- Email: [email protected]
A study of flame propagation mechanisms in lycopodium dust clouds based on dust particles' behavior
Journal of Loss Prevention in the Process Industries 14 (2001) 153–160 www.elsevier.com/locate/jlp
A study of flame propagation mechanisms in lycopodium dust clouds based on dust particles’ behavior Ou-Sup Han b
a,*
, Masaaki Yashima a, Toei Matsuda a, Hidenori Matsui a, Atsumi Miyake b, Terushige Ogawa b
a National Institute of Industrial Safety, Ministry of Labor, 1-4-6 Umezono, Kiyose-si, Tokyo 204-0024, Japan Department of Safety Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Abstract Following our earlier study on the behavior of lycopodium dust flames, further experiments using a particle image velocimetry system with a high-resolution video camera have been conducted to clarify the mechanisms of laminar dust flame propagation in a vertical duct. Lycopodium, a nearly equal-sized particle, has been recognized as being monodispersed, but it was found that an actual lycopodium dust cloud consisted of individual and agglomerated particles. Corresponding to the particle forms, the reaction zone showed a double flame structure, consisting of enveloped diffusion flames (spot flame) of individual particles and diffusion flames (independent flame) surrounding some particles. Due to the convective flow caused by a flame, part of the gravitational settling particles was shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front of the flame. Such particle movement causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentration rather than the mean concentration. Although the flame moved discontinuously on a micro scale, an overall constant flame velocity was found, presumably due to the dynamic variation in dust concentration and induced flow ahead of the flame. Judging from the above-mentioned movement of single particles in front of the flame, a residence time of the unburnt particle in the preheating zone is needed to form combustible gases close to the particle. This residence time depends on the preheating zone thickness, the particle velocity and the flame propagation velocity. The observation of the movement of a single particle suggested a flame propagation mechanism where an enveloped and diffusion lycopodium dust flame discontinuously propagates from one particle to those adjacent in a laminar suspension. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dust flame; Flame propagation; Lycopodium; PIV
1. Introduction Many experimental and theoretical studies on the properties of flame propagation in dust clouds have been conducted (Krazinski, Buckius & Krier, 1979; Hertzberg & Cashdollar, 1987; Berlad, Ross, Facca & Tangirala, 1990; Sun, Dobashi & Hirano, 1998). Nevertheless, knowledge on fundamental phenomenon of dust flame propagation is still imprecise and insufficient to explain the mechanism of flame propagation in dust clouds. Smoot and Horton (1977) showed that the flame propagation mechanism of a laminar coal-air flame is diffusion controlled as a result of the combustion of volatiles. Pro-
* Corresponding author. Tel.: +81-424-91-4512; fax: +81-424-917846. E-mail address: [email protected] (O.-S. Han).
ust and Veyssiere (1988) reported that, from temperature profile and luminosity of a starch dust flame, there is no temperature rise associated with measurable radiative flux. These results indicate that laminar dust flame propagation mechanism is controlled by thermal and molecular diffusion into the preheating zone. Ballal (1983) investigated that the effect of radiative heat losses from dust particles on the burning velocity in dust clouds in microgravity. Ju, Dobashi and Hirano (1998) reported that the flame propagation mechanism of stearic acid particles is similar to that of a usual hydrocarbon-air premixed flame, although the reaction zone thickness is larger than that of a premixed flame. Recently the study of propagation mechanisms based on dust flame structure has become of interest, but very little fundamental information has been given regarding the mechanisms of dust flame propagation. In our previous study (Han et al., 2000), the structure
0950-4230/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 0 0 ) 0 0 0 4 9 - 8
154
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
of flame propagating through lycopodium dust particles in a vertical duct was examined experimentally. As a result, it was found that the flame front shows a double flame structure, which consists of isolated individual burning particles and ball-shaped flames surrounding some particles. The ball-shaped flame has been defined as an independent flame in our study. Although the independent flames form irregularly in the individual burning particles ahead of flame, the leading edge of the lycopodium flame moves at nearly constant velocity for the whole range of the dust concentration examined. However, it has not been clarified yet how independent flames could be formed and how the combustion zone moves in a lycopodium dust cloud. Since the dust flame depends on the behavior of dust particles, it is necessary to explore the movement of particles near the flame for the elucidation of dust flame propagation mechanism. Many studies on dust explosion and flame propagation in lycopodium dust clouds have been reported (Eckhoff, 1997), and thus lycopodium dust was selected in this study. This paper reports the further study of laminar flame propagation in a lycopodium dust cloud. Visualization of movements of the particles near the flame during flame propagation in a vertical duct is analyzed using a PIV (particle image velocimetry) measurement system. From these results, the mechanism of laminar flame propagation through lycopodium dust particles is discussed.
2. Experimental 2.1. Experimental apparatus and procedure The vertical duct of 1800 mm height with 150 mm × 150 mm square cross-section and experimental arrangement are the same as that described in the previous study (Han et al., 2000). Here only the experimental procedure and the visualization method of dust particles using a PIV system are explained. The experimental procedure is as follows. Lycopodium particles having nearly equal particle sizes of mean diameter of 32 µm were layered on a fine porous plate supported by a metallic grid at the bottom of the duct. With the condition of the duct top and bottom open, an airflow with an adjustable flow rate was introduced through the porous plate which acted as a flow rectifier to disperse the dust. When the dust particles have filled the duct, a time controlling system stops the airflow and removes a fluidized bed from the bottom of the duct. After closing the shutter at the top end of duct, the dust cloud was ignited at the bottom of the duct by an electric spark lasting 0.3 s in duration. Mean dust concentrations were determined by measuring the decrease in dust mass in the movable fluidized bed. As shown in Fig. 1, the movement of dust particles near the front of the flame during flame propagation was
recorded by a high-resolution video camera (1018 × 1008 pixel) and analyzed by a PIV system (TSI Inc., USA). In order to obtain a good visualization of the particles’ movement, quartz glass windows were set up on the duct side walls and a vertical laser light sheet was established in the middle of the vertical combustion duct. 2.2. Visualization of the dust particles’ movements PIV is an instantaneous, non-intrusive full field flow measuring technique, and provides a velocity field of the flow being the most powerful tool to date (Adrian, 1991; Hesselink, 1998). A scheme of the PIV measurement system is illustrated in Fig. 1. It consists mainly of two parts of a flow visualization system and a digital image processing system for the quantitative measurements of instantaneous particle velocities. In order to disclose the particles’ motion near the flames, the flow in flame propagation is seeded by the lycopodium (combustible particles themselves) that will scatter the pulsing laser sheet, and photographed twice from the side with the high-resolution camera. The PIV parameters used for the experiment were as follows: object size, 60×60 mm; time between pulsed lasers; 1 and 10 ms. The resulting double-exposed image, taken in a known time interval (time between pulsed lasers), provides a displacement record of the particles within a measurement plane, which is then analyzed and scaled to velocity by numerical processing. A PIV image will be divided up into some thousands of interrogation areas with each producing a local value of displacement when an analysis algorithm is applied. A local interrogation area of a PIV image is smaller than the smallest recorded scale of 1 mm in an analysis algorithm of PIV (Reuss, Adrian, Landreth, French & Fansler, 1989). The correlation processing technique was used to determine the correct pairings within an interrogation region.
3. Results and discussions 3.1. Development of independent flames At the concentration range of 47–592 g/m3, there existed a local decrease (10–20 g/m3) in the dust concentration with the height of the vertical duct. In spite of these variations in dust concentration, the leading edge of flame in each concentration moves at a constant velocity in the whole duct (Han et al., 2000). This result could be interpreted in terms of dynamic variation in dust concentration in front of a propagation flame. The behavior of the lycopodium particles was then examined in detail to give the spatial distribution of particles in dust clouds near and far the flame front. An example of the laser light scattering images of the particles is shown in Fig. 2, which has been recorded for a laser light sheet
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
Fig. 1.
155
Schema of the PIV measurement system.
Fig. 2. Aspect of the instantaneous vertical sectional distribution of lycopodium dust particles in the laser light sheet 0.5 mm thick. C=122 g/m3, time interval of pulsed lasers=10 ms.
0.5 mm thick using a band-pass filter of 532 nm. The band-pass filter was used to eliminate the luminosity of the dust flame. The image in the Fig. 2 shows both individual particles and a large number of groups of lycopodium particles, whose regional size is about 1–3 mm in diameter. It is believed that lycopodium dust, often used as a reference dust, consists of monodispersed particles, having the nearly same size in particle diameter.
However, its cloud was, in fact, composed of two forms, that is, the individual dust particles and particle agglomerations. These particle agglomerations may be attributable to static electricity by friction and conflict between the particles due to a continuous agitating flow in a fluidized bed. Comparison of the size over the diameter (2– 4 mm) of independent flames with that (1–3 mm) of an agglomerate of dust particles gives the reason why independent flames are formed. Namely, individual particles are ignited first and then the agglomerates of the particles will be heated to release pyrolyzed gases before ignition. Luminosities of the independent flames change with time. It seems that the variation of luminosities for independent flames will be related to three stages of thermal decomposition characteristics of lycopodium spores (Han et al., 2000). Initially, a faint ball-shaped flame appears around an agglomerate. Although the lycopodium spores used included moisture content of 3.3– 3.7 wt%, it could be mainly due to burning of the easyreleased volatile corresponding to the ca 7% weight loss of lycopodium (first stage of weight loss) in a premixed flame. Then each particle of the agglomerate will be separated with evolution of pyrolyzed gases, presumably due to the weight loss which is more than half the weight of lycopodium (second stage of weight loss). The combustible pyrolyzed gases close to each particle in an independent flame are consumed in diffusion combustion with each enveloped flame. The cellulose residues of the spore will be pyrolyzed completely (third stage of weight loss). Then, the independent flame would become luminous with diffusion-controlled reactions. The rate of heating of particles in the cases of thermal analysis and flame is greatly different, so that pyrolyzed characteristics of particle will be changed. However, the above explanation for flame luminosity will be supported from the observation of independent flame. As observed in our previous study, microscopic flame in lycopodium dust cloud is not continuous. This means that pyrolyzed combustible gases stay close or near to single or agglomerated particles before ignition.
156
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
3.2. Behavior of dust particles around the flame Fig. 3 shows an instantaneous two-dimensional visualization image [Fig. 3(a)] and the velocity distribution [Fig. 3(b)] of particles. The image [Fig. 3(a)], illuminated by vertical laser light sheet of 0.5 mm in thickness, was taken using a high-resolution video camera with the band-pass filter. This picture shows the left hand part of the flame and the vector arrows of the particle velocity obtained. The velocity distribution of the particle [Fig. 3(b)] was calculated from the PIV image [Fig. 3(a)]. In Fig. 3(b), it can be seen that particles at about 8 mm above the top of flame move horizontally and particles far from the flame shift obliquely from the top to the left. The velocity of particles around the flame takes a maximum value of 0.55 m/s near the duct wall, and a minimum value of 0.07 m/s at about 8 mm from the top of the flame. Also, it is seen that most of the unburnt particles which have passed through the front of the flame disappear, and only a few particles exist in the combustion zone. The variation in velocities of the moving particles in the center axis of flame has been measured and is shown in Fig. 4 along with the distance from the leading edge of the flame. In the case of initial dust concentration of C0=47 g/m3, unburnt particles at a distance from the flame front of ⬎11 mm in the vertical center axis (y) settle down at a velocity of about 0.04 m/s. At y=9 mm, the unburnt particle velocity becomes zero and, ⬍9 mm, they start to be accelerated in the same direction as the upward flame propagation. Thereafter, they take a maximum velocity (0.29 m/s) just before the dust particles are caught up with in the reaction zone. In the case of C=122 g/m3, at yⲏ13 mm, the unburnt particles fall gravitationally at a velocity of 0.06 m/s. Then the unburnt particles start to be accelerated at y⬇11 mm, and
Fig. 4. Dust particle velocity with change of distance from the leading edge of the reaction zone at the center axis of the flame. Vf=0.39 m/s (47 g/m3), Vf=0.47 m/s (122 g/m3).
the maximum velocity (0.38 m/s) of a moving particle is then attained at the leading edge of the parabolicshaped flame. From the PIV measurements, we have a schematic representation of the movement of the particles near a propagating flame in Fig. 5. The flow pattern for laminar flame propagation in a gas-fuel and oxidizer mixture is well known (Lewis & Von Elbe, 1961). A propagating gaseous flame produces an upward and convective flow ahead of the flame. In the dust-air flame, part of the gravitationally settling particles are shifted to the surrounding sides due to the convection flow and the rest of the particles change their movement upward due to
Fig. 3. Instantaneous PIV image (a), and velocity distributions (b) of vertical two-dimensional dust particles corresponding to the interrogated PIV photograph of flame propagation in a 122 g/m3 lcopodium–air mixture. Time interval of pulsed lasers=10 ms, Vf=0.41 m/s.
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
Fig. 5.
157
Movement of dust particles near the flame front during flame propagation.
the induced flow just in front of the flame. When these upward moving particles are blown up at a maximum speed, they will be ignited and then involved in the reaction. At 47–122 g/m3, the upward moving particles meet with settling particles at about 8 mm (Sa in Fig. 5) from the leading edge of the parabolic-shaped flame, and the mean vertical velocity of the particles at Sa approaches zero. Accordingly this region would become a stagnation area for the particles with an enriched dust concentration, which could be estimated from the flame speed, the particle sedimentation speed and the average dust concentration. Mason and Saunders (1975) showed that the settling dust particles relative to the air would cause concentration changes at the flow boundaries wherever the air velocity had a horizontal component. Conduction and convection heat transfer to particles would be attained after the particles had entered the stagnation area, and the leading edge of the preheating zone would be suggested from the particle velocities. Further, it is found that unburnt particles near the duct wall move downward and then turn to the rear region of the parabolic-shaped flame along with a convective flow (Han et al., 1999).
observation area (4×4 mm) of the PIV image. Fig. 6 shows the variation in dust ratio concentration with a laser light sheet of 0.5 mm thick along the vertical line of the flame center. In Fig. 6, the preheating zones are included, which have been measured using a schlieren system, an ion probe and a video camera. As, in the case of C=47 g/m3, the dust concentration in the unburnt region reaches a peak about 9 mm from the leading edge of the reaction zone and C/C0 reaches a peak at y=8 mm and C=122 g/m3, the maximum value of C/C0 appears just ahead of the preheating zone, presumably corresponding to an enriched dust concentration in the stagnation area (Fig. 4). After the maximima, the ratios (C/C0) of dust concentration in the preheating zone decrease to a value lower than the original unburnt dust concentration. Since the particle velocity reaches its
3.3. Estimated dust concentration near flame The variation in dust concentration ahead of the flame was estimated from laser scattering images of lycopodium particles, although they would give erroneous overlapped shadow images. Dust concentration (C) was assumed to be proportional to A/A0 [the ratio of the total shadow area (A) for the particles to that (A0) for one single particle]. They were obtained with the aid of an image processor and the observed shadow area for one single particle was 1.30×10⫺3 mm2 in the unburnt area. The estimated dust concentrations at 18 mm from the flame front and a vertical distance (y) from the flame front are represented by C0 and C, respectively, in an
Fig. 6. Variation in dust concentration ratio with distance from the flame front at the center axis of flame. Vf =0.40 m/s (47 g/m3), Vf=0.45 m/s (122 g/m3).
158
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
maximum just ahead of the flame (Fig. 4), the variation in the dust concentration will be considered in relation to the moving speed of the particle in the vertical line of the flame center. Namely, it is thought that the particles in the preheating zone are blown up by the flame and that their movement creates the differences in dust concentrations. Such a movement of particles causes a dynamic variation in dust concentration ahead of the flame, which propagates at a lower dust concentration rather than the mean concentration. Further a dynamic variation in dust concentration is repeated and the local dust concentration ahead of the flame would be averaged over the whole duct. Thus, the flame could propagate at a constant velocity in a vertical duct. When a particle below several micrometers in diameter exists in the region of a large temperature gradient, it is well known that the particle moves to low temperature region due to a thermophoresis effect. In this study, however, since the temperature gradient in the lycopodium dust flame is about 398 K/mm and the diameters of the lycopodium particles are large (32 µm), the calculated thermophoresis velocity is very small (0.3 mm/s) (Talbot, Cheng, Schefer & Willis, 1980). Accordingly, the thermophoresis effect in the appearance of the dust concentration maximum can be negligible. 3.4. Flame propagation mechanisms Supposing a mono-uniformed arrangement of particles, the mean distance (L) between the particles near the flame is given by L=[(pD3prs/6Cd)(T/T0)]1/3, in which rs, Dp, Cd, T and T0 are the particle density, the particle diameter, the dust concentration, the flame temperature and the initial temperature, respectively. When particles approach the flame, L increases as does temperature. For example, the inter-particle distance will vary from 0.51 mm in the unburnt region to 1.73 mm in the burnt region at a dust concentration of 47 g/m3. The dust concentration at the spot the flames (0.5–1.0 mm in diameter) combine is about 230 g/m3 and then the propagating flame in front would become continuous at higher concentrations. Since particle agglomerations exists in dust clouds, spot flames at concentrations ⬍230 g/m3 may combine with each other. The distance between particles at the front of the preheating zone increases slightly over that in the unburnt region due to the increase in dust concentration; for example, 0.48–0.51 mm at 47 g/m3 and 0.34–0.37 mm at 120 g/m3, respectively. From this consideration, the distance between particles is useful for understanding of the flame structure and the development of the independent flame. Based on the particle behavior, a flame propagation mechanism in lycopodium dust clouds is illustrated in Fig. 7. An unburnt particle (Particle 1 in Fig. 7) far from the flame settles down gravitationally. When it approaches the leading edge of the preheating zone, its
vertical velocity decreases to nearly zero. The particle that enters the preheating zone becomes heated, but it moves upward in the same direction as flame propagation, as can be seen in particles 2, 3, 4 in Fig. 7. The particle is accelerated to maximum velocity near the leading edge of the reaction zone. Since such particle movement increases its residence time in the preheating zone, the particle will be sufficiently heated to cause pyrolysis. The residence time will be estimated by dpr/(Vf⫺Vp), in which Vf, Vp and dpr are the flame velocity, the particle velocity and preheating zone thickness, respectively. The released combustible gases close to the particle will diffuse towards the surrounding atmosphere. When the diffused combustible gases reach ignition temperature (Ti), the particle is ignited and an enveloped diffusion flame (particle 5 in Fig. 7) forms around it at time t0. The minimum ignition temperatures between 425 and 460°C for a lycopodium dust cloud in air has been found (Eckhoff, 1997). When the temperature around the ignited particle reaches flame temperature (Tf), the leading edge of the preheating zone at t0 moves upward due to thermal diffusion and convection from the flame and then the new temperature profile becomes T1. Particle 4 has maximum moving velocity and is then ignited at t1 to form the leading edge of reaction zone (particle 4⬙ in Fig. 7). As a result, the leading edge of the reaction zone moves discontinuously from particle 5 to particle 4⬙ with an ignition delay time (tig=L/(Vf⫺Vp)). In this manner, particle 3 will be ignited at t2 with the delay time. The flame velocity (Vf) is larger than the particle moving velocity (Vp). A microscopic view of this flame propagation mechanism will show that the lycopodium dust flame discontinuously propagates from one particle to those adjacent in a laminar dust cloud. However, the proposed model has been made by emphasizing the movement of a single particle. In the following, a mechanism of flame propagation is further considered in terms of agglomerates and dust concentration. Fig. 8 shows the schematic explanation of the mechanisms for flame propagation in a lycopodium dust cloud, where individual particles and many agglomerates exist. Since individual particles in the dust cloud will be heated more quickly than the agglomerates, they will be ignited first and form spot flames. At this stage of flame propagation, these spot flames will sustain the flame. Thereafter, a few volatile gases will be formed and diffused around the agglomerates, making a blue flame [A in Fig. 8(a)]. When each particle in an agglomerate is separated by the release of pyrolyzed gases, they will burn, making an independent flame [B in Fig. 8(a)]. After the combustion of pyrolyzed gases inside the independent flame, the particles are then pyrolyzed completely and the independent flame would become luminous and disappear [C in Fig. 8(b)]. The burning time of these independent flames is 4–6 ms and their move-
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
Fig. 7.
159
Scheme of the flame propagation mechanism based on the movement of one dust particle.
Fig. 8. Schematic explanation of the mechanisms of flame propagation in lycopodium dust clouds. da=distance between the agglomerates of dust particles.
ment in space during the movement of leading flame edge is limited to a distance of 1–2 mm. A richer dust concentration generates a larger combustion heat, thereby causing speedy flame propagation by high heat transfer. In this case, the distance between the particles become closer with an increase in dust concentration, so that spot flames and independent flames will merge with ease into a thick and continuous flame [D in Fig. 8(c)]. The oxygen for the independent flame behind the continuous flame would be supplied from the fresh region ahead of the flame front.
4. Conclusions For better understanding of flame propagation mechanisms using lycopodium dust clouds, an experimental study on the behavior of the particles near flame was conducted. The movement of particles was examined by a PIV measurement system, with the following conclusions being drawn. 1. Although lycopodium dust is well known for its monodispersed and narrow particle size distribution, the
160
2.
3.
4.
5.
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
actual dust distribution in a dust cloud involved a large number of particle agglomerates as well as individually suspended particles. The double flame structure, specific to the lycopodium flame, in which spot and independent flames existed, will be created, corresponding to these particle forms in the dust cloud. The free-falling lycopodium particles caused by gravitational forces were shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front of the flame. The particle velocity in the center axis of the flame reached a maximum just ahead of the flame front. When the particles meet with settling particles, there exists a stagnation area with their velocities approaching zero and their dust concentration being enriched. The preheating zone of the flame was estimated to start from the center of the stagnation area. Movement of the particles causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentrations rather than at the mean concentration. Dynamic variation in dust concentration is repeated and the local dust concentration ahead of the flame would be averaged over the whole duct. This will be considered one of the reasons why the flame could propagate at a constant velocity in a vertical duct. The analysis of the movement of a single particle indicated a flame propagation mechanism where an enveloped and diffusion lycopodium dust flame discontinuously propagates from one particle to another in a laminar dust cloud. A mechanism of flame propagation was considered in terms of agglomerates and dust concentration. Individual particles are ignited first and form spot flames, and then each particle in an agglomerate forms an independent flame. The richer dust concentration generates a larger heat of combustion and the distance between the particles become closer, so that spot and independent flames are merged into a continuous flame.
References Adrian, R. J. (1991). Particle imaging technique for experimental fluid mechanics. A. Rev. Fluid Mech., 23, 261–304.
Ballal, D.R. (1983) Further studies on the ignition and flame quenching of quiescent dust clouds, Proceedings of the Royal Society of London, Series A: Mathematical and Physical Sciences, 385, 1–19. Berlad, A. L., Ross, H., Facca, L., & Tangirala, V. (1990). Particle cloud flames in acoustic fields. Combust. Flame, 82, 448–450. Eckhoff, R. K. (1997). Dust explosion in the process industries. (2nd ed.). Oxford: Butterworth Heinemann. Han, O.-S., Yashima, M., Matsuda, T., Matsui, H., Miyake, A., & Ogawa, T. (1999). Mechanism of development of post flames during the dust flame propagation in a vertical combustion duct. In The 1st Conference of the Association of Korean–Japanese Safety Engineering Society, Korean Safety Engineering Institute, (pp. 111–114). Han, O.-S., Yashima, M., Matsuda, T., Matsui, H., Miyake, A., & Ogawa, T. (2000). Behavior of flame propagating through lycopodium dust clouds in a vertical duct. J. Loss Prev. Process Ind., 13 (6), 449–457. Hertzberg, M., & Cashdollar, K. L. (1987). Introduction to dust explosions. Industrial dust explosions. In ASTM Special Technical Publication No. 958 (pp. 5–32). Philadelphia: ASTM. Hesselink, L. (1998). Digital image processing in flow visualization. A. Rev. Fluid Mech., 20, 421–485. Ju, W. J., Dobashi, R., & Hirano, T. (1998). Reaction zone structures and propagation mechanisms of flames in steric acid particle clouds. J. Loss Prev. Process Ind., 11, 423–430. Krazinski, J. L., Buckius, R. O., & Krier, H. (1979). Coal dust flames; a review and development of a model for flame propagation. Prog. Energy Combust Sci., 5, 31–71. Lewis, B., & Von Elbe, G. (1961). In Combustion flames and explosions of gases (2nd ed.) (pp. 292–294). New York: Academic Press. Mason, W. E., & Saunders, K. V. (1975). Recirculating flow in vertical columns of gas–solid suspension. J. Appl. Phys., 8, 1674–1685. Proust, Ch., & Veyssiere, B. (1988). Fundamental properties of flames propagating in starch dust–air mixtures. Comb. Sci. Tech., 62, 149–172. Reuss, D.L., Adrian, R.J., Landreth, C.C., French, D.T., & Fansler, T.D. (1989). Instantaneous planar measurements of velocity and large-scale vorticity and strain rate in an engine using PIV. The Engineering Society for Advanced Mobility Land Sea Air and Space, USA, SAE Paper 890616. Smoot, L. D., & Horton, M. D. (1977). Propagation of laminar pulverized coal-air flame. Prog. Energy Combust. Sci., 3, 235–258. Sun, J. H., Dobashi, R., & Hirano, T. (1998). Structure of flame propagation through metal particle cloud and behavior of particles. In 27th Symposium (International) on Combustion (pp. 2405–2411). Pittsburgh: The Combustion Institute. Talbot, L., Cheng, R. K., Schefer, R. W., & Willis, D. R. (1980). J. Fluid Mech., 101, 737.
A study of flame propagation mechanisms in lycopodium dust clouds based on dust particles’ behavior Ou-Sup Han b
a,*
, Masaaki Yashima a, Toei Matsuda a, Hidenori Matsui a, Atsumi Miyake b, Terushige Ogawa b
a National Institute of Industrial Safety, Ministry of Labor, 1-4-6 Umezono, Kiyose-si, Tokyo 204-0024, Japan Department of Safety Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Abstract Following our earlier study on the behavior of lycopodium dust flames, further experiments using a particle image velocimetry system with a high-resolution video camera have been conducted to clarify the mechanisms of laminar dust flame propagation in a vertical duct. Lycopodium, a nearly equal-sized particle, has been recognized as being monodispersed, but it was found that an actual lycopodium dust cloud consisted of individual and agglomerated particles. Corresponding to the particle forms, the reaction zone showed a double flame structure, consisting of enveloped diffusion flames (spot flame) of individual particles and diffusion flames (independent flame) surrounding some particles. Due to the convective flow caused by a flame, part of the gravitational settling particles was shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front of the flame. Such particle movement causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentration rather than the mean concentration. Although the flame moved discontinuously on a micro scale, an overall constant flame velocity was found, presumably due to the dynamic variation in dust concentration and induced flow ahead of the flame. Judging from the above-mentioned movement of single particles in front of the flame, a residence time of the unburnt particle in the preheating zone is needed to form combustible gases close to the particle. This residence time depends on the preheating zone thickness, the particle velocity and the flame propagation velocity. The observation of the movement of a single particle suggested a flame propagation mechanism where an enveloped and diffusion lycopodium dust flame discontinuously propagates from one particle to those adjacent in a laminar suspension. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dust flame; Flame propagation; Lycopodium; PIV
1. Introduction Many experimental and theoretical studies on the properties of flame propagation in dust clouds have been conducted (Krazinski, Buckius & Krier, 1979; Hertzberg & Cashdollar, 1987; Berlad, Ross, Facca & Tangirala, 1990; Sun, Dobashi & Hirano, 1998). Nevertheless, knowledge on fundamental phenomenon of dust flame propagation is still imprecise and insufficient to explain the mechanism of flame propagation in dust clouds. Smoot and Horton (1977) showed that the flame propagation mechanism of a laminar coal-air flame is diffusion controlled as a result of the combustion of volatiles. Pro-
* Corresponding author. Tel.: +81-424-91-4512; fax: +81-424-917846. E-mail address: [email protected] (O.-S. Han).
ust and Veyssiere (1988) reported that, from temperature profile and luminosity of a starch dust flame, there is no temperature rise associated with measurable radiative flux. These results indicate that laminar dust flame propagation mechanism is controlled by thermal and molecular diffusion into the preheating zone. Ballal (1983) investigated that the effect of radiative heat losses from dust particles on the burning velocity in dust clouds in microgravity. Ju, Dobashi and Hirano (1998) reported that the flame propagation mechanism of stearic acid particles is similar to that of a usual hydrocarbon-air premixed flame, although the reaction zone thickness is larger than that of a premixed flame. Recently the study of propagation mechanisms based on dust flame structure has become of interest, but very little fundamental information has been given regarding the mechanisms of dust flame propagation. In our previous study (Han et al., 2000), the structure
0950-4230/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 0 0 ) 0 0 0 4 9 - 8
154
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
of flame propagating through lycopodium dust particles in a vertical duct was examined experimentally. As a result, it was found that the flame front shows a double flame structure, which consists of isolated individual burning particles and ball-shaped flames surrounding some particles. The ball-shaped flame has been defined as an independent flame in our study. Although the independent flames form irregularly in the individual burning particles ahead of flame, the leading edge of the lycopodium flame moves at nearly constant velocity for the whole range of the dust concentration examined. However, it has not been clarified yet how independent flames could be formed and how the combustion zone moves in a lycopodium dust cloud. Since the dust flame depends on the behavior of dust particles, it is necessary to explore the movement of particles near the flame for the elucidation of dust flame propagation mechanism. Many studies on dust explosion and flame propagation in lycopodium dust clouds have been reported (Eckhoff, 1997), and thus lycopodium dust was selected in this study. This paper reports the further study of laminar flame propagation in a lycopodium dust cloud. Visualization of movements of the particles near the flame during flame propagation in a vertical duct is analyzed using a PIV (particle image velocimetry) measurement system. From these results, the mechanism of laminar flame propagation through lycopodium dust particles is discussed.
2. Experimental 2.1. Experimental apparatus and procedure The vertical duct of 1800 mm height with 150 mm × 150 mm square cross-section and experimental arrangement are the same as that described in the previous study (Han et al., 2000). Here only the experimental procedure and the visualization method of dust particles using a PIV system are explained. The experimental procedure is as follows. Lycopodium particles having nearly equal particle sizes of mean diameter of 32 µm were layered on a fine porous plate supported by a metallic grid at the bottom of the duct. With the condition of the duct top and bottom open, an airflow with an adjustable flow rate was introduced through the porous plate which acted as a flow rectifier to disperse the dust. When the dust particles have filled the duct, a time controlling system stops the airflow and removes a fluidized bed from the bottom of the duct. After closing the shutter at the top end of duct, the dust cloud was ignited at the bottom of the duct by an electric spark lasting 0.3 s in duration. Mean dust concentrations were determined by measuring the decrease in dust mass in the movable fluidized bed. As shown in Fig. 1, the movement of dust particles near the front of the flame during flame propagation was
recorded by a high-resolution video camera (1018 × 1008 pixel) and analyzed by a PIV system (TSI Inc., USA). In order to obtain a good visualization of the particles’ movement, quartz glass windows were set up on the duct side walls and a vertical laser light sheet was established in the middle of the vertical combustion duct. 2.2. Visualization of the dust particles’ movements PIV is an instantaneous, non-intrusive full field flow measuring technique, and provides a velocity field of the flow being the most powerful tool to date (Adrian, 1991; Hesselink, 1998). A scheme of the PIV measurement system is illustrated in Fig. 1. It consists mainly of two parts of a flow visualization system and a digital image processing system for the quantitative measurements of instantaneous particle velocities. In order to disclose the particles’ motion near the flames, the flow in flame propagation is seeded by the lycopodium (combustible particles themselves) that will scatter the pulsing laser sheet, and photographed twice from the side with the high-resolution camera. The PIV parameters used for the experiment were as follows: object size, 60×60 mm; time between pulsed lasers; 1 and 10 ms. The resulting double-exposed image, taken in a known time interval (time between pulsed lasers), provides a displacement record of the particles within a measurement plane, which is then analyzed and scaled to velocity by numerical processing. A PIV image will be divided up into some thousands of interrogation areas with each producing a local value of displacement when an analysis algorithm is applied. A local interrogation area of a PIV image is smaller than the smallest recorded scale of 1 mm in an analysis algorithm of PIV (Reuss, Adrian, Landreth, French & Fansler, 1989). The correlation processing technique was used to determine the correct pairings within an interrogation region.
3. Results and discussions 3.1. Development of independent flames At the concentration range of 47–592 g/m3, there existed a local decrease (10–20 g/m3) in the dust concentration with the height of the vertical duct. In spite of these variations in dust concentration, the leading edge of flame in each concentration moves at a constant velocity in the whole duct (Han et al., 2000). This result could be interpreted in terms of dynamic variation in dust concentration in front of a propagation flame. The behavior of the lycopodium particles was then examined in detail to give the spatial distribution of particles in dust clouds near and far the flame front. An example of the laser light scattering images of the particles is shown in Fig. 2, which has been recorded for a laser light sheet
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
Fig. 1.
155
Schema of the PIV measurement system.
Fig. 2. Aspect of the instantaneous vertical sectional distribution of lycopodium dust particles in the laser light sheet 0.5 mm thick. C=122 g/m3, time interval of pulsed lasers=10 ms.
0.5 mm thick using a band-pass filter of 532 nm. The band-pass filter was used to eliminate the luminosity of the dust flame. The image in the Fig. 2 shows both individual particles and a large number of groups of lycopodium particles, whose regional size is about 1–3 mm in diameter. It is believed that lycopodium dust, often used as a reference dust, consists of monodispersed particles, having the nearly same size in particle diameter.
However, its cloud was, in fact, composed of two forms, that is, the individual dust particles and particle agglomerations. These particle agglomerations may be attributable to static electricity by friction and conflict between the particles due to a continuous agitating flow in a fluidized bed. Comparison of the size over the diameter (2– 4 mm) of independent flames with that (1–3 mm) of an agglomerate of dust particles gives the reason why independent flames are formed. Namely, individual particles are ignited first and then the agglomerates of the particles will be heated to release pyrolyzed gases before ignition. Luminosities of the independent flames change with time. It seems that the variation of luminosities for independent flames will be related to three stages of thermal decomposition characteristics of lycopodium spores (Han et al., 2000). Initially, a faint ball-shaped flame appears around an agglomerate. Although the lycopodium spores used included moisture content of 3.3– 3.7 wt%, it could be mainly due to burning of the easyreleased volatile corresponding to the ca 7% weight loss of lycopodium (first stage of weight loss) in a premixed flame. Then each particle of the agglomerate will be separated with evolution of pyrolyzed gases, presumably due to the weight loss which is more than half the weight of lycopodium (second stage of weight loss). The combustible pyrolyzed gases close to each particle in an independent flame are consumed in diffusion combustion with each enveloped flame. The cellulose residues of the spore will be pyrolyzed completely (third stage of weight loss). Then, the independent flame would become luminous with diffusion-controlled reactions. The rate of heating of particles in the cases of thermal analysis and flame is greatly different, so that pyrolyzed characteristics of particle will be changed. However, the above explanation for flame luminosity will be supported from the observation of independent flame. As observed in our previous study, microscopic flame in lycopodium dust cloud is not continuous. This means that pyrolyzed combustible gases stay close or near to single or agglomerated particles before ignition.
156
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
3.2. Behavior of dust particles around the flame Fig. 3 shows an instantaneous two-dimensional visualization image [Fig. 3(a)] and the velocity distribution [Fig. 3(b)] of particles. The image [Fig. 3(a)], illuminated by vertical laser light sheet of 0.5 mm in thickness, was taken using a high-resolution video camera with the band-pass filter. This picture shows the left hand part of the flame and the vector arrows of the particle velocity obtained. The velocity distribution of the particle [Fig. 3(b)] was calculated from the PIV image [Fig. 3(a)]. In Fig. 3(b), it can be seen that particles at about 8 mm above the top of flame move horizontally and particles far from the flame shift obliquely from the top to the left. The velocity of particles around the flame takes a maximum value of 0.55 m/s near the duct wall, and a minimum value of 0.07 m/s at about 8 mm from the top of the flame. Also, it is seen that most of the unburnt particles which have passed through the front of the flame disappear, and only a few particles exist in the combustion zone. The variation in velocities of the moving particles in the center axis of flame has been measured and is shown in Fig. 4 along with the distance from the leading edge of the flame. In the case of initial dust concentration of C0=47 g/m3, unburnt particles at a distance from the flame front of ⬎11 mm in the vertical center axis (y) settle down at a velocity of about 0.04 m/s. At y=9 mm, the unburnt particle velocity becomes zero and, ⬍9 mm, they start to be accelerated in the same direction as the upward flame propagation. Thereafter, they take a maximum velocity (0.29 m/s) just before the dust particles are caught up with in the reaction zone. In the case of C=122 g/m3, at yⲏ13 mm, the unburnt particles fall gravitationally at a velocity of 0.06 m/s. Then the unburnt particles start to be accelerated at y⬇11 mm, and
Fig. 4. Dust particle velocity with change of distance from the leading edge of the reaction zone at the center axis of the flame. Vf=0.39 m/s (47 g/m3), Vf=0.47 m/s (122 g/m3).
the maximum velocity (0.38 m/s) of a moving particle is then attained at the leading edge of the parabolicshaped flame. From the PIV measurements, we have a schematic representation of the movement of the particles near a propagating flame in Fig. 5. The flow pattern for laminar flame propagation in a gas-fuel and oxidizer mixture is well known (Lewis & Von Elbe, 1961). A propagating gaseous flame produces an upward and convective flow ahead of the flame. In the dust-air flame, part of the gravitationally settling particles are shifted to the surrounding sides due to the convection flow and the rest of the particles change their movement upward due to
Fig. 3. Instantaneous PIV image (a), and velocity distributions (b) of vertical two-dimensional dust particles corresponding to the interrogated PIV photograph of flame propagation in a 122 g/m3 lcopodium–air mixture. Time interval of pulsed lasers=10 ms, Vf=0.41 m/s.
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
Fig. 5.
157
Movement of dust particles near the flame front during flame propagation.
the induced flow just in front of the flame. When these upward moving particles are blown up at a maximum speed, they will be ignited and then involved in the reaction. At 47–122 g/m3, the upward moving particles meet with settling particles at about 8 mm (Sa in Fig. 5) from the leading edge of the parabolic-shaped flame, and the mean vertical velocity of the particles at Sa approaches zero. Accordingly this region would become a stagnation area for the particles with an enriched dust concentration, which could be estimated from the flame speed, the particle sedimentation speed and the average dust concentration. Mason and Saunders (1975) showed that the settling dust particles relative to the air would cause concentration changes at the flow boundaries wherever the air velocity had a horizontal component. Conduction and convection heat transfer to particles would be attained after the particles had entered the stagnation area, and the leading edge of the preheating zone would be suggested from the particle velocities. Further, it is found that unburnt particles near the duct wall move downward and then turn to the rear region of the parabolic-shaped flame along with a convective flow (Han et al., 1999).
observation area (4×4 mm) of the PIV image. Fig. 6 shows the variation in dust ratio concentration with a laser light sheet of 0.5 mm thick along the vertical line of the flame center. In Fig. 6, the preheating zones are included, which have been measured using a schlieren system, an ion probe and a video camera. As, in the case of C=47 g/m3, the dust concentration in the unburnt region reaches a peak about 9 mm from the leading edge of the reaction zone and C/C0 reaches a peak at y=8 mm and C=122 g/m3, the maximum value of C/C0 appears just ahead of the preheating zone, presumably corresponding to an enriched dust concentration in the stagnation area (Fig. 4). After the maximima, the ratios (C/C0) of dust concentration in the preheating zone decrease to a value lower than the original unburnt dust concentration. Since the particle velocity reaches its
3.3. Estimated dust concentration near flame The variation in dust concentration ahead of the flame was estimated from laser scattering images of lycopodium particles, although they would give erroneous overlapped shadow images. Dust concentration (C) was assumed to be proportional to A/A0 [the ratio of the total shadow area (A) for the particles to that (A0) for one single particle]. They were obtained with the aid of an image processor and the observed shadow area for one single particle was 1.30×10⫺3 mm2 in the unburnt area. The estimated dust concentrations at 18 mm from the flame front and a vertical distance (y) from the flame front are represented by C0 and C, respectively, in an
Fig. 6. Variation in dust concentration ratio with distance from the flame front at the center axis of flame. Vf =0.40 m/s (47 g/m3), Vf=0.45 m/s (122 g/m3).
158
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
maximum just ahead of the flame (Fig. 4), the variation in the dust concentration will be considered in relation to the moving speed of the particle in the vertical line of the flame center. Namely, it is thought that the particles in the preheating zone are blown up by the flame and that their movement creates the differences in dust concentrations. Such a movement of particles causes a dynamic variation in dust concentration ahead of the flame, which propagates at a lower dust concentration rather than the mean concentration. Further a dynamic variation in dust concentration is repeated and the local dust concentration ahead of the flame would be averaged over the whole duct. Thus, the flame could propagate at a constant velocity in a vertical duct. When a particle below several micrometers in diameter exists in the region of a large temperature gradient, it is well known that the particle moves to low temperature region due to a thermophoresis effect. In this study, however, since the temperature gradient in the lycopodium dust flame is about 398 K/mm and the diameters of the lycopodium particles are large (32 µm), the calculated thermophoresis velocity is very small (0.3 mm/s) (Talbot, Cheng, Schefer & Willis, 1980). Accordingly, the thermophoresis effect in the appearance of the dust concentration maximum can be negligible. 3.4. Flame propagation mechanisms Supposing a mono-uniformed arrangement of particles, the mean distance (L) between the particles near the flame is given by L=[(pD3prs/6Cd)(T/T0)]1/3, in which rs, Dp, Cd, T and T0 are the particle density, the particle diameter, the dust concentration, the flame temperature and the initial temperature, respectively. When particles approach the flame, L increases as does temperature. For example, the inter-particle distance will vary from 0.51 mm in the unburnt region to 1.73 mm in the burnt region at a dust concentration of 47 g/m3. The dust concentration at the spot the flames (0.5–1.0 mm in diameter) combine is about 230 g/m3 and then the propagating flame in front would become continuous at higher concentrations. Since particle agglomerations exists in dust clouds, spot flames at concentrations ⬍230 g/m3 may combine with each other. The distance between particles at the front of the preheating zone increases slightly over that in the unburnt region due to the increase in dust concentration; for example, 0.48–0.51 mm at 47 g/m3 and 0.34–0.37 mm at 120 g/m3, respectively. From this consideration, the distance between particles is useful for understanding of the flame structure and the development of the independent flame. Based on the particle behavior, a flame propagation mechanism in lycopodium dust clouds is illustrated in Fig. 7. An unburnt particle (Particle 1 in Fig. 7) far from the flame settles down gravitationally. When it approaches the leading edge of the preheating zone, its
vertical velocity decreases to nearly zero. The particle that enters the preheating zone becomes heated, but it moves upward in the same direction as flame propagation, as can be seen in particles 2, 3, 4 in Fig. 7. The particle is accelerated to maximum velocity near the leading edge of the reaction zone. Since such particle movement increases its residence time in the preheating zone, the particle will be sufficiently heated to cause pyrolysis. The residence time will be estimated by dpr/(Vf⫺Vp), in which Vf, Vp and dpr are the flame velocity, the particle velocity and preheating zone thickness, respectively. The released combustible gases close to the particle will diffuse towards the surrounding atmosphere. When the diffused combustible gases reach ignition temperature (Ti), the particle is ignited and an enveloped diffusion flame (particle 5 in Fig. 7) forms around it at time t0. The minimum ignition temperatures between 425 and 460°C for a lycopodium dust cloud in air has been found (Eckhoff, 1997). When the temperature around the ignited particle reaches flame temperature (Tf), the leading edge of the preheating zone at t0 moves upward due to thermal diffusion and convection from the flame and then the new temperature profile becomes T1. Particle 4 has maximum moving velocity and is then ignited at t1 to form the leading edge of reaction zone (particle 4⬙ in Fig. 7). As a result, the leading edge of the reaction zone moves discontinuously from particle 5 to particle 4⬙ with an ignition delay time (tig=L/(Vf⫺Vp)). In this manner, particle 3 will be ignited at t2 with the delay time. The flame velocity (Vf) is larger than the particle moving velocity (Vp). A microscopic view of this flame propagation mechanism will show that the lycopodium dust flame discontinuously propagates from one particle to those adjacent in a laminar dust cloud. However, the proposed model has been made by emphasizing the movement of a single particle. In the following, a mechanism of flame propagation is further considered in terms of agglomerates and dust concentration. Fig. 8 shows the schematic explanation of the mechanisms for flame propagation in a lycopodium dust cloud, where individual particles and many agglomerates exist. Since individual particles in the dust cloud will be heated more quickly than the agglomerates, they will be ignited first and form spot flames. At this stage of flame propagation, these spot flames will sustain the flame. Thereafter, a few volatile gases will be formed and diffused around the agglomerates, making a blue flame [A in Fig. 8(a)]. When each particle in an agglomerate is separated by the release of pyrolyzed gases, they will burn, making an independent flame [B in Fig. 8(a)]. After the combustion of pyrolyzed gases inside the independent flame, the particles are then pyrolyzed completely and the independent flame would become luminous and disappear [C in Fig. 8(b)]. The burning time of these independent flames is 4–6 ms and their move-
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
Fig. 7.
159
Scheme of the flame propagation mechanism based on the movement of one dust particle.
Fig. 8. Schematic explanation of the mechanisms of flame propagation in lycopodium dust clouds. da=distance between the agglomerates of dust particles.
ment in space during the movement of leading flame edge is limited to a distance of 1–2 mm. A richer dust concentration generates a larger combustion heat, thereby causing speedy flame propagation by high heat transfer. In this case, the distance between the particles become closer with an increase in dust concentration, so that spot flames and independent flames will merge with ease into a thick and continuous flame [D in Fig. 8(c)]. The oxygen for the independent flame behind the continuous flame would be supplied from the fresh region ahead of the flame front.
4. Conclusions For better understanding of flame propagation mechanisms using lycopodium dust clouds, an experimental study on the behavior of the particles near flame was conducted. The movement of particles was examined by a PIV measurement system, with the following conclusions being drawn. 1. Although lycopodium dust is well known for its monodispersed and narrow particle size distribution, the
160
2.
3.
4.
5.
O.-S. Han et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 153–160
actual dust distribution in a dust cloud involved a large number of particle agglomerates as well as individually suspended particles. The double flame structure, specific to the lycopodium flame, in which spot and independent flames existed, will be created, corresponding to these particle forms in the dust cloud. The free-falling lycopodium particles caused by gravitational forces were shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front of the flame. The particle velocity in the center axis of the flame reached a maximum just ahead of the flame front. When the particles meet with settling particles, there exists a stagnation area with their velocities approaching zero and their dust concentration being enriched. The preheating zone of the flame was estimated to start from the center of the stagnation area. Movement of the particles causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentrations rather than at the mean concentration. Dynamic variation in dust concentration is repeated and the local dust concentration ahead of the flame would be averaged over the whole duct. This will be considered one of the reasons why the flame could propagate at a constant velocity in a vertical duct. The analysis of the movement of a single particle indicated a flame propagation mechanism where an enveloped and diffusion lycopodium dust flame discontinuously propagates from one particle to another in a laminar dust cloud. A mechanism of flame propagation was considered in terms of agglomerates and dust concentration. Individual particles are ignited first and form spot flames, and then each particle in an agglomerate forms an independent flame. The richer dust concentration generates a larger heat of combustion and the distance between the particles become closer, so that spot and independent flames are merged into a continuous flame.
References Adrian, R. J. (1991). Particle imaging technique for experimental fluid mechanics. A. Rev. Fluid Mech., 23, 261–304.
Ballal, D.R. (1983) Further studies on the ignition and flame quenching of quiescent dust clouds, Proceedings of the Royal Society of London, Series A: Mathematical and Physical Sciences, 385, 1–19. Berlad, A. L., Ross, H., Facca, L., & Tangirala, V. (1990). Particle cloud flames in acoustic fields. Combust. Flame, 82, 448–450. Eckhoff, R. K. (1997). Dust explosion in the process industries. (2nd ed.). Oxford: Butterworth Heinemann. Han, O.-S., Yashima, M., Matsuda, T., Matsui, H., Miyake, A., & Ogawa, T. (1999). Mechanism of development of post flames during the dust flame propagation in a vertical combustion duct. In The 1st Conference of the Association of Korean–Japanese Safety Engineering Society, Korean Safety Engineering Institute, (pp. 111–114). Han, O.-S., Yashima, M., Matsuda, T., Matsui, H., Miyake, A., & Ogawa, T. (2000). Behavior of flame propagating through lycopodium dust clouds in a vertical duct. J. Loss Prev. Process Ind., 13 (6), 449–457. Hertzberg, M., & Cashdollar, K. L. (1987). Introduction to dust explosions. Industrial dust explosions. In ASTM Special Technical Publication No. 958 (pp. 5–32). Philadelphia: ASTM. Hesselink, L. (1998). Digital image processing in flow visualization. A. Rev. Fluid Mech., 20, 421–485. Ju, W. J., Dobashi, R., & Hirano, T. (1998). Reaction zone structures and propagation mechanisms of flames in steric acid particle clouds. J. Loss Prev. Process Ind., 11, 423–430. Krazinski, J. L., Buckius, R. O., & Krier, H. (1979). Coal dust flames; a review and development of a model for flame propagation. Prog. Energy Combust Sci., 5, 31–71. Lewis, B., & Von Elbe, G. (1961). In Combustion flames and explosions of gases (2nd ed.) (pp. 292–294). New York: Academic Press. Mason, W. E., & Saunders, K. V. (1975). Recirculating flow in vertical columns of gas–solid suspension. J. Appl. Phys., 8, 1674–1685. Proust, Ch., & Veyssiere, B. (1988). Fundamental properties of flames propagating in starch dust–air mixtures. Comb. Sci. Tech., 62, 149–172. Reuss, D.L., Adrian, R.J., Landreth, C.C., French, D.T., & Fansler, T.D. (1989). Instantaneous planar measurements of velocity and large-scale vorticity and strain rate in an engine using PIV. The Engineering Society for Advanced Mobility Land Sea Air and Space, USA, SAE Paper 890616. Smoot, L. D., & Horton, M. D. (1977). Propagation of laminar pulverized coal-air flame. Prog. Energy Combust. Sci., 3, 235–258. Sun, J. H., Dobashi, R., & Hirano, T. (1998). Structure of flame propagation through metal particle cloud and behavior of particles. In 27th Symposium (International) on Combustion (pp. 2405–2411). Pittsburgh: The Combustion Institute. Talbot, L., Cheng, R. K., Schefer, R. W., & Willis, D. R. (1980). J. Fluid Mech., 101, 737.