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Experimental observation of radiative ignition mechanisms
COMBUSTION A N D F L A M E 3 4 : 2 3 1 - 2 4 4 (1979)
231
Experimental Observation of Radiative Ignition Mechanisms TAKASHI KASHIWAGI National Bureau of Standards, Washington, D. C. 20234
Radiative ignition experiments were conducted on PMMA and red oak using a CO2 laser with incident flux up to about 20 W/cm2 under autoignition and piloted ignition in air. The laser irradiated perpendicular to the horizontal sample surface. It was observed that there was strong attenuation of the incident laser radiation by the plume consisting of decomposition products in the gas phase. This was also observed using an electric coil heater as a radiant source. It is postulated that, under autoignition, PMMA ignites by the absorption of the incident radiation by the decomposition products in the gas phase, and red oak by a similar absorption at high-incident flux and at medium flux, aided by high surface temperature acting as an induced pilot.
INTRODUCTION The ultimate goal o f fire research is to provide the scientific and engineering knowledge that can be applied to reduce fire hazards. Since ignition is the initiation of the fire, it is very useful to know how combustible solids ignite and, using the knowledge, to reduce the chance of ignition. The most common fire starts in a room of a residential dwelling and may spread to other rooms and buildings. Since it is known that thermal radiation is a primary mode o f energy transfer in a room fire [1], the ignition o f combustible solids under a well-defined external radiation is being studied. In a room fire, thermal radiation from flames, hot gases, and particulates under the ceiling and also from the ceiling itself may heat and ignite combustible materials on the floor such as furniture and floor coverings. Since these materials may be exposed to thermal radiation over a wide range of angles incident to their surfaces, it is useful to study the mechanism o f radiative ignition under a range of configurations. Although the external radiation imposed horizontally on a vertical sample surface has been extensively studied [2-6], other cases with different incident angles have not been systematically studied. Basic differences in Copyright © 1979 by The National Bureau of Standards Published by Elsevier North Holland, Inc.
the orientation of the sample surface with respect to incident radiation would cause differences in the heat transfer of surfaces and in the flow pattern of decomposition products and their mixing with entrained air. At present, the effect of the orientation on ignition is not well known. As a first step, this article reports the observations obtained using a configuration in which vertical radiation is imposed downward on a horizontally oriented sample surface. In the radiative ignition of combustible materials the important process is the absorption of sufficient external radiation to heat the material near the surface above its decomposition temperature and to release combustible gases and particles. The amount of energy absorbed by the solid material and by the decomposition products is determined by the emission spectrum of the external radiation [6] and the surface reflectance and absorption coefficient of the materials [6-8]. Generally, the emission spectra of flames, particulates, and hot surfaces are broad [6, 9, 10], and the absorption spectra of the materials and decomposition products consist of many absorption bands. A change in the external radiant flux produced by a change in the temperature of the emitter shifts the emission spectrum. This changes the amount of energy
232 actually absorbed by the materials and also, in diathermic materials, the depth over which this energy is absorbed. Thus changes in the external radiant flux due to source temperature changes make it very difficult to measure precisely how much energy is absorbed into the material and to understand the already complicated ignition mechanism. To simplify this complexity, a continuous-wave CO2 laser is used in this study. Its essentially monochromatic emission avoids the above-mentioned problem with the use of conventional radiant heat sources. A laser source makes theoretical modeling easier and comparison of theoretical predictions with experimental data more precise, thus helping to clarify the complicated ignition mechanism. Once the ignition mechanism is understood using the CO2 laser source, the model can be extended to more common radiant heat sources representing fire by including the appropriate wavelength dependency of radiative properties. There are two modes of radiative ignition: one is autoignition and the other is piloted ignition. The former mode applies to ignition resulting solely from external heating and without any addition of a high-temperature source on or near the material surface. The latter applies to ignition under external heating with the addition of a hightemperature "pilot" such as a hot wire, hot particles, or a small flame. Also, the latter mode applies to flame spread under external radiation viewed as successive piloted ignitions [7]. It is important to understand these two modes of ignition in order to develop predictive models. In autoignition, two necessary conditions must be satisfied simultaneously for ignition to occur: (1) sufficient amounts of fuel and oxygen must be available in the gas phase, and (2) the gas-phase temperature must be high enough to initiate and accelerate the gasphase chemical reaction. On the other hand, only the first condition must be satisfied for piloted ignition, provided that the pilot temperature is high enough to initiate the chemical reaction. Therefore, the difference between the two ignition modes would appear to be the heating process of the gas phase. There are two questions to be addressed in this heating process: (1) how the gas phase is heated
TAKASHI KASHIWAGI and (2) how rapidly the gas phase can be heated. For the first question, there are two ways to heat the gas phase during the ignition period in the radiative ignition mode: (1) convection-conduction from the heated surface, and (2) absorption of the incident radiation. In general, the first process has been considered as the main mechanism to heat the gas phase [11-13]. However, there have been no direct measurements to exclude the possibility of the second mechanism, especially in the case of vertical radiation onto a horizontally mounted sample surface. For the second question, the effectiveness of the heating process of the gas phase can be obtained by comparing the ignition delay times for autoignition and piloted ignition. There are some published data available for which these comparisons were made using the same sample size and the same external radiant source, for vertical materials irradiated horizontally [14]. Comparable data for horizontally mounted materials irradiated vertically or at other angles have not been available. The present study has been designed to answer these questions.
EXPERIMENTAL APPARATUS A schematic illustration of the experimental apparatus is provided in Fig. 1. The COz laser (Coherent Radiation Model 41) 1 emits an approximately 3-mm-diameter beam whose power can be varied from 240 W to 340 W by adjusting the current through the laser-discharge tube. This range of the power is selected to maintain the fundamental mode (TEMoo), for which the power distribution across the beam is Gaussian. A diffuser lens with a focal length of 6.2 cm is used to expand the incident laser beam to obtain a more uniform flux distribution. The incident flux distribution at the sample surface position is measured prior to each ignition experiment by traversing a Gardon-type 1 Certain commercial equipment, instruments, or materials are identified in this article in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the NationM Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
% CO2 LASER
SWITCHABLE ROTATING MIRROR SHUTTER
\
233
MIRROR
ETECTOR - DIFFUSER LENS
CHAMBER
L SAMPLE
Fig. 1. Schematicillustration of the experimental apparatus.
calorimeter with a 2.5-mm sensing diameter. The flux distribution is typically uniform within 10% over a circle of approximately 2-3 cm diameter. The average radiant flux for each experiment is very closely approximated by a value equal to 95% of the maximum flux. A shutter consisting of a switchable mirror is used to provide a step-function irradiance and remains open through the ignition event. The sample is mounted horizontally, and the incident laser beam irradiates perpendicularly to the sample surface. Ignition is determined by the first light emission, which is measured by an EMI 7656 photomultiplier, the output of which is displayed on a visicorder. At the onset of flaming, a step-function-like output of the photomultiplier is observed. This output is used as unambiguous determination of the ignition event. For piloted ignition tests a 250-tam-diameter by 7.5-cm-length platinum wire is stretched 1.5 cm above and parallel to the sample surface. This wire is very rapidly heated, electrically ('~ 20,000°C/ sec), to a temperature previously set, which is kept constant during the ignition period. The wire temperature can be varied from the room temper-
ature to about 950°C; it was kept at 950°C in this study. The energy input to the wire is at most 0.1 W to maintain this temperature. The relationship between the resistance of the wire and wire temperature was calibrated by using welldefined melting temperatures of 10 materials ranging from 170°C to 900°C. The resistance of the wire is continuously recorded during the ignition period for each experiment to confirm the setting of the wire. The lower part of the experimental chamber is 22.5 cm in diameter and 50 cm in height. The upper part of the chamber is 30 cm in diameter and 33 cm in height. The distance between the top of the chamber and the sample surface is 70 cm. Although the pressure in the chamber can be varied from vacuum to one atmosphere and the gas composition from an inert gas to 100% oxygen, air at atmospheric pressure was used in the tests reported here. Two different materials were used in this study, polymethylmethacrylate (PMMA) and red oak. The PMMA was cut into 7.5-cm squares, 1.2 cm thick. Red oak samples were dried at 60°C for 24 hr and left in a conditioned room (50% humidity at 20°C) to reach an equilibrium mois-
234
TAKASHI KASHIWAGI
(a)
(b) Fig. 2. Decomposedpattern of PMMAsurface after laser irradition.
ture content (determined by weighing) prior to the experiment. The dimensions of the oak samples were 7.5 cm square by 1.7 cm thickness. EXPERIMENTAL RESULTS AND DISCUSSION After heating the material above its decomposition temperature by external radiation, decomposition products rise from the surface by buoyancy. For vertical materials irradiated horizontally, the flow pattern of boundary layer forms along the sample surface. The thickness of this layer remains relatively thin with time and the attenuation of the incident radiation crossing the layer should usually be small. However, for horizontal materials irradiated vertically, the decomposition products form a rising plume and the height of the plume grows with time. The incident radiation passes through the growing plume, and its intensity may be significantly attenuated by the plume. Some experimental evidence to support the importance of this attenuation is described below. After a test it was sometimes found that the ablated surface of PMMA showed a distinct horseshoe shape such as that shown in Fig. 2c. This frequently occurred when PMMA was not ignited with autoignition. In Fig. 2c the center part decomposed little and the surrounding white zone
(e)
showed a considerable amount of decomposition. However, this horseshoe shape disappeared by blowing room-temperature air (speed ~ 30 cm/sec) toward the sample and parallel to its surface during the laser radiation. A picture of the sample tested in this way is shown in Fig. 2a. To eliminate the possibility of the enhancement of surface oxidation by blowing air, room-temperature argon was blown over the sample under the same experimental condition. The decomposition pattern was the same as the air-blown case (Fig. 2b). The effect of air or argon blowing on the sample is to blow the decomposed gas plume away from the incident laser beam and to reduce the interaction between the plume and the beam. This suggests that the horseshoe decomposition pattern is caused by this interaction. To find the mechanism of the interaction between the incident beam and the plume containing decomposition products, the attenuation of the incident radiation was measured during the ignition period for PMMA and red oak under various radiant fluxes. The measurement was made using a water-cooled flux meter with a 2.5-mm-diameter sensing area, mounted facing upward beneath the sample. The sample had a 4-mm-diameter hole through its center, through which the flux meter could measure the radiation reaching the sample
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
235
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Fig. 3. Normalized transmission of the incident flux as a function of time under various initial incident radiant fluxes for PMMA autoignition mode.
surface through the decomposed gas column. Since the thickness of PMMA sample was 1.2 cm and red oak 1.7 cm and the experimental time was at most 30 sec, the heat conduction through either sample was negligible. With this configuration, the flux meter initially measured the incident radiant flux and then the flux transmitted through the plume after the decomposition started. The results for PMMA are shown in Fig. 3 and for red oak, in Fig. 4. The ordinate is the transmitted radiant flux, I, normalized by the initial incident flux, Io. Therefore, the ratio I[I o stays at one until the decomposition starts. The fluctuation of the curves could
be caused by the wandering of the plume. The curves are roughly exponential with rates that agree roughly with the rates of growth in the height of the plume. The sudden increases in the normalized flux for red oak at Io = 16.3 and 13.4 W/cm z were caused by ignition. This could be caused by the changes in the gas composition and particulate size distribution before and after ignition. However, no sudden increase in the transmission was observed for PMMA at ignition at Io = 17.4 W/cm z. Therefore, the cause of the transmission change after ignition is not clear at present. The sudden drop in the transmission after the sud-
236
TAKASHI KASHIWAGI 1 . 0 ~
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IGNITION
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Fig. 4. Normalized transmission of the incident radiant flux as a function of time under various initial incident radiant fluxes for red oak autoignition mode.
den increase for red oak resulted when the incident laser beam was turned off. Figures 3 and 4 both show, at high radiant flux, that the attenuation of the incident radiation is very strong and that 70-80% of the incident radiant flux is attenuated by the decomposition products within 5 sec for PMMA and red oak. The amount of the attenuation decreases with a decrease in the incident radiant flux for both samples, presumably because of lower sample temperature and reduced rate of generation of the decomposition products. However, even at low incident flux, the attenuation is still at least 20% and is certainly not negligible, especiaUy for PMMA.
From the observations just described, the importance of the attenuation of the incident radiation by the decomposition products is evident, for the case of vertically irradiated ignition of the horizontally placed sample. However, the attenuation characteristics depend strongly on the spectrum of the incident radiation. The wavelength of the incident radiation in this study is 10.6/am, from a CO2 laser. This wavelength is much longer than those typical of emission by fires. Therefore, it is important to investigate whether the observed importance of the attenuation of the incident radiation by the decompostion products is due to the use of the CO2 laser or whether it is also applica-
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
237
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Fig. 5. Normalized transmission of the incident radiant flux as a function of time at 4 W/cm2 from electric coil heater. ble for a more conventional radiant source in a fire. To clarify this, the attenuation of the incident radiation by the decomposed products from both PMMA and red oak was measured using an electricaUy heated coil similar to that proposed as International Standard Organization ignition test method [15]. This heater approximates a gray-body radiator. It has a conical shape with dimensions of: 20 cm bottom diameter, 7 cm top diameter, and 7.5 cm height. There was a 9-turn electric heating coil over the inside wall facing downward. The top was open for the exhaust of the decomposition products. A sample 8 cm in diameter and 1.2 cm in thickness was located in the center 5 cm below the bottom of the conical heater. The incident flux on the sample surface was about 4 W/cm 2 with 700°C coil temperature. A sensitive watercooled flux meter with a 9-mm sensing diameter was located facing upward beneath a thin-layer,
water-cooled shield below the sample to avoid any heat conduction and transmitted radiation through the sample. The sensing element collected radiation through 9-mm-diameter holes of the cooled shield and the sample. The timewise change of the transmitted flux normalized by the initial incident flux is shown in Fig. 5 for PMMA and red 'oak. Shortly after exposure to the radiation, the surface of red oak charred and it started to decompose. The attenuation of the incident radiation became significant shortly after the exposure and it reached about 35% attenuation. However, no attenuation was observed until roughly 2 min had elapsed for PMMA and then the attenuation increased rapidly. It also reached about 35%. Since the absorption of the clear solid PMMA is less in the wavelength range below 2 jum, part of tlae incident radiation is reflected and part is absorbed in-depth in the PMMA. Also, the value of KpC,
238 where K is thermal conductivity, p density, and C specific heat, for PMMA is roughly twice that of red oak. Thus, under the same incident radiation, PMMA is heated more uniformly in-depth than red oak so that its surface temperature is lower and the onset of the decomposition is delayed. Therefore, PMMA takes a longer heating time to attenuate the incident radiation than red oak, which absorbs most of the incident radiation close to the surface. It is clearly shown in Fig. 5 that the attenuation of the incident radiation by the decomposition products is important with the electric heating coil, with approximately graybody emittance, as well as with the CO2 laser. Therefore, it is reasonable to conclude that attenuation of vertically incident radiation is important during the decomposition after the preheating period for horizontally placed combustible solids in fires. With the understanding of the interaction mechanism of the plume of the decomposition products with the incident radiation, the appearance of the horseshoe-ablation pattern described previously can be explained as follows. After the laser beam heats the sample, the decomposition products rise and form the plume. As shown in Fig. 6, the column center sometimes differs slightly from the center of the incident beam because of a slight deviation of the plume from the vertical (or a little nonsymmetry in the flux distribution). Since the decomposition products significantly attenuate the incident beam, the amount of the incident energy that reaches the surface through the column declines around the center of the column. This causes the nonuniform flux distribution at the surface, low around the column center, and causes the appearance of the horseshoe decomposition pattern. In several cases, a doughnut-shaped decomposition pattern was also observed after the experiment. This was formed when the center of the plume was located close to the center of the incident beam. When the heating time becomes very long without ignition, this horseshoe pattern gradually disappears and eventually the entire irradiation surface starts to decompose fairly uniformly. This is due to the heat conduction in the solid with the long heating time at the low-flux range and wandering of the plume that averages out the effect. This horseshoe or doughnut-shaped
TAKASHI KASHIWAGI
II
II
AREALASER IRRADIATES CROSS SECTIONOF PLUME
Fig. 6. Schematic illustration of the cross sections of laser beam and the plume.
decomposition pattern was not so clear for red oak as for PMMA, but still there existed the trend of the nonuniform decomposition pattern for red oak. It indicates that the red oak surface could char at the low flux, even though a part of the incident flux is attentuated in the gas phase. The attenuation of the incident radiation consists of both absorption and scattering by gas molecules and particulates. Since the absorption heats the gas phase thermally 2 and the scattering merely deflects the incident radiation, it is impor. tant to find which process causes the observed attenuation. Since the Rayleigh scattering cross section of molecules is much smaller than the incident wavelength, the scattering from molecules is negligible compared with other attenuation processes in this experiment. The decomposition products of both samples were collected in the plume 2 cm above the surface under laser irradiation without flaming. The particulate size distribution of these samples was measured using an electrical aerosol analyzer (Thermo-Systems, Inc., Model 3030). Preliminary results show that there are significant 2 Since the range of the incident flux is very low in this study, the multiphoton process [16, 17] found recently is negligible in this study.
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS" numbers of particles. The measured peak of the volume particle size distributions 8 of PMMA and red oak are at least 0.75/am, which is unfortunately above the instrument capacity. Further study is planned to measure the volume size distribution with a different instrument. Bankston [18] measured the volume size distribution of similar materials under the smoldering condition and found that their peaks were about 0.8 /am. Therefore, it is expected that the peak volume size distributions for PMMA and red oak would not be far from 0.75/am. Then, X = (2rra/X) ~ 0.2, where a is a radius of the particle at the peak volume size distribution and X is the laser wavelength of 10.6 /am. With X "~ 0.2, mainly Rayleigh scattering is expected and the ratio of scattering to absorption coefficient is proportional to X a [19] and is a function of the real and imaginary parts of refractive index. At this early stage of the program, values of the refractive index of particulates produced from the decomposition of PMMA and red oak during the ignition period have not been measured and the role of particulates in the attenuation of the incident radiation is not clear. Further study is necessary. To measure the absorption characteristics of the decomposition gases, evolved gases were drawn from 2 cm above the sample surface into an evacuated gas cell through a small sampling probe. Their transmission spectrum at 2.5-20/am was measured with a Perkin-Elmer Model 180 infrared photometer. The spectrum of the decomposition gases from PMMA is very close to the spectrum of the solid PMMA, which is expected from the unzipping decomposition characteristics. The spectrum shows a significant absorption around the incident wavelength of 10.6 /am. It is considered that the carbon-carbon double bond, which has numerous absorption bands at 10-11 /am, absorbs the incident laser beam. A similar observation was obtained with the decomposition gases from red oak. 3 The volume of particulates in the range of particle diameters log D and log D + d(log D) is d V . Then, d V = v(D)d(log D).
This equation defines the volume particle-size distribution v(D).
239
From the above discussion it is qualitatively found that some part of the attenuation of the incident radiation is due to absorption by the decomposition products. It is plausible that the absorption in the gas phase might cause autoignition instead of the convection-conduction from the heated surface that has been considered the key ignition mechanism by external radiation. To investigate whether the absorption of the external radiation is the key mechanism, both red oak and PMMA samples were heated by the electric heating coil at about 4 W/cm 2 and the CO s laser radiation was passed through the plume of decomposition products, approximately 3 cm above and paraUel to the sample surface. Ignition was observed with a radiant flux about 10 times larger than used in the preceding experiments, probably because of a lower concentration of decomposition products at lower incident flux. This indicates that the absorption of the incident radiation can ignite the decomposition products if the incident radiant flux is high and the concentration of absorbers is high. There are three possible mechanisms for ignition of the decomposition products by the absorption of the incident radiation: (1) absorption by decomposition gases to heat the gas phase and to initiate and accelerate the gas-phase chemical reactions, (2) absorption by particulates which heat the gas phase by convection/conduction to accelerate gas-phase chemical reactions, and (3) absorption by particulates that ignite by heterogeneous chemical reaction on their surfaces. Since the density of a gas is about 1000 times less than that of a solid and the value of the specific heat of the gas is about the same as that of the solid, the gas-phase heating requires far less absorbed energy for ignition than does ignition by particulates. Also, if particulates are formed by condensation of molecules, they have to go through the endothermic vaporization process. This favors the first mechanism. However, if the absorptivity of particulates is much higher than that of gases, or if particulates are formed directly from the decomposition of the sample and they decompose exothermally, particulates might be heated more efficiently than gases. To find the correct process, we need detailed information on the absorptivity of particulates at the wavelength of the incident radiation, the
240
TAKASHI KASHIWAGI
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Fig. 7. Relationship b e t w e e n ignition surface t e m p e r a t u r e and intial i n c i d e n t radiant flux for PMMA.
chemical nature and activity of particulates, the particulate formation mechanism, and the chemical composition of decomposition gases. At present, we do not have this information and it is not understood which process causes ignition for PMMA and red oak. Surface temperature was measured during the ignition period by using a 25-/am-diameter AlumelChromel thermocouple. The absorption of the incident radiation by the thermocouple wire was investigated by measuring the output of the thermocouple exposed to the incident radiation in air. No change in the output was observed with and without the incident radiation, and it is concluded that the thermocouple does not absorb the incident radiation in the range of the radiant flux used in this study. The thermocouple wire was electrically heated by a small voltage and melted the PMMA surface to make good contact. For red oak, the wire was pressed onto the surface by a ceramic pestle. Surface temperature at ignition was determined by simultaneous measurements of temperature and of emission from the onset of flaming,
using the photomultiplier. The observed surface temperatures are shown in Fig. 7 for PMMA and in Fig. 8 for red oak under various incident fluxes. There is some scatter in the data, but the trend is clearly shown in both figures. The ignition surface temperature of PMMA is fairly constant in the range 375°-410°C over a wide range of the incident flux. Since ignition surface temperature does not differ between the pilot-ignition mode and the autoignition mode, gas-phase temperature would not differ between the two ignition modes prior to ignition. The only difference is the existence of the 950°C pilot wire for the pilot-ignition mode. Therefore, the autoignition mode must raise the gas-phase temperature high enough to initiate and accelerate chemical reaction. From the previous discussion of this study, this key mechanism of gas-phase heating for auto-ignition of PMMA is considered to be the absorption of the incident radiation by the decomposition products. On reducing the incident radiant flux, the sample temperature rises more slowly and the concentration of the ,decomposition products in the plume be-
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
241
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Fig. 8. Relationship between ignition surface temperature and initial incident radiant flux for red oak.
comes less. The ignition delay time becomes longer and the plume becomes taller. The absorbed energy from the incident radiation is spread over the entire height of the plume and the local absorbed intensity is correspondingly less. Therefore, to attain autoignition, high-incident radiant flux is required in order to produce high concentrations of decomposition products during plume forming. These absorb the energy from the incident radiation locally and intensively. This is observed for autoignition of PMMA as shown in Fig. 9. The minimum incident radiant flux was 16 W/cm 2 or greater. However, such behavior was not observed for autoignition of red oak, (Fig. 10). The minimum incident radiant flux for red oak was 8 W/cm z4, much lower than that for PMMA. Careful observation revealed that, under the long ignition delay time at the low-incident flux range, the red oak surface glowed faintly with a reddish 4 This value might be overestimated by the limit of the experimental time (~45 sees) due to the decomposition products f'fllingthe chamber.
color prior to ignition. This could be seen only in a darkened room. Supporting this observation, Fig. 8 shows that the ignition surface temperature of red oak can reach 550°C or greater, which probably would be high enough to initiate and accelerate gas-phase chemical reactions for ignition. This appears to be the reason why red oak can ignite at lower incident flux than PMMA under the autoignition mode. The important characteristic of this figure is that the ignition surface temperature of red oak is not constant and increases significantly with decrease in the incident radiant flux for both of the ignition modes. This observation is quite different from the previous results measured with a vertical sample under horizontal external irradiation [2, 3, 20]. The trend, in which the ignition surface temperature under the autoignition mode is higher than that under the pilot ignition mode, agrees with previous results [2]. Considering the low-ignition surface temperature and noting that about 75% of the incident flux is absorbed (Fig. 4), it appears that, at high-
242
TAKASHI KASHIWAGI 32 • AUTOIGNITION o PILOT IGNITION WITH HEATEDWIRE
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Fig. 9. Relationship between ignition delay time and initial incident radiant flux for PMMA.
incident flux levels, the absorption of the incident radiation by the decomposition products will ignite red oak, but, at low flux levels, red oak will be ignited by the high-temperature surface as an induced pilot. This complex ignition mechanism of red oak is mainly a result of its complicated decomposition mechanism, including char formation. More detailed study of the decomposition of red oak during the ignition period is planned in the future. The effect of the attenuation of the incident beam on the pilot ignition mode is not as significant as in the case of the auto igniton mode. However, the effect is not negligible. The attenuation at the onset of pilot ignition was derived as follows: pilot ignition delay times were obtained from Fig. 9 for PMMA and from Fig. 10 for red oak for each of the three fluxes. The attenuation
at these times was read from Fig. 3 and 4, respectively. It is assumed that there are no changes in attenuation produced by the pilot ignition mode compared with the autoignition mode. For PMMA, roughly 30-40% of the incident flux is attenuated near the ignition and roughly 10-20% for red oak. Therefore, the attenuation of the incident flux caused by absorption and/or scattering by the decomposition products must be included in a model of piloted ignition. At extremely high incident flux, ignition delay times of both ignition modes tend to be equal (Figs. 9 and 10). This indicates that the absorption of the incident flux by the decomposition products rather than gas-phase heating by the pilot would cause ignition under these conditions. The relationships between ignition delay time and incident radiant flux obtained in the study are
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
243
• AUTOIGHITION
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o PILOT IGHITIOH WITH HEATED WIRE • AUTO IGNITION WITH TUHGSTEH LAMP JREF.2)
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INCIDEHT RAOIAHT FLUX IW/CM 2)
Fig. 10. Relationship between ignition delay time and intial incident radiant flux for red oak. compared with previously published results in Figs. 9 and 10. The results for PMMA were obtained for vertically mounted samples with benzene flames as an external radiant source [6], and those for red oak were obtained for vertically mounted samples with a tungsten filament lamp for autoignition [2] and with a gas-fired radiant panel for pilot ignition [3]. Since there are so many differences in experimental parameters among them, the effect of the sample orientation on ignition delay time is not clear. For this reason, a further study is planned to measure the amount of attenuation of the incident radiation and the ignition delay times under various incident angles of external radiation on the sample surface.
CONCLUSION Ignition experiments were conducted using a CO9. laser to irradiate PMMA and red oak samples under the autoignition mode and the pilot-ignition mode with a heated wire. Results obtained from the early phase of the program are reported in this paper. In this study, the sample was mounted horizontally and the laser beam irradiated downward perpendicular to the sample surface. It was observed that there were strong interactions between the rising plume of the decomposition products from the sample and the incident radiation during the ignition period. The incident flux was attenuated as much as 75-80% for both samples. At lower levels of incident flux the
244 amount of the attenuation was less but was still not negligible, even at the lowest flux. This attenuation was not peculiar to using a CO2 laser as a radiant source but was also observed with the electric coil heater whose emission spectrum is not too dissimilar from typical radiant sources encountered in fires. Therefore, attenuation of the incident radiation by the decomposition products in fires under this experimental configuration is important during the ignition period. It appears that PMMA can only ignite by absorption of the incident radiation by the decomposition products in the gas phase under the autoignition mode in this experimental configuration. This requires high-incident flux, a minimum of 16 W/cm 2 based on measurements from this study. Red oak can ignite by the absorption in the gas phase at high flux and also by the high-temperature surfaces acting as an induced pilot at low flux. However, the detailed mechanism of ignition, such as absorption by decomposition gases or by particulates or both and subsequent ignition by gases or particulates, is not clearly understood and further studies are necessary to clarify this mechanism.
The author would like to thank Mr. William Wooden for his assistance in conducting experiments and Mr. James Raines and Mrs. Margaret Harkleroad for their help in obtaining attenuation results by a minicomputer. REFERENCES 1. Quintiere, J., Fifteenth Symposium {International) on Combustion, The Combustion Institute, Pittsburgh, 1975, p. 163. 2. Simms, D. L., Combust. Flame 5,369 (1961). 3. Simms, D. L., Combust. Flame 7,253 (1963). 4. Alvares, N. J. and Wiltshire, L. L., Ignition and Fire Spread in a Thermal Radiation Field, USNRDLTR-68-56 (1968).
TAKASHI KASHIWAGI 5. Alvares, N. J. and Martin, S. B., Thirteenth Symposium (International} on Combustion, The Combustion Institute, Pittsburgh, 1971, p. 905. 6. Hallman, J., Welker, J. R., and Sliepievich, C. M., SPEJ. 28, 43 (1972). 7. Kashiwagi, T., J. Fire Flam. Consumer Prod. Flam. 1,367 (1974). 8. Ohlemiller, T. J., Radiative Ignition of Polymeric Fuels in an Oxidizing Gas, Ph.D. thesis, Dept. Aerospace and Mechanical Science, Princeton University 1969. 9. Comeford, J. J., Combust. Flame 18, 125 (1972). 10. Buckius, R. O. and Tien, C. L., Int. J. Heat Mass Transfer 20, 93 (1977). 11. Price, E. W., Bradley, H. H., Jr., Dehority, G. L., and Ibiricu, M. M.,AIAA J., 4, 1153 (1966). 12. OhlemiUer, T. J. and Summerfield, M., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1971, p. 1087. 13. Kashiwagi, T., Combust. Sci. Technol. 8,225 (1974). 14. Lawson, D. I. and Simms, D. L. Brt. J. Appl. Phys. 3,288 (1952). 15. International Standard Organization, Technical Committee 92, Draft Proposal 5657. 16. Zitter, R. N. and Koster, D. F., Laser Induced Reaction Rate and Optical Absorption at Various Frequencies on the Absorption Bands of CF2C1 CF2C1 and CHaCF2C1 , paper presented at 173rd A.C.S. Meeting, New Orleans, La., March 21-25, 1977. 17. Grant, E. R., Coggiola, M. J., Schultz, P. A., Lee, Y. T., and Sher, Y. R., A Molecular Beam Study of the Multiphoton Dissociation of SF6, presented at 173rd A.C.S. Meeting, New Orleans, La., March 21-25, 1977. 18. Bankston, C. P., Determination of the Physical Characteristics of Smoke Particulates Generated by Burning Polymers, Ph.D. thesis, Georgia Institute of Technology, Atlanta, March 1976. 19. Van De Hulst, H. C., Light Scattering by Small Particles, Wiley, New York, 1957, p. 66. 20. Alvares, N. J., Measurements of the Temperature of the Thermally Irradiated Surface of Alpha-Cellulose, USNRDL TR-735, March 1964.
Received 1 March 1978
231
Experimental Observation of Radiative Ignition Mechanisms TAKASHI KASHIWAGI National Bureau of Standards, Washington, D. C. 20234
Radiative ignition experiments were conducted on PMMA and red oak using a CO2 laser with incident flux up to about 20 W/cm2 under autoignition and piloted ignition in air. The laser irradiated perpendicular to the horizontal sample surface. It was observed that there was strong attenuation of the incident laser radiation by the plume consisting of decomposition products in the gas phase. This was also observed using an electric coil heater as a radiant source. It is postulated that, under autoignition, PMMA ignites by the absorption of the incident radiation by the decomposition products in the gas phase, and red oak by a similar absorption at high-incident flux and at medium flux, aided by high surface temperature acting as an induced pilot.
INTRODUCTION The ultimate goal o f fire research is to provide the scientific and engineering knowledge that can be applied to reduce fire hazards. Since ignition is the initiation of the fire, it is very useful to know how combustible solids ignite and, using the knowledge, to reduce the chance of ignition. The most common fire starts in a room of a residential dwelling and may spread to other rooms and buildings. Since it is known that thermal radiation is a primary mode o f energy transfer in a room fire [1], the ignition o f combustible solids under a well-defined external radiation is being studied. In a room fire, thermal radiation from flames, hot gases, and particulates under the ceiling and also from the ceiling itself may heat and ignite combustible materials on the floor such as furniture and floor coverings. Since these materials may be exposed to thermal radiation over a wide range of angles incident to their surfaces, it is useful to study the mechanism o f radiative ignition under a range of configurations. Although the external radiation imposed horizontally on a vertical sample surface has been extensively studied [2-6], other cases with different incident angles have not been systematically studied. Basic differences in Copyright © 1979 by The National Bureau of Standards Published by Elsevier North Holland, Inc.
the orientation of the sample surface with respect to incident radiation would cause differences in the heat transfer of surfaces and in the flow pattern of decomposition products and their mixing with entrained air. At present, the effect of the orientation on ignition is not well known. As a first step, this article reports the observations obtained using a configuration in which vertical radiation is imposed downward on a horizontally oriented sample surface. In the radiative ignition of combustible materials the important process is the absorption of sufficient external radiation to heat the material near the surface above its decomposition temperature and to release combustible gases and particles. The amount of energy absorbed by the solid material and by the decomposition products is determined by the emission spectrum of the external radiation [6] and the surface reflectance and absorption coefficient of the materials [6-8]. Generally, the emission spectra of flames, particulates, and hot surfaces are broad [6, 9, 10], and the absorption spectra of the materials and decomposition products consist of many absorption bands. A change in the external radiant flux produced by a change in the temperature of the emitter shifts the emission spectrum. This changes the amount of energy
232 actually absorbed by the materials and also, in diathermic materials, the depth over which this energy is absorbed. Thus changes in the external radiant flux due to source temperature changes make it very difficult to measure precisely how much energy is absorbed into the material and to understand the already complicated ignition mechanism. To simplify this complexity, a continuous-wave CO2 laser is used in this study. Its essentially monochromatic emission avoids the above-mentioned problem with the use of conventional radiant heat sources. A laser source makes theoretical modeling easier and comparison of theoretical predictions with experimental data more precise, thus helping to clarify the complicated ignition mechanism. Once the ignition mechanism is understood using the CO2 laser source, the model can be extended to more common radiant heat sources representing fire by including the appropriate wavelength dependency of radiative properties. There are two modes of radiative ignition: one is autoignition and the other is piloted ignition. The former mode applies to ignition resulting solely from external heating and without any addition of a high-temperature source on or near the material surface. The latter applies to ignition under external heating with the addition of a hightemperature "pilot" such as a hot wire, hot particles, or a small flame. Also, the latter mode applies to flame spread under external radiation viewed as successive piloted ignitions [7]. It is important to understand these two modes of ignition in order to develop predictive models. In autoignition, two necessary conditions must be satisfied simultaneously for ignition to occur: (1) sufficient amounts of fuel and oxygen must be available in the gas phase, and (2) the gas-phase temperature must be high enough to initiate and accelerate the gasphase chemical reaction. On the other hand, only the first condition must be satisfied for piloted ignition, provided that the pilot temperature is high enough to initiate the chemical reaction. Therefore, the difference between the two ignition modes would appear to be the heating process of the gas phase. There are two questions to be addressed in this heating process: (1) how the gas phase is heated
TAKASHI KASHIWAGI and (2) how rapidly the gas phase can be heated. For the first question, there are two ways to heat the gas phase during the ignition period in the radiative ignition mode: (1) convection-conduction from the heated surface, and (2) absorption of the incident radiation. In general, the first process has been considered as the main mechanism to heat the gas phase [11-13]. However, there have been no direct measurements to exclude the possibility of the second mechanism, especially in the case of vertical radiation onto a horizontally mounted sample surface. For the second question, the effectiveness of the heating process of the gas phase can be obtained by comparing the ignition delay times for autoignition and piloted ignition. There are some published data available for which these comparisons were made using the same sample size and the same external radiant source, for vertical materials irradiated horizontally [14]. Comparable data for horizontally mounted materials irradiated vertically or at other angles have not been available. The present study has been designed to answer these questions.
EXPERIMENTAL APPARATUS A schematic illustration of the experimental apparatus is provided in Fig. 1. The COz laser (Coherent Radiation Model 41) 1 emits an approximately 3-mm-diameter beam whose power can be varied from 240 W to 340 W by adjusting the current through the laser-discharge tube. This range of the power is selected to maintain the fundamental mode (TEMoo), for which the power distribution across the beam is Gaussian. A diffuser lens with a focal length of 6.2 cm is used to expand the incident laser beam to obtain a more uniform flux distribution. The incident flux distribution at the sample surface position is measured prior to each ignition experiment by traversing a Gardon-type 1 Certain commercial equipment, instruments, or materials are identified in this article in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the NationM Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
% CO2 LASER
SWITCHABLE ROTATING MIRROR SHUTTER
\
233
MIRROR
ETECTOR - DIFFUSER LENS
CHAMBER
L SAMPLE
Fig. 1. Schematicillustration of the experimental apparatus.
calorimeter with a 2.5-mm sensing diameter. The flux distribution is typically uniform within 10% over a circle of approximately 2-3 cm diameter. The average radiant flux for each experiment is very closely approximated by a value equal to 95% of the maximum flux. A shutter consisting of a switchable mirror is used to provide a step-function irradiance and remains open through the ignition event. The sample is mounted horizontally, and the incident laser beam irradiates perpendicularly to the sample surface. Ignition is determined by the first light emission, which is measured by an EMI 7656 photomultiplier, the output of which is displayed on a visicorder. At the onset of flaming, a step-function-like output of the photomultiplier is observed. This output is used as unambiguous determination of the ignition event. For piloted ignition tests a 250-tam-diameter by 7.5-cm-length platinum wire is stretched 1.5 cm above and parallel to the sample surface. This wire is very rapidly heated, electrically ('~ 20,000°C/ sec), to a temperature previously set, which is kept constant during the ignition period. The wire temperature can be varied from the room temper-
ature to about 950°C; it was kept at 950°C in this study. The energy input to the wire is at most 0.1 W to maintain this temperature. The relationship between the resistance of the wire and wire temperature was calibrated by using welldefined melting temperatures of 10 materials ranging from 170°C to 900°C. The resistance of the wire is continuously recorded during the ignition period for each experiment to confirm the setting of the wire. The lower part of the experimental chamber is 22.5 cm in diameter and 50 cm in height. The upper part of the chamber is 30 cm in diameter and 33 cm in height. The distance between the top of the chamber and the sample surface is 70 cm. Although the pressure in the chamber can be varied from vacuum to one atmosphere and the gas composition from an inert gas to 100% oxygen, air at atmospheric pressure was used in the tests reported here. Two different materials were used in this study, polymethylmethacrylate (PMMA) and red oak. The PMMA was cut into 7.5-cm squares, 1.2 cm thick. Red oak samples were dried at 60°C for 24 hr and left in a conditioned room (50% humidity at 20°C) to reach an equilibrium mois-
234
TAKASHI KASHIWAGI
(a)
(b) Fig. 2. Decomposedpattern of PMMAsurface after laser irradition.
ture content (determined by weighing) prior to the experiment. The dimensions of the oak samples were 7.5 cm square by 1.7 cm thickness. EXPERIMENTAL RESULTS AND DISCUSSION After heating the material above its decomposition temperature by external radiation, decomposition products rise from the surface by buoyancy. For vertical materials irradiated horizontally, the flow pattern of boundary layer forms along the sample surface. The thickness of this layer remains relatively thin with time and the attenuation of the incident radiation crossing the layer should usually be small. However, for horizontal materials irradiated vertically, the decomposition products form a rising plume and the height of the plume grows with time. The incident radiation passes through the growing plume, and its intensity may be significantly attenuated by the plume. Some experimental evidence to support the importance of this attenuation is described below. After a test it was sometimes found that the ablated surface of PMMA showed a distinct horseshoe shape such as that shown in Fig. 2c. This frequently occurred when PMMA was not ignited with autoignition. In Fig. 2c the center part decomposed little and the surrounding white zone
(e)
showed a considerable amount of decomposition. However, this horseshoe shape disappeared by blowing room-temperature air (speed ~ 30 cm/sec) toward the sample and parallel to its surface during the laser radiation. A picture of the sample tested in this way is shown in Fig. 2a. To eliminate the possibility of the enhancement of surface oxidation by blowing air, room-temperature argon was blown over the sample under the same experimental condition. The decomposition pattern was the same as the air-blown case (Fig. 2b). The effect of air or argon blowing on the sample is to blow the decomposed gas plume away from the incident laser beam and to reduce the interaction between the plume and the beam. This suggests that the horseshoe decomposition pattern is caused by this interaction. To find the mechanism of the interaction between the incident beam and the plume containing decomposition products, the attenuation of the incident radiation was measured during the ignition period for PMMA and red oak under various radiant fluxes. The measurement was made using a water-cooled flux meter with a 2.5-mm-diameter sensing area, mounted facing upward beneath the sample. The sample had a 4-mm-diameter hole through its center, through which the flux meter could measure the radiation reaching the sample
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
235
1.0
0.9
0.8
o71- 1
V
0.6
:11.6W/CM2 1
0.4
Io:79W/CM2
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0.3
0.2
m
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0.0 0
I 4
l 8
I 12
I 16
I 20
I 24
TIME (SEC)
Fig. 3. Normalized transmission of the incident flux as a function of time under various initial incident radiant fluxes for PMMA autoignition mode.
surface through the decomposed gas column. Since the thickness of PMMA sample was 1.2 cm and red oak 1.7 cm and the experimental time was at most 30 sec, the heat conduction through either sample was negligible. With this configuration, the flux meter initially measured the incident radiant flux and then the flux transmitted through the plume after the decomposition started. The results for PMMA are shown in Fig. 3 and for red oak, in Fig. 4. The ordinate is the transmitted radiant flux, I, normalized by the initial incident flux, Io. Therefore, the ratio I[I o stays at one until the decomposition starts. The fluctuation of the curves could
be caused by the wandering of the plume. The curves are roughly exponential with rates that agree roughly with the rates of growth in the height of the plume. The sudden increases in the normalized flux for red oak at Io = 16.3 and 13.4 W/cm z were caused by ignition. This could be caused by the changes in the gas composition and particulate size distribution before and after ignition. However, no sudden increase in the transmission was observed for PMMA at ignition at Io = 17.4 W/cm z. Therefore, the cause of the transmission change after ignition is not clear at present. The sudden drop in the transmission after the sud-
236
TAKASHI KASHIWAGI 1 . 0 ~
0.9
Io--8.4W/CM2
0.8 Io =13.4W/CM2 0.7 0.6
Io-'16.3W/CM2
0.4 IGNITION
0.3 r
0.2 ~--
0.1 t
0.0 0
IGNITION
I 3
I 6
I 9 TIME (SECI
I
I
I
12
15
18
I
Fig. 4. Normalized transmission of the incident radiant flux as a function of time under various initial incident radiant fluxes for red oak autoignition mode.
den increase for red oak resulted when the incident laser beam was turned off. Figures 3 and 4 both show, at high radiant flux, that the attenuation of the incident radiation is very strong and that 70-80% of the incident radiant flux is attenuated by the decomposition products within 5 sec for PMMA and red oak. The amount of the attenuation decreases with a decrease in the incident radiant flux for both samples, presumably because of lower sample temperature and reduced rate of generation of the decomposition products. However, even at low incident flux, the attenuation is still at least 20% and is certainly not negligible, especiaUy for PMMA.
From the observations just described, the importance of the attenuation of the incident radiation by the decomposition products is evident, for the case of vertically irradiated ignition of the horizontally placed sample. However, the attenuation characteristics depend strongly on the spectrum of the incident radiation. The wavelength of the incident radiation in this study is 10.6/am, from a CO2 laser. This wavelength is much longer than those typical of emission by fires. Therefore, it is important to investigate whether the observed importance of the attenuation of the incident radiation by the decompostion products is due to the use of the CO2 laser or whether it is also applica-
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
237
PMMA 1.0 0.9 0.8 0.7 0.6 o
0.5
B
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I
i
I
I
I
I
I
I
30
60
90
120
150
180
210
240
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Fig. 5. Normalized transmission of the incident radiant flux as a function of time at 4 W/cm2 from electric coil heater. ble for a more conventional radiant source in a fire. To clarify this, the attenuation of the incident radiation by the decomposed products from both PMMA and red oak was measured using an electricaUy heated coil similar to that proposed as International Standard Organization ignition test method [15]. This heater approximates a gray-body radiator. It has a conical shape with dimensions of: 20 cm bottom diameter, 7 cm top diameter, and 7.5 cm height. There was a 9-turn electric heating coil over the inside wall facing downward. The top was open for the exhaust of the decomposition products. A sample 8 cm in diameter and 1.2 cm in thickness was located in the center 5 cm below the bottom of the conical heater. The incident flux on the sample surface was about 4 W/cm 2 with 700°C coil temperature. A sensitive watercooled flux meter with a 9-mm sensing diameter was located facing upward beneath a thin-layer,
water-cooled shield below the sample to avoid any heat conduction and transmitted radiation through the sample. The sensing element collected radiation through 9-mm-diameter holes of the cooled shield and the sample. The timewise change of the transmitted flux normalized by the initial incident flux is shown in Fig. 5 for PMMA and red 'oak. Shortly after exposure to the radiation, the surface of red oak charred and it started to decompose. The attenuation of the incident radiation became significant shortly after the exposure and it reached about 35% attenuation. However, no attenuation was observed until roughly 2 min had elapsed for PMMA and then the attenuation increased rapidly. It also reached about 35%. Since the absorption of the clear solid PMMA is less in the wavelength range below 2 jum, part of tlae incident radiation is reflected and part is absorbed in-depth in the PMMA. Also, the value of KpC,
238 where K is thermal conductivity, p density, and C specific heat, for PMMA is roughly twice that of red oak. Thus, under the same incident radiation, PMMA is heated more uniformly in-depth than red oak so that its surface temperature is lower and the onset of the decomposition is delayed. Therefore, PMMA takes a longer heating time to attenuate the incident radiation than red oak, which absorbs most of the incident radiation close to the surface. It is clearly shown in Fig. 5 that the attenuation of the incident radiation by the decomposition products is important with the electric heating coil, with approximately graybody emittance, as well as with the CO2 laser. Therefore, it is reasonable to conclude that attenuation of vertically incident radiation is important during the decomposition after the preheating period for horizontally placed combustible solids in fires. With the understanding of the interaction mechanism of the plume of the decomposition products with the incident radiation, the appearance of the horseshoe-ablation pattern described previously can be explained as follows. After the laser beam heats the sample, the decomposition products rise and form the plume. As shown in Fig. 6, the column center sometimes differs slightly from the center of the incident beam because of a slight deviation of the plume from the vertical (or a little nonsymmetry in the flux distribution). Since the decomposition products significantly attenuate the incident beam, the amount of the incident energy that reaches the surface through the column declines around the center of the column. This causes the nonuniform flux distribution at the surface, low around the column center, and causes the appearance of the horseshoe decomposition pattern. In several cases, a doughnut-shaped decomposition pattern was also observed after the experiment. This was formed when the center of the plume was located close to the center of the incident beam. When the heating time becomes very long without ignition, this horseshoe pattern gradually disappears and eventually the entire irradiation surface starts to decompose fairly uniformly. This is due to the heat conduction in the solid with the long heating time at the low-flux range and wandering of the plume that averages out the effect. This horseshoe or doughnut-shaped
TAKASHI KASHIWAGI
II
II
AREALASER IRRADIATES CROSS SECTIONOF PLUME
Fig. 6. Schematic illustration of the cross sections of laser beam and the plume.
decomposition pattern was not so clear for red oak as for PMMA, but still there existed the trend of the nonuniform decomposition pattern for red oak. It indicates that the red oak surface could char at the low flux, even though a part of the incident flux is attentuated in the gas phase. The attenuation of the incident radiation consists of both absorption and scattering by gas molecules and particulates. Since the absorption heats the gas phase thermally 2 and the scattering merely deflects the incident radiation, it is impor. tant to find which process causes the observed attenuation. Since the Rayleigh scattering cross section of molecules is much smaller than the incident wavelength, the scattering from molecules is negligible compared with other attenuation processes in this experiment. The decomposition products of both samples were collected in the plume 2 cm above the surface under laser irradiation without flaming. The particulate size distribution of these samples was measured using an electrical aerosol analyzer (Thermo-Systems, Inc., Model 3030). Preliminary results show that there are significant 2 Since the range of the incident flux is very low in this study, the multiphoton process [16, 17] found recently is negligible in this study.
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS" numbers of particles. The measured peak of the volume particle size distributions 8 of PMMA and red oak are at least 0.75/am, which is unfortunately above the instrument capacity. Further study is planned to measure the volume size distribution with a different instrument. Bankston [18] measured the volume size distribution of similar materials under the smoldering condition and found that their peaks were about 0.8 /am. Therefore, it is expected that the peak volume size distributions for PMMA and red oak would not be far from 0.75/am. Then, X = (2rra/X) ~ 0.2, where a is a radius of the particle at the peak volume size distribution and X is the laser wavelength of 10.6 /am. With X "~ 0.2, mainly Rayleigh scattering is expected and the ratio of scattering to absorption coefficient is proportional to X a [19] and is a function of the real and imaginary parts of refractive index. At this early stage of the program, values of the refractive index of particulates produced from the decomposition of PMMA and red oak during the ignition period have not been measured and the role of particulates in the attenuation of the incident radiation is not clear. Further study is necessary. To measure the absorption characteristics of the decomposition gases, evolved gases were drawn from 2 cm above the sample surface into an evacuated gas cell through a small sampling probe. Their transmission spectrum at 2.5-20/am was measured with a Perkin-Elmer Model 180 infrared photometer. The spectrum of the decomposition gases from PMMA is very close to the spectrum of the solid PMMA, which is expected from the unzipping decomposition characteristics. The spectrum shows a significant absorption around the incident wavelength of 10.6 /am. It is considered that the carbon-carbon double bond, which has numerous absorption bands at 10-11 /am, absorbs the incident laser beam. A similar observation was obtained with the decomposition gases from red oak. 3 The volume of particulates in the range of particle diameters log D and log D + d(log D) is d V . Then, d V = v(D)d(log D).
This equation defines the volume particle-size distribution v(D).
239
From the above discussion it is qualitatively found that some part of the attenuation of the incident radiation is due to absorption by the decomposition products. It is plausible that the absorption in the gas phase might cause autoignition instead of the convection-conduction from the heated surface that has been considered the key ignition mechanism by external radiation. To investigate whether the absorption of the external radiation is the key mechanism, both red oak and PMMA samples were heated by the electric heating coil at about 4 W/cm 2 and the CO s laser radiation was passed through the plume of decomposition products, approximately 3 cm above and paraUel to the sample surface. Ignition was observed with a radiant flux about 10 times larger than used in the preceding experiments, probably because of a lower concentration of decomposition products at lower incident flux. This indicates that the absorption of the incident radiation can ignite the decomposition products if the incident radiant flux is high and the concentration of absorbers is high. There are three possible mechanisms for ignition of the decomposition products by the absorption of the incident radiation: (1) absorption by decomposition gases to heat the gas phase and to initiate and accelerate the gas-phase chemical reactions, (2) absorption by particulates which heat the gas phase by convection/conduction to accelerate gas-phase chemical reactions, and (3) absorption by particulates that ignite by heterogeneous chemical reaction on their surfaces. Since the density of a gas is about 1000 times less than that of a solid and the value of the specific heat of the gas is about the same as that of the solid, the gas-phase heating requires far less absorbed energy for ignition than does ignition by particulates. Also, if particulates are formed by condensation of molecules, they have to go through the endothermic vaporization process. This favors the first mechanism. However, if the absorptivity of particulates is much higher than that of gases, or if particulates are formed directly from the decomposition of the sample and they decompose exothermally, particulates might be heated more efficiently than gases. To find the correct process, we need detailed information on the absorptivity of particulates at the wavelength of the incident radiation, the
240
TAKASHI KASHIWAGI
PMMA 450
? 0 I--
0
400
0 o
0
o
o
0
o
o o
i,,iJ
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INCIDENT RADIANT FLUX lo (w/cm 2)
Fig. 7. Relationship b e t w e e n ignition surface t e m p e r a t u r e and intial i n c i d e n t radiant flux for PMMA.
chemical nature and activity of particulates, the particulate formation mechanism, and the chemical composition of decomposition gases. At present, we do not have this information and it is not understood which process causes ignition for PMMA and red oak. Surface temperature was measured during the ignition period by using a 25-/am-diameter AlumelChromel thermocouple. The absorption of the incident radiation by the thermocouple wire was investigated by measuring the output of the thermocouple exposed to the incident radiation in air. No change in the output was observed with and without the incident radiation, and it is concluded that the thermocouple does not absorb the incident radiation in the range of the radiant flux used in this study. The thermocouple wire was electrically heated by a small voltage and melted the PMMA surface to make good contact. For red oak, the wire was pressed onto the surface by a ceramic pestle. Surface temperature at ignition was determined by simultaneous measurements of temperature and of emission from the onset of flaming,
using the photomultiplier. The observed surface temperatures are shown in Fig. 7 for PMMA and in Fig. 8 for red oak under various incident fluxes. There is some scatter in the data, but the trend is clearly shown in both figures. The ignition surface temperature of PMMA is fairly constant in the range 375°-410°C over a wide range of the incident flux. Since ignition surface temperature does not differ between the pilot-ignition mode and the autoignition mode, gas-phase temperature would not differ between the two ignition modes prior to ignition. The only difference is the existence of the 950°C pilot wire for the pilot-ignition mode. Therefore, the autoignition mode must raise the gas-phase temperature high enough to initiate and accelerate chemical reaction. From the previous discussion of this study, this key mechanism of gas-phase heating for auto-ignition of PMMA is considered to be the absorption of the incident radiation by the decomposition products. On reducing the incident radiant flux, the sample temperature rises more slowly and the concentration of the ,decomposition products in the plume be-
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
241
RED OAK 550
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6
8
10
12
14
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INCIDENT RADIANT FLUX Io (w/cm 2)
Fig. 8. Relationship between ignition surface temperature and initial incident radiant flux for red oak.
comes less. The ignition delay time becomes longer and the plume becomes taller. The absorbed energy from the incident radiation is spread over the entire height of the plume and the local absorbed intensity is correspondingly less. Therefore, to attain autoignition, high-incident radiant flux is required in order to produce high concentrations of decomposition products during plume forming. These absorb the energy from the incident radiation locally and intensively. This is observed for autoignition of PMMA as shown in Fig. 9. The minimum incident radiant flux was 16 W/cm 2 or greater. However, such behavior was not observed for autoignition of red oak, (Fig. 10). The minimum incident radiant flux for red oak was 8 W/cm z4, much lower than that for PMMA. Careful observation revealed that, under the long ignition delay time at the low-incident flux range, the red oak surface glowed faintly with a reddish 4 This value might be overestimated by the limit of the experimental time (~45 sees) due to the decomposition products f'fllingthe chamber.
color prior to ignition. This could be seen only in a darkened room. Supporting this observation, Fig. 8 shows that the ignition surface temperature of red oak can reach 550°C or greater, which probably would be high enough to initiate and accelerate gas-phase chemical reactions for ignition. This appears to be the reason why red oak can ignite at lower incident flux than PMMA under the autoignition mode. The important characteristic of this figure is that the ignition surface temperature of red oak is not constant and increases significantly with decrease in the incident radiant flux for both of the ignition modes. This observation is quite different from the previous results measured with a vertical sample under horizontal external irradiation [2, 3, 20]. The trend, in which the ignition surface temperature under the autoignition mode is higher than that under the pilot ignition mode, agrees with previous results [2]. Considering the low-ignition surface temperature and noting that about 75% of the incident flux is absorbed (Fig. 4), it appears that, at high-
242
TAKASHI KASHIWAGI 32 • AUTOIGNITION o PILOT IGNITION WITH HEATEDWIRE
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Fig. 9. Relationship between ignition delay time and initial incident radiant flux for PMMA.
incident flux levels, the absorption of the incident radiation by the decomposition products will ignite red oak, but, at low flux levels, red oak will be ignited by the high-temperature surface as an induced pilot. This complex ignition mechanism of red oak is mainly a result of its complicated decomposition mechanism, including char formation. More detailed study of the decomposition of red oak during the ignition period is planned in the future. The effect of the attenuation of the incident beam on the pilot ignition mode is not as significant as in the case of the auto igniton mode. However, the effect is not negligible. The attenuation at the onset of pilot ignition was derived as follows: pilot ignition delay times were obtained from Fig. 9 for PMMA and from Fig. 10 for red oak for each of the three fluxes. The attenuation
at these times was read from Fig. 3 and 4, respectively. It is assumed that there are no changes in attenuation produced by the pilot ignition mode compared with the autoignition mode. For PMMA, roughly 30-40% of the incident flux is attenuated near the ignition and roughly 10-20% for red oak. Therefore, the attenuation of the incident flux caused by absorption and/or scattering by the decomposition products must be included in a model of piloted ignition. At extremely high incident flux, ignition delay times of both ignition modes tend to be equal (Figs. 9 and 10). This indicates that the absorption of the incident flux by the decomposition products rather than gas-phase heating by the pilot would cause ignition under these conditions. The relationships between ignition delay time and incident radiant flux obtained in the study are
"EXPERIMENTAL OBSERVATION OF RADIATIVE INGITION MECHANISMS"
243
• AUTOIGHITION
48
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Fig. 10. Relationship between ignition delay time and intial incident radiant flux for red oak. compared with previously published results in Figs. 9 and 10. The results for PMMA were obtained for vertically mounted samples with benzene flames as an external radiant source [6], and those for red oak were obtained for vertically mounted samples with a tungsten filament lamp for autoignition [2] and with a gas-fired radiant panel for pilot ignition [3]. Since there are so many differences in experimental parameters among them, the effect of the sample orientation on ignition delay time is not clear. For this reason, a further study is planned to measure the amount of attenuation of the incident radiation and the ignition delay times under various incident angles of external radiation on the sample surface.
CONCLUSION Ignition experiments were conducted using a CO9. laser to irradiate PMMA and red oak samples under the autoignition mode and the pilot-ignition mode with a heated wire. Results obtained from the early phase of the program are reported in this paper. In this study, the sample was mounted horizontally and the laser beam irradiated downward perpendicular to the sample surface. It was observed that there were strong interactions between the rising plume of the decomposition products from the sample and the incident radiation during the ignition period. The incident flux was attenuated as much as 75-80% for both samples. At lower levels of incident flux the
244 amount of the attenuation was less but was still not negligible, even at the lowest flux. This attenuation was not peculiar to using a CO2 laser as a radiant source but was also observed with the electric coil heater whose emission spectrum is not too dissimilar from typical radiant sources encountered in fires. Therefore, attenuation of the incident radiation by the decomposition products in fires under this experimental configuration is important during the ignition period. It appears that PMMA can only ignite by absorption of the incident radiation by the decomposition products in the gas phase under the autoignition mode in this experimental configuration. This requires high-incident flux, a minimum of 16 W/cm 2 based on measurements from this study. Red oak can ignite by the absorption in the gas phase at high flux and also by the high-temperature surfaces acting as an induced pilot at low flux. However, the detailed mechanism of ignition, such as absorption by decomposition gases or by particulates or both and subsequent ignition by gases or particulates, is not clearly understood and further studies are necessary to clarify this mechanism.
The author would like to thank Mr. William Wooden for his assistance in conducting experiments and Mr. James Raines and Mrs. Margaret Harkleroad for their help in obtaining attenuation results by a minicomputer. REFERENCES 1. Quintiere, J., Fifteenth Symposium {International) on Combustion, The Combustion Institute, Pittsburgh, 1975, p. 163. 2. Simms, D. L., Combust. Flame 5,369 (1961). 3. Simms, D. L., Combust. Flame 7,253 (1963). 4. Alvares, N. J. and Wiltshire, L. L., Ignition and Fire Spread in a Thermal Radiation Field, USNRDLTR-68-56 (1968).
TAKASHI KASHIWAGI 5. Alvares, N. J. and Martin, S. B., Thirteenth Symposium (International} on Combustion, The Combustion Institute, Pittsburgh, 1971, p. 905. 6. Hallman, J., Welker, J. R., and Sliepievich, C. M., SPEJ. 28, 43 (1972). 7. Kashiwagi, T., J. Fire Flam. Consumer Prod. Flam. 1,367 (1974). 8. Ohlemiller, T. J., Radiative Ignition of Polymeric Fuels in an Oxidizing Gas, Ph.D. thesis, Dept. Aerospace and Mechanical Science, Princeton University 1969. 9. Comeford, J. J., Combust. Flame 18, 125 (1972). 10. Buckius, R. O. and Tien, C. L., Int. J. Heat Mass Transfer 20, 93 (1977). 11. Price, E. W., Bradley, H. H., Jr., Dehority, G. L., and Ibiricu, M. M.,AIAA J., 4, 1153 (1966). 12. OhlemiUer, T. J. and Summerfield, M., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1971, p. 1087. 13. Kashiwagi, T., Combust. Sci. Technol. 8,225 (1974). 14. Lawson, D. I. and Simms, D. L. Brt. J. Appl. Phys. 3,288 (1952). 15. International Standard Organization, Technical Committee 92, Draft Proposal 5657. 16. Zitter, R. N. and Koster, D. F., Laser Induced Reaction Rate and Optical Absorption at Various Frequencies on the Absorption Bands of CF2C1 CF2C1 and CHaCF2C1 , paper presented at 173rd A.C.S. Meeting, New Orleans, La., March 21-25, 1977. 17. Grant, E. R., Coggiola, M. J., Schultz, P. A., Lee, Y. T., and Sher, Y. R., A Molecular Beam Study of the Multiphoton Dissociation of SF6, presented at 173rd A.C.S. Meeting, New Orleans, La., March 21-25, 1977. 18. Bankston, C. P., Determination of the Physical Characteristics of Smoke Particulates Generated by Burning Polymers, Ph.D. thesis, Georgia Institute of Technology, Atlanta, March 1976. 19. Van De Hulst, H. C., Light Scattering by Small Particles, Wiley, New York, 1957, p. 66. 20. Alvares, N. J., Measurements of the Temperature of the Thermally Irradiated Surface of Alpha-Cellulose, USNRDL TR-735, March 1964.
Received 1 March 1978