Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA

Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA

Journal of Loss Prevention in the Process Industries xxx (2017) 1e7 Contents lists available at ScienceDirect Journal of Loss Prevention in the Proc...

2MB Sizes 10 Downloads 62 Views

Journal of Loss Prevention in the Process Industries xxx (2017) 1e7

Contents lists available at ScienceDirect

Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp

Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA Yojiro Yuzuriha a, **, Wei Gao a, b, *, Toshio Mogi a, Ritsu Dobashi a a b

Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan School of Chemical Machinery and Safety Engineering, Dalian University of Technology, Dalian, Liaoning 116024, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 November 2016 Received in revised form 1 June 2017 Accepted 20 June 2017 Available online xxx

A dust explosion occurs when an ignition source such as static electricity gives energy to a cloud of combustible particles. The flame propagates at high speed and the pressure rises up drastically. To take appropriate measures preventing dust explosions accidents, it is necessary to understand the phenomenon scientifically, in particular, to elucidate the effects of particle size distributions on flame propagating behavior. The purpose of this study is to investigate the effects of particle size distributions systematically. On this account, experiments were performed, in which PMMA particles with a very narrow particle size distribution (monodispersed) and blended samples of these monodispersed particles in various ratios were used. Flame propagation behavior of blended samples was compared with that of monodispersed samples of 3, 10, 20, and 30 mm diameters. As a result, it was found that flame propagation behavior varied according to the particle size distributions even if Sauter mean diameter was same. In particular, flame propagated very fast in small and monodispersed particles which didn't contain large particles. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Industrial dust explosions Particle size distributions Flame propagation behavior Flame propagation velocity

1. Introduction As a high risk industrial disaster, dust explosion is a complex phenomenon that flame propagates in the heterogeneous medium, where particles undergo heating, vaporization/pyrolysis, mixing with oxidizer, ignition, burning, and flame extinction. In this phenomenon, the flame propagates at high speed and the pressure rises up drastically. It is predicted that the potential risk of the dust explosion will increase as industrial powder has recently become smaller (Dobashi, 2008). To take appropriate measures preventing dust explosions accidents, it is necessary to understand the phenomenon scientifically, in particular, to elucidate the effects of particle size distributions on flame propagation behavior. Mechanisms of flame propagation through octadecanol particle clouds were revealed by Chen et al. (1996). It was concluded that in the schlieren front smaller particles rapidly gasified, while the

* Corresponding author. School of Chemical Machinery and Safety Engineering, Dalian University of Technology, Dalian, Liaoning 116024, China. ** Corresponding author. Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail addresses: [email protected] (Y. Yuzuriha), [email protected] dlut.edu.cn (W. Gao).

gasification of particles with a diameter larger than 80 mm was delayed, and vapour lumps were formed behind the schlieren front. These lumps then burned to form circular dispersed blue flames. Jun et al. (1998) observed that when the mass density of smaller particles was high, the flame propagation mechanism was similar to that of a usual hydrocarboneair premixed flame; when the mass density of smaller particles was low, the flame propagation was supported by the heat release due to combustion at the blue spots. Huang et al. (2009). found that as the particle diameter decreased from the micron to the nano range, the flame speed increased and the combustion transited from a diffusion-controlled to a kinetically controlled mode. A. Di Benedetto et al. (Benedetto et al., 2010) developed a novel model to quantify the effect of particle size on dust reactivity in an explosion. It was found that varying the dust size could establish different regimes depending on the values of the characteristic time of each step and of several dimensionless numbers. Wei Gao et al. (Gao et al., 2013) revealed that flame propagated in the dust cloud with a smaller particle size was characterized by a regular shape and spatially continuous combustion zone structure, which was similar to the premixed gas explosions. On the contrary, when flame propagated through the dust cloud with a larger particle size, discrete blue luminous spots appeared surrounding the yellow luminous zone. Castellanos et al.

http://dx.doi.org/10.1016/j.jlp.2017.06.011 0950-4230/© 2017 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Yuzuriha, Y., et al., Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA, Journal of Loss Prevention in the Process Industries (2017), http://dx.doi.org/10.1016/j.jlp.2017.06.011

2

Y. Yuzuriha et al. / Journal of Loss Prevention in the Process Industries xxx (2017) 1e7

(2014). indicated that the explosion hazard characterization was effected by surface median diameter and particle size dispersity. Taking surface area into consideration, Harris et al. (2015). proved that dust particle size had the greatest influence on the propagation and inhibition of dust explosions. Unfortunately, to date the effects of particle size distributions on flame propagation behavior have not been sufficiently studied. The purpose of this study is to investigate the effects of particle size distributions systematically. On this account, the experiments were performed, in which PMMA particles with a very narrow particle size distribution (monodispersed) and blended particles of small, middle, and large ones in various ratios were used. Flame propagation behavior of blended samples was compared with that of monodispersed samples with 3, 10, 20, and 30 mm diameters. 2. Experiments 2.1. Experimental apparatus The dust explosion experiment apparatus, Air-Blowing-Type is schematically shown in Fig. 1. PMMA particles were set in the bottom of the rectangular duct (7  7  30 cm) and dispersed by compressed air of 0.07 MPa to form dust cloud. To observe the flame structures in detail, the rectangular duct was constructed with quartz glasses. The photograph of the rectangular duct is shown in Fig. 2. A mesh was attached to the top of the duct, which prevented the particles out of the duct. The equivalent ratio of dust cloud was 2 (concentration: 290 g/m3) in all conditions, at which the particles were successfully fully dispersed and burnt completely. Direct observation of the propagating flames could be made clearly. 0.2s later, the 15 kV neon transformer was discharged and the suspended particles were ignited. The ignition duration was controlled by the pulse generator (Quantum Composer Sapphire 9200 Series). The pressure of the compressed air and the ignition duration conformed to Japanese Industrial Standards Z 8818: 2002. The flame propagation behavior was recorded directly by a high-speed camera (Photron FASTCAM SA2).

Fig. 2. Rectangular duct.

2.2. PMMA particles 3, 10, 20, 30 mm PMMA particles provided by Soken Chemical Co.,Ltd. of Japan Ministry were used as samples. The product name was MX Series, which exhibited a very narrow particle size distribution (monodispersed) with regular spherical shape. The SEM image of the smallest monodispersed particles (3 mm PMMA particles) is shown in Fig. 3, from which it could be proved that there must be no cohesion in other larger diameter particles due to the weaker interparticle forces for them. The quantiles of the

Fig. 3. SEM image of 3 mm PMMA particles.

volumetric distributions of a production lot were as follows. d32 (Sauter mean diameter) ¼ 2.781 mm, d (0.5) (the diameter under which the percentage in volume is 50%) ¼ 2.709 mm, d(0.9) ¼ 3.496 mm. 2.3. Blend conditions Blended samples A ~ D were prepared by blending two or three monodispersed particles. The blend conditions of PMMA particles are shown in Table 1. Sauter mean diameter d32 was calculated by particle diameter

Table 1 Blend conditions.

Fig. 1. Experimental apparatus.

Sample

3 mm[%]

10 mm[%]

30 mm[%]

Sauter mean diameter[mm]

Blend Blend Blend Blend

75.0 50.0 22.2 e

e e e 25.0

25.0 50.0 77.8 75.0

3.9 5.5 10.0 20.0

A B C D

Please cite this article in press as: Yuzuriha, Y., et al., Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA, Journal of Loss Prevention in the Process Industries (2017), http://dx.doi.org/10.1016/j.jlp.2017.06.011

Y. Yuzuriha et al. / Journal of Loss Prevention in the Process Industries xxx (2017) 1e7

d and mass fraction w.

d32 ¼ 1=Sðw=dÞ

(1)

Different with the monodispersed particles, Fig. 4 presents the agglomeration effects in the blend conditions before dispersion. 2.4. Dispersion When Air-Blowing-Type was introduced to form a dust cloud, dispersion must have important effects on the propagation behavior. Dispersions of the 3 mm and 10 mm particles at the ignition time observed by laser sheet is shown in Fig. 5. It seemed that both particles were thrown up in a small vortex at almost the same speed and reached the top of the duct at the ignition time. From this, suspended particles were found to be relatively uniform. The suspended particle size distributions of 3, 30 mm and blend PMMA particles were real-time measured by the particle size analyzer. The results are shown in Fig. 6. Although agglomeration appeared in the blend conditions before dispersion, the suspended particles still exhibited monodispersed features. 3. Results and discussions 3.1. Flame propagation behavior Flame propagation behavior of 3, 30 mm PMMA particles is

3

shown in Fig. 7. It was clearly different. For 3 mm PMMA particles, the flame front was relatively smooth, and the flame structure was similar with flame propagating thorough flammable pre-mixed gases. Flame propagated faster than 30 mm PMMA particles. On the other hand, for 30 mm PMMA particles, the division of flame was observed in parts and the flame structure exhibited a discontinuous shape. The flame bright was apparently different with 3 mm PMMA particles. The flame propagation behavior of 10, 20 mm PMMA particles was similar with that of 30 mm PMMA particles. Flame configurations of blended samples at t ¼ 25 ms are shown in Fig. 8. The flame structures of Blend A and Blend B were alike. The flame front seemed light in color. Compared with the flame front of 3 mm particles, it was suggested that the thin flame area was mainly supported by combustion of 3 mm particles. Likewise, the flame structures of Blend C and Blend D were alike and were similar with that of 30 mm particles. The flame front exhibited strong luminous flame. 3.2. Flame propagation velocities Flame propagation velocity was obtained by chasing the point of the flame front. Flame propagation velocities of monodispersed particles are shown in Fig. 9 (a). It was found that the smaller particle size decreased, the faster flame propagated. This result was caused by the property that heating and devolatilization characteristic time to release flammable gases was shorter in small particles. Flame propagating velocities of blended particles are shown

Fig. 4. Agglomeration effects in the blend conditions before dispersion.

Fig. 5. Dispersions at the ignition time.

Please cite this article in press as: Yuzuriha, Y., et al., Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA, Journal of Loss Prevention in the Process Industries (2017), http://dx.doi.org/10.1016/j.jlp.2017.06.011

4

Y. Yuzuriha et al. / Journal of Loss Prevention in the Process Industries xxx (2017) 1e7

in Fig. 9 (b). It was also suggested that Blend A and B were alike and Blend C and D were alike. As Blend C and D included almost the same and large proportion (75%) of 30 mm particles, it was suggested that the combustion of Blend C and D was mainly controlled by plenty of large particles. In addition, flame propagated faster in Blend A and B particles than in Blend C and D particles.

volume is same. This results indicate the particle surface area is not the governing parameter of the flame propagation. Comparing monodispersed 3 mm with Blend A, it was found that flame propagated very fast in small and monodispersed particles which didn't contain large particles. The result was because heat of combustion was mainly consumed for the heating and devolatilization of large particles.

3.3. Comparison of the similar Sauter mean diameter particles 3.4. Flame structures The comparison of the flame propagation velocities through particles with similar Sauter mean diameter is shown in Fig. 10. It was found that flame propagating velocities varied according to the particle size distributions even if Sauter mean diameter was same. The characteristic became more remarkable when similar Sauter mean diameter was small. The same Sauter mean diameter represents that the total particle surface area is same when the particle

Assumed flame structures with different particle size distributions are shown in Fig. 11. As for the monodispersed particles, the effect of particle size simply appears on the flame structure. For monodispersed small particles, particles vaporized immediately heated by flame front and flammable gas were formed so that flame could propagate continuously. In this case, the flame front was

Fig. 6. Suspended particle size distributions of 3, 30 mm and blend PMMA particles.

Please cite this article in press as: Yuzuriha, Y., et al., Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA, Journal of Loss Prevention in the Process Industries (2017), http://dx.doi.org/10.1016/j.jlp.2017.06.011

Y. Yuzuriha et al. / Journal of Loss Prevention in the Process Industries xxx (2017) 1e7

5

Fig. 7. Flame structures of monodispersed particles.

vaporize and preheating zone were thick. Flame propagated slowly and the flame structure exhibited a discontinuous shape. The flame exhibited strong luminous flame because some unburnt particles existed inside flame. For Blend A and Blend B which contained a large number of small particles and a few large particles, flame propagated much more slowly than 3 mm particles because large particles deprived heat. For Blend C and Blend D which contained a lot of large particles, the flame structures were similar with that of €hler number to 30 mm particles. Gao et al. (2015). defined Damko characterize different flame propagation mechanisms.

Heating and devolatilisation characteristic time Combustion reaction characteristic time C33 rd D232 ðTi  Tu ÞVf2 Tu2   ¼   8C1 l cp g f lnð1 þ BÞðl=C rÞ Tf  Ti Ti2

Da ¼

Fig. 8. Flame structures of blended samples at t ¼ 25 ms.

(2)

€hler numbers for monodispersed 3, 10, 20, The calculated Damko 30 mm and blend A ~ D PMMA particles were respectively 1.2, 8.9,

Fig. 9. Flame propagation velocities.

relatively smooth, and the flame structure was similar with flame propagating thorough flammable pre-mixed gases. For monodispersed large particles, it took a lot of time for particles to

26.5, 35.5, 11.8, 9.6, 15.2 and 15.9. The relationship between the €hler number and flame structure is shown in calculated Damko €hler number was close to 1, Fig. 12. It was found that when Damko

Please cite this article in press as: Yuzuriha, Y., et al., Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA, Journal of Loss Prevention in the Process Industries (2017), http://dx.doi.org/10.1016/j.jlp.2017.06.011

6

Y. Yuzuriha et al. / Journal of Loss Prevention in the Process Industries xxx (2017) 1e7

Fig. 10. Comparison of the flame propagation velocities through particles with similar Sauter mean diameter.

Fig. 11. Assumed flame structures with different particle size distributions.

the flame front was smooth and the flame structure was similar with flame propagating thorough flammable pre-mixed gases; on € hler number was away from 1, the flame the contrary, when Damko structure exhibited a discontinuous shape. 4. Conclusions PMMA particles with a very narrow particle size distribution (monodispersed) and blended samples of these monodispersed particles in various ratios were used to examine the effect on flame propagation behavior experimentally. The flame propagation

€hler number and flame Fig. 12. The relationship between the calculated Damko structure.

Please cite this article in press as: Yuzuriha, Y., et al., Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA, Journal of Loss Prevention in the Process Industries (2017), http://dx.doi.org/10.1016/j.jlp.2017.06.011

Y. Yuzuriha et al. / Journal of Loss Prevention in the Process Industries xxx (2017) 1e7

behavior of blended samples A ~ D was compared with that of 3, 10, 20, and 30 mm monodispersed particles. As a result, it was found that flame propagation behavior varied according to particle size distributions even if Sauter mean diameter was same. In particular, flame propagated very fast in small and monodispersed particles which didn't contain large particles. Acknowledgments The authors gratefully 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 (2), 772e779.

7

Castellanos, D., Carreto-Vazquez, V.H., Mashuga, C.V., Trottier, R., Mejia, A.F., Mannan, M.S., 2014. The effect of particle size polydispersity on the explosibility characteristics of aluminium dust. Powder Technol. 254, 331e337. Chen, J.L., Dobashi, R., Hirano, T., 1996. Mechanisms of flame propagation through combustible particle clouds. Loss Prev. Process Ind. 9 (3), 225e229. Dobashi, R., 2008. Risk of dust explosions combustible nanomaterials. J. Phys. Conf. Ser. IOP Publ. 170. Gao, W., Mogi, T., Sun, J.H., Yu, J.L., Dobashi, R., 2013. Effects of particle size distributions on flame propagation mechanism during octadecanol dust explosions. Powder Technol. 249, 168e174. Gao, W., Mogi, Yu, J.L., Yan, X.Q., Sun, J.H., Dobashi, R., 2015. Flame propagation mechanisms in dust explosions. Loss Prev. Process Ind. 36, 186e194. Harris, M.L., Sapko, M.J., Zlochower, I.A., Perera, I.E., Weiss, E.S., 2015. Particle size and surface area effects on explosibility using a 20-l chamber. Loss Prev. Process Ind. 37, 33e38. Huang, Y., Risha, G.A., Yang, V., Yetter, R.A., 2009. Effect of particle size on combustion of aluminum particle dust in air. Combust. Flame 156 (1), 5e13. Jun, W.J., Dobashi, R., Hirano, T., 1998. Dependence of flammability limits of a combustible particle cloud on particle diameter distribution. Loss Prev. Process Ind. 11 (3), 177e185.

Please cite this article in press as: Yuzuriha, Y., et al., Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA, Journal of Loss Prevention in the Process Industries (2017), http://dx.doi.org/10.1016/j.jlp.2017.06.011