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Flame propagation behaviors of nano- and micro-scale PMMA dust explosions
Journal of Loss Prevention in the Process Industries 40 (2016) 101e111
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Flame propagation behaviors of nano- and micro-scale PMMA dust explosions Xinyan Zhang a, Jianliang Yu a, Xingqing Yan a, Qiaofeng Xie b, Wei Gao a, c, * a
School of Chemical Machinery, Dalian University of Technology, Dalian, 116024, China School of Aerospace Engineering, Tsinghua University, Beijing, 100084, China c Institute of Fluid Science, Tohoku University, Sendai, Miyagi, 980-8577, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 8 November 2015 Accepted 15 December 2015 Available online 18 December 2015
Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m3 to 217 g/m3 for 100 nm PMMA particles, and from 72 g/m3 to 170 g/m3 for 30 mm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 mm PMMA dust clouds. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dust explosion Nano- and micro-PMMA particles Flame structure Propagation mechanism
1. Introduction Accompanying the wide use of nano-particles in a number of fields such as solar cell materials, electrode materials, high-density magnetic recording materials, high toughness ceramic materials, catalyst materials, and body repair materials in recent years, nanosafety has been attracted as an urgent concern. As the core aspect of nano-safety, dust explosion is a phenomenon that a flame is propagating in combustible nanoparticles dispersed in the air. The explosion intensity of nano-particles is typically stronger than micro-particles. Studies on nano-dust explosion have been extensively conducted, most of which were focused on the explosion sensitivity (Minimum Explosible Concentration (MEC), Minimum Ignition Energy (MIE) and Minimum Ignition Temperature (MIT)) and the explosion severity (Maximum Explosion Pressure (Pmax),
* Corresponding author. School of Chemical Machinery, Dalian University of Technology, Dalian, 116024, China. E-mail address: [email protected] (W. Gao). http://dx.doi.org/10.1016/j.jlp.2015.12.010 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
Maximum Rate of Pressure-rise ((dP/dt)max), and Deflagration Index (KSt)) (Krietsch et al., 2015; Mittal, 2014; Boilard et al., 2013; Eckhoff, 2012; Li et al., 2011; Wu et al., 2009). Though the likelihood of an explosion increased significantly as the particle size decreased into the nano-range (Mittal, 2014; Boilard et al., 2013), particles in nano-range were not expected to exhibit more severe than micro-powders (Mittal, 2014; Eckhoff, 2012) due to the serious agglomeration effect of nano-dust clouds. Flame propagation mechanisms through combustible particle clouds for better understanding dust explosions, were mainly proposed for microparticles (Gao et al., 2014, 2015; Cao et al., 2014; Dobashi and Senda, 2006; Proust, 2006; Sun et al., 2000). Gao et al. (Gao et al., 2013) demonstrated that the flame front structure changed along with varying the particle size distributions in micro-range. Ju et al. (Ju et al., 1998) pointed out that the flame structure and propagating velocity were depended on the mass density and the diameter distribution of micro particles strongly. But were the flame structure and propagation mechanism changed when moving from micro-to nano-scale? Until now, the current understanding of flame propagation mechanism in nano-dust cloud is
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still in its rudimentary state. In this study, the flame structures and propagation behaviors were systematically observed through a high-speed camera to reveal the differences between nano- and micro-PMMA dust clouds with different mass densities in an open-space dust explosion apparatus. In addition, simple physical models were developed to explain the different microstructures and flame propagation mechanisms in burning nano- and micro-PMMA dust clouds. 2. Experiments 2.1. Experimental apparatus The open-space dust explosion experimental apparatus, which consisted of combustion tubes, a dispersion system, a gas supplying system, an ignition system, a high-speed photography system, and a time controller system, was schematically shown in Fig. 1. The transparent cylindrical combustion tubes included a top tube with an open end, a movable middle tube, and a bottom tube connected to the stainless steel basement. The movable tube kept the dust cloud free from the turbulence caused by surrounding air movement during dispersion. Just before ignition, the movable tube was moved down to provide an open space for the combustible dust clouds, so that the flame could propagated in an open field without the confinement of the tube wall, and direct observation of flame structure and propagation behavior could be realized. A gas nozzle and a dispersing cone were mounted in the chamber to disperse the experimental dust particles similar to the traditional Hartman basement. The gas supplying unit were composed of a buffer vessel filled with compressed air, a solenoid valve and air pipe lines. The ignition system was composed of a high voltage transformer and a pair of tungsten wire electrodes with the diameter of 0.4 mm. Two high-speed cameras (Photron FASTCAM SA4 and Photron FASTCAM SA5), one installed with a normal lens of Nikon AF Nikkor 50 mm f/ 1.2, and the other installed with a microscopic lens of Nikon AF
Micro 200 mm f/4D, were used to record synchronously the flame propagation behavior and microstructure, respectively. The time when the movable tube started to be moved, the on-off time of the solenoid valve, the startup times of the high-speed video cameras and the action time of the high voltage transformer, were controlled by a programmable logic controller (OMRON CPM1A). Before experiments, the particles were dried in a drying oven for at least 24 h to reduce the liquid bridge force between particles caused by the water content. The experimental steps were as follows: weighed a certain amount of particles and put them in the sample dish uniformly; opened the pressure reducing valve to make sure that the gas pressure in the buffer vessel was 0.5 MPa; turned on the high-speed cameras and started the time controller system. The subsequent processes were controlled by the programmable logic controller. The time the programmable logic controller was triggered was set to be 0 s. From 0.5 s to 1 s, the solenoid valve was opened. The particles were dispersed in the chamber by the ejected high pressure air to form the homogeneous dust cloud. 0.2 s later, the electrical stoppers were triggered and the movable tube was moved down to provide the open combustion space. From 1.4 s, the high speed cameras were triggered to record the flame microstructure and propagation behavior. When the dust cloud was relatively stable and uniform at about 1.5 s, the 15 kV high voltage transformer was discharged for 0.01 s. The suspended particles were ignited and the flame propagated in the open space. To determine the real mass densities of PMMA dust clouds, a measurement illustrated in Fig. 2 was designed to capture the suspended particles in the middle tube space. The measurement system was composed of the particle dispersion unit, the particle capture unit and the particle weigh unit. The particle dispersion conditions were the same as aforementioned those of combustion experiments. A sample capture orifice controlled by an air driven cylinder was inserted into the gap between the tubes. The driven pressure of the cylinder was 0.26 MPa. When there was almost no particles suspended in the middle space, the sample capture orifice
Fig. 1. Experimental apparatus.
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Fig. 2. Mass densities measurement system.
was retracted. Particles which fell on the bottom board were collected and weighted. As a result the mass densities of particles in the middle tube space were determined according to the volume of the middle space. In this study, the real mass densities of 30 mm and 100 nm PMMA particles were listed in Table 1. 2.2. Experimental materials 30 mm and 100 nm PMMA particles provided by Soken Chemical Co., Ltd. of Japan Ministry were selected as the experimental samples. Particle size distributions of 30 mm and 100 nm PMMA particles measured by a Malvern Mastersizer 2000 laser diffraction analyzer were shown in Fig. 3. The size distribution of 30 mm PMMA particles was consistent satisfactorily with the value provided by Soken Chemical Co., Ltd. of Japan Ministry. While the size distribution of 100 nm PMMA particles was obviously larger than the provided value due to the serious agglomeration effect. The volume diameter D[4,3], the Sauter diameter D[3,2], and the parameters Dv (10), Dv (50), and Dv (90) which were the quantiles of the volumetric distribution are listed in Table 2. It was indicated that despite of the agglomeration effect, the Sauter diameter of 100 nm PMMA particles was still much smaller than that of 30 mm PMMA particles, which proved a larger specific surface area for 100 nm PMMA particles. Scanning electron microscope (SEM) was used to observe the static microstructures and the particle sizes of 30 mm and 100 nm PMMA particles by tens of thousands times amplification. It was observed from Fig. 4 that both 30 mm and 100 nm PMMA particles Table 1 Mass densities of 30 mm and 100 nm PMMA particles. Name
MZ-30H (30 mm)
MP-300 (100 nm)
Mass density (g/m3)
72 138 170
105 168 217
Fig. 3. Particle size distributions of 30 mm and 100 nm PMMA particles.
Table 2 Characteristic diameters of 30 mm and 100 nm PMMA particles. Diameters
MZ-30H (30 mm)
MP-300 (100 nm)
D[4,3] (mm) D[3,2] (mm) Dv (10) (mm) Dv (50) (mm) Dv (90) (mm)
28.974 24.080 16.509 24.523 43.186
24.877 10.486 6.151 22.543 46.764
were in regularly spherical shape. The 30 mm PMMA particles were dispersedly distributed, whereas 100 nm PMMA particles exhibited serious agglomeration effect due to the considerable inter-particle forces. The inter-particle forces are mainly composed of van der Waals'
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Fig. 4. SEM images of 30 mm and 100 nm PMMA particles.
forces, electrostatic forces and the liquid bridge forces. The theoretical van der Waals’ force Fw between two spherical particles was
Fw ¼
A d1 d2 a2 d1 þ d2
(1)
where A was a constant, a was the smallest distance between the sphere surfaces, d1 and d2 were the diameters of the two spheres (Eckhoff, 2012). Assumed that the sizes of the contacted particles were the same, the equation was reduced to
Fw ¼
Ad 2a2
(2)
which indicated that the Van der Waals' force was enhanced with decreasing particle diameter. For PMMA particles used in this study, the electrostatic contact force was negligible. In addition, because the particles were dried in a drying oven for at least 24 h before experiments, the liquid bridge force between particles caused by the water content could also be negligible. The inter-particle forces probably mostly depended on the Van der Waals’ forces, which meant that the inter-particle forces of PMMA particles were increasing as decreasing the particle diameter. The agglomeration between particles mainly depended on the dimensionless number C0:
C0 ¼
F mg
(3)
where F was the inter-particle forces, m was the mass of the particles, and g was the gravitational acceleration. It can be concluded that the smaller the particle diameter, the larger the agglomeration dimensionless number. For 100 nm PMMA particles, the inter-particle forces were of significance and the agglomerates were easily formed. These results confirmed the larger particle size distribution than the provided value of 100 nm PMMA particles as shown in Fig. 3.
3. Results and discussion 3.1. Effects of particle diameter scales on flame structures and propagation behaviors A series of dust explosion experiments of 100 nm PMMA particles with a mass density of 168 g/m3 and 30 mm PMMA particles with a mass density of 170 g/m3 were conducted in the open-space experimental apparatus. The flame propagation processes were presented in Fig. 5. It was obvious that the flame structures and propagation behaviors of particles with different diameter scales were significantly different. For 100 nm PMMA dust cloud, the flame structure was smooth and continuous with the spherical shape similar to the premixed gas explosions. It could be inferred that the particles were
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Fig. 5. Flame propagations of 100 nm PMMA dust cloud with mass density of 168 g/m3 (a) and 30 mm PMMA dust cloud with mass density of 170 g/m3 (b).
evaporated or pyrolyzed to form the premixed combustible gases before the flame front passed through. While the flame structure of 30 mm PMMA dust cloud presented the clusters and irregular flame fronts. Compared with 30 mm PMMA particles, 100 nm PMMA particles had a larger specific surface area, which led to the faster pyrolysis rate, diffusing and mixing with oxygen. As a result, the flame exhibited a homogenous and smooth structure similar to the premixed flame propagating in the homogeneous combustible gas. Whereas for 30 mm PMMA particles, the larger particle sizes resulted in the slower pyrolysis and decomposition. It had been demonstrated previously that the flame was firstly propagated towards the smaller particles nearby, and when these smaller particles were completely pyrolyzed, a local premixed flame was formed and developed to heat the adjacent larger particles, thus establishing the local diffusion flame (Gao et al., 2013). In this situation, the combustion reaction only occurred in the regions where the gaseous pyrolyzate of dust particles was available, which resulted in clusters of flame fronts. Additionally, 100 nm PMMA exhibited a stronger combustion luminous intensity due to the more violent nature of the combustion reaction. Using Roberts operator which is an image edge detection algorithm programmed by Matlab, the flame edges were exactly extracted. The original flames and the corresponding flame contours of 100 nm and 30 mm PMMA dust clouds with different mass densities listed in Table 1 were illustrated in Fig. 6. The flame front positions from ignition point to the highest point of the flame front and the flame propagation velocities with time after ignition of 100 nm PMMA dust cloud with mass density of 168 g/m3 and 30 mm
PMMA dust cloud with mass density of 170 g/m3 were obtained and showed in Fig. 7. In these experiments, the flame front of 100 nm PMMA dust cloud reached the upper boundary of the open combustion space at 60 ms after ignition, while the 30 mm PMMA dust cloud kept burning until 70 ms. It was indicated from Fig. 7(a) that the flame front of 100 nm PMMA dust cloud moved upward nearly linearly after ignition, whereas for 30 mm PMMA dust clouds, the flame front position exhibited a different trend due to the complex propagation mechanism that the combustion reaction only occurred in the regions where the gaseous pyrolyzate of dust particles was available, resulting in the diffusion flame combined with the local premixed flame and clusters of flame fronts. In the early combustion stage of 30 mm PMMA dust cloud, the distance of flame front from the ignition point to the highest point of the flame front showed a pattern of first decreasing, keeping constant, and then increasing. This was indicated that the combustion reaction first occurred in the ignition area. Because the effect of particle sedimentation of unburnt particles was more significant than that of thermophoresis force in the early stage of combustion, the flame propagated on other directions rather than upward direction, thus the flame front position could keep constant or even move downward. With more particles participated in the combustion reaction, more heat was released and the thermophoresis was accelerated. The flame propagated upward where the gaseous pyrolyzate of dust particles was available, and the distance of flame front was increased. It was notable that in the later combustion process the flame front propagation exhibited the tendency of “increasingkeeping constant-increasing”, which represented the flame
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Fig. 6. The original flames and the corresponding flame contours of 100 nm and 30 mm PMMA dust clouds with different mass densities.
Fig. 7. Flame front positions from ignition point and flame propagation velocities with time after ignition of 100 nm PMMA dust cloud with mass density of 168 g/m3 and 30 mm PMMA dust cloud with mass density of 170 g/m3.
propagation process of propagating to the adjacent area where the gaseous pyrolyzate of particles was available-the flame in this area developed mostly in other directions rather than upward in a very short time - the flame propagating to another adjacent area. This result was precisely demonstrated by the micro flame photographs showed in Fig. 12. Fig. 7(b) exhibited that the flame propagation velocities were fluctuated in the combustion processes of 100 nm and 30 mm PMMA dust clouds. It was obvious that the flame
propagation velocity of 30 mm PMMA dust cloud was fluctuated more seriously than that of the 100 nm PMMA dust cloud. This phenomenon was caused by the discontinuous propagation fronts of 30 mm PMMA dust cloud and the turbulence induced by heat and mass transfer between particles. The average flame propagation velocities of 100 nm PMMA dust cloud with mass density of 168 g/ m3 and 30 mm PMMA dust cloud with mass density of 170 g/m3 were 0.91 m/s and 0.93 m/s, respectively.
X. Zhang et al. / Journal of Loss Prevention in the Process Industries 40 (2016) 101e111
3.2. Effects of dust mass densities on flame structures and propagation behaviors To analyze the effects of dust mass densities on flame structures and propagation behaviors, 100 nm and 30 mm PMMA dust clouds with different mass densities listed in Table 1 were arranged, and the combustion processes were depicted in Figs. 8e9. For 100 nm PMMA dust clouds illustrated in Fig. 8, when the dust mass density was 105 g/m3, the flame propagated outward almost spherically from the ignition point in the early stage (0e30 ms after ignition). At about 40 ms after ignition, a small flame was formed below the main flame and developed slowly as the combustion proceeded. This was caused by the turbulence induced by particle sedimentation and buoyancy effect distorted the flame. Additionally, the relative smaller flame propagation velocity made the turbulence effect more significant. As a whole, the flame of 100 nm dust clouds with different mass densities were all approximately in regular spherical shape. Increasing the mass density from 105 g/m3 to 217 g/m3, more particles participated in the combustion reaction. Hence, more heat was released to enhance the pyrolysis and decomposition of the unburnt particles. The flame luminous intensity became stronger, and the flame scale became larger. For 30 mm PMMA dust cloud shown in Fig. 9, the flame propagation was considerable weaker when the mass density was 72 g/m3. With the mass density increased to 170 g/m3, the flame scale became larger and the flame front became more irregular. It could be also seen that the flame of 30 mm PMMA dust cloud was positioned on the upper area of the photos compared with that of 100 nm PMMA dust cloud, which indicated that the effect of buoyancy on flame propagation was dominant so that the upward flame propagation velocity was enhanced. The flame front positions that from ignition point to the highest point of the flame front and the flame propagation velocities with time after ignition of 100 nm and 30 mm PMMA dust clouds with different mass densities were shown in Fig. 10. It could be seen that with time going, the flame fronts of 100 nm PMMA dust clouds with
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different mass densities were all quickly moved upward. At 60 ms after ignition, the flame fronts of 100 nm PMMA dust clouds with mass densities of 168 g/m3 and 217 g/m3 were already reached the upper boundary of the open space. The flame propagation velocities were fluctuated in a certain range. With the mass density increasing, the extent of velocity fluctuation became larger. This might be because that with larger mass density, more particles participated in the combustion reaction, so that the turbulence caused by heat transfer and mass transfer between particles was accelerated, thus induced the velocity fluctuation. Another reason was that the agglomeration effect was enhanced along with the increasing concentration. The average flame propagation velocities of 100 nm PMMA dust clouds with mass density of 105 g/m3, 168 g/ m3 and 217 g/m3 were 0.71 m/s, 0.91 m/s and 0.97 m/s, respectively. This indicated that the flame propagation was accelerated with mass density increased in the experimental range. Due to the different propagation mechanism of 30 mm PMMA dust clouds, the flame front positions and flame front propagation velocities exhibited a different change tendency, as shown in Fig 10(c,d). The distance of flame fronts from the ignition point showed the tendency of discontinuous propagation process of 30 mm PMMA dust clouds. The flame propagation velocities of 30 mm PMMA dust clouds were fluctuated due to a discontinuous propagation type and the turbulence caused by heat transfer and mass transfer between particles. The flame velocity fluctuation of 170 g/m3 dust cloud was most violent. The average flame propagation velocities of 30 mm PMMA dust clouds with mass density of 72 g/m3, 138 g/m3 and 170 g/m3 were 0.31 m/s, 0.41 m/s and 0.93 m/s, respectively, which revealed that the average flame propagation was accelerated with mass density increased in the experimental range. 3.3. Flame structures and propagation mechanisms The flame propagation mechanisms through combustible dust clouds are essential for better understanding of dust explosions. By
Fig. 8. Flame propagations of 100 nm PMMA dust clouds with mass densities of 105 g/m3, 168 g/m3 and 217 g/m3.
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Fig. 9. Flame propagations of 30 mm PMMA dust clouds with mass densities of 72 g/m3, 138 g/m3 and 170 g/m3.
premixed flame. In addition, it could be speculated that smaller particles maintained the leading part of propagating flame and governed the combustion process. To quantify the effect of particle size on dust reactivity in the PMMA dust explosion, the dimensionless characteristic number Bi was discussed. The Biot number (Bi) is a measure of internal heat conduction time with respect to external heat transfer time. External heat transfer to the combustion of PMMA particles mainly depended on heat convection and radiation. So Bi could be defined as (Benedetto et al., 2010; Blasi, 1999):
Bi ¼
hc Lc Qr Lc 1 þ ¼ Lc hc þ εsDTi3 l lDTi l
(4)
where ε was the emissivity, s was the StefaneBoltzmann constant, l was the thermal conductivity, DTi was the temperature difference between particle and surrounding gas, hc was the heat transfer coefficient, and Lc was the particle characteristic length. The heat transfer coefficient hc could be defined as:
hc ¼
lNu Lc
(5)
where Nu was the Nusselt number. The formula for the Nusselt number was: 1
1
=
Nu ¼ 2 þ 0:6Re 2 Pr
=
using the high-speed camera with the microscopic lens, the flame structures were more clearly observed. Flame propagation through 100 nm PMMA dust cloud with mass density of 168 g/m3 was depicted in Fig. 11(a), and the corresponding flame structure was illustrated in Fig. 11(b). After ignition by the electrical spark, the flame kernel was formed. The adjacent suspended particles absorbed the released heat and were pyrolyzed and decomposed to form the combustible gas mixture. The continuous flame was developed. The glowing particles were soot particles produced in the combustion reaction. Then the particles in the preheated zone were heated for the subsequent oxidation reaction. It is important to point out that there were a number of luminous spot flames in the combustion zone. These luminous spot flames might be the agglomerates existed in 100 nm PMMA dust clouds. The combustion reaction first occurred on the surface of the agglomerates where the pyrolysis gases were available. As the combustion proceeding, the agglomerates were completely pyrolyzed and gasified with more heat transfer, and the flame then exhibited homogenous and smooth structure. This result indicated that the smaller particles played an important role in the flame propagation. Flame propagation and structure through 30 mm PMMA dust cloud with mass density of 138 g/m3 were shown in Fig. 12, which indicated an obvious different propagation type compared with that of 100 nm PMMA dust cloud. The flame propagating through 30 mm PMMA particles formed a complicated structure. After ignition by the electrical spark, the particles around the electrodes were rapidly pyrolyzed and reacted with oxygen to release the heat. At the same time, the particles in the preheat zone were heated through thermal radiation and heat conduction. As the direct flame photographs showed that the flame propagated to one area, developed, and then to another area. As the combustion proceeded, clusters of flames were formed where the gaseous pyrolyzate of dust particles was available, then combined and propagated as the flame propagating outward. The propagation of such clusters of flame was a diffusion flame accompanying the local
3
(6)
where Re was the Reynolds number, and Pr was the Prandtl number. In the experiments, both 100 nm and 30 mm PMMA particles were in regular spherical shape, so the particle characteristic length Lc was:
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Fig. 10. Flame front positions from ignition point and flame propagation velocities with time after ignition of 100 nm and 30 mm PMMA dust clouds with different mass densities.
LC ¼ D
(7)
where D was the diameter of PMMA dust particles. So the Biot number could be calculated as: 1
=
1
=
Bi ¼ 2 þ 0:6Re 2 Pr
3
þ
DεsDTi3 l
was similar to that of a premixed gas explosion coupled with the solid surface combustion. For 30 mm PMMA particles, the flame was characterized by clusters of flames and irregular flame front. The flame firstly propagated towards the smaller particles nearby, and until these smaller particles were pyrolyzed, the local premixed flame was formed and developed to heat the adjacent larger particles, thus establishing the diffusion flame.
(8)
In the present case, the size distribution of 30 mm PMMA particles was consistent satisfactorily with the value provided while the size distribution of 100 nm PMMA particles was obviously larger than the provided value due to the serious agglomeration effect. But the Sauter diameter of 100 nm PMMA particles was still much smaller than that of 30 mm PMMA particles, which indicated that the Biot number of 100 nm PMMA particles was smaller than that of 30 mm PMMA particles. So for 100 nm PMMA particles, the particle size was smaller enough so that the heating and pyrolysis steps were very fast, leading to gas combustion controlling the dust explosion. But for 30 mm PMMA particles, other phenomena such as pyrolysis and particle heating controlled the explosion process. The flame structures of PMMA dust clouds with different diameter scales could be categorized into two regimes: for 100 nm PMMA particles, the flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with numerous luminous spot flames which corresponding to the burning of the agglomerates. The flame propagation mechanism
4. Conclusions Flame propagation behaviors of 100 nm and 30 mm PMMA dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed cameras with normal and microscopic lenses were used to record the combustion behaviors and flame microstructures. The conclusions obtained were as follows: 1) For 100 nm PMMA dust clouds, the flames grew symmetrically and propagated spherically. Whereas for 30 mm PMMA dust clouds, clusters of flames were formed and propagated in the local regions where the gaseous pyrolyzate of dust particles was available with the irregular flame front. 2) The flame luminous intensities of 100 nm PMMA dust clouds were stronger than those of 30 mm PMMA dust clouds due to the faster reaction with oxygen. 3) Increasing the mass density from 105 g/m3 to 217 g/m3 for 100 nm PMMA particles, and from 72 g/m3 to 170 g/m3 for
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Fig. 11. Flame propagation and structure through 100 nm PMMA dust cloud with mass density of 168 g/m3.
Fig. 12. Flame propagation and structure of 30 mm PMMA dust cloud with mass density of 138 g/m3.
30 mm PMMA particles, the flame luminous intensity and the average flame propagation velocity were enhanced. The flame front became more irregular for 30 mm PMMA dust clouds.
4) For 100 nm PMMA particles, the flame propagation mechanism was similar to that of a premixed gas explosion coupled with the solid surface combustion. For 30 mm PMMA particles, the flame
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was composed of the diffusion flame accompanying the local premixed flame. It could be speculated that smaller particles maintained the leading part of propagating flame and governed the combustion process. Acknowledgment The authors appreciate the financial support by the National Natural Science Foundation of China (NO. 51406023), Project funded by China Postdoctoral Science Foundation (NO. 2014M560213) and the open fund of State Key Laboratory of Fire Science (NO. HZ2015-KF01). The authors also thank Soken Chemical Co., Ltd. of Japan Ministry for providing the experimental PMMA particles. References Benedetto, A.D., Russo, P., Amyotte, P., Marchand, N., 2010. Modelling the effect of particle size on dust explosions. Chem. Eng. Sci. 65, 772e779. Blasi, C.D., 1999. Transition between regimes in the degradation of thermoplastic polymers. Polym. Degrad. Stab. 64, 359e367. Boilard, S.P., Amyotte, P.R., Khan, F.I., Dastidar, A.G., Eckhoff, R.K., 2013. Explosibility of micron- and nano-size titanium powders. J. Loss Prev. Process Ind. 26, 1646e1654. Cao, W.G., Gao, W., Liang, J.Y., Xu, S., Pan, F., 2014. Flame-propagation behavior and a dynamic model for the thermal-radiation effects in coal-dust explosions. J. Loss Prev. Process Ind. 29, 65e71.
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Flame propagation behaviors of nano- and micro-scale PMMA dust explosions Xinyan Zhang a, Jianliang Yu a, Xingqing Yan a, Qiaofeng Xie b, Wei Gao a, c, * a
School of Chemical Machinery, Dalian University of Technology, Dalian, 116024, China School of Aerospace Engineering, Tsinghua University, Beijing, 100084, China c Institute of Fluid Science, Tohoku University, Sendai, Miyagi, 980-8577, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 8 November 2015 Accepted 15 December 2015 Available online 18 December 2015
Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m3 to 217 g/m3 for 100 nm PMMA particles, and from 72 g/m3 to 170 g/m3 for 30 mm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 mm PMMA dust clouds. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dust explosion Nano- and micro-PMMA particles Flame structure Propagation mechanism
1. Introduction Accompanying the wide use of nano-particles in a number of fields such as solar cell materials, electrode materials, high-density magnetic recording materials, high toughness ceramic materials, catalyst materials, and body repair materials in recent years, nanosafety has been attracted as an urgent concern. As the core aspect of nano-safety, dust explosion is a phenomenon that a flame is propagating in combustible nanoparticles dispersed in the air. The explosion intensity of nano-particles is typically stronger than micro-particles. Studies on nano-dust explosion have been extensively conducted, most of which were focused on the explosion sensitivity (Minimum Explosible Concentration (MEC), Minimum Ignition Energy (MIE) and Minimum Ignition Temperature (MIT)) and the explosion severity (Maximum Explosion Pressure (Pmax),
* Corresponding author. School of Chemical Machinery, Dalian University of Technology, Dalian, 116024, China. E-mail address: [email protected] (W. Gao). http://dx.doi.org/10.1016/j.jlp.2015.12.010 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
Maximum Rate of Pressure-rise ((dP/dt)max), and Deflagration Index (KSt)) (Krietsch et al., 2015; Mittal, 2014; Boilard et al., 2013; Eckhoff, 2012; Li et al., 2011; Wu et al., 2009). Though the likelihood of an explosion increased significantly as the particle size decreased into the nano-range (Mittal, 2014; Boilard et al., 2013), particles in nano-range were not expected to exhibit more severe than micro-powders (Mittal, 2014; Eckhoff, 2012) due to the serious agglomeration effect of nano-dust clouds. Flame propagation mechanisms through combustible particle clouds for better understanding dust explosions, were mainly proposed for microparticles (Gao et al., 2014, 2015; Cao et al., 2014; Dobashi and Senda, 2006; Proust, 2006; Sun et al., 2000). Gao et al. (Gao et al., 2013) demonstrated that the flame front structure changed along with varying the particle size distributions in micro-range. Ju et al. (Ju et al., 1998) pointed out that the flame structure and propagating velocity were depended on the mass density and the diameter distribution of micro particles strongly. But were the flame structure and propagation mechanism changed when moving from micro-to nano-scale? Until now, the current understanding of flame propagation mechanism in nano-dust cloud is
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still in its rudimentary state. In this study, the flame structures and propagation behaviors were systematically observed through a high-speed camera to reveal the differences between nano- and micro-PMMA dust clouds with different mass densities in an open-space dust explosion apparatus. In addition, simple physical models were developed to explain the different microstructures and flame propagation mechanisms in burning nano- and micro-PMMA dust clouds. 2. Experiments 2.1. Experimental apparatus The open-space dust explosion experimental apparatus, which consisted of combustion tubes, a dispersion system, a gas supplying system, an ignition system, a high-speed photography system, and a time controller system, was schematically shown in Fig. 1. The transparent cylindrical combustion tubes included a top tube with an open end, a movable middle tube, and a bottom tube connected to the stainless steel basement. The movable tube kept the dust cloud free from the turbulence caused by surrounding air movement during dispersion. Just before ignition, the movable tube was moved down to provide an open space for the combustible dust clouds, so that the flame could propagated in an open field without the confinement of the tube wall, and direct observation of flame structure and propagation behavior could be realized. A gas nozzle and a dispersing cone were mounted in the chamber to disperse the experimental dust particles similar to the traditional Hartman basement. The gas supplying unit were composed of a buffer vessel filled with compressed air, a solenoid valve and air pipe lines. The ignition system was composed of a high voltage transformer and a pair of tungsten wire electrodes with the diameter of 0.4 mm. Two high-speed cameras (Photron FASTCAM SA4 and Photron FASTCAM SA5), one installed with a normal lens of Nikon AF Nikkor 50 mm f/ 1.2, and the other installed with a microscopic lens of Nikon AF
Micro 200 mm f/4D, were used to record synchronously the flame propagation behavior and microstructure, respectively. The time when the movable tube started to be moved, the on-off time of the solenoid valve, the startup times of the high-speed video cameras and the action time of the high voltage transformer, were controlled by a programmable logic controller (OMRON CPM1A). Before experiments, the particles were dried in a drying oven for at least 24 h to reduce the liquid bridge force between particles caused by the water content. The experimental steps were as follows: weighed a certain amount of particles and put them in the sample dish uniformly; opened the pressure reducing valve to make sure that the gas pressure in the buffer vessel was 0.5 MPa; turned on the high-speed cameras and started the time controller system. The subsequent processes were controlled by the programmable logic controller. The time the programmable logic controller was triggered was set to be 0 s. From 0.5 s to 1 s, the solenoid valve was opened. The particles were dispersed in the chamber by the ejected high pressure air to form the homogeneous dust cloud. 0.2 s later, the electrical stoppers were triggered and the movable tube was moved down to provide the open combustion space. From 1.4 s, the high speed cameras were triggered to record the flame microstructure and propagation behavior. When the dust cloud was relatively stable and uniform at about 1.5 s, the 15 kV high voltage transformer was discharged for 0.01 s. The suspended particles were ignited and the flame propagated in the open space. To determine the real mass densities of PMMA dust clouds, a measurement illustrated in Fig. 2 was designed to capture the suspended particles in the middle tube space. The measurement system was composed of the particle dispersion unit, the particle capture unit and the particle weigh unit. The particle dispersion conditions were the same as aforementioned those of combustion experiments. A sample capture orifice controlled by an air driven cylinder was inserted into the gap between the tubes. The driven pressure of the cylinder was 0.26 MPa. When there was almost no particles suspended in the middle space, the sample capture orifice
Fig. 1. Experimental apparatus.
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Fig. 2. Mass densities measurement system.
was retracted. Particles which fell on the bottom board were collected and weighted. As a result the mass densities of particles in the middle tube space were determined according to the volume of the middle space. In this study, the real mass densities of 30 mm and 100 nm PMMA particles were listed in Table 1. 2.2. Experimental materials 30 mm and 100 nm PMMA particles provided by Soken Chemical Co., Ltd. of Japan Ministry were selected as the experimental samples. Particle size distributions of 30 mm and 100 nm PMMA particles measured by a Malvern Mastersizer 2000 laser diffraction analyzer were shown in Fig. 3. The size distribution of 30 mm PMMA particles was consistent satisfactorily with the value provided by Soken Chemical Co., Ltd. of Japan Ministry. While the size distribution of 100 nm PMMA particles was obviously larger than the provided value due to the serious agglomeration effect. The volume diameter D[4,3], the Sauter diameter D[3,2], and the parameters Dv (10), Dv (50), and Dv (90) which were the quantiles of the volumetric distribution are listed in Table 2. It was indicated that despite of the agglomeration effect, the Sauter diameter of 100 nm PMMA particles was still much smaller than that of 30 mm PMMA particles, which proved a larger specific surface area for 100 nm PMMA particles. Scanning electron microscope (SEM) was used to observe the static microstructures and the particle sizes of 30 mm and 100 nm PMMA particles by tens of thousands times amplification. It was observed from Fig. 4 that both 30 mm and 100 nm PMMA particles Table 1 Mass densities of 30 mm and 100 nm PMMA particles. Name
MZ-30H (30 mm)
MP-300 (100 nm)
Mass density (g/m3)
72 138 170
105 168 217
Fig. 3. Particle size distributions of 30 mm and 100 nm PMMA particles.
Table 2 Characteristic diameters of 30 mm and 100 nm PMMA particles. Diameters
MZ-30H (30 mm)
MP-300 (100 nm)
D[4,3] (mm) D[3,2] (mm) Dv (10) (mm) Dv (50) (mm) Dv (90) (mm)
28.974 24.080 16.509 24.523 43.186
24.877 10.486 6.151 22.543 46.764
were in regularly spherical shape. The 30 mm PMMA particles were dispersedly distributed, whereas 100 nm PMMA particles exhibited serious agglomeration effect due to the considerable inter-particle forces. The inter-particle forces are mainly composed of van der Waals'
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Fig. 4. SEM images of 30 mm and 100 nm PMMA particles.
forces, electrostatic forces and the liquid bridge forces. The theoretical van der Waals’ force Fw between two spherical particles was
Fw ¼
A d1 d2 a2 d1 þ d2
(1)
where A was a constant, a was the smallest distance between the sphere surfaces, d1 and d2 were the diameters of the two spheres (Eckhoff, 2012). Assumed that the sizes of the contacted particles were the same, the equation was reduced to
Fw ¼
Ad 2a2
(2)
which indicated that the Van der Waals' force was enhanced with decreasing particle diameter. For PMMA particles used in this study, the electrostatic contact force was negligible. In addition, because the particles were dried in a drying oven for at least 24 h before experiments, the liquid bridge force between particles caused by the water content could also be negligible. The inter-particle forces probably mostly depended on the Van der Waals’ forces, which meant that the inter-particle forces of PMMA particles were increasing as decreasing the particle diameter. The agglomeration between particles mainly depended on the dimensionless number C0:
C0 ¼
F mg
(3)
where F was the inter-particle forces, m was the mass of the particles, and g was the gravitational acceleration. It can be concluded that the smaller the particle diameter, the larger the agglomeration dimensionless number. For 100 nm PMMA particles, the inter-particle forces were of significance and the agglomerates were easily formed. These results confirmed the larger particle size distribution than the provided value of 100 nm PMMA particles as shown in Fig. 3.
3. Results and discussion 3.1. Effects of particle diameter scales on flame structures and propagation behaviors A series of dust explosion experiments of 100 nm PMMA particles with a mass density of 168 g/m3 and 30 mm PMMA particles with a mass density of 170 g/m3 were conducted in the open-space experimental apparatus. The flame propagation processes were presented in Fig. 5. It was obvious that the flame structures and propagation behaviors of particles with different diameter scales were significantly different. For 100 nm PMMA dust cloud, the flame structure was smooth and continuous with the spherical shape similar to the premixed gas explosions. It could be inferred that the particles were
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Fig. 5. Flame propagations of 100 nm PMMA dust cloud with mass density of 168 g/m3 (a) and 30 mm PMMA dust cloud with mass density of 170 g/m3 (b).
evaporated or pyrolyzed to form the premixed combustible gases before the flame front passed through. While the flame structure of 30 mm PMMA dust cloud presented the clusters and irregular flame fronts. Compared with 30 mm PMMA particles, 100 nm PMMA particles had a larger specific surface area, which led to the faster pyrolysis rate, diffusing and mixing with oxygen. As a result, the flame exhibited a homogenous and smooth structure similar to the premixed flame propagating in the homogeneous combustible gas. Whereas for 30 mm PMMA particles, the larger particle sizes resulted in the slower pyrolysis and decomposition. It had been demonstrated previously that the flame was firstly propagated towards the smaller particles nearby, and when these smaller particles were completely pyrolyzed, a local premixed flame was formed and developed to heat the adjacent larger particles, thus establishing the local diffusion flame (Gao et al., 2013). In this situation, the combustion reaction only occurred in the regions where the gaseous pyrolyzate of dust particles was available, which resulted in clusters of flame fronts. Additionally, 100 nm PMMA exhibited a stronger combustion luminous intensity due to the more violent nature of the combustion reaction. Using Roberts operator which is an image edge detection algorithm programmed by Matlab, the flame edges were exactly extracted. The original flames and the corresponding flame contours of 100 nm and 30 mm PMMA dust clouds with different mass densities listed in Table 1 were illustrated in Fig. 6. The flame front positions from ignition point to the highest point of the flame front and the flame propagation velocities with time after ignition of 100 nm PMMA dust cloud with mass density of 168 g/m3 and 30 mm
PMMA dust cloud with mass density of 170 g/m3 were obtained and showed in Fig. 7. In these experiments, the flame front of 100 nm PMMA dust cloud reached the upper boundary of the open combustion space at 60 ms after ignition, while the 30 mm PMMA dust cloud kept burning until 70 ms. It was indicated from Fig. 7(a) that the flame front of 100 nm PMMA dust cloud moved upward nearly linearly after ignition, whereas for 30 mm PMMA dust clouds, the flame front position exhibited a different trend due to the complex propagation mechanism that the combustion reaction only occurred in the regions where the gaseous pyrolyzate of dust particles was available, resulting in the diffusion flame combined with the local premixed flame and clusters of flame fronts. In the early combustion stage of 30 mm PMMA dust cloud, the distance of flame front from the ignition point to the highest point of the flame front showed a pattern of first decreasing, keeping constant, and then increasing. This was indicated that the combustion reaction first occurred in the ignition area. Because the effect of particle sedimentation of unburnt particles was more significant than that of thermophoresis force in the early stage of combustion, the flame propagated on other directions rather than upward direction, thus the flame front position could keep constant or even move downward. With more particles participated in the combustion reaction, more heat was released and the thermophoresis was accelerated. The flame propagated upward where the gaseous pyrolyzate of dust particles was available, and the distance of flame front was increased. It was notable that in the later combustion process the flame front propagation exhibited the tendency of “increasingkeeping constant-increasing”, which represented the flame
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Fig. 6. The original flames and the corresponding flame contours of 100 nm and 30 mm PMMA dust clouds with different mass densities.
Fig. 7. Flame front positions from ignition point and flame propagation velocities with time after ignition of 100 nm PMMA dust cloud with mass density of 168 g/m3 and 30 mm PMMA dust cloud with mass density of 170 g/m3.
propagation process of propagating to the adjacent area where the gaseous pyrolyzate of particles was available-the flame in this area developed mostly in other directions rather than upward in a very short time - the flame propagating to another adjacent area. This result was precisely demonstrated by the micro flame photographs showed in Fig. 12. Fig. 7(b) exhibited that the flame propagation velocities were fluctuated in the combustion processes of 100 nm and 30 mm PMMA dust clouds. It was obvious that the flame
propagation velocity of 30 mm PMMA dust cloud was fluctuated more seriously than that of the 100 nm PMMA dust cloud. This phenomenon was caused by the discontinuous propagation fronts of 30 mm PMMA dust cloud and the turbulence induced by heat and mass transfer between particles. The average flame propagation velocities of 100 nm PMMA dust cloud with mass density of 168 g/ m3 and 30 mm PMMA dust cloud with mass density of 170 g/m3 were 0.91 m/s and 0.93 m/s, respectively.
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3.2. Effects of dust mass densities on flame structures and propagation behaviors To analyze the effects of dust mass densities on flame structures and propagation behaviors, 100 nm and 30 mm PMMA dust clouds with different mass densities listed in Table 1 were arranged, and the combustion processes were depicted in Figs. 8e9. For 100 nm PMMA dust clouds illustrated in Fig. 8, when the dust mass density was 105 g/m3, the flame propagated outward almost spherically from the ignition point in the early stage (0e30 ms after ignition). At about 40 ms after ignition, a small flame was formed below the main flame and developed slowly as the combustion proceeded. This was caused by the turbulence induced by particle sedimentation and buoyancy effect distorted the flame. Additionally, the relative smaller flame propagation velocity made the turbulence effect more significant. As a whole, the flame of 100 nm dust clouds with different mass densities were all approximately in regular spherical shape. Increasing the mass density from 105 g/m3 to 217 g/m3, more particles participated in the combustion reaction. Hence, more heat was released to enhance the pyrolysis and decomposition of the unburnt particles. The flame luminous intensity became stronger, and the flame scale became larger. For 30 mm PMMA dust cloud shown in Fig. 9, the flame propagation was considerable weaker when the mass density was 72 g/m3. With the mass density increased to 170 g/m3, the flame scale became larger and the flame front became more irregular. It could be also seen that the flame of 30 mm PMMA dust cloud was positioned on the upper area of the photos compared with that of 100 nm PMMA dust cloud, which indicated that the effect of buoyancy on flame propagation was dominant so that the upward flame propagation velocity was enhanced. The flame front positions that from ignition point to the highest point of the flame front and the flame propagation velocities with time after ignition of 100 nm and 30 mm PMMA dust clouds with different mass densities were shown in Fig. 10. It could be seen that with time going, the flame fronts of 100 nm PMMA dust clouds with
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different mass densities were all quickly moved upward. At 60 ms after ignition, the flame fronts of 100 nm PMMA dust clouds with mass densities of 168 g/m3 and 217 g/m3 were already reached the upper boundary of the open space. The flame propagation velocities were fluctuated in a certain range. With the mass density increasing, the extent of velocity fluctuation became larger. This might be because that with larger mass density, more particles participated in the combustion reaction, so that the turbulence caused by heat transfer and mass transfer between particles was accelerated, thus induced the velocity fluctuation. Another reason was that the agglomeration effect was enhanced along with the increasing concentration. The average flame propagation velocities of 100 nm PMMA dust clouds with mass density of 105 g/m3, 168 g/ m3 and 217 g/m3 were 0.71 m/s, 0.91 m/s and 0.97 m/s, respectively. This indicated that the flame propagation was accelerated with mass density increased in the experimental range. Due to the different propagation mechanism of 30 mm PMMA dust clouds, the flame front positions and flame front propagation velocities exhibited a different change tendency, as shown in Fig 10(c,d). The distance of flame fronts from the ignition point showed the tendency of discontinuous propagation process of 30 mm PMMA dust clouds. The flame propagation velocities of 30 mm PMMA dust clouds were fluctuated due to a discontinuous propagation type and the turbulence caused by heat transfer and mass transfer between particles. The flame velocity fluctuation of 170 g/m3 dust cloud was most violent. The average flame propagation velocities of 30 mm PMMA dust clouds with mass density of 72 g/m3, 138 g/m3 and 170 g/m3 were 0.31 m/s, 0.41 m/s and 0.93 m/s, respectively, which revealed that the average flame propagation was accelerated with mass density increased in the experimental range. 3.3. Flame structures and propagation mechanisms The flame propagation mechanisms through combustible dust clouds are essential for better understanding of dust explosions. By
Fig. 8. Flame propagations of 100 nm PMMA dust clouds with mass densities of 105 g/m3, 168 g/m3 and 217 g/m3.
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Fig. 9. Flame propagations of 30 mm PMMA dust clouds with mass densities of 72 g/m3, 138 g/m3 and 170 g/m3.
premixed flame. In addition, it could be speculated that smaller particles maintained the leading part of propagating flame and governed the combustion process. To quantify the effect of particle size on dust reactivity in the PMMA dust explosion, the dimensionless characteristic number Bi was discussed. The Biot number (Bi) is a measure of internal heat conduction time with respect to external heat transfer time. External heat transfer to the combustion of PMMA particles mainly depended on heat convection and radiation. So Bi could be defined as (Benedetto et al., 2010; Blasi, 1999):
Bi ¼
hc Lc Qr Lc 1 þ ¼ Lc hc þ εsDTi3 l lDTi l
(4)
where ε was the emissivity, s was the StefaneBoltzmann constant, l was the thermal conductivity, DTi was the temperature difference between particle and surrounding gas, hc was the heat transfer coefficient, and Lc was the particle characteristic length. The heat transfer coefficient hc could be defined as:
hc ¼
lNu Lc
(5)
where Nu was the Nusselt number. The formula for the Nusselt number was: 1
1
=
Nu ¼ 2 þ 0:6Re 2 Pr
=
using the high-speed camera with the microscopic lens, the flame structures were more clearly observed. Flame propagation through 100 nm PMMA dust cloud with mass density of 168 g/m3 was depicted in Fig. 11(a), and the corresponding flame structure was illustrated in Fig. 11(b). After ignition by the electrical spark, the flame kernel was formed. The adjacent suspended particles absorbed the released heat and were pyrolyzed and decomposed to form the combustible gas mixture. The continuous flame was developed. The glowing particles were soot particles produced in the combustion reaction. Then the particles in the preheated zone were heated for the subsequent oxidation reaction. It is important to point out that there were a number of luminous spot flames in the combustion zone. These luminous spot flames might be the agglomerates existed in 100 nm PMMA dust clouds. The combustion reaction first occurred on the surface of the agglomerates where the pyrolysis gases were available. As the combustion proceeding, the agglomerates were completely pyrolyzed and gasified with more heat transfer, and the flame then exhibited homogenous and smooth structure. This result indicated that the smaller particles played an important role in the flame propagation. Flame propagation and structure through 30 mm PMMA dust cloud with mass density of 138 g/m3 were shown in Fig. 12, which indicated an obvious different propagation type compared with that of 100 nm PMMA dust cloud. The flame propagating through 30 mm PMMA particles formed a complicated structure. After ignition by the electrical spark, the particles around the electrodes were rapidly pyrolyzed and reacted with oxygen to release the heat. At the same time, the particles in the preheat zone were heated through thermal radiation and heat conduction. As the direct flame photographs showed that the flame propagated to one area, developed, and then to another area. As the combustion proceeded, clusters of flames were formed where the gaseous pyrolyzate of dust particles was available, then combined and propagated as the flame propagating outward. The propagation of such clusters of flame was a diffusion flame accompanying the local
3
(6)
where Re was the Reynolds number, and Pr was the Prandtl number. In the experiments, both 100 nm and 30 mm PMMA particles were in regular spherical shape, so the particle characteristic length Lc was:
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Fig. 10. Flame front positions from ignition point and flame propagation velocities with time after ignition of 100 nm and 30 mm PMMA dust clouds with different mass densities.
LC ¼ D
(7)
where D was the diameter of PMMA dust particles. So the Biot number could be calculated as: 1
=
1
=
Bi ¼ 2 þ 0:6Re 2 Pr
3
þ
DεsDTi3 l
was similar to that of a premixed gas explosion coupled with the solid surface combustion. For 30 mm PMMA particles, the flame was characterized by clusters of flames and irregular flame front. The flame firstly propagated towards the smaller particles nearby, and until these smaller particles were pyrolyzed, the local premixed flame was formed and developed to heat the adjacent larger particles, thus establishing the diffusion flame.
(8)
In the present case, the size distribution of 30 mm PMMA particles was consistent satisfactorily with the value provided while the size distribution of 100 nm PMMA particles was obviously larger than the provided value due to the serious agglomeration effect. But the Sauter diameter of 100 nm PMMA particles was still much smaller than that of 30 mm PMMA particles, which indicated that the Biot number of 100 nm PMMA particles was smaller than that of 30 mm PMMA particles. So for 100 nm PMMA particles, the particle size was smaller enough so that the heating and pyrolysis steps were very fast, leading to gas combustion controlling the dust explosion. But for 30 mm PMMA particles, other phenomena such as pyrolysis and particle heating controlled the explosion process. The flame structures of PMMA dust clouds with different diameter scales could be categorized into two regimes: for 100 nm PMMA particles, the flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with numerous luminous spot flames which corresponding to the burning of the agglomerates. The flame propagation mechanism
4. Conclusions Flame propagation behaviors of 100 nm and 30 mm PMMA dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed cameras with normal and microscopic lenses were used to record the combustion behaviors and flame microstructures. The conclusions obtained were as follows: 1) For 100 nm PMMA dust clouds, the flames grew symmetrically and propagated spherically. Whereas for 30 mm PMMA dust clouds, clusters of flames were formed and propagated in the local regions where the gaseous pyrolyzate of dust particles was available with the irregular flame front. 2) The flame luminous intensities of 100 nm PMMA dust clouds were stronger than those of 30 mm PMMA dust clouds due to the faster reaction with oxygen. 3) Increasing the mass density from 105 g/m3 to 217 g/m3 for 100 nm PMMA particles, and from 72 g/m3 to 170 g/m3 for
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Fig. 11. Flame propagation and structure through 100 nm PMMA dust cloud with mass density of 168 g/m3.
Fig. 12. Flame propagation and structure of 30 mm PMMA dust cloud with mass density of 138 g/m3.
30 mm PMMA particles, the flame luminous intensity and the average flame propagation velocity were enhanced. The flame front became more irregular for 30 mm PMMA dust clouds.
4) For 100 nm PMMA particles, the flame propagation mechanism was similar to that of a premixed gas explosion coupled with the solid surface combustion. For 30 mm PMMA particles, the flame
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was composed of the diffusion flame accompanying the local premixed flame. It could be speculated that smaller particles maintained the leading part of propagating flame and governed the combustion process. Acknowledgment The authors appreciate the financial support by the National Natural Science Foundation of China (NO. 51406023), Project funded by China Postdoctoral Science Foundation (NO. 2014M560213) and the open fund of State Key Laboratory of Fire Science (NO. HZ2015-KF01). The authors also thank Soken Chemical Co., Ltd. of Japan Ministry for providing the experimental PMMA particles. References Benedetto, A.D., Russo, P., Amyotte, P., Marchand, N., 2010. Modelling the effect of particle size on dust explosions. Chem. Eng. Sci. 65, 772e779. Blasi, C.D., 1999. Transition between regimes in the degradation of thermoplastic polymers. Polym. Degrad. Stab. 64, 359e367. Boilard, S.P., Amyotte, P.R., Khan, F.I., Dastidar, A.G., Eckhoff, R.K., 2013. Explosibility of micron- and nano-size titanium powders. J. Loss Prev. Process Ind. 26, 1646e1654. Cao, W.G., Gao, W., Liang, J.Y., Xu, S., Pan, F., 2014. Flame-propagation behavior and a dynamic model for the thermal-radiation effects in coal-dust explosions. J. Loss Prev. Process Ind. 29, 65e71.
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