ML18213A076

Joint EPRI/NRC-RES Fire PRA Workshop - Module III - Fire Analysis -Fire Fundamentals: Fires in the Open and Fully Ventilated Fires

ML18213A076
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Issue date:	07/31/2018
From:	Tammie Rivera NRC/RES/DRA/FRB, Electric Power Research Institute
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Joint EPRI/NRC

-RES Fire PRA WorkshopAugust 6-10, 2018Module III

-Fire AnalysisFire Fundamentals: Fires in the Open and Fully Ventilated FiresA Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES) 2Recall: Fuel limited firesA fire where the fuel burning rate is limited only by the surface burning rate of the material. Sufficient air is always available for the fire (plenty of oxygen to support burning)Fire generates hot gases (convective fraction) and emits radiative heat (radiative fraction)Generally applies to fires in the open or fires in large compartments

-A nuclear power plant has lots of large compartments-3Heat Release Rate (HRR)For a simple fire, the HRR can be estimated using the following equation:

-2)-H cis the net*heat of combustion (kJ/kg)

-Ais the burning area (m 2)So HRR ends up as kJ/s or kW* "net" heat of combustion implies that a burnefficiency has been included

-fuels don't burnat 100% efficiency in real fires c H A m Q&&m&EnergyReleased RateFuel m&q&

4Heat Release RateHRR can be estimated experimentally using oxygen consumption calorimetrywhere:~ 13.1 MJ/kg O2for many common fuels

)/(2 2 O c OkgkJ H m Q&&c H 5FlamesLaminar -very small firesTurbulent

-most real firesFuel OxygenReaction Zone 6Ignition of GasesWith aspark orsmallflame (pilot)present,ignitionisbased onwhetherthegaseousfuelconcentrationisbetweentheupper(rich)andlower(lean)flammabilitylimits.-Thefuel-air(oxidizer)mixture issaid to beflammable if aflamewillpropagate inthismixture.With nopilotpresent, agaseousfuelinaircanstilligniteifthemixtureis at orabovetheauto-ignitiontemperature

.-Theauto-ignitiontemperature isusuallymeasuredfor astoichiometricmixture-justtherightmix sothat nofuel oroxygenremainsafterthereaction.

7Ignition of LiquidsFor a liquid to ignite, it must first evaporatesufficiently to form a flammable mixture of gaseous fuel and oxygen-This occurs at a liquid temperature called a flash-pointtemperature.

-In general, this temperature can be called the piloted ignition temperatureand the same term carries over to solids. -The flash-point is the temperature at which the amount of liquid evaporated from the surface achieves the lower flammable limit.If no pilot is present, the mixture must be heated to the auto-ignition temperature in order to ignite. The auto-ignition temperature of a gas will be higher than the boiling point of the liquid.LiquidsEvaporatingfuel*Spark 8Ignition of SolidsSolids do not evaporate like liquids when heated. Solids form gaseous decomposition compounds, generally leaving behind char, in a process called pyrolysis. At some point, the gases reach the lower flammability limit and may ignite by piloted ignition or, if hot enough, auto-ignition.Typically, piloted ignition temperatures for solids range from 250°C (~480°F)to 450°C(~840°F).Auto-ignition temperatures can exceed 500

°C (~930°F). -For a given material, these temperatures are not constants and can change with the nature of heating.

-For practical purposes, a (piloted) ignition temperature (T ig) may be treated as a property of a combustible solid.We shall consider thin (less than

~1 mm) and thick solids to have different time responses to ignition when exposed to impinging heat fluxHot SurfaceSolidsRadiantHeatPyrolysis products*Spark 9Flame SpreadMotion of vaporization front at the ignition temperature for solids and liquids-The surface is heated by the existing flames

-More material pyrolyzes(or evaporates) ahead of the flame front-The existing flame acts as the pilot-The flame (fire) spreads-Cable trayFire x p z f 10Typical Flame Spread RatesIt is very difficult to compute flame spread rates because formulas are not completely available, rates may not be steady, and fundamental fuel properties are not generally available.Nevertheless, we can estimate approximate magnitudes for spread rates for various cases.Spread caseSpread Rate (cm/s)Smoldering solids0.001 to 0.01Lateral or downward spread on thick solids0.1Upward spread on thick solids1.0 to 100. (0.022 to 2.2 mph)Horizontal spread on liquids1.0 to 100.Premixed flames (gaseous)10. to 100.(laminar)10 5(detonations) 11Zone of InfluenceRegions near the fire where damage or fire propagation is expected.For fires in the open we consider:

-Flame Radiation

-Convection, especially inside the fire plume xTarget q&Target 12Buoyant FlowTemperature rise causes a decrease in gas densityPotential energy converted into kinetic energy

-gasses flow upwards Z V DBuoyant plumeUnit volume at plume gas at density and temperature TUnit volume of air at density aand temperature T a 13Turbulent EntrainmentEntrainmentis air drawn into the fire plume by upward movement of the buoyant plume

-Engulfing air from the surroundings into the fire plumeEddies: fluctuating and rotating balls of fluid, large scale rolling fluid motion on the edge of the plume.Buoyant forceFlameEddies 14Turbulent Fire PlumeVery low initial fuel velocityEntrainment and flame height controlled by buoyancy 15Fire Plume Temperature Along the Centerline 16Example Case

-Zone-of-Influence CalculationFlame Height and Plume TemperatureHeskdestad'sFlame Height Correlation Input D -Fire diameter [m]

0.6 Q f-HRR [kW]250Result L -Flame height [m]

1.5where:Heskestad'sPlume Temperature Correlation Input Tamb-Ambient temp. [C]

20 Q f-HRR [kW]250 F e-Fire elevation [m]

0 H p-Target Elevation [m]

3.7 D-Fire Diameter [m]

1 k f-Location factor 1 (-2 or 4)

X r-Radiative Fraction 0.4Result Tpl-Plume Temp [C]

328 D Q L f 02.1 235.0 5 2&3 5 5 2 1 25o e p r f famb pl z F H Q k T T&D Q z f o 02.1 083.0 5 2&

17Example Case

-Zone-of-Influence CalculationRadiation Heat FluxFlame Radiation: Point Source Model 2 4 R Q q r f irr&&Input Parameters: Q f: Fire heat release rate (kW) R: Distance from flames (m) X r: Radiative fraction (FIVE recommends 0.4)D: Fire diameter (m) 18Example Case

-Zone-of-Influence CalculationRadiation Heat Flux 2 4 R Q q r f irr&&Point Source Flame Radiation ModelInputsFire heat release rate [kW]

317Radiation fraction0.40Distance from flames [m]1.5ResultsHeat flux [kW/m2]4.5