《火灾科学与城市安全》课程教学资源（参考教材）James G. Quintiere - Principles of Fire Behavior（1997）

Chapter2CombustioninNatural FiresLUMINOUSYELLOWZONEDARKZONE"STAND-OFF"SPACEGLOWING,CLIPPEDWICKMELTEDWAXBLUETIPFigure 2-8Characteristics of acandle flame.thecandleflame,buttheflow path is small enough so that laminarflow ismain-tained.However, as the hot combustion gases rise from the flame, they becometurbulent.This phenomenon can be seen by projecting a collimated light (from aslideprojector or strong flashlight)at the flame and onto a whitescreen.You willseeashadowimage of theflowonthescreen asaresultof therefractionof lightrays caused by the hot and cold flows.Note that thefilament-like plume fromtheflamebecomesturbulent atabout1ftabove theflame.If you observe moreclose-INOTEly,you will see the shadow of the wick and the shadow of a part of the flameregion. What might cause this flame shadow? (Some solid ingredients blockingCombustion reactionsthelightintheflame.)can only be sustained ifNow place a metal screen (typical window screen mesh,but not aluminum)the flame temperatureinto the flame as shown in Figure 2-9.The metal screen cuts the flame, extin-is high enough,guishing further combustion.Why? Describe the luminous flame cut.typically greater thanCombustion reactions can onlybe sustained if theflametemperatureishigh1300°℃.enough,typicallygreater than 1,300°C.As the height of the screen is varied in theBLACKORWHITE"SMOKE'Figure2-9 Metalscreen cuttingflame

Figure 2-8 Characteristics of a candle flame. ■ NOTE combustion reactions can only be sustained if the flame temperature is high enough, typically greater than 1500°c. Figure 2-9 Metal screen cutting flame. Chapter 2 combustion in Natural Fires LUMINOUS YELLOW ZONE GLOWING, CLIPPED WICK the candle flame, but the flow path is small enough so that laminar flow is main￾tained. However, as the hot combustion gases rise from the flame, they become turbulent. This phenomenon can be seen by projecting a collimated light (from a slide projector or strong flashlight) at the flame and onto a white screen. You will see a shadow image of the flow on the screen as a result of the refraction of light rays caused by the hot and cold flows. Note that the filament-like plume from the flame becomes turbulent at about 1 ft above the flame. If you observe more close￾ly, you will see the shadow of the wick and the shadow of a part of the flame region. What might cause this flame shadow? (Some solid ingredients blocking the light in the flame.) Now place a metal screen (typical window screen mesh, but not aluminum) into the flame as shown in Figure 2-9. The metal screen cuts the flame, extin￾guishing further combustion. Why? Describe the luminous flame cut. Combustion reactions can only be sustained if the flame temperature is high enough, typically greater than 1,300°C. As the height of the screen is varied in the ) BLACK OR WHITE "SMOKE

Chapter 2CombustioninNaturalFiresDROPPERFigure 2-10Extractinginteriorflamegases.flame,black smoke is released with the screen high inthe flameand white orgrey"smoke"with the screen near thetop ofthe wick.What dothese colored smokesimply? Collect the black smoke on a piece of white paper,being careful not toinsert the paper in the flame. What is collected on the paper? Where do these blackparticles originate?Find that region in Figure 2-8.Blow out the flame and observethecolorandcharacterof thesmokethatmomentarilyemanatesfromthewickHowdoesthis comparetothewhitesultingfromthelowscreenposition?Relightthecandle.Aftertheflais,insertaglassdropperintothesteadiesregion where you think the white smokeiginated.Extractthewhitesmokeintothe dropper tube. Squirt this whitethetopoftheflame(Figure2-10).Itshouldigniteandresembleasmallflamwer.What were these whitevapors?etniTo confirm your conclusion, blow out the candle flame again, and quickly trytoignite the white vapors coming from the wick.You should see a flame propagatedown this vapor trail and anchor on the wick.This propagating flame is a pre-mixed flame; the anchored flame is, of course, our diffusion flame.Review all of the information deduced about the candle flame and youshould have a good understanding of the diffusion flame and fire.You may wishto consult Faraday's textfor more informative experiments with the candle.Anatomyofa Diffusion FlameMore sophisticated experiments with a laminar diffusion flame using methane(CH)as thefuel havebeen conductedbyKermitSmythandcoworkers.2.3Theirflameis formedfrom controlled flows of CH and air,as shown in Figure 2-11.Theburneris a slotburner, so itis planar incharactercompared tothe cylindrical char-acter of the candle,but the flame dynamics are the same.Various point measure-menttechniques were used intheflameto quantitativelyreveal its anatomy.Wecan relate these measurements to what we have deduced from the candle experi-ments.Now ourqualitativeobservations canbequantitativelypresented.Thesequantitative results will be displayed for a height 9mm above the burner

_ch_a_Pt_er_2_c_o_m_bu_s_tio_n_i_n _Na_t_ur_a1_F_ir_es _ ~- Figure2-10 Extracting interior flame gases. DROPPER flame, black smoke is released with the screen high in the flame and white or grey "smoke" with the screen near the top of the wick. What do these colored smokes imply? Collect the black smoke on a piece of white paper, being careful not to insert the paper in the flame. What is collected on the paper? Where do these black particles originate? Find that region in Figure 2-8. Blow out the flame and observe the color and character of the smoke that momentarily emanates from the wick. How does this compare to the white smoke resulting from the low screen position? Relight the candle. After the flame steadies, insert a glass dropper into the region where you think the white smoke originated. Extract the white smoke into the dropper tube. Squirt this white gas across the top of the flame (Figure 2-10). It should ignite and resemble a small flame thrower. What were these white vapors? To confirm your conclusion, blow out the candle flame again, and quickly try to ignite the white vapors coming from the wick. You should see a flame propagate down this vapor trail and anchor on the wick. This propagating flame is a pre￾mixed flame; the anchored flame is, of course, our diffusion flame . Review all of the information deduced about the candle flame and you should have a good understanding of the diffusion flame and fire. You may wish to consult Faraday's text for more informative experiments with the candle. 1 Anatomy of a Diffusion Flame More sophisticated experiments with a laminar diffusion flame using methane (CH4 ) as the fuel have been conducted by Kermit Smyth and coworkers. 2 •3 Their flame is formed from controlled flows of CH4 and air, as shown in Figure 2-11. The burner is a slot burner, so it is planar in character compared to the cylindrical char￾acter of the candle, but the flame dynamics are the same. Various point measure￾ment techniques were used in the flame to quantitatively reveal its anatomy. We can relate these measurements to what we have deduced from the candle experi￾ments. Now our qualitative observations can be quantitatively presented. These quantitative results will be displayed for a height 9 mm above the burner

Chapter2Combustion in Natural FiresSTABILIZINGSCREENSMONOCHROMATORLASERLIL2FLAMEZONESFigure2-11Arrange-ment of slotburnerflame and pointmeasurementtech-niques.From Smyth4et al., Ref. 2.Thermocouple measurements of temperature across theflame region arethermocoupleshown in Figure2-12 atthe9-mm height,Otherheights within the luminousdevicemadeoftwoflame region will be similar.These 9-mdissimilarmetalwiresurementsareindicativeofthetwotomeasuresides of the diffusion flame.It is difficult to accurately measure the highest tem-temperatureperatures indicativeofthereactionzonebecausethethermocoupleprobewill losesome heat and slightlycool theflame.Therefore,the high temperatures ofapprox-imately1,700°Carelikelytobemorelike1,900°to2,000°C.Nevertheless,thisveryhigh temperature region is indicative of the chemical reaction zone of the flame.Thiswas theluminousyellowshell of the candleflame.The interiortemperaturesare due to heat conducted inward.The outer-wing temperature should eventual-ly become the temperature of the pure air.Thecorrespondingvertical flow speed is shown inFigure2-13.Theseflowspeeds are principally due to buoyancy.radicalsUsing special laser-optical techniques Smyth and coworkers were able toshort-livedunstableimage the visibleflame,OH radicals,and soot.The OH radicals are indicative ofmolecules such as OHshort-lived chemical species in the heart of the combustion reaction.They extendto the air side of the flame.The soot, or black smoke of the candle experiments,sootcomesfrom withinthe (luminous)flame zone.This is clearlyseen intheirimagescarbonaceousparticlesillustrated inFigure2-14.producedinflamesThe soot is formed on the fuel side of the diffusion flame. It results from acomplex process as the original fuel (CH)is heated on diffusing toward the flamecrackingThisintenseheatingresultsincrackingtheCH,moleculesintomanyotherhydropyrolysis; breakingcarbonmolecules.As acetylene (C,H2)and other precursorsform, soot (mainlyCgaseousmoleculesintoothermoleculesatoms) is produced.The soot (a solid formedfrom gases)migrates throughthe reac

_ c_h_ap_t_er_2_c_om_b_u_st_io_n_in_N_a_tu_ra_1_Fi_re_s Figure 2-11 Arrange￾ment of slot burner flame and point measurement tech￾niques. From Smyth et al., Ref 2. thermocouple device made of two dissimilar metal wires to measure temperature radicals short-lived unstable molecules such as OH soot carbonaceous particles produced in flames cracking pyrolysis; breaking gaseous molecules into other molecules MONOCHROMATOR FLAME ZONES Air l1 STABILIZING SCREENS Thermocouple measurements of temperature across the flame region are shown in Figure 2-12 at the 9-mm height. Other heights within the luminous flame region will be similar. These 9-mm measurements are indicative of the two sides of the diffusion flame. It is difficult to accurately measure the highest tem￾peratures indicative of the reaction zone because the thermocouple probe will lose some heat and slightly cool the flame. Therefore, the high temperatures of approx￾imately 1,700°C are likely to be more like 1,900° to 2,000°C. Nevertheless, this very high temperature region is indicative of the chemical reaction zone of the flame. This was the luminous yellow shell of the candle flame. The interior temperatures are due to heat conducted inward. The outer-wing temperature should eventual￾ly become the temperature of the pure air. The corresponding vertical flow speed is shown in Figure 2-13. These flow speeds are principally due to buoyancy. Using special laser-optical techniques Smyth and coworkers were able to image the visible flame, OH radicals, and soot. The OH radicals are indicative of short-lived chemical species in the heart of the combustion reaction. They extend to the air side of the flame. The soot, or black smoke of the candle experiments, comes from within the (luminous) flame zone. This is clearly seen in their images illustrated in Figure 2-14. The soot is formed on the fuel side of the diffusion flame. It results from a complex process as the original fuel (CH4) is heated on diffusing toward the flame. This intense heating results in cracking the CH4 molecules into many other hydro￾carbon molecules. As acetylene (C2H2) and other precursors form, soot (mainly C atoms) is produced. The soot (a solid formed from gases) migrates through the reac-

Chapter2Combustion in Natural Fires2000172716001327?812009278005279mm400127Figure2-12Temper-aturesacrossalaminardiffusion-10-50105flame.FromSmythet al.,Ref.2.LATERALPOSITION,mm80409mmFigure2-13 Verticalvelocityin a lami-0nar diffusion flame.10-50510From Smythet al.,Ref.2.LATERALPOSITION,mm

Chapter 2 combustion in Natural Fires Figure 2-12 Temper￾atures across a laminar diffusion flam e. From Smyth et al., Ref 2. ~ 0 u1 c.:: ::::) ~ c.:: w 0. 2 w I- <I) - E u ~ u 0 _J w > 1600 1200 800 400 -10 80 5 40 ~ w > • 9mm -5 0 5 LATERAL POSITION, mm • 9mm Figure 2·13 Vertical velocity in a lami￾nar diffusion flame . From Smyth et al., Ref 2. O~-~~ -10 ~ 0 5 10 LATERAL POSITION, mm 1327 \-;-> w· 927 c.:: ::::) ~ c.:: w 0. 2 w I- 527 127 10

Chapter2CombustioninNatural Firestion (flame) zone, is oxidized (mainly by OH),and is consumed.We see this oxi-dationby the yellow (incandescent)glow ofthe visibleflame. In thisflame,as wellas the candleflame,nearly all the soot is consumed before the reaction zone ends.Thevisibleflame height nearly coincides with the end of the soot height (Figure2-14) and the OH zone is slightly above this height. If we "tear" the flame by"stretching"it (e.g.,abruptlypull thecandleflame-becareful nottospill meltedwax), soot can escape through the tear.Again, this illustrates that the soot forma-tion is on thefuel side of theflame.For a heavily sootingfuel, all the soot may notbeoxidizedbeforetheendofthereactionzone;thensootescapesasblacksmokeLetus examinethe othercompoundsthatoccur in thisflameprocess.Someof these are shown inFigure2-15:The measurements were taken by Smyth et al.2at the same9mm above thefuel port.The concentrations aregiven inmolefrac-tion, which is the same as the fraction byvolume,For example,extract the oxygen at some point, let it come into balance with the air pressure; the ratio of itsequilibrated volumeto thevolume of the mixtureofgas is the same as themolefraction, Note the oxygen (O2) in pure air is at 0.21 with the nitrogen (N2) makingup the remainder,0.79.These values are measured at the extreme left and rightwings in Figure 2-15. The fuel (methane, CH,) is nearly 1 (all fuel) at the center.We see that the fuel and oxygen reach zero at approximately 6mm,the peak of thecombustion zone.It is probablyless than1mm in width at aheightof 9mm abovethefuel source.Thewatervapor(H,O)isalsoatmaximumhere.Alongwithcar-bon dioxide (CO2),it isa principal compound formed and released intothe atmos-phere as a productofcombustion.Figure 2-16 shows theresults forCO,along with128mm96mmVISIBLE FLAME HEIGHT(79mm)64mmSOOT32mmFigure 2-14 Soot andOHflame (reaction)zone locations indlaminardiffusion0mmflame.AfterSmyth-10to10et al., Ref.3

_._ _ ch_ap_t_e,_2_co_m_bu_st_io_n_in_Na_tu_r_a1_F_1re_s Figure 2-14 Soot and flame (reaction) zone locations in a laminar diffusion flame . After Smyth et al., Ref. 3. tion (flame) zone, is oxidized (mainly by OH), and is consumed. We see this oxi￾dation by the yellow (incandescent) glow of the visible flame. In this flame, as well as the candle flame, nearly all the soot is consumed before the reaction zone ends. The visible flame height nearly coincides with the end of the soot height (Figure 2-14) and the OH zone is slightly above this height. If we "tear" the flame by "stretching" it (e.g., abruptly pull the candle flame-be careful not to spill melted wax), soot can escape through the tear. Again, this illustrates that the soot forma￾tion is on the fuel side of the flame. For a heavily sooting fuel, all the soot may not be oxidized before the end of the reaction zone; then soot escapes as black smoke. Let us examine the other compounds that occur in this flame process. Some ofthese are shown in Figure 2-15: The measurements were taken by Smyth et al. 2 at the same 9 mm above the fuel port. The concentrations are given in mole frac￾tion, which is the same as the fraction by volume. For example, extract the oxy￾gen at some point, let it come into balance with the air pressure; the ratio of its equilibrated volume to the volume of the mixture of gas is the same as the mole fraction. Note the oxygen (02) in pure air is at 0.21 with the nitrogen (N2) making up the remainder, 0.79. These values are measured at the extreme left and right wings in Figure 2-15. The fuel (methane, CH4) is nearly 1 (all fuel) at the center. We see that the fuel and oxygen reach zero at approximately 6 mm, the peak of the combustion zone. It is probably less than 1 mm in width at a height of 9 mm above the fuel source. The water vapor (H20) is also at maximum here. Along with car￾bon dioxide (CO2), it is a principal compound formed and released into the atmos￾phere as a product of combustion. Figure 2-16 shows the results for CO2 along with 128 mm -10 0 10 VISIBLE FLAME HEIGHT (79mm)