoichiometric)Tr,admrAHFlameToTf.ad = To +Ecp.pmpIn most cases,theoxygen (orfuel)would notbecompletely consumedafter combustion,sothecombustionisfuel-lean (orfuel-rich)Fuel +Product+CombustionmrAHcExtraairunburnt air>Tmin ~1350KTf,ad = To +ECp.pmp+ CpmexFlameT。Tr.ad10FireScienceDrXinyan Huang
Fire Science 10 Dr Xinyan Huang Review: Adiabatic Flame Temperature (𝑇𝑓,𝑎𝑑) ▪ 𝑇𝑓,𝑎𝑑 is the temperature of gas product from a combustion process without heat loss. 𝑚𝐹∆𝐻𝑐 = 𝑐𝑝,𝑝 𝑚𝑝 𝑇𝑓,𝑎𝑑 − 𝑇0 = 𝑚𝑂2 ∆𝐻𝑂2 𝑇𝑓,𝑎𝑑 = 𝑇0 + 𝑚𝐹∆𝐻𝑐 σ 𝑐𝑝,𝑝𝑚𝑝 𝑇𝑓,𝑎𝑑 = 𝑇0 + 𝑚𝐹∆𝐻𝑐 σ 𝑐𝑝,𝑝𝑚𝑝 + 𝑐𝑝𝑚𝑒𝑥 > 𝑇𝑚𝑖𝑛 ≈ 1350K ▪ In most cases, the oxygen (or fuel) would not be completely consumed after combustion, so the combustion is fuel-lean (or fuel-rich). Fuel + Air (stoichiometric) 𝑇𝑜 Product 𝑇𝑓,𝑎𝑑 Combustion Flame Fuel + Extra air 𝑇𝑜 Product + unburnt air 𝑇𝑓,𝑎𝑑 Combustion Flame