1Chapter2Combustion in Natural FiresTypes of FireWe can categorize fireintofour distinct phenomena:1. Diffusion flames,2.Smoldering,3.Spontaneouscombustion,4.Premixed flamesDiffusion flames represent the predominant category.It is thebuilding fire,thefor-est fire, the lit match. Smoldering can be the birth or follow the death of a diffu-sionflame.It is theglowing embers ofthetragic house fire,theresultofalightningstriketotheforest bed,ortheglow ofa blown outmatchflame.Spontaneous com-bustion is the incubation of a chemical reaction that leads to smoldering or (dif-fusion) flaming.It can occur in oily cotton rags (e.g.,linseed oil),ahaystack,orapile ofwood chips.Premixed flames represent controlled combustion processessuch as in the gasoline internal combustion engine with spark ignition or thediffusion flamediesel engine with autoignition. It also represents the incipient flame in the igni-aflame inwhichthetion of solids and liquids before a diffusion flame emerges.fuelandoxygenaretransported (diffused)fromoppositesidesofDIFFUSION FLAMESthe reaction zoneA diffusion flame is a combustion process in which the fuel gas and oxygen are(flame)transported into the reaction zone due to concentration differences.This transportdiffusionprocess is calleddiffusion and is governedbyFick's Law,which says thatagivenprocessofspeciesspecies (e.g.,in connection withfire,oxygen,fuel,CO,)will move fromahigh totransportfroma highlowconcentration inthemixture.Adropofblueinkinaglassofwaterwill even-tolowconcentrationtually diffuse into the water to give a blue tinge.Oxygen in air will move to theflamewhereithasaconcentraticas it is consumed in thereaction.Fuel11enspeciesis transported into the opposite side of the flameby the same process.The com-anothernameforbustionproductsdiffuseawayneinbothdirections.Thisprocessistheadistinctchemicalillustrated in Figure 2-2.Most natural flaming fires are diffusion flames.A com-compoundsandothermonexampleistheflameofamatchorcandle(Figure2-3).Inacandletheflamemolecular structures inmelts the wax,which is transported upthe wickby capillaryaction.Theflamea mixture, usuallythen vaporizes the wax, and the gaseous fuel diffuses into the flame where itgasesFUEL (GAS)OXYGENFigure 2-2Schematic ofaREACTIONdiffusion flame.ZONE
_ch_a_Pte_,_2_c_o_m_bu_s_tio_n_i_n _Na_t_u,_a1_F_ir_es _ ~- diffusion flame a flame in which the fuel and oxygen are transported (diffused) from opposite sides of the reaction zone (flame) diffusion process of species transport from a high to low concentration species another name for distinct chemical compounds and other molecular structures in a mixture, usually gases Figure 2-2 Schematic of a diffusion flame. Types of Fire We can categorize fire into four distinct phenomena: 1. Diffusion flames, 2. Smoldering, 3. Spontaneous combustion, 4. Premixed flames. Diffusion flames represent the predominant category. It is the building fire, the forest fire, the lit match. Smoldering can be the birth or follow the death of a diffu . sion flame. It is the glowing embers of the tragic house fire, the result of a lightning strike to the forest bed, or the glow of a blown out match flame. Spontaneous combustion is the incubation of a chemical reaction that leads to smoldering or (diffusion) flaming. It can occur in oily cotton rags (e.g., linseed oil), a haystack, or a pile of wood chips. Premixed flames represent controlled combustion processes such as in the gasoline internal combustion engine with spark ignition or the diesel engine with autoignition. It also represents the incipient flame in the ignition of solids and liquids before a diffusion flame emerges. DIFFUSION FLAMES A diffusion flame is a combustion process in which the fuel gas and oxygen are transported into the reaction zone due to concentration differences. This transport process is called diffusion and is governed by Fick's Law, which says that a given species (e.g., in connection with fire, oxygen, fuel, CO2) will move from a high to low concentration in the mixture. A drop of blue ink in a glass of water will eventually diffuse into the water to give a blue tinge. Oxygen in air will move to the flame where it has a concentration of zero as it is consumed in the reaction. Fuel is transported into the opposite side of the flame by the same process. The combustion products diffuse away from the flame in both directions. This process is illustrated in Figure 2-2. Most natural flaming fires are diffusion flames. A common example is the flame of a match or a candle (Figure 2-3). In a candle the flame melts the wax, which is transported up the wick by capillary action. The flame then vaporizes the wax, and the gaseous fuel diffuses into the flame where it FUEL (GAS) -~ REACTION ZONE OXYGEN
Chapter2Combustion in Natural Firespyrolysismeets oxygen. For the wooden match the wood is decomposed by the heat of theaprocessofbreakingflame into gaseous fuel and char. This decomposition process is called pyrolysis.upasubstanceintoAcandle flame is an example ofa laminar diffusion flamegoverned by pure mol-othermolecules asaeculardiffusion.Anyflamehigherthan approximately1ft will naturallypossessresultofheating;alsorandomfluid mechanical unsteadiness,illustrated byvisibleeddies in the smokeknownasthermalandflame.This is called turbulence,and sowehaveturbulentdiffusion flamesdecomposition(Figure2-4).Smokefrom alargechimneyhas this turbulentcharacter,andasmokefilament from a cigarette in a still room begins as a laminar flow then clear-laminarly breaks up after rising about 1 ft to become turbulent (Figure 2-5).refers to orderly.Gravity influences the shape of diffusionflames and profoundly affects fireunfluctuating fluidprocesses in general.Because fire creates high temperatures,the hotter (lighter)motiongases rise as a result of buoyancy caused by gravity.The ensuing flow distorts theturbulentflame andeventuallyflow instabilities cause turbulence.Turbulence is due to dis-refers to randomlyturbances, arising naturally, that excite the flow at its natural frequency.Thisfluctuatingfluidsamephenomenon occurs when an unbalanced tire begins to vibrate erratically atmotionaroundaa speed associated with the wheel's naturalfrequency.Themechanical constraintsmeanflowrestrict the wheel from ripping off the vehicle. Similarly,fluid friction keeps theerratic fluid's turbulent motion within limits.Accordingly,buoyancy and turbu-buoyancylence aretwofactors that control fire and its associated flows.It is interesting toan effectiveforce oncontemplatethebehaviorof diffusionflames in a space ship in which gravity andfluidduetodensityorthereforebuoyancyisnegligible.temperatureThe general shapes of diffusion flames are illustrated in Figure 2-6 on pagedifferences ina28.Naturalfires involving liquid or solid fuels have very low velocities at thefuelgravitational fieldINOTEBuoyancy andturbulence are twofactors thatcontrol fireand its associated flow.Figure2-3Acandle,anexampleofadiffusion flame
pyrolysis a process of breaking up a substance into other molecules as a result of heating; also known as thermal decomposition laminar refers to orderly, unfluctuating fluid motion turbulent refers to randomly fluctuating fluid motion around a mean flow buoyancy an effective force on fluid due to density or temperature differences in a gravitational field ■ NOTE Buoyancy and turbulence are two factors that control fire and its associated flow. Figure 2-3 A candle, an example of a diffusion flame. Chapter 2 combustion in Natural Fires meets oxygen. For the wooden match the wood is decomposed by the heat of the flame into gaseous fuel and char. This decomposition process is called pyrolysis. A candle flame is an example of a laminar diffusion flame governed by pure molecular diffusion. Any flame higher than approximately 1 ft will naturally possess random fluid mechanical unsteadiness, illustrated by visible eddies in the smoke and flame. This is called turbulence, and so we have turbulent diffusion flames (Figure 2-4). Smoke from a large chimney has this turbulent character, and a smoke filament from a cigarette in a still room begins as a laminar flow then clearly breaks up after rising about 1 ft to become turbulent (Figure 2-5). Gravity influences the shape of diffusion flames and profoundly affects fire processes in general. Because fire creates high temperatures, the hotter (lighter) gases rise as a result of buoyancy caused by gravity. The ensuing flow distorts the flame and eventually flow instabilities cause turbulence. Turbulence is due to disturbances, arising naturally, that excite the flow at its natural frequency. This same phenomenon occurs when an unbalanced tire begins to vibrate erratically at a speed associated with the wheel's natural frequency. The mechanical constraints restrict the wheel from ripping off the vehicle. Similarly, fluid friction keeps the erratic fluid's turbulent motion within limits. Accordingly, buoyancy and turbulence are two factors that control fire and its associated flows. It is interesting to contemplate the behavior of diffusion flames in a space ship in which gravity and therefore buoyancy is negligible. The general shapes of diffusion flames are illustrated in Figure 2-6 on page 28. Natural fires involving liquid or solid fuels have very low velocities at the fuel
Chapter2CombustioninNaturalFiresFigure 2-4Turbulent diffusionflame.base (-1 cm/s),buta highpressuregaseousfuel source can initiatefuelatmuchhigher velocities.For such flames,buoyancy effects can benegligible,and at a highjet flameenough velocity,the jetflamereaches afixed height.Fires with characteristics asflamedueto a highshown inFigure2-6b,more representative ofnormalburning commoditiesvelocityfuelsupplyachieveheights associated with their fuel supply and air supply drawn in by thebuoyancy of the fire.For fires over large areas,such as forestfires or city confla-grations,theairflowcanbedrawn down from aboveaswell as in fromthe sides.Figure 2-6b will bethe focusof our attentionand represents diffusion flamesassociatedwithbuildingfires
Chapter 2 combustion in Natural Fires Figure 2-4 Turbulent diffusion flame. jet flame flame due to a high velocity fuel supply base (-1 cm/s), but a high pressure gaseous fuel source can initiate fuel at much higher velocities. For such flames, buoyancy effects can be negligible, and at a high enough velocity, the jet flame reaches a fixed height. Fires with characteristics as shown in Figure 2-6b, more representative of normal burning commodities, achieve heights associated with their fuel supply and air supply drawn in by the buoyancy of the fire. For fires over large areas, such as forest fires or city conflagrations, the air flow can be drawn down from above as well as in from the sides. Figure 2-6b will be the focus of our attention and represents diffusion flames associated with building fires
Chapter2 Combustion in Natural FiresFigure2-5Cigarettesmoke.Figure2-6Diffusionflameshapes:a.jetflame,b.liquidspill(a)(b)(c)fire,c.forestfire
_ ._ _ ch_a_Pt_er_2_c_o_m_bu_s_tio_n_1_n _Na_t_ur_a1_F_ir_es Figure 2-5 Cigarette smoke. Figure 2-6 Diffusion flame shapes: a. jet flame, b. liquid spill fire, c. forest fire . <a> (b) (C) 1
Chapter2Combustion in Natural FiresThe Scientific Discoveries of Michael Faraday1820discoveredtwounknownchloridesofcarbon1821discoveredelectromagneticrotation—setupanexperiment in which a wire carrying an electriccurrentrotated in the field ofa horseshoe magnet1823liquifiedchlorine1825isolatedbenzene1831demonstrated theprincipleof electromagneticinduction-the production of electric current byFigure 2-7 Michaela change in magneticintensityFaraday and his1844discoveredtherotationoftheplaneof polarizascientific discover-tion of light in a magnetic field.ies,After Ref.1.Candle FlameThe candle flame provides most of the essentialfeatures of natural fires and a dif-fusion flame. It will be our learning tool.We are not original in this approachbecause we shall repeat some of the experiments created by thenineteenth cen-tury scientist,Michael Faraday (Figure 2-7).These experiments were presented atthe Royal Institution in London as a science show for the public.Known as theChristmas Lectures,they were very popular with children and havebeen archivedinabookthatis still availabletoday.1Someof theselecturesare remarkable intheirsimplicity,yetpowerfulintheirillustrationofthebasicprinciplesofflamesIt isastonishing to those who study fire to seehowFaradayappreciated theworkings of a candle and its implication to the science offire.These thoughts areexpressed in his opening remarks at the convening of the Christmas Lectures:I propose to bring before you, in the course of these lectures, the ChemicalHistoryofaCandle.Thereisnotalawunderwhichanypartofthisuniverseis governed which does not come into play and is touched upon in these phe-nomena.There is no better, there is no more open door by which you canenter into the study ofnatural philosophy than by considering the physicaphenomena ofacandle.Light a candle and observe the processes that create the sustained flame.Asillustrated in the sketch in Figure 2-8,you can see several things. The yellow andblue zones constitutetheflame (theformer,diffusion;the latter,premixed).Thewick is actuallydesigned to curve so that theflame"clips"off the wick,and lim-its its height. Why isn't the wick destroyed throughout the flame? What is the pur-pose of the wick? With some thought, it can be deduced that the heat of the flamemelts thewax,themeltsoaks thewick (by capillaryaction,likewaterfeedingtreeleaves),evaporates,and supplies the fuel gasto diffuse into the luminous zonewhere itfinds oxygenhaving diffused from theotherside.Buoyantflow elongates
Chapter 2 combustion in Natural Fires Figure 2·7 Michael Faraday and his scientific discoveries. After Ref 1. candle Flame The Scientific Discoveries of Michael Faraday 1820 discovered two unknown chlorides of carbon 1821 discovered electromagnetic rotation-set up an experiment in which a wire carrying an electric current rotated in the field of a horseshoe magnet 1823 liquified chlorine 1825 isolated benzene 1831 demonstrated the principle of electromagnetic induction-the production of electric current by a change in magnetic intensity 1844 discovered the rotation of the plane of polarization of light in a magnetic field. The candle flame provides most of the essential features of natural fires and a diffusion flame. It will be our learning tool. We are not original in this approach because we shall repeat some of the experiments created by the nineteenth century scientist, Michael Faraday (Figure 2-7). These experiments were presented at the Royal Institution in London as a science show for the public. Known as the Christmas Lectures, they were very popular with children and have been archived in a book that is still available today. I Some of these lectures are remarkable in their simplicity, yet powerful in their illustration of the basic principles of flames. It is astonishing to those who study fire to see how Faraday appreciated the workings of a candle and its implication to the science of fire. These thoughts are expressed in his opening remarks at the convening of the Christmas Lectures: I propose to bring before you, in the course of these lectures, the Chemical History of a Candle. There is not a law under which any part of this universe is governed which does not come into play and is touched upon in these phenomena. There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle. I Light a candle and observe the processes that create the sustained flame. As illustrated in the sketch in Figure 2-8, you can see several things. The yellow and blue zones constitute the flame (the former, diffusion; the latter, premixed). The wick is actually designed to curve so that the flame "clips" off the wick, and limits its height. Why isn't the wick destroyed throughout the flame? What is the purpose of the wick? With some thought, it can be deduced that the heat of the flame melts the wax, the melt soaks the wick (by capillary action, like water feeding tree leaves), evaporates, and supplies the fuel gas to diffuse into the luminous zone where it finds oxygen having diffused from the other side. Buoyant flow elongates