Explosion severity of micro-sized aluminum dust and its flame propagation properties in 20 L spherical vessel

Powder Technology 301 (2016) 1299–1308

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Explosion severity of micro-sized aluminum dust and its flame propagation properties in 20 L spherical vessel Qingzhao Li ⁎, Ke Wang, Yuannan Zheng, Xiaoning Mei, Baiquan Lin School of Safety Engineering, China University of Mining and Technology, Key Laboratory of Gas and Fire Control for Coal Mines of Ministry of Education, State Key Laboratory of Coal Resources and Safe Mining, Xuzhou 221116, Jiangsu Province, PR China

a r t i c l e

i n f o

Article history: Received 23 May 2016 Received in revised form 16 July 2016 Accepted 7 August 2016 Available online 8 August 2016 Keywords: Aluminum dust explosion Particle size Surface area Dust concentration Flame propagation

a b s t r a c t Using 20 L explosion vessel, the present research work reports some experimental results which elucidate the effects of dust concentration, particle size and specific surface areas on aluminum dust explosion severity. Dust flame propagation property and explosion products were analyzed systemically. Six kinds of aluminum powders with different size distributions were selected and specially prepared for explosion tests. Results show that explosion parameters present an increasing and then decreasing trend with dust concentrations. The optimum explosion concentrations for all the selected aluminum dusts are similarly equal to 500 g/m3. Maximum explosion pressures would be quadratic correlated with the decreasing of particle size. However, (dP/dt)max would be exponentially increasing with particle size decreasing. Aluminum dust flame propagation speed would be increased and the combustion mechanism would be transited from diffusion-controlled mode to kinetically controlled mode with particles diameter decreasing. For the finer dust cloud (dS less than 10 μm), dust flame propagation speed would be exponential increased with dust concentration. However, there is a power function relation between flame propagation speeds and dust concentration for the coarse aluminum dust cloud (dS larger than 10 μm). At the experimental condition, flame propagation speed presents an exponentially decreased trend with the increasing of particle size. Explosion products obtained at different conditions present different apparent morphology. The higher explosion pressure, the finer explosion fragments and nano-scale spherical structure would be produced. For coarse aluminum particles, explosion products present fluffy flocculent structure. Furthermore, alumina produced under lower explosion pressure conditions are mainly γ-Al2O3 components. Under high explosion pressure condition (such as 0.56 MPa), there are notable bulges corresponding to αAl2O3 are produced. Present research results may have important significance for understanding the explosion mechanism of aluminum-air dust/air cloud. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Aluminum powder is widely used in various applications, such as propulsion and pyrotechnic compounds because of the high burning rate of micro-particles [1–3]. For instance, aluminum dust can be used to improve the optical properties of pigments [4,5], enhance the combustion and reactivity in propellants [6,7]. However, aluminum powder is a very reactive metal and its oxidation can cause serious explosions disaster if aluminum dusts are well dispersed in the air of confined space with an ignition source. Due to its severity, aluminum dust explosion accidents are often considered as “fatal” or “devastating” [8–12]. About 19% of these dust explosions are caused due to the metal oxidation and aluminum dust account for most of these events [13]. Dust explosion risk is always presenting in

⁎ Corresponding author. E-mail address: [email protected] (Q. Li).

http://dx.doi.org/10.1016/j.powtec.2016.08.012 0032-5910/© 2016 Elsevier B.V. All rights reserved.

the process where combustible dusts are handled. So, risk analysis must be applied to these dust processing industries [8]. Safety handling of aluminum powder requires a complete risk analysis for the plants, which would provide adequate prevention and protection methods and cause risk awareness to the employees [14,15]. Therefore, mitigation and prevention technology is needed in the processing industries including manufacture, use or handle combustible dusts, and an accurate knowledge of dust explosion hazard is quite essential [16,17]. Aluminum dust explosion is a complex surface-area dependent process that involves simultaneous momentum, energy, and mass transport in a reactive multi-phase dust/air cloud system. The metal mainly features the surface heterogeneous oxidation, as they are melt and burn as discrete entities [18,19]. Series of studies could be found in previous articles and the results showed that the initiation and subsequent explosion processes are governed by numerous factors, such as dust concentration [20–22], particle size [23,24], oxidant concentration [25, 26], ignition energy [16,27], initial pressure [28–30], moisture content [31–33], uniformity of dust cloud [34,35] and cloud turbulence [36,37].

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For metallic materials, there are two combustion regimes that are either kinetically controlled for small size particles, or diffusion controlled for the large size particles [38]. As particle size decreasing to 10 μm, reactions would be occurring at or near the particle surface rather than in a detached diffusion flame [39]. Ohkura et al. [40] found that the flash ignition occurred via melt-dispersion mechanism when the aluminum powders had suitable diameter and sufficient packing density to cause a temperature rise above their ignition temperatures. Particle surface chemistry may be contribute to promoting the melt dispersion mechanism and be responsible for propagating energy in aluminum reactions [41]. Under oxidative conditions, it is proposed that the oxidation rate is limited by oxygen diffusion through the amorphous alumina layer. Despite the smaller particle size of powder with higher reactivity, the thickness of the alumina layer is rather independent on particle size [42]. And, the effect of oxygen concentration on the combustion time of micro-sized aluminum particles was weak [6]. Levitas et al. [43] found that some micro particles flame speed could reach about 58% lower than that of nanoparticles. As the particle diameter decreases from micro to nano-scale, the flame speed increases and the combustion mechanism would transit from a diffusion-controlled mode to a kinetically controlled mode. For micro or even larger particles, the flame speed is correlated with the particle size respectively [44,45]. Previous research showed that particle size distributions and ignition temperature played a major role in aluminum reaction rate [46,47]. The flame temperatures are observed to increase with particle size for the entire range of particle sizes considered [48]. Interestingly, the flame structure may display either an overlapping or a separated configuration, depending on the combustion properties of aluminum particles at different scales [49]. Generally, many natural and industrial metal dusts always present a wide particle size distribution. Castellanos et al. [50] reported that the particle size polydispersity (σd) had great effects on the explosion severity of aluminum dust. And the surface weighted mean diameter of aluminum dust could provide a better description of the average sample size and could be adequately related to the real hazard potential of aluminum dust [31,50]. For ordinary dust explosion mitigation device, flame propagation speed is a key parameter for the designing of protecting measures. Julien [51,52] found that the scale of experimental set-ups have great influence on the isobaric and freely propagating dust flames. Safe handling of aluminum powder requires a complete risk analysis, which would provide adequate means for dust processing industries. Generally, most dust explosion tests are performed in constant volume vessels and explosion pressure and its rise rate are the key parameters typically measured in most experiments. However, dust explosion flame propagation speed had never recorded due to the difficulties of extracting flame signals in the 20 L constant volume sphere. In order to understand the effect of particle properties on the explosibility and flame propagation properties of aluminum dust cloud, special designed experiments with explosion products analysis had been conducted in the present research work. 2. Experimental procedure 2.1. Apparatus and methods In present research work, aluminum dust explosion experiments were performed in a standard 20 L stainless steel spherical vessel (Fig.1) according to the international standard ISO6184-1. Before dust explosion test, a pre-weighted amount of aluminum dust was first placed in dust container (volume: 0.6 L), the centrally mounted chemical igniter was connected with the ignition leads, and the explosion chamber was closed safely. Firstly, the explosion chamber was partially vacuumed to 0.06 MPa (Gauge) and the dispersing air pressure was set to 2 MPa (Gauge). When the solenoid valve (Made by Kuhner AG

Fig. 1. 20 L explosion sphere. 1-Sealing cover; 2-Outer jacket; 3-Inner jacket; 4-Vacuum gauge; 5-Water Inlet; 6-Outlet Valve; 7-Base; 8-Peep hole; 9-Exhaust port; 10-Rebound Nozzle; 11-Dust Container; 12-Pressure gauge; 13-Pressure Sensor; 14-Water Outlet; 15-Safety limit switch; 16-Spark rod.

Company, Switzerland) between the dust storage container and the test chamber was opened automatically, the air and coal dust were dispersed into the explosion chamber and the chemical igniter was energized after a 60 ms time delay. After the test was finished, the explosion chamber and dust container were thoroughly cleaned with compressed air for the next test [53,54]. It should be pointed out that no noticeable changes of the particle-size distribution have been observed due to the particles dispersion through the rebound nozzle. During the experiments, the aluminum dust/air cloud in the vessel is ignited by electrically activated pyrotechnical igniter, which is prepared in accordance with the principle of zero-oxygen balance. The igniter consists of zirconium, barium nitrate and barium peroxide by the weight ratio of 4:3:3, and the energy release of 0.48 g chemical igniter is corresponding to 2 kJ. Aluminum dust concentration varied among 30–1250 g/m3 ranges. Each test of aluminum dust explosion was performed in three replications and the explosion pressure evolutions were measured by a pressure sensor (produced by Dytran Company, America) installed in the vessel wall and recorded by a data acquisition system for each run. These data yielded values of the maximum explosion pressure (Pm) and maximum rate of pressure rise (dP/dt) max. 2.2. Experimental materials' characteristics Aluminum powders were purchased from the commercial manufacturers. Particles size and their volumetric distributions were determined by a laser diffraction analyzer and characterized by d10, d50, d90, and surface mean diameter (dS, corresponds to the diameter of a sphere with equal surface area), which are presented separately in Fig.2. Generally, particle shape and the surface roughness always have great influence on aluminum powders oxidation properties. Scanning electron microscope (SEM, FEI Quanta TM 250) was used to measure the morphology of particles' surface and the results are shown in Fig.3. It can be seen that almost all the particles shapes are rather ellipsoidal than spherical, but their surface is quite smooth. Before experiments, the aluminum samples were pre-dried about 2 h at 50 °C under inert atmosphere for the removal of adsorbed moisture. Aluminum particle size polydispersity (σd) characterized by the span of the size distribution is calculated using the following equation [24]: σ d ¼ ðd90 −d10 Þ=d50

ð1Þ

And, the polydispersity (σd) of each aluminum samples are presented on the SEM pictures of Fig.3.

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Fig. 2. Particle size distributions of aluminum dust.

3. Results and discussion 3.1. Influences of dust concentration on aluminum dusts explosion Experiments aluminum dust explosion were conducted with nominal dust concentrations of 30, 60, 125, 250, 500, 750, and 1000 g/m3. Almost all aluminum samples explosion processes present the similar pressure profiles with different maximum explosion pressure (Pm)

and maximum pressure rise rate ((dP/dt) max) separately. For each sample, explosion tests were repeated for three times and the explosion parameters (Pm and (dP/dt) max) were averaged correspondingly. Among all the tests, the results are presented as a function of dust concentration, which are shown in Fig.4. It can be seen that the explosion pressure (Pm) would increase with the dust concentration at poor dust/air mixtures until dust concentration up to about 500 g/m3. However, when dust concentration is higher than 500 g/m3, explosion pressure shows

Fig. 3. SEM pictures of six aluminum powders (classified by surface area based diameter, dS).

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Fig. 4. Aluminum dust explosion parameters vs. dust concentrations.

Table 1 Regression equation of explosion parameters vs. dust concentrations (cdust). Explosion parameters

Particle diameter dS (μm)

Maximum explosion pressure (MPa)

0.992

P m ¼ 0:177 þ 0:001cdust −1:076  10

8.576

P m ¼ 0:172 þ 9:067  10−4 cdust −7:568  10−7 cdust 2

0.937

17.393

P m ¼ 0:159 þ 5:275  10−4 cdust −4:419  10−7 cdust 2

0.821

33.940

P m ¼ 0:114 þ 5:467  10−4 cdust −4:589  10−7 cdust 2

0.812

0.992

ðdP=dtÞ ;max ¼ −21:667 þ 0:921cdust −7:203  10−4 cdust 2

0.944

8.576

ðdP=dtÞ ;max ¼ 12:575 þ 0:262cdust −1:807  10−4 cdust 2

0.939

17.393

ðdP=dtÞ ;max ¼ 4:186 þ 0:058cdust −3:781  10−5 cdust 2

0.874

33.940

ðdP=dtÞ ;max ¼ 2:906 þ 0:039cdust −2:854  10−5 cdust 2

0.893

Maximum rate of pressure rise (MPa/s)

Regression equation

a decreasing trend. Interestingly, the optimum explosion concentration for all the selected aluminum dust is very similarly, which is equal to 500 g/m3. For aluminum oxidation in the air, chemical reaction could be estimated by considering the following equation: 4AlðsÞ þ 3O2 ðgÞ→2Al2 O3 ðsÞ

ð2Þ

Therefore, at the typical condition of temperature (15 °C) and 1 atm pressure, the stoichiometric concentration is 315 g/m3, which is obviously lower than the experimental result of 500 g/m3. The outstanding

R [2] −6

cdust

2

0.866

deviation between the stoichiometric and worst-case means that aluminum dust explosion is actually an incomplete reaction. Excessive increase of dust concentration after the optimum explosion concentration (500 g/m3) may cause a negative effect on explosion pressure and pressure rise rate [31]. The surface heterogeneous oxidation rate of aluminum particles are mainly governed by two different processes of particle melting and oxygen diffusion mechanisms. Under poor dust concentrations, oxygen diffusion process is very fast due to the sufficient oxygen contents in 20 L vessel. With the increasing of inter particle space and less efficient heat transfer, particle melting rate becomes a main limiting factor in the poor dust conditions. Conversely, when

Fig. 5. Aluminum dust explosion parameters vs. particle diameters.

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Table 2 Regression equation of explosion parameters vs. particle diameters (dS). Explosion parameters

Dust concentration c (g/m3)

Maximum explosion pressure (MPa)

250

P m ¼ 0:465−0:013dS þ 1:729  10

500 750

Maximum rate of pressure rise (MPa/s)

250 500 750

particle melting rate is promoted to an enough high degree with the increase of dust concentration, oxygen surrounding particles may be completely consumed and the combustion rate of aluminum dust would be governed by the oxygen diffusion process. In addition, it is found that there is a quite similar increasing rules of (dP/dt)max for all samples in the poor dust clouds. Excessive increase of dust concentration after the optimum explosion concentration may have no obvious influence on (dP/dt)max. Through fitting analysis on the experiment data, influences of dust concentration on the maximum explosion parameters are listed as empirical formulas in Table 1. It can be seen that explosion parameters of Pm and (dP/dt)max are all presented a parabolic functions with the increasing of dust concentrations. And also, the fitting correlation coefficients are remarkable. For the selected aluminum powder, almost all the clouds with dust concentration of 500 g/ m3 are presenting the relative higher explosion parameters. It can be deduced that dust/air cloud with optimum dust explosion concentration would be contributing to aluminum deflagration flame propagations under the present experimental conditions in 20 L explosion vessel. 3.2. Influences of particle diameter on aluminum dusts explosion Castellanos et al. [24] found that surface mean diameter (dS) would provide a better description of the averaged aluminum sample size, because that is adequately related to the real potential hazard of aluminum dust. In the present research work, relations between explosion parameters (Pm and (dP/dt)max) with surface mean diameter (dS) had been investigated and the results are presented in Fig. 5. It can be seen that the maximum explosion pressure (Pm) and maximum rate of pressure ((dP/dt)max) will greatly increase with the reducing of particle size, which is agree with the results of Dufaud et al. [31]. Regression equations of explosion parameters vs. particle diameters (dS) are listed in Table 2. It can be seen that the influence of particle size on Pm is approximate quadratic correlations for the specific dust/air cloud with dust

Regression equation

R [2] −4

dS

2

0.951

P m ¼ 0:573−0:019dS þ 3:093  10−4 dS

2

0.951

2

0.896

−4

P m ¼ 0:509−0:017dS þ 2:595  10 dS (dP/dt)max = 2.738 + 190.649 exp (−dS/8.132) (dP/dt)max = 12.486 + 286.367 exp (−dS/6.564) (dP/dt)max = 1.363 + 278.712 exp (−dS/8.129)

0.989 0.998 0.991

concentrations of 250, 500, and 750 g/m3. However, (dP/dt)max would be exponentially increasing with the decreasing of particle size (dS). Actually, coarse particles in the aluminum dust/air cloud would be greatly affected by particles own gravity. At the same mass concentrations, much finer aluminum particles contents would be contributed to their remarkable increase of specific surface areas and sharply reduce interparticles distance, which is contributed to the heating process of finer aluminum particles and cause a different particles reaction kinetics. In fact, for the ignition of aluminum particles, the activation energy and flame speed among dust dust/air cloud are closely related to particle size, especially to the finer particles. Decreasing of dS would result in an increasing of heat exchange efficiency between particles and the surrounding environment, which promotes the aluminum particles molten process and contributes to cause a serious explosion severity. Considering that the maximum explosion pressure is mainly affected by the total energy released by the aluminum mixtures, it could lead to a higher explosion pressures for aluminum dust/air cloud with optimum explosion concentrations. Therefore, with the decreasing of particles diameter, aluminum dust flame speed would increase and the particle combustion would transit from diffusion-controlled mode to kinetically controlled mode to some extent. The results are consistent with that previous researches reported by Levitas [43]. 3.3. Influences of particle surface areas on aluminum dusts explosion Compared with large-sized aluminum particles, it is well known that smaller ones exhibiting lower ignition temperature, lower heat diffusion time, and faster burning rate. Results of particle size decreasing would cause great increasing of specific surface areas, which would strengthen heat transfer to aluminum cores [55] and accelerate particles oxidation process. Therefore, it can be deduced that reducing particle diameter is mostly contributed to the flame propagation in aluminum hybrid dust mixtures. Fig. 6 shows us the correlations between the particle surface areas vs. explosion parameters and the specific quantitative

Fig. 6. Aluminum dust explosion parameters vs. particle surface areas.

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Table 3 Regression equation of explosion parameters vs. particle surface areas (SP). Explosion parameters Dust concentration c (g/m3) Maximum explosion pressure (MPa) Maximum rate of pressure rise (MPa/s)

Regression equation

Table 4 Regression equation of flame propagation speed (vF) vs. dust concentration (cdust). R [2]

250

P m ¼ 0:345  SP 0:165

0.837

500

P m ¼ 0:407  SP 0:186

0.882

750

P m ¼ 0:349  SP 0:192

0.826

250

ðdP=dtÞ ;max ¼ 54:89  SP 0:638

0.918

500

ðdP=dtÞ ;max ¼ 82:64  SP 0:639

0.936

750

ðdP=dtÞ ;max ¼ 72:66  SP 0:685

0.945

Particle diameter dS (μm)

Regression equation

R [2]

0.992 8.576 17.393 22.822 27.441 33.940

vF = 30.79 −42.01 exp (−cdust/111.12) vF = 21.56 −24.12 exp (−cdust/156.51) v F ¼ 0:13cdust 0:64 v F ¼ 0:18cdust 0:55 v F ¼ 0:14cdust 0:55 v F ¼ 0:10cdust 0:55

0.967 0.969 0.963 0.994 0.973 0.992

particle size and their surface area are a set of coupling factors affecting aluminum particles deflagration characteristics. The higher surface areas, the greater flammability of the aluminum dust/air cloud is. 3.4. Flame propagation during aluminum dusts explosion in 20 L sphere

Fig. 7. Typical pressure evolution curve during aluminum dust explosion.

relationships are listed in Table 3. Results show that, with the increasing of specific surface area, there is a sharply increase for the explosion parameters under the near optimum dust concentration ranges from 250 g/m3 to 750 g/m3. However, for the samples with ds larger than 17.393, particles specific surface area is greatly reduced and the effect of surface area on the explosion parameters is becoming weak comparatively. Furthermore, there is no obvious correlations between the aluminum explosion intensity and particles surface area under poor dust conditions when dust concentration is much lower than 500 g/m3. Meanwhile, with the reducing of particle size, the oxide layer has a higher curvature and experiences higher internal pressures which increase the propensity to rupture. At the same time, the reduced particle size caused by the rupture will also lead a large surface area and increase gasification and combustion rate of aluminum particles. Therefore,

For dust explosion in the 20 L vessel, flame propagation speed is very difficult to be determined due to properties of stainless steel sphere. However, for dust explosion in the constant volume, particles burning rate and flame propagations would cause dramatically explosion pressures and sharp pressure rise rates. It can be believed that dust flame propagation speed could be deduced from the explosion pressure evolution curves based on the specified 20 L explosion vessel. In the present work, dust cloud burning time (tc) is defined as the time span from dust ignition point to reach the maximum explosion pressure (Pm) point. Fig. 7 shows a typical pressure profile as a function of time during aluminum dust explosion test, where Pm, (dP/dt)max, and tc can be obtained at a specific dust concentration condition. So, dust flame propagation speed can be estimated according to the burning time and the radius of 20 L explosion sphere, and that is shown in Eq.(3). vF ¼

8.576 µm 27.441 µm

Flame propagation speed vF(m/s)

Flame propagation speed vF(m/s)

40

Particle diameter dS: 0.992 µm 22.822 µm

17.393 µm 33.940 µm

30 25 20 15 10 5 0 0

200

400

ð3Þ

where, vF is the flame propagation speed of aluminum dust explosion. R20L ‐ Sphere is the radius of 20 L explosion sphere, and tc is the dust cloud burning time. Based on all the explosion pressure evolutions curves, burning time (tc) of aluminum dust clouds were determined and dust flame propagation speeds were calculated and presented in Fig.8. The fitting correlations are listed in Table 4 and Table 5. Results show the influencing results of aluminum particle diameter (dS) and dust concentration

40 35

R20L‐Sphere ðm=sÞ tc

600

800

1000 3

Dust concentration Cdust (g/m )

(a) vF vs. Cdust

1200

Dust concentration:

35 30 25 20

3

60 g/m 3 125 g/m 3 250 g/m 3 500 g/m 3 750 g/m 3 1000 g/m

15 10 5 0 0

6

12

18

24

Particle diameter dS (µm)

(b) vF vs. dS

Fig. 8. Aluminum dust/air cloud explosion flame propagation speed under different conditions.

30

36

Q. Li et al. / Powder Technology 301 (2016) 1299–1308 Table 5 Regression equation of flame propagation speed (vF) vs. diameter (dS). Dust concentration / (g/m3)

Regression equation

R [2]

60 125 250 500 750 1000

vF = − 0.71 +6.85 exp (−dS/23.19) vF = − 1.03 +21.76 exp (−dS/13.29) vF = − 1.09 +28.79 exp (−dS/13.25) vF = − 0.77 +32.58 exp (−dS/14.34) vF = − 0.74 +34.08 exp (−dS/15.61) vF = − 2.09 +37.19 exp (−dS/18.24)

0.956 0.960 0.962 0.957 0.972 0.989

(Cdust) on the flame propagation velocity under the same ignition conditions. Interestingly, it can be found that dust flame propagation speed (vF) shows different rules with the increasing of concentrations for various sizes aluminum dusts from Fig.8 (a). For dust clouds with finer aluminum particles (less than 10 μm), dust flame speed (vF) would be exponential increased (vF = A − B exp (− cdust/C)) with the increasing of dust concentration (Cdust). However, there is a power function (v F ¼ A  cdust B) between flame speed (vF) vs. dust concentration (Cdust) for the coarse aluminum powders explosion. Fig.8 (b) shows that dust clouds flame speed (vF) would be exponentially decreased (vF = − A + B exp (−cdust/C)) with the gradually increasing of particle size and the fitting correlation parameters are list in Table 5. It is interesting to

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note that dust clouds with finer aluminum particles and higher dust concentrations would contribute to dust flame propagations due to the higher heat exchange efficiency between aluminum particles and surrounding environments. These effects can be explained from the role played by the interactions between aluminum particles during the dust clouds combustion process, which is enhanced by the reduction of the inter-particles spacing under rich finer dusts environments. However, for coarse aluminum particles, slow heating up process and non-uniform dust distribution due to their own gravity would decrease dust flame propagation speed. 3.5. Characteristics of aluminum dusts explosion products Aluminum oxidation mechanism of is very complex, which is greatly depended on particles' temperature and oxidation condition. Rai et al. [45] proposed that there were two competition processes during aluminum oxidation: one was governed by the diffusion in gas phase, and the other was controlled by the heterogeneous kinetics. Generally, the oxidation would occur in the condensed phase when the environmental temperature increases to aluminum and alumina melting temperature. The increasing volatility of alumina caused by the presence of molten aluminum would promote oxygen and Al3 + ions diffuse through the alumina layer [56]. In addition, increased temperature of alumina

Fig. 9. SEM pictures of raw aluminum particles and their different explosion products.

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would cause the oxide layer becoming thinner and divide in multiple parts. If this process was sufficiently quick, a burst of the oxide shell would lead to aluminum ejection and gas phase combustion [57]. Based on aluminum oxidation mechanism, it can be deduced that different aluminum dust explosion kinetics would cause various structure of explosion products. In other words, analysis on the structure of explosion products would provide some valuable information about aluminum particles burning mechanism to some extent. The scanning electron microscope (SEM) pictures of raw aluminum particles and their explosion products corresponding to different explosion severities are shown in Fig.9. It can be seen that all the explosion products are presented different apparent morphology. The higher explosion pressures get, the finer explosion fragments are produced. Furthermore, products produced under higher explosion pressure environments shows nanoscale spherical structure. For coarse aluminum particles, their explosion products present fluffy flocculent structure. Based on the energy dispersive spectrometer (EDS), the main elements (Al & O) and their contents in explosion products were analyzed combined with SEM and the results are presented in Fig.10. Obviously, the higher oxidation degree of aluminum, the higher oxygen content is, that is proportional to the explosion pressure. Fig.11 shows X-ray diffraction (XRD) results of raw aluminum particles and their explosion products. It can be seen for Fig.11 that only several bulges corresponding to pure aluminum located at XRD curve of raw aluminum particles. Compared with XRD curve of raw aluminum particles, diffraction peaks corresponding to pure aluminum are decreased and diffraction peaks of alumina are detected. Based on the crystal phase analysis, it can be informed those alumina

produced under lower explosion pressures are mainly γ-Al2O3 and their diffraction peaks corresponding to γ-Al2O3 (220) and (440) reflections. Under high explosion pressure condition (such as 0.56 MPa), there are notable bulges corresponding to α-Al2O3 reflection are detected. It can be deduced that aluminum dust explosion was so strong at this condition that it reached the proper temperature for the formation of α phase Al2O3 [12]. 4. Conclusions Using 20 L explosion vessel, the present research work reports some experimental results which elucidate the effects of dust concentration (Cdust), particle size (dS) and specific surface areas (Sp) on aluminum dust explosion severity. Dust flame propagation property and explosion products were analyzed systemically. Six kinds of aluminum powders with different size distributions were selected and specially prepared for explosion tests. Results show that explosion parameters present an increasing and then decreasing trend with dust concentrations. The optimum explosion concentrations for all the selected aluminum dusts are similarly equal to 500 g/m3. Maximum explosion pressures would be quadratic correlated with the decreasing of particle size. However, (dP/dt)max would be exponentially increasing with particle size decreasing. Aluminum dust flame propagation speed (vF) would be increased and the combustion mechanism would be transited from diffusion-controlled mode to kinetically controlled mode with particles diameter decreasing. For the finer dust cloud (dS less than 10 μm), dust flame propagation speed (vF) would be exponential increased with dust

Fig. 10. EDS results of raw aluminum particles and their different explosion products.

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Fig. 11. XRD results of raw aluminum particles and their different explosion products.

concentration (Cdust). However, there is a power function relation between flame propagation speeds (vF) and dust concentration (Cdust) for the coarse aluminum dust cloud (dS larger than 10 μm). At the present experimental condition, flame propagation speed (vF) presents an exponentially decreased trend with the increasing of particle size (dS). Explosion products obtained at different conditions present different apparent morphology. The higher explosion pressure, the finer explosion fragments and nano-scale spherical structure would be produced. For coarse aluminum particles, explosion products present fluffy flocculent structure. Furthermore, alumina produced under lower explosion pressure conditions are mainly γ-Al2O3 components. Under high explosion pressure condition (such as 0.56 MPa), there are notable bulges corresponding to α-Al2O3 are produced. Present research results may have important significance for understanding the explosion mechanism of aluminum-air dust/air cloud. Acknowledgements Financial supports from the National Natural Science Foundation of China (Grant No. 51574230), the Fundamental Research Funds for the Central Universities (Grant No. 2015XKMS010, 2014QNA04, 2014XT02), the Fund for Innovation Team of CUMT (2014QN001), a project funded by the priority academic program development of Jiangsu higher education institutions, the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13098) and the Natural Science Foundation of Jiangsu Province (Grant No. BK20131115) are sincerely acknowledged. References [1] H. Shalom, M. Aped, D. Kivity, The effect of nanosized aluminum on composite propellant properties, 41st AIAA/ASME/SAE/ASEE, Joint Propuls. Conf. & Exhib, Tucson, Arizona, 2005. [2] S.F. Li, Z.K. Lin, T.F. Wang, Y.L. Sun, X. Han, The effects of aluminum size on the combustion characteristics of high energy propellants with higher burning rate, 42nd AIAA/ASME/SAE/ASEE, Joint Propuls. Conf. & Exhib., Sacramento, California, 2006. [3] M. Kearns, Development and applications of ultrafine aluminum powders, Mater. Sci. Eng. 375-377 (2004) 120–126. [4] F.J. Maile, G. Pfaff, P. Reynders, Effect pigments-past, present and future, Prog. Org. Coat. 54 (2005) 150–163. [5] Y. Zhang, H. Ye, H. Liu, K. Han, Preparation and characterization of aluminum pigments coated with silica for corrosion protection, Corros. Sci. 53 (2011) 1694–1699.

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