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air fuels using experimental and numerical approach
Journal Pre-proof Hazard evaluation of explosion venting behaviors for aluminum powder/air fuels using experimental and numerical approach
Sen Xu, Ji Wang, Hongsong Wang, Rongpei Jiang, Yun Zhang, Mengke Zhao, Yuyan Li, Tianlu Shi, Weiguo Cao PII:
S0032-5910(20)30081-4
DOI:
https://doi.org/10.1016/j.powtec.2020.01.069
Reference:
PTEC 15138
To appear in:
Powder Technology
Received date:
4 May 2019
Revised date:
30 November 2019
Accepted date:
23 January 2020
Please cite this article as: S. Xu, J. Wang, H. Wang, et al., Hazard evaluation of explosion venting behaviors for aluminum powder/air fuels using experimental and numerical approach, Powder Technology(2019), https://doi.org/10.1016/j.powtec.2020.01.069
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© 2019 Published by Elsevier.
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Hazard evaluation of explosion venting behaviors for aluminum powder/air fuels using experimental and numerical approach Sen Xua , Ji Wangb, Hongsong Wangc, Rongpei Jiangd, Yun Zhange, Mengke Zhaoe, Yuyan Lia, Tianlu Shie, Weiguo Caoa,
e
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China
b
Shanghai Space Propulsion Technology Research Institute, Shanghai 201109, PR China
c
Nanjing Customs Testing Center for Dangerous Goods and Packaging, Changzhou, Jiangsu 213022, PR China
d
Beijing Institute of Aerospace Technology, Beijing 100074, PR China
e
School of Environmental and Safety Engineering, North University of China, Taiyuan, Shanxi 030051, PR China
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ABSTRACT
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a
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In this paper, modified 20 L sphere was used for the determination of the explosion venting of aluminum powder/air fuels, Fluent software was used for simulation to reveal the combustion mechanism. The results showed that the airtight accumulation effect gradually strengthened with the increase of the cracking pressure. Meanwhile, the maximum reduced explosion pressure and the flame length increased gradually accompanied by the flame changed from secondary/multiple flame type to single flame type. Additionally, the pressure field was reproduced by simulation. It demonstrated that the under-expansion jet was formed due to sudden change of pressure, density, temperature and Mach number between internal and outer of the vessel, which caused the turbulent kinetic energy at the vent Corresponding authors. E-mail address: [email protected] (S. Xu). E-mail address: [email protected] (W.G. Cao). *
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to be much higher than that inside the vessel. The results at the conditions with vents of 70 mm and 50
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Pr
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pr
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mm in diameter had better accuracy and safety margin according to theoretical calculation.
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1 Introduction Aluminum powders are extensively used in hybrid explosives and industrial production for their excellent combustion and detonation properties with the improvement of ultrafine technology. Trzciski et al. [1, 2] studied the detonation properties of open field and confinement field of the hybrid
oo
f
explosives with different content of micro n aluminum powders. The results showed that the explosion
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heat of the hybrid explosives went up with the increasing concentration of aluminum powder, reaching a maximum value at the optimum concentration of 30 %, and then decreased gradually at the higher
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concentration. Chen et al. [3-5] and Cheng et al. [6] compared the detonation properties of aluminum
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powder with metal hydride, and characterized the properties from the thermodynamic and kinetics.
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According to the study of Zachariah et al. [7,8], the combustion effect of aluminum composite particles
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prepared by electrostatic spray method was better than that in the almost same size of particles.
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However, although aluminum powders have excellent combustion and explosion performance substantial number of dust explosion accidents were triggered due to its low ignition energy (0.02 mJ). For example, an extremely serious aluminum dust explosion occurred in Kunshan, Jiangsu Province in August 2014, resulting in about 100 deaths and the direct economic loss of 351 million yuan [9]. To better understand the hazardous of dust explosion, many recent studies had carried out the characteristics of sensitivity and severity. Zhang et al. [10] studied the explosion characteristics of aluminum powder in confined space by means of experiment and numerical simulation. It was found that turbulence was the main factor affecting the uniformity of aluminum powder/air explosion at lower concentration, while at higher concentration, the suspension uniformity of aluminum dust was dominated by the explosion process. Li et al. [11] used a 20 L sphere to study the explosion *
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characteristics of micron-sized aluminum powder with different particle sizes. It was concluded that the explosion with smaller particle sizes was mainly affected by oxygen diffusion, while the larger one affected by the particles melting state under the same concentration. Gao et al. [12,13] systematically studied the explosion characteristics of aluminum powders by mixing inert materials. The results showed that the doping of NaHCO 3 and NH4 H2 PO 4 could obviously inhibit the flame propagation.
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Amyotte et al. [14-16] and Proust et al. [17,18] clarified the inertia and suppression methods of dust
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explosion, and analysed the related explosion mechanism by evaluating the prevention and suppression
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of inert materials in the process of dust explosion..
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The above researches actually justified the essence of the dust explosion suppression. To alleviate
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the severity of accidental explosion, explosion venting technology can be used in industrial production as significant as the suppression technology. Solberg et al. [19] and Bradly et al. [20] summarized the
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experience of explosion venting process through a large amount of experimental results, and put
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forward relevant empirical formulas. Yu et al. [21,22] tested the explosion venting of lycopodium
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powder under various vent diameters. The results showed that the secondary explosion frequently occurred outside the explosion pressure vent with the increase of the vent diameter, and then design reasonably for dust explosion venting was proposed. Based on the previous statistics, Rangwala et al. [23,24] established the relevant explosion calculation model. In addition, Di Sarli et al. [25,26], Di Benedetto et al. [27], Bidabadi et al. [28], and Chow et al. [29] conducted numerous studies on dust/gas explosion through experimental and simulation approach. The results provided a basis data for preventing or reducing dust/gas explosion accidents. Cao et al. [30,31] delved into the test of gas explosion venting, and showed the tremendous variations by comparing the experimental results with the calculated ones from the standards NFPA 68 and EN14491.
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In this paper, the aluminum powder/air fuels (APAF) venting experiment was studied via experiment and numerical simulation. The dynamic parameters in the venting process of APAF were tested through the modified 20 L sphere, with high-speed video camera and pressure testing device put in place to capture the process of explosion venting. Besides, numerical simulation was carried out with Fluent 19.0 software to discuss the combustion reaction characteristics and mechanism, which could
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facilitate the calculation of the appropriate safety design and theoretical basis in industrial explosion.
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2 Experimental
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2.1 Materials
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Fig. 1 showed the scanning electron microscope (SEM) photo of aluminum powders. As can be seen from Fig. 1(a), the morphology of aluminum powders was basically spherical with regular powder
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size distribution. The aluminum powders morphology had a certain degree of irregularity, as shown in
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Fig. 1(b), obtained by enlarged photo. Combined with the diameter distribution analyzed in Fig.2, it
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was in good agreement with the SEM photo. The powder size was distributed within the range of 1 to 10 μm, showing a normal distribution, and D50 (medium diameter), D10 and D90 of the powders were 3.0, 1.1 and 6.1 μm, respectively.
Before the test, all aluminum powders were vacuum packed. The experiments were completed within 24 hours after the aluminum powder packaging bag was opened to prevent the influence of aluminum powder oxidation on the results. All the experiments were tested at room temperature about 20– 30 °C and humidity less than ca.30 %.
2.2 Apparatus *
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The explosion venting device, added on the basis of the standard 20 L sphere, was composed through the blind flange with different vent diameters. As illustrated in Fig. 3, the vent membranes of various cracking pressure (Pcra) were placed between the vessel wall and the flange. The vent diameters in each test equal to the diameter of the blind flange were 70, 50 and 30 mm. Before experiment, the internal pressure of the vessel was firstly extracted to -0.06 MPa (gauge pressure). After two phase
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valve was opened, 2 MPa high-pressure air connected with the dust chamber dispersed aluminum
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powder into the vessel to form APAF until the internal pressure of the vessel was 0 MPa (gauge
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pressure, i.e. 1 atm). The aluminum powders were carried by high-pressure gas for turbulent motion for
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60 ms before ignition, and the pressure sensor was used to record the explosion pressure. The detail
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experiment process was introduced in our previous studies [32]. When the pressure in the vessel reached Pcra of the vent membrane, the vessel began to vent. Meanwhile, the high-speed video camera,
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which was placed at 5 m away from the vessel, was used to shoot the process of APAF venting flame.
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3 Numerical models
Fluent, the software of computational fluid dynamics, has been widely used to calculate the process of dust explosion [33,34]. The governing equations (mass conservation equation, momentum conservation equation, and energy conservation equation), and the k-ε turbulence model [35] (derived from Navier-Stokes equations) were applied in this study. Moreover, the models of discrete phase (aluminum powders), surface combustion, and diffusion- limited surface reaction rate are given below from equation (1) to (10). Fluent 19.0 predicts the trajectories of a discrete aluminum powders (derived from Lagrangian reference frame) by integrating the aluminum powder force balance equation, torque balance, and spherical drag law. The powder force balance equation: 6
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du p dt
u up
r
u up
r
g p
p
F
(1)
is the drag force per unit powder mass, and r is the powder relaxation time, which is
defined as:
p d p2 24 r 18 CdRe
(2)
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f
Here F is an additional acceleration term, is the fluid phase density, p is the aluminum dust density, and dp is the aluminum dust diameter; u p is the aluminum dust velocity, u is the fluid
(3)
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The powder torque balance equation:
d p up u
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Re
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velocity, is the fluid molecular viscosity. Re is the Reynolds number, which can be written as:
Powder rotation is a natural part of powder motion and could have an important influence on the
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trajectories of powders moving in a fluid. To account for powder rotation, an additional ordinary
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differential equation for the angular momentum is solved: Ip
d p dt
f dp
5
C T 2 2
(4)
Here I p is the inertia moment, p is the powder angular velocity, f is the fluid density, C is the rotational drag coefficient, T is the torque applied to powders in a fluid domain, and is the relative powder-fluid angular velocity calculated by: 1 = u f p 2
For a spherical powder, the moment of inertia
I p is calculated as: Ip=
π p d p5 60
The spherical drag law: The drag coefficient, CD , for smooth powders can be taken from *
(5)
7
(6)
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CD =a1
a a2 32 Re Re
(7)
Here, a1 , a2 , and a3 are constants that apply over several ranges of Re given by Morsi and Alexander[36]. The surface combustion: The combustion process of aluminum powder is very complex, involving the internal melting of aluminum particles, the formation of external oxide layer, and the mutual radiation between particles.
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Therefore, we simplified the calculation of chemical reaction of aluminum powder. The aluminum
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powder was heated up to a certain temperature, then some vapor was generated on the surface of the particles. After the vapour was evolved, a surface reaction begins that consumes the combustible
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fraction, f comb , of the particle. Equation (8) is therefore active (for a combusting particle) after the
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vapour was evolved:
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mp 1 f v,0 1 f w,0 mp,0
till the combustible fraction is consumed:
(9)
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mp 1 f v,o f comb 1 f w,0 mp,0
(8)
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With the exception of the multiple surface reactions model, the surface combustion models
surface reaction:
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consume the reactive content of the particle as governed by the stoichiometric requirement of the
Particle Sb Product
(10)
Here, Sb is defined in terms of mass of oxidant per mass of particle. The diffusion- limited surface reaction rate model: The diffusion- limited rate model, the kinetics/diffusion-limited rate model, the intrinsic model, and the multiple surface reactions model are commonly applied to heterogeneous surface reaction rate models for combusting particles. In this work, the diffusion- limited rate model was used. The diffusion- limited surface reaction rate model assumes that the surface reaction proceeds at a rate determined by the diffusion of the gaseous oxidant to the surface of the particle:
dmp dt
4πd p Di,m
8
YoxT Sb TP T
(11)
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Here, Di ,m is diffusion coefficient for oxidant in the bulk, Yox is mass fraction of oxidant in the gas, is gas density. As can be depicted from Figs. 1 and 2, aluminum powders were basically spherical, presenting a normal distribution. Therefore, to ensure the consistency of powders diameter distribution between the simulation process and the experimental process, the user define function (UDF) was independently written by C language, and the normal distribution function law of aluminum powder size was adopted
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f
in the simulation process.
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According to the test conditions, the initial pressure of modified 20 L sphere and dust chamber
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was set as -0.06 MPa (gauge pressure) and 2 MPa (overpressure) respectively. Considering that 2 MPa high-pressure air dispersed aluminum powders into the vessel during the experiment, the temperature
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changed slightly. Therefore, Peng-Robinson (PR) equation was chosen in the simulation process.
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The PR equation is given below:
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Here:
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p
a T RT 2 v b v 2bv b 2
b 0.0778
(12)
RTc Pc
T a T a0 1 n 1 Tc
(13) 2
(14)
Here:
a0 0.45724
R 2Tc 2 Pc
(15)
4 Grid descriptions As schematized in Fig. 4, the modified 20 L sphere and the external discharge space were selected *
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to establish the computational domain, including the blue part and the green part of the vent area. The whole computational domain was divided into triangle and quadrilateral meshes, and the boundary layer was encrypted. The minimum mesh size near the wall was 0.1 mm and the mesh growth ratio was 1.1. To accurately capture the under-expansion jet formed in the explosion venting process, the area near the explosion vent and the external area were also encrypted, and the minimum mesh size around
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the under-expansion jet area was 0.2 mm. The number of global grids was 260,000 in total. After the
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division, the shape of the grid was further smoothed and the mesh quality was checked to improve the
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computational convergence and ensure the accuracy.
5.1 Behaviors of explosion pressure
Pr
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5 Results and discussion
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The test of explosion venting was conducted in the modified 20 L sphere, and the vent diameters
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were 70, 50 and 30 mm, respectively. The APAF venting parameters were tested with various Pcra. As
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can be observed from Fig. 5, the explosion process can be divided into four stages [32]: I-the stable stage of holding pressure after vacuuming the 20 L sphere (from t 0 to t 1 ), II-APAF dispersed stage before ignition (from t 1 to t 2 ), III- explosion pressure rising stage after ignition (from t 2 to t 3 ), and IV-explosion pressure declining stage (from t 3 to t 4 ). In our early study [37], the explosion pressure of aluminum powders with different concentrations was systematically studied, and it was found that the explosion pressure increased with increasing concentration until reached the maximum value (Pmax ) at the optimum concentration (500 g/m3 ), and then decreased at the higher concentration. Therefore, the stage III was focused on studying the process of pressure venting at 500 g/m3 . As displayed in Fig. 6, the curves showed Pmax (no venting at 500 g/m3 ), reached 1.03 MPa, and Pred (the maximum reduced 10
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explosion pressure with various Pcra) of APAF. When the vent diameter was 70 mm, Pred reached the maximum value in the vessel with different initial conditions of Pcra. The ignition time (t2, in Fig. 5) was defined as 0 ms. With the increase of Pcra, the airtight accumulation effect of the explosion process in the vessel gradually enhanced, and the Pred boosted. When Pcra went up from 0.10 MPa to 0.30 MPa, Pred climbed from 0.29 MPa to 0.44 MPa,
f
increasing by 1.51 times, while (dP/dt) growing from 9.95 to 37.17 MPa / s with 3.74 times increase.
oo
Therefore, the influence of Pcra on (dP/dt) was greater than venting pressure. The test value of Pred was
pr
obtained by changing the vent diameters and Pcra. Pred in the vessel with vent diameters of 70, 50 and
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30 mm were attained, as shown in Table 1.
Pr
The diagonal line in the Fig. 7 represented the equilibrium release line, and the point on this line meant that the explosion pressure in the vessel immediately decreased when the vent membranes were
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opened, and Pred was equal to Pcra, achieving the equilibrium release [22]. It can be seen from the Fig. 7
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that Pred rose with the growth of Pcra, and then fell on the equilibrium release line when P
cra
increased.
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The variation trend of Pred with Pcra under different vent diameters was substantial agreement. With the increase of Pcra, the Pred overall presented rising trend, and the growth rate of Pred gradually slowed down and was close to the equilibrium release line. The smaller venting pressure was obtained with the smaller Pcra and larger vent diameter. Pcra in the state of equilibrium venting decreased as the vent diameter increased, till Pcra was larger than Pmax in the sealed vessel, the explosion venting device would not activate without the function of explosion venting.
5.2 Flame venting Behaviors Fig. 8 plotted the process of APAF flame venting by a high-speed video camera with the vent diameter of 70 mm and Pcra of 0.30 MPa. The flame venting mainly presented two kinds of regimes: *
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single flame type and secondary/multiple flame type. Fig. 8(a) showed the process of the flame venting at the aluminum powders concentration of 500 g/m3 . The flame venting time was determined at about 10 ms according to Fig. 6 and the synchronous control test system. It can be clearly seen from the Fig. 8(a) that the flame front increased rapidly, reaching the maximum value of 1.08 m at 31 ms, then gradually decreased, and completely extinguished after 166 ms, which was defined as single flame type
f
here. Fig. 8(b) displayed the process in which the flame venting gradually extinguished at 750 g/m3 ,
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and then secondary/multiple flame type occurred in the venting area about 10 ms after the first flame
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extinguishment.
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Furthermore, the flame length during the venting process had the relationship with the Pcra and
Pr
concentration of aluminum powders. Fig. 9 showed the flame length at the vent diameter of 70 mm. The flame length rose with the increase of Pcra and powder concentration, and the flame length reached
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a maximum value of 1.25 m at 750 g/m3 and 0.30 MPa. As the concentration increased, a large amount
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of unburned powders was released to the external vessel along with the rupture of the vent membrane.
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And this provided more fuels for the entire venting process, further extended the flame venting duration and enlarged the flame length. Besides, as Pcra increased, the pressure difference between the internal and external of the vessel was widened during venting indicating an augmentation of the flame length. Based on above analysis, it can be concluded that the concentration was a primary factor that determined the flame type during the venting process. This was mainly because the unburned aluminum powders spayed from the vessel at the higher concentration, leading to the flame transition from single flame type to secondary/multiple flame type. Additionally, the vent diameter had a significant influence on the flame type as well. As illustrated in Fig. 10, the flame types with different concentrations and vent diameters were summarized.
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It can be seen from Fig. 10 that the critical concentration of secondary/multiple flame was in the range of 500 to 750 g/m3 , which was slightly higher than the optimum explosion concentration. Secondary/multiple flame typically occurred with the lower Pcra, larger concentration and vent diameters. The emergence of secondary/multiple flame happened when the APAF was rightly beyond the minimum explosible concentration (MEC) near the explosion vent during the unburned powders
f
releasing from the vessel. Subsequently, the APAF was re- ignited outside the vessel. With the increase
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of Pcra, the decrease of concentration and vent diameters, the explosion reaction time in the vessel after
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the break of vent membrane was elongated, which leads to the reaction intensity in the vessel increased
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and the amount of unburned powders released during the venting process reduced. Furthermore, with
Pr
the enlargement of the pressure difference between the internal and external of the vessel, injection distance of the releasing powders was amplified. Thus, it is more difficult to reach MEC near the
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explosion.
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vent diameters during venting.
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In general, the flame exhibited different regimes depending mainly on the Pcra, concentration and
5.3 Results of numerical simulation
To better understand the combustion mechanism in the venting process, the variation of the venting characteristics of APAF in the modified 20 L sphere was discussed when the aluminum powder concentration was 500 g/m3 . Fig. 11 showed the variation of the venting characteristics obtained by numerical simulation at the Pcra of 0.30 MPa and the vent diameter of 70 mm. It can be seen from Fig. 6 and Fig. 11(a) that the pressure error was less than 10%, i.e. the variation of the explosion pressure obtained by simulation and the test results were in acceptable agreement. And then the variation of the pressure field during the whole process was intuitively displayed. When the internal pressure of the *
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vessel reached 0.30 MPa, the vent membrane ruptured. Nevertheless, the internal pressure kept going up until it reached a maximum value of 0.45 MPa at 32 ms. Moreover, according to the temperature field in the venting process displayed in Fig. 11(b), it can be seen that the maximum temperature approached to 2500 K, and the maximum flame front length of the entire high temperature zone reached 1.20 m. The flame front length was slightly larger than that captured by high-speed video
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camera in Fig. 8(a). Moreover, the distribution of characteristics by the simulation, such as powders
oo
temperature field, gas- flow velocity field and turbulent kinetic energy field, which were hardly
pr
obtained in the experiment, could compensate for the limitations of test methods. It could provide
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valuable information for the mechanism of explosion venting through analyzing the coupling effect of
Pr
the venting characteristics.
Fig. 11(c) showed the variation of the powders temperature field. The powders were in a
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high-speed turbulence due to that the high-pressure air dispersed aluminum powders into the vessel.
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Therefore, the heat exchange between the unburned and burned powders could be effectively improved
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during the process of combustion. As a consequence, the explosion severity was remarkably enhanced. Compared to the variation of the gas temperature field, the high temperature zone of the powders was narrow, and the high temperature zone in the whole combustion process was located at the center of the vessel. The probable reason was that the entire combustion time was shorter (less than 200 ms), causing the heat transfer was insufficient, especially in the venting zone, the energy supplied to the portion of unburned powders did not reach the activation energy required for combustion due to the heat loss to the atmosphere. Eventually, the high temperature zone of the powders was significantly narrower than that of the gas temperature field. Fig. 11 (d) displayed the distribution profile for gas- flow over the entire calculation domain. It
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revealed that the gas-flow was free to propagate around the vessel center in a laminar way within 3 ms after ignition. After the gas-flow reached the vessel wall, the gas began to reflect due to the vessel limitation, colliding with the laminar gas-flow propagating in the center to form a turbulent phenomenon. From 3 to 10 ms, the explosion pressure was less than 0.30 MPa and the maximum gas- flow velocity did not exceed 200 m/s. As the reaction continued, the venting began when the
f
pressure in the vessel reached 0.30 MPa. At 11 ms, the gas-flow velocity of the explosion vent
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exceeded 300 m/s with the airflow velocity close to 1 Ma. After that, the under-expansion jet
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phenomenon near the explosion vent began to form. At 26 ms, the gas- flow velocity surpassed 1500
e-
m/s due to the pressure difference between the internal and external of the vessel. The under-expansion
Pr
jet phenomenon formed clearly near the explosion vent (it can be clearly seen through the air flow temperature field, pressure field and turbulent kinetic energy field at 32ms), and the velocity on the axis
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of the explosion vent increased along the direction of the jet. In addition, in the turbulent mode,
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turbulence consisted of eddy in various scales. The large eddy carried and transferred energy, and the
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turbulent kinetic energy was related to turbulent velocity and fluid quality. The turbulence intensity was estimated by the turbulent kinetic energy. Fig. 11(e) presented that the turbulent kinetic energy in the vessel was sufficiently low before the explosion venting. After the vent membrane was broken, high- intensity convection was formed on both sides of the high-speed jet zone. Due to the difference of pressure, density and temperature between the internal and external of the vessel, the turbulent kinetic energy at the explosion vent was much higher than that in the vessel. Ultimately, the maximum velocity of the jet was on the venting center axis, a most hazardous area which needed special attention during the venting process. It was discussed in details through Equation (16).
5.4 Comparison of formula calculation and experimental results *
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According to the experiment and simulation results, the explosion venting design was discussed. In recent years, the researchers proposed variety of empirical formula for the design of dust explosion venting, and NFPA68 were widely used [38]. As shown in formula (16). 43 As 104 1 1.54 Pcra V 1 3 ddpt V 3 4 PPmax 1 max red
(16)
Where, As was the venting area calculated according to the standard, m2 ; Pcra was the cracking
oo
f
pressure, bar; V was the vessel volume, m3 ; Pmax was the maximum explosion pressure, bar; Pred was the maximum reduced explosion pressure, bar. Uniform units were required for calculation.
pr
The predicted value of the venting area was calculated by substituting the characteristics which
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were measured under different test conditions into the formula. As ⁄ Ae was used as a characteristic to
Pr
compare the calculated results with the experimental values. Ae was the value of the venting area in the
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experiment, Ae =π∙D2 ⁄4. If As ⁄ Ae>1, the calculation result was too large, and the design of the venting
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experiment was relatively safe; if As ⁄ Ae <1, the prediction result was too small and the design of the
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venting experiment was dangerous.
As schematically shown in Fig. 12, the result of venting experiment under different venting conditions was As ⁄ Ae >1, thus the corresponding experimental design was safety. The design has better accuracy and safety margin when 1
6 Conclusion 16
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This study was evaluated the explosion venting behaviors of aluminum powder/air fuels using experimental and numerical approach. It was shown that the theoretical results of the existing standards, which were quite different from the test results, have certain limited scope. Therefore, concerning the risk in the explosion venting test, an effective safety evaluation and design can be carried out through a combination of a large amount of numerical models and a few test verification, rather than just through
f
the calculations by explosion venting standards. Furthermore, the data accumulated by the modified 20
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L sphere (including test data and simulation data) can provide reference for revising the corresponding
pr
dust explosion venting standards. The results reveal the following:
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1) With the increase of Pcra, the airtight accumulation effect of the explosion process in the vessel
Pr
gradually enhanced, and so did the Pred. When the vent diameter was 70 mm, Pcra rose from 0.10 MPa to 0.30 MPa, Pred increased by 1.51 times, while (dP/dt) increased by 3.74 times. Compared with Pred,
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Pcra had a greater impact on (dP/dt). The Pred usually increased with the increase of Pcra at different
rn
diameters gradually approaching the equilibrium release line.
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2) Both single flame type and secondary/multiple flame type were observed in the test. Secondary/multiple flame type usually appeared with lower Pcra, larger concentration and vent diameters. The flame length grew with the increase of Pcra and powder concentration, and conversely, broadened with the reduction of vent diameters. 3) The explosion venting process of APAF was reproduced by numerical simulation. It was concluded that the pressure variation and the high temperature zone obtained by the simulation were in accord with the test results. Moreover, the distributions of temperature, airflow and turbulence kinetic energy, hardly obtained in the experiment process, were analysed. It indicated that the phenomenon of under-expansion jet was formed due to sudden change of pressure, density, temperature and Mach *
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number between internal and outer of the vessel, which caused the turbulent kinetic energy at the explosion pressure vent to be much higher than that inside the vessel. The maximum velocity of the gas jet was on the venting center axis, a critical area of safety concern in explosion venting. 4) The comparison of the venting area between theoretical calculation and experiments revealed that explosion venting of 70 mm and 50 mm in diameter had greater accuracy and safety margin
f
according to the standards, which was suitable for the practical design of industrial venting explosion
pr
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safety.
Acknowledgments
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The authors appreciate the financial support from the Natural Science Foundation of China
Pr
(11802272), the China Postdoctoral Science Foundation (2019M651085), the Cultivation Programs for
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Young Scientific Research Personnel of Higher Education Institutions in Shanxi Province
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(1912200059MZ), the Science and Technology Innovation Project of University in Shanxi Province
References
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(201802079), and the Natural Science Foundation of Shanxi Province (201801D121285).
[1] W.A. Trzciński, S. Cudziło, L. Szymańczyk, Studies of detonation characteristics of aluminium enriched RDX compositions, Propellants, Explos. Pyrotech. 32 (2007) 392– 400. [2] W.A. Trzciński, S. Cudziło, J. Paszula, Studies of free field and confined explosions of aluminium enriched RDX compositions, Propellants, Explos. Pyrotech. 32 (2007) 502– 508. [3] Y. Chen, X. Chen, M.X. Xu, S. Xu, D.B. Liu, Properties of dust clouds of novel hydrogen-containing alloys. Combust. Explos. Shock Waves 51 (2015) 313– 318.
18
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[4] Y. Chen, X. Chen, D.J. Wu, S. Xu, D.B. Liu, Underwater explosion analysis of hexogen enriched novel hydrogen storage alloy. J. Energ. Mater. 34 (2016) 49– 61. [5] Y. Chen, S. Xu, D.J. Wu, D.B. Liu, Experimental study of the explosion of aluminized explosives in air. Cent. Eur. J. Energ. Mater. 13 (2016) 117– 134. [6] Y.F. Cheng, X.R. Meng, H.H. Ma, S.H. Liu, Q. Wang, C.M. Shu, Z.W. Shen, W.J. Liu, S.X. Song, F. Hua, Flame
f
propagation behaviors and influential factors of TiH2 dust explosions at a constant pressure, Int. J. Hydrogen Energy,
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43 (2018) 16355– 16363.
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[7] H.Y. Wang, G.Q. Jian, S. Yan, J.B. Delisio, C. Huang, M.R. Zachariah, Electrospray formation of gelled
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nano-aluminium microspheres with superior reactivity. ACS Appl. Mater. Interfaces, 5 (2013) 6797– 6801.
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[8] H.Y. Wang, G.Q. Jian, G.C. Egan, M.R. Zachariah, Assembly and reactive properties of Al/CuO based nanothermite microparticles. Combust. Flame 161 (2014) 2203– 2208.
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[9] Y. Zhang, W.G. Cao, G.N. Rao, L. Liu, H.D. Zhou, Y.X. Tan, Experiment-based investigations on the variation
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laws of functional groups on ignition energy of coal dusts. Combust. Sci. Technol. 190 (2018) 1850– 1860.
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[10] Q. Zhang, L. Liu, S. Shen, Effect of turbulence on explosion of aluminium dust at various concentrations in air. Powder Technol. 325 (2018) 467– 475.
[11] Q.Z. Li, K. Wang, Y.N. Zheng, X.N. Mei, B.Q. Lin, Explosion severity of micro-sized aluminium dust and its flame propagation properties in 20 L spherical vessel. Powder Technol. 301 (2016) 1299– 1308. [12] H.P. Jiang, M.S. Bi, B. Li, D.Q. Ma, W. Gao, Flame inhibition of aluminium dust explosion by NaHCO3 and NH4 H2 PO4 . Combust. Flame 200 (2019) 97– 114. [13] H.P. Jiang, M.S. Bi, B. Li, D.W. Zhang, W. Gao, Inhibition evaluation of ABC powder in aluminium dust explosion. J. Hazard. Mater. 361 (2019) 273– 282.
*
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[14] P.R. Amyotte, Solid inertants and their use in dust explosion prevention and mitigatio. J. Loss Prev. Process Ind. 19 (2006) 161– 173. [15] P.R. Amyotte, M.J. Pegg, F.I. Khan, Application of inherent safety principles to dust explosion prevention and mitigation. Process Saf. Environ. Prot. 87 (2009) 35– 39. [16] P.R. Amyotte, R.K. Eckhoff, Dust explosion causation, prevention and mitigation: an overview, J. Chem. Health
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Saf. 17 (2010) 15– 28.
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[17] C. Proust, Turbulent flame propagation in large dust clouds, J. Loss Prev. Process Ind. 49 (2017) 859 – 869.
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[18] R.B. Moussa, C. Proust, M. Guessasma, K. Saleh, J. Fortin, Physical mechanisms involved into the flame
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propagation process through aluminium dust-air clouds: A review, J. Loss Prev. Process Ind. 45 (2016) 9 – 28.
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[19] D.M. Solberg, J.A Pappas, E. Skramstad, Observations of flame instabilities in large scale vented gas explos ions, Symp. Combust. 18 (1981) 1607– 1614.
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[20] D. Bradley, A. Mitcheson, The venting of gaseous explosions in spherical vessels. I-Theory, Combust. Flame 32
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(1978) 221– 236.
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[21] X.Q. Yan, J.L. Yu, W. Gao, Duct-venting of dust explosions in a 20 L sphere at elevated static activation overpressure. J. Loss Prev. Process Ind. 32 (2014) 63– 69. [22] X.Q. Yan, J.L. Yu, Dust explosion venting of small vessels at the elevated static activation overpressure. Powder Technol. 261 (2014) 250– 256. [23] O.J. Ugarte, V. Akkerman, A.S. Rangwala, A computational platform for gas explosion venting, Process Saf. Environ. Prog. 99 (2016) 167– 174. [24] H. Sezer, F. Kronz, V. Akkerman, A.S. Rangwala, Methane-induced explosions in vented enclosures, J. Loss Prevent. Proc. 48 (2017) 199– 206. [25] V.D. Sarli, A.D. Benedetto, E.J. Long, Time-resolved particle image velocimetry of dynamic interactions
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between hydrogen-enriched methane/air premixed flames and toroidal vortex structures, Int. J. Hydrogen Energy 37 (2012) 16201– 16213. [26] V.D. Sarli, A.D. Benedetto, Effects of non-equidiffusion on unsteady propagation of hydrogen-enriched methane/air premixed flames. Int. J. Hydrogen Energy 38 (2013) 7510– 7518. [27] A.D. Benedetto, P. Russo, Thermo-kinetic modelling of dust explosions, J. Loss Prev. Process Ind. 20 (2007)
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303– 309.
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[28] M. Bidabadi, A. Haghiri, A. Rahbari, The effect of Lewis and Damköhler numbers on the flame propagation
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through micro-organic dust particles, Int. J. Therm. Sci. 49 (2010) 534– 542.
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[29] S.K. Chow, R.P. Cleaver, M. Fairweather, D.G. Walker, An experimental study of vented explosions in a 3:1
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aspect ratio cylindrical vessel, Process Saf. Environ. Prot. 78 (2000) 425 – 433. [30] Y. Cao, J. Guo, K.L. Hu, L.F. Xie, B. Li, Effect of ignition location on external explosion in hydrogen–air
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explosion venting, Int. J. Hydrogen Energy 42 (2017) 10547– 10554.
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vessel, Energy 151 (2018) 26– 32.
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[31] Y. Cao, B. Li, K. H. Gao, Pressure characteristics during vented explosion of ethylene-air mixtures in a square
[32] W.G. Cao, Q.F. Qin, W. Cao, Y.H. Lan, T. Chen, S. Xu, X. Cao. Experimental and numerical studies on the explosion severities of coal dust/air mixtures in a 20-L spherical vessel, Powder Technol. 310 (2017) 17– 23. [33] Z. Salamonowicz, M. Kotowski, M. Półka, W. Barnat, Numerical simulation of dust explosion in the spherical 20L vessel, Bull. Pol. Acad. Sci. Tech. Sci. 63 (2015) 289– 293. [34] C. Murillo, O. Dufaud, N. Bardin-Monnier, O. López, F. Munoz, L. Perrin, Dust explosions: CFD modeling as a tool to characterize the relevant parameters of the dust dispersion, Chem. Eng. Sci. 18 (2013) 103–116. [35] S.A. Orszag, V. Yakhot, W.S. Flannery, F. Boysan, D. Choudhury, J. Maruzewski, B. Patel, Renormalization group modeling and turbulence simulations. In International Conference on Near-Wall Turbulent Flows, Tempe, *
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Arizona. 1993. [36] S.A. Morsi, A.J. Alexander, An investigation of particle Trajectories in two-phase flow systems, J. Fluid Mech. 55 (1972) 198– 208. [37] T.L. Shi, Y.X. Tan, Y.Y. Li, C.J. Yu, Y.M. Ye, W.G. Cao, Effect of oxidation rate of aluminium on dust explosion properties, Fire Sci. Tech. 36 (2017) 1194– 1196.
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[38] NFPA 68. Standard on explosion protection by deflagration venting. Quincy, MA, USA: National Fire Protection
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Association. 2013.
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H IG H LI GH T S
Severity parameters were evaluated for the aluminum powder explosion venting.
Single and secondary/multiple venting flame types were discussed.
Transient explosion venting parameters were revealed by simulation results.
Under-expansion jet phenomenon was formed in the venting process .
Formula calculation of venting area was used to verify the experimental results .
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*
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Table caption
Table 1 Test results of cracking pressure (P cra) and maximum reduced pressure (P red ). Table 1 Test results of cracking pressure (P cra) and maximum reduced pressure (P red ).
D=50 mm
P red / MPa P cra / MPa D=70 mm
P red / MPa
0.
40 2
4
6
0.3 4
0.4
0.5
2
3
0.
0.
20 0.
30
0.
0.
36
44
0.6
Pr al rn Jo u 24
0.7
0.7
40
0.
50
55
0.8
0.7 6
0.8 5
6
0.
70
0. 62
0.9
0.
60
0.
0.9 6
0.
50
04
4
0.
1.
92
2
0.
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Note: D is the vent diameter.
0
1
0.
10 29
8
04 0.
82
0.6
1.
91 0.
78
0.4
0.
78 0.
72
0.3
0.
65 0.
61
0.2
0.
52 0.
51
0.1
0.
39
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0.
P red / MPa
0.
26
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D=30 mm
P cra / MPa
0.
13
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0.
P cra / MPa
80
0. 71
0. 80
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Figure captions Fig. 1. Scanning electron microscope (SEM) photo of aluminum powders. Fig. 2. Diameter distribution of aluminum powders. Fig. 3. Modified 20 L sphere test system. Fig. 4. Hybrid grids of the computational domain. Fig. 5. Typical pressure history of aluminum powder/air fuels (APAF) explosion.
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Fig. 6. Pressure rising of aluminum powder/air fuels (APAF) explosion.
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Fig. 7. Relationship between maximum reduced explosion pressure (Pred) and cracking pressure (Pcra).
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Fig. 8. High speed photographs of Flame venting. (a) Single flame type, (b) Secondary/multiple flame type.
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Fig. 9. Relationship between maximum flame front distance and cracking pressure (Pcra).
(c) Vent diameter: 30 mm.
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Fig. 10. Flame type with different concentration. (a) Vent diameter: 70 mm, (b) Vent diameter: 50 mm,
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Fig.11. Spatial distribution of explosion venting characteristics. (a) Pressure (MPa) field in the process
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of explosion, (b) Gas temperature (K) field in the process of explosion, (c) Powders temperature (K) field in the process of explosion, (d) Gas-flow velocity (m/s) field in the process of explosion, (e) Turbulent kinetic energy (m2 /s2 ) field in the process of explosion. Fig. 12. Ratio of standard calculated area to experimental area (As /Ae) with different cracking pressures (Pcra)
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Fig. 1. Scanning electron microscope (SEM) photo of aluminum powders.
26
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Fig. 2. Diameter distribution of aluminum powders.
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Fig. 3. Modified 20 L sphere test system.
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Fig. 4. Hybrid grids of the computational domain.
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Fig. 5. Typical pressure history of aluminum powder/air fuels (APAF) explosion.
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Fig. 6. Pressure rising of aluminum powder/air fuels (APAF) explosion.
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Fig. 7. Relationship between maximum reduced explosion pressure (Pred) and cracking pressure (Pcra).
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(a) Single flame type
(b) Secondary / multiple flame type
Fig. 8. High speed photographs of Flame venting.
*
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Fig. 9. Relationship between maximum flame front distance and cracking pressure (Pcra).
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(a) Vent diameter: 70 mm
(b) Vent diameter: 50 mm
(c) Vent diameter: 30 mm Fig. 10. Flame type with different concentration. *
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(a) Pressure (MPa) field in the process of explosion
(b) Gas temperature (K) field in the process of explosion 36
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(c) Powders temperature (K) field in the process of explosion
(d) Gas-flow velocity (m/s) field in the process of explosion *
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(e) Turbulent kinetic energy (m2 /s2 ) field in the process of explosion
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Fig.11. Spatial distribution of explosion venting characteristics.
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Fig. 12. Ratio of standard calculated area to experimental area (As /Ae) with different cracking pressures
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Pr
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(Pcra)
*
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Declaration of interest statement The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
40
Sen Xu, Ji Wang, Hongsong Wang, Rongpei Jiang, Yun Zhang, Mengke Zhao, Yuyan Li, Tianlu Shi, Weiguo Cao PII:
S0032-5910(20)30081-4
DOI:
https://doi.org/10.1016/j.powtec.2020.01.069
Reference:
PTEC 15138
To appear in:
Powder Technology
Received date:
4 May 2019
Revised date:
30 November 2019
Accepted date:
23 January 2020
Please cite this article as: S. Xu, J. Wang, H. Wang, et al., Hazard evaluation of explosion venting behaviors for aluminum powder/air fuels using experimental and numerical approach, Powder Technology(2019), https://doi.org/10.1016/j.powtec.2020.01.069
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© 2019 Published by Elsevier.
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Hazard evaluation of explosion venting behaviors for aluminum powder/air fuels using experimental and numerical approach Sen Xua , Ji Wangb, Hongsong Wangc, Rongpei Jiangd, Yun Zhange, Mengke Zhaoe, Yuyan Lia, Tianlu Shie, Weiguo Caoa,
e
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China
b
Shanghai Space Propulsion Technology Research Institute, Shanghai 201109, PR China
c
Nanjing Customs Testing Center for Dangerous Goods and Packaging, Changzhou, Jiangsu 213022, PR China
d
Beijing Institute of Aerospace Technology, Beijing 100074, PR China
e
School of Environmental and Safety Engineering, North University of China, Taiyuan, Shanxi 030051, PR China
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ABSTRACT
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a
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In this paper, modified 20 L sphere was used for the determination of the explosion venting of aluminum powder/air fuels, Fluent software was used for simulation to reveal the combustion mechanism. The results showed that the airtight accumulation effect gradually strengthened with the increase of the cracking pressure. Meanwhile, the maximum reduced explosion pressure and the flame length increased gradually accompanied by the flame changed from secondary/multiple flame type to single flame type. Additionally, the pressure field was reproduced by simulation. It demonstrated that the under-expansion jet was formed due to sudden change of pressure, density, temperature and Mach number between internal and outer of the vessel, which caused the turbulent kinetic energy at the vent Corresponding authors. E-mail address: [email protected] (S. Xu). E-mail address: [email protected] (W.G. Cao). *
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to be much higher than that inside the vessel. The results at the conditions with vents of 70 mm and 50
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mm in diameter had better accuracy and safety margin according to theoretical calculation.
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1 Introduction Aluminum powders are extensively used in hybrid explosives and industrial production for their excellent combustion and detonation properties with the improvement of ultrafine technology. Trzciski et al. [1, 2] studied the detonation properties of open field and confinement field of the hybrid
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explosives with different content of micro n aluminum powders. The results showed that the explosion
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heat of the hybrid explosives went up with the increasing concentration of aluminum powder, reaching a maximum value at the optimum concentration of 30 %, and then decreased gradually at the higher
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concentration. Chen et al. [3-5] and Cheng et al. [6] compared the detonation properties of aluminum
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powder with metal hydride, and characterized the properties from the thermodynamic and kinetics.
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According to the study of Zachariah et al. [7,8], the combustion effect of aluminum composite particles
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prepared by electrostatic spray method was better than that in the almost same size of particles.
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However, although aluminum powders have excellent combustion and explosion performance substantial number of dust explosion accidents were triggered due to its low ignition energy (0.02 mJ). For example, an extremely serious aluminum dust explosion occurred in Kunshan, Jiangsu Province in August 2014, resulting in about 100 deaths and the direct economic loss of 351 million yuan [9]. To better understand the hazardous of dust explosion, many recent studies had carried out the characteristics of sensitivity and severity. Zhang et al. [10] studied the explosion characteristics of aluminum powder in confined space by means of experiment and numerical simulation. It was found that turbulence was the main factor affecting the uniformity of aluminum powder/air explosion at lower concentration, while at higher concentration, the suspension uniformity of aluminum dust was dominated by the explosion process. Li et al. [11] used a 20 L sphere to study the explosion *
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characteristics of micron-sized aluminum powder with different particle sizes. It was concluded that the explosion with smaller particle sizes was mainly affected by oxygen diffusion, while the larger one affected by the particles melting state under the same concentration. Gao et al. [12,13] systematically studied the explosion characteristics of aluminum powders by mixing inert materials. The results showed that the doping of NaHCO 3 and NH4 H2 PO 4 could obviously inhibit the flame propagation.
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Amyotte et al. [14-16] and Proust et al. [17,18] clarified the inertia and suppression methods of dust
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explosion, and analysed the related explosion mechanism by evaluating the prevention and suppression
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of inert materials in the process of dust explosion..
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The above researches actually justified the essence of the dust explosion suppression. To alleviate
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the severity of accidental explosion, explosion venting technology can be used in industrial production as significant as the suppression technology. Solberg et al. [19] and Bradly et al. [20] summarized the
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experience of explosion venting process through a large amount of experimental results, and put
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forward relevant empirical formulas. Yu et al. [21,22] tested the explosion venting of lycopodium
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powder under various vent diameters. The results showed that the secondary explosion frequently occurred outside the explosion pressure vent with the increase of the vent diameter, and then design reasonably for dust explosion venting was proposed. Based on the previous statistics, Rangwala et al. [23,24] established the relevant explosion calculation model. In addition, Di Sarli et al. [25,26], Di Benedetto et al. [27], Bidabadi et al. [28], and Chow et al. [29] conducted numerous studies on dust/gas explosion through experimental and simulation approach. The results provided a basis data for preventing or reducing dust/gas explosion accidents. Cao et al. [30,31] delved into the test of gas explosion venting, and showed the tremendous variations by comparing the experimental results with the calculated ones from the standards NFPA 68 and EN14491.
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In this paper, the aluminum powder/air fuels (APAF) venting experiment was studied via experiment and numerical simulation. The dynamic parameters in the venting process of APAF were tested through the modified 20 L sphere, with high-speed video camera and pressure testing device put in place to capture the process of explosion venting. Besides, numerical simulation was carried out with Fluent 19.0 software to discuss the combustion reaction characteristics and mechanism, which could
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facilitate the calculation of the appropriate safety design and theoretical basis in industrial explosion.
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2 Experimental
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2.1 Materials
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Fig. 1 showed the scanning electron microscope (SEM) photo of aluminum powders. As can be seen from Fig. 1(a), the morphology of aluminum powders was basically spherical with regular powder
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size distribution. The aluminum powders morphology had a certain degree of irregularity, as shown in
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Fig. 1(b), obtained by enlarged photo. Combined with the diameter distribution analyzed in Fig.2, it
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was in good agreement with the SEM photo. The powder size was distributed within the range of 1 to 10 μm, showing a normal distribution, and D50 (medium diameter), D10 and D90 of the powders were 3.0, 1.1 and 6.1 μm, respectively.
Before the test, all aluminum powders were vacuum packed. The experiments were completed within 24 hours after the aluminum powder packaging bag was opened to prevent the influence of aluminum powder oxidation on the results. All the experiments were tested at room temperature about 20– 30 °C and humidity less than ca.30 %.
2.2 Apparatus *
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The explosion venting device, added on the basis of the standard 20 L sphere, was composed through the blind flange with different vent diameters. As illustrated in Fig. 3, the vent membranes of various cracking pressure (Pcra) were placed between the vessel wall and the flange. The vent diameters in each test equal to the diameter of the blind flange were 70, 50 and 30 mm. Before experiment, the internal pressure of the vessel was firstly extracted to -0.06 MPa (gauge pressure). After two phase
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valve was opened, 2 MPa high-pressure air connected with the dust chamber dispersed aluminum
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powder into the vessel to form APAF until the internal pressure of the vessel was 0 MPa (gauge
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pressure, i.e. 1 atm). The aluminum powders were carried by high-pressure gas for turbulent motion for
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60 ms before ignition, and the pressure sensor was used to record the explosion pressure. The detail
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experiment process was introduced in our previous studies [32]. When the pressure in the vessel reached Pcra of the vent membrane, the vessel began to vent. Meanwhile, the high-speed video camera,
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which was placed at 5 m away from the vessel, was used to shoot the process of APAF venting flame.
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3 Numerical models
Fluent, the software of computational fluid dynamics, has been widely used to calculate the process of dust explosion [33,34]. The governing equations (mass conservation equation, momentum conservation equation, and energy conservation equation), and the k-ε turbulence model [35] (derived from Navier-Stokes equations) were applied in this study. Moreover, the models of discrete phase (aluminum powders), surface combustion, and diffusion- limited surface reaction rate are given below from equation (1) to (10). Fluent 19.0 predicts the trajectories of a discrete aluminum powders (derived from Lagrangian reference frame) by integrating the aluminum powder force balance equation, torque balance, and spherical drag law. The powder force balance equation: 6
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du p dt
u up
r
u up
r
g p
p
F
(1)
is the drag force per unit powder mass, and r is the powder relaxation time, which is
defined as:
p d p2 24 r 18 CdRe
(2)
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Here F is an additional acceleration term, is the fluid phase density, p is the aluminum dust density, and dp is the aluminum dust diameter; u p is the aluminum dust velocity, u is the fluid
(3)
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The powder torque balance equation:
d p up u
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Re
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velocity, is the fluid molecular viscosity. Re is the Reynolds number, which can be written as:
Powder rotation is a natural part of powder motion and could have an important influence on the
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trajectories of powders moving in a fluid. To account for powder rotation, an additional ordinary
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differential equation for the angular momentum is solved: Ip
d p dt
f dp
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C T 2 2
(4)
Here I p is the inertia moment, p is the powder angular velocity, f is the fluid density, C is the rotational drag coefficient, T is the torque applied to powders in a fluid domain, and is the relative powder-fluid angular velocity calculated by: 1 = u f p 2
For a spherical powder, the moment of inertia
I p is calculated as: Ip=
π p d p5 60
The spherical drag law: The drag coefficient, CD , for smooth powders can be taken from *
(5)
7
(6)
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CD =a1
a a2 32 Re Re
(7)
Here, a1 , a2 , and a3 are constants that apply over several ranges of Re given by Morsi and Alexander[36]. The surface combustion: The combustion process of aluminum powder is very complex, involving the internal melting of aluminum particles, the formation of external oxide layer, and the mutual radiation between particles.
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Therefore, we simplified the calculation of chemical reaction of aluminum powder. The aluminum
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powder was heated up to a certain temperature, then some vapor was generated on the surface of the particles. After the vapour was evolved, a surface reaction begins that consumes the combustible
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fraction, f comb , of the particle. Equation (8) is therefore active (for a combusting particle) after the
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vapour was evolved:
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mp 1 f v,0 1 f w,0 mp,0
till the combustible fraction is consumed:
(9)
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mp 1 f v,o f comb 1 f w,0 mp,0
(8)
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With the exception of the multiple surface reactions model, the surface combustion models
surface reaction:
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consume the reactive content of the particle as governed by the stoichiometric requirement of the
Particle Sb Product
(10)
Here, Sb is defined in terms of mass of oxidant per mass of particle. The diffusion- limited surface reaction rate model: The diffusion- limited rate model, the kinetics/diffusion-limited rate model, the intrinsic model, and the multiple surface reactions model are commonly applied to heterogeneous surface reaction rate models for combusting particles. In this work, the diffusion- limited rate model was used. The diffusion- limited surface reaction rate model assumes that the surface reaction proceeds at a rate determined by the diffusion of the gaseous oxidant to the surface of the particle:
dmp dt
4πd p Di,m
8
YoxT Sb TP T
(11)
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Here, Di ,m is diffusion coefficient for oxidant in the bulk, Yox is mass fraction of oxidant in the gas, is gas density. As can be depicted from Figs. 1 and 2, aluminum powders were basically spherical, presenting a normal distribution. Therefore, to ensure the consistency of powders diameter distribution between the simulation process and the experimental process, the user define function (UDF) was independently written by C language, and the normal distribution function law of aluminum powder size was adopted
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in the simulation process.
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According to the test conditions, the initial pressure of modified 20 L sphere and dust chamber
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was set as -0.06 MPa (gauge pressure) and 2 MPa (overpressure) respectively. Considering that 2 MPa high-pressure air dispersed aluminum powders into the vessel during the experiment, the temperature
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changed slightly. Therefore, Peng-Robinson (PR) equation was chosen in the simulation process.
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The PR equation is given below:
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Here:
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p
a T RT 2 v b v 2bv b 2
b 0.0778
(12)
RTc Pc
T a T a0 1 n 1 Tc
(13) 2
(14)
Here:
a0 0.45724
R 2Tc 2 Pc
(15)
4 Grid descriptions As schematized in Fig. 4, the modified 20 L sphere and the external discharge space were selected *
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to establish the computational domain, including the blue part and the green part of the vent area. The whole computational domain was divided into triangle and quadrilateral meshes, and the boundary layer was encrypted. The minimum mesh size near the wall was 0.1 mm and the mesh growth ratio was 1.1. To accurately capture the under-expansion jet formed in the explosion venting process, the area near the explosion vent and the external area were also encrypted, and the minimum mesh size around
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the under-expansion jet area was 0.2 mm. The number of global grids was 260,000 in total. After the
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division, the shape of the grid was further smoothed and the mesh quality was checked to improve the
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computational convergence and ensure the accuracy.
5.1 Behaviors of explosion pressure
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5 Results and discussion
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The test of explosion venting was conducted in the modified 20 L sphere, and the vent diameters
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were 70, 50 and 30 mm, respectively. The APAF venting parameters were tested with various Pcra. As
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can be observed from Fig. 5, the explosion process can be divided into four stages [32]: I-the stable stage of holding pressure after vacuuming the 20 L sphere (from t 0 to t 1 ), II-APAF dispersed stage before ignition (from t 1 to t 2 ), III- explosion pressure rising stage after ignition (from t 2 to t 3 ), and IV-explosion pressure declining stage (from t 3 to t 4 ). In our early study [37], the explosion pressure of aluminum powders with different concentrations was systematically studied, and it was found that the explosion pressure increased with increasing concentration until reached the maximum value (Pmax ) at the optimum concentration (500 g/m3 ), and then decreased at the higher concentration. Therefore, the stage III was focused on studying the process of pressure venting at 500 g/m3 . As displayed in Fig. 6, the curves showed Pmax (no venting at 500 g/m3 ), reached 1.03 MPa, and Pred (the maximum reduced 10
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explosion pressure with various Pcra) of APAF. When the vent diameter was 70 mm, Pred reached the maximum value in the vessel with different initial conditions of Pcra. The ignition time (t2, in Fig. 5) was defined as 0 ms. With the increase of Pcra, the airtight accumulation effect of the explosion process in the vessel gradually enhanced, and the Pred boosted. When Pcra went up from 0.10 MPa to 0.30 MPa, Pred climbed from 0.29 MPa to 0.44 MPa,
f
increasing by 1.51 times, while (dP/dt) growing from 9.95 to 37.17 MPa / s with 3.74 times increase.
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Therefore, the influence of Pcra on (dP/dt) was greater than venting pressure. The test value of Pred was
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obtained by changing the vent diameters and Pcra. Pred in the vessel with vent diameters of 70, 50 and
e-
30 mm were attained, as shown in Table 1.
Pr
The diagonal line in the Fig. 7 represented the equilibrium release line, and the point on this line meant that the explosion pressure in the vessel immediately decreased when the vent membranes were
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opened, and Pred was equal to Pcra, achieving the equilibrium release [22]. It can be seen from the Fig. 7
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that Pred rose with the growth of Pcra, and then fell on the equilibrium release line when P
cra
increased.
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The variation trend of Pred with Pcra under different vent diameters was substantial agreement. With the increase of Pcra, the Pred overall presented rising trend, and the growth rate of Pred gradually slowed down and was close to the equilibrium release line. The smaller venting pressure was obtained with the smaller Pcra and larger vent diameter. Pcra in the state of equilibrium venting decreased as the vent diameter increased, till Pcra was larger than Pmax in the sealed vessel, the explosion venting device would not activate without the function of explosion venting.
5.2 Flame venting Behaviors Fig. 8 plotted the process of APAF flame venting by a high-speed video camera with the vent diameter of 70 mm and Pcra of 0.30 MPa. The flame venting mainly presented two kinds of regimes: *
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single flame type and secondary/multiple flame type. Fig. 8(a) showed the process of the flame venting at the aluminum powders concentration of 500 g/m3 . The flame venting time was determined at about 10 ms according to Fig. 6 and the synchronous control test system. It can be clearly seen from the Fig. 8(a) that the flame front increased rapidly, reaching the maximum value of 1.08 m at 31 ms, then gradually decreased, and completely extinguished after 166 ms, which was defined as single flame type
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here. Fig. 8(b) displayed the process in which the flame venting gradually extinguished at 750 g/m3 ,
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and then secondary/multiple flame type occurred in the venting area about 10 ms after the first flame
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extinguishment.
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Furthermore, the flame length during the venting process had the relationship with the Pcra and
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concentration of aluminum powders. Fig. 9 showed the flame length at the vent diameter of 70 mm. The flame length rose with the increase of Pcra and powder concentration, and the flame length reached
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a maximum value of 1.25 m at 750 g/m3 and 0.30 MPa. As the concentration increased, a large amount
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of unburned powders was released to the external vessel along with the rupture of the vent membrane.
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And this provided more fuels for the entire venting process, further extended the flame venting duration and enlarged the flame length. Besides, as Pcra increased, the pressure difference between the internal and external of the vessel was widened during venting indicating an augmentation of the flame length. Based on above analysis, it can be concluded that the concentration was a primary factor that determined the flame type during the venting process. This was mainly because the unburned aluminum powders spayed from the vessel at the higher concentration, leading to the flame transition from single flame type to secondary/multiple flame type. Additionally, the vent diameter had a significant influence on the flame type as well. As illustrated in Fig. 10, the flame types with different concentrations and vent diameters were summarized.
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It can be seen from Fig. 10 that the critical concentration of secondary/multiple flame was in the range of 500 to 750 g/m3 , which was slightly higher than the optimum explosion concentration. Secondary/multiple flame typically occurred with the lower Pcra, larger concentration and vent diameters. The emergence of secondary/multiple flame happened when the APAF was rightly beyond the minimum explosible concentration (MEC) near the explosion vent during the unburned powders
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releasing from the vessel. Subsequently, the APAF was re- ignited outside the vessel. With the increase
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of Pcra, the decrease of concentration and vent diameters, the explosion reaction time in the vessel after
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the break of vent membrane was elongated, which leads to the reaction intensity in the vessel increased
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and the amount of unburned powders released during the venting process reduced. Furthermore, with
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the enlargement of the pressure difference between the internal and external of the vessel, injection distance of the releasing powders was amplified. Thus, it is more difficult to reach MEC near the
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explosion.
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vent diameters during venting.
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In general, the flame exhibited different regimes depending mainly on the Pcra, concentration and
5.3 Results of numerical simulation
To better understand the combustion mechanism in the venting process, the variation of the venting characteristics of APAF in the modified 20 L sphere was discussed when the aluminum powder concentration was 500 g/m3 . Fig. 11 showed the variation of the venting characteristics obtained by numerical simulation at the Pcra of 0.30 MPa and the vent diameter of 70 mm. It can be seen from Fig. 6 and Fig. 11(a) that the pressure error was less than 10%, i.e. the variation of the explosion pressure obtained by simulation and the test results were in acceptable agreement. And then the variation of the pressure field during the whole process was intuitively displayed. When the internal pressure of the *
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vessel reached 0.30 MPa, the vent membrane ruptured. Nevertheless, the internal pressure kept going up until it reached a maximum value of 0.45 MPa at 32 ms. Moreover, according to the temperature field in the venting process displayed in Fig. 11(b), it can be seen that the maximum temperature approached to 2500 K, and the maximum flame front length of the entire high temperature zone reached 1.20 m. The flame front length was slightly larger than that captured by high-speed video
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camera in Fig. 8(a). Moreover, the distribution of characteristics by the simulation, such as powders
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temperature field, gas- flow velocity field and turbulent kinetic energy field, which were hardly
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obtained in the experiment, could compensate for the limitations of test methods. It could provide
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valuable information for the mechanism of explosion venting through analyzing the coupling effect of
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the venting characteristics.
Fig. 11(c) showed the variation of the powders temperature field. The powders were in a
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high-speed turbulence due to that the high-pressure air dispersed aluminum powders into the vessel.
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Therefore, the heat exchange between the unburned and burned powders could be effectively improved
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during the process of combustion. As a consequence, the explosion severity was remarkably enhanced. Compared to the variation of the gas temperature field, the high temperature zone of the powders was narrow, and the high temperature zone in the whole combustion process was located at the center of the vessel. The probable reason was that the entire combustion time was shorter (less than 200 ms), causing the heat transfer was insufficient, especially in the venting zone, the energy supplied to the portion of unburned powders did not reach the activation energy required for combustion due to the heat loss to the atmosphere. Eventually, the high temperature zone of the powders was significantly narrower than that of the gas temperature field. Fig. 11 (d) displayed the distribution profile for gas- flow over the entire calculation domain. It
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revealed that the gas-flow was free to propagate around the vessel center in a laminar way within 3 ms after ignition. After the gas-flow reached the vessel wall, the gas began to reflect due to the vessel limitation, colliding with the laminar gas-flow propagating in the center to form a turbulent phenomenon. From 3 to 10 ms, the explosion pressure was less than 0.30 MPa and the maximum gas- flow velocity did not exceed 200 m/s. As the reaction continued, the venting began when the
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pressure in the vessel reached 0.30 MPa. At 11 ms, the gas-flow velocity of the explosion vent
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exceeded 300 m/s with the airflow velocity close to 1 Ma. After that, the under-expansion jet
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phenomenon near the explosion vent began to form. At 26 ms, the gas- flow velocity surpassed 1500
e-
m/s due to the pressure difference between the internal and external of the vessel. The under-expansion
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jet phenomenon formed clearly near the explosion vent (it can be clearly seen through the air flow temperature field, pressure field and turbulent kinetic energy field at 32ms), and the velocity on the axis
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of the explosion vent increased along the direction of the jet. In addition, in the turbulent mode,
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turbulence consisted of eddy in various scales. The large eddy carried and transferred energy, and the
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turbulent kinetic energy was related to turbulent velocity and fluid quality. The turbulence intensity was estimated by the turbulent kinetic energy. Fig. 11(e) presented that the turbulent kinetic energy in the vessel was sufficiently low before the explosion venting. After the vent membrane was broken, high- intensity convection was formed on both sides of the high-speed jet zone. Due to the difference of pressure, density and temperature between the internal and external of the vessel, the turbulent kinetic energy at the explosion vent was much higher than that in the vessel. Ultimately, the maximum velocity of the jet was on the venting center axis, a most hazardous area which needed special attention during the venting process. It was discussed in details through Equation (16).
5.4 Comparison of formula calculation and experimental results *
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According to the experiment and simulation results, the explosion venting design was discussed. In recent years, the researchers proposed variety of empirical formula for the design of dust explosion venting, and NFPA68 were widely used [38]. As shown in formula (16). 43 As 104 1 1.54 Pcra V 1 3 ddpt V 3 4 PPmax 1 max red
(16)
Where, As was the venting area calculated according to the standard, m2 ; Pcra was the cracking
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pressure, bar; V was the vessel volume, m3 ; Pmax was the maximum explosion pressure, bar; Pred was the maximum reduced explosion pressure, bar. Uniform units were required for calculation.
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The predicted value of the venting area was calculated by substituting the characteristics which
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were measured under different test conditions into the formula. As ⁄ Ae was used as a characteristic to
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compare the calculated results with the experimental values. Ae was the value of the venting area in the
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experiment, Ae =π∙D2 ⁄4. If As ⁄ Ae>1, the calculation result was too large, and the design of the venting
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experiment was relatively safe; if As ⁄ Ae <1, the prediction result was too small and the design of the
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venting experiment was dangerous.
As schematically shown in Fig. 12, the result of venting experiment under different venting conditions was As ⁄ Ae >1, thus the corresponding experimental design was safety. The design has better accuracy and safety margin when 1
6 Conclusion 16
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This study was evaluated the explosion venting behaviors of aluminum powder/air fuels using experimental and numerical approach. It was shown that the theoretical results of the existing standards, which were quite different from the test results, have certain limited scope. Therefore, concerning the risk in the explosion venting test, an effective safety evaluation and design can be carried out through a combination of a large amount of numerical models and a few test verification, rather than just through
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the calculations by explosion venting standards. Furthermore, the data accumulated by the modified 20
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L sphere (including test data and simulation data) can provide reference for revising the corresponding
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dust explosion venting standards. The results reveal the following:
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1) With the increase of Pcra, the airtight accumulation effect of the explosion process in the vessel
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gradually enhanced, and so did the Pred. When the vent diameter was 70 mm, Pcra rose from 0.10 MPa to 0.30 MPa, Pred increased by 1.51 times, while (dP/dt) increased by 3.74 times. Compared with Pred,
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Pcra had a greater impact on (dP/dt). The Pred usually increased with the increase of Pcra at different
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diameters gradually approaching the equilibrium release line.
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2) Both single flame type and secondary/multiple flame type were observed in the test. Secondary/multiple flame type usually appeared with lower Pcra, larger concentration and vent diameters. The flame length grew with the increase of Pcra and powder concentration, and conversely, broadened with the reduction of vent diameters. 3) The explosion venting process of APAF was reproduced by numerical simulation. It was concluded that the pressure variation and the high temperature zone obtained by the simulation were in accord with the test results. Moreover, the distributions of temperature, airflow and turbulence kinetic energy, hardly obtained in the experiment process, were analysed. It indicated that the phenomenon of under-expansion jet was formed due to sudden change of pressure, density, temperature and Mach *
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number between internal and outer of the vessel, which caused the turbulent kinetic energy at the explosion pressure vent to be much higher than that inside the vessel. The maximum velocity of the gas jet was on the venting center axis, a critical area of safety concern in explosion venting. 4) The comparison of the venting area between theoretical calculation and experiments revealed that explosion venting of 70 mm and 50 mm in diameter had greater accuracy and safety margin
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according to the standards, which was suitable for the practical design of industrial venting explosion
pr
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safety.
Acknowledgments
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The authors appreciate the financial support from the Natural Science Foundation of China
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(11802272), the China Postdoctoral Science Foundation (2019M651085), the Cultivation Programs for
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Young Scientific Research Personnel of Higher Education Institutions in Shanxi Province
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(1912200059MZ), the Science and Technology Innovation Project of University in Shanxi Province
References
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(201802079), and the Natural Science Foundation of Shanxi Province (201801D121285).
[1] W.A. Trzciński, S. Cudziło, L. Szymańczyk, Studies of detonation characteristics of aluminium enriched RDX compositions, Propellants, Explos. Pyrotech. 32 (2007) 392– 400. [2] W.A. Trzciński, S. Cudziło, J. Paszula, Studies of free field and confined explosions of aluminium enriched RDX compositions, Propellants, Explos. Pyrotech. 32 (2007) 502– 508. [3] Y. Chen, X. Chen, M.X. Xu, S. Xu, D.B. Liu, Properties of dust clouds of novel hydrogen-containing alloys. Combust. Explos. Shock Waves 51 (2015) 313– 318.
18
Journal Pre-proof
[4] Y. Chen, X. Chen, D.J. Wu, S. Xu, D.B. Liu, Underwater explosion analysis of hexogen enriched novel hydrogen storage alloy. J. Energ. Mater. 34 (2016) 49– 61. [5] Y. Chen, S. Xu, D.J. Wu, D.B. Liu, Experimental study of the explosion of aluminized explosives in air. Cent. Eur. J. Energ. Mater. 13 (2016) 117– 134. [6] Y.F. Cheng, X.R. Meng, H.H. Ma, S.H. Liu, Q. Wang, C.M. Shu, Z.W. Shen, W.J. Liu, S.X. Song, F. Hua, Flame
f
propagation behaviors and influential factors of TiH2 dust explosions at a constant pressure, Int. J. Hydrogen Energy,
oo
43 (2018) 16355– 16363.
pr
[7] H.Y. Wang, G.Q. Jian, S. Yan, J.B. Delisio, C. Huang, M.R. Zachariah, Electrospray formation of gelled
e-
nano-aluminium microspheres with superior reactivity. ACS Appl. Mater. Interfaces, 5 (2013) 6797– 6801.
Pr
[8] H.Y. Wang, G.Q. Jian, G.C. Egan, M.R. Zachariah, Assembly and reactive properties of Al/CuO based nanothermite microparticles. Combust. Flame 161 (2014) 2203– 2208.
al
[9] Y. Zhang, W.G. Cao, G.N. Rao, L. Liu, H.D. Zhou, Y.X. Tan, Experiment-based investigations on the variation
rn
laws of functional groups on ignition energy of coal dusts. Combust. Sci. Technol. 190 (2018) 1850– 1860.
Jo u
[10] Q. Zhang, L. Liu, S. Shen, Effect of turbulence on explosion of aluminium dust at various concentrations in air. Powder Technol. 325 (2018) 467– 475.
[11] Q.Z. Li, K. Wang, Y.N. Zheng, X.N. Mei, B.Q. Lin, Explosion severity of micro-sized aluminium dust and its flame propagation properties in 20 L spherical vessel. Powder Technol. 301 (2016) 1299– 1308. [12] H.P. Jiang, M.S. Bi, B. Li, D.Q. Ma, W. Gao, Flame inhibition of aluminium dust explosion by NaHCO3 and NH4 H2 PO4 . Combust. Flame 200 (2019) 97– 114. [13] H.P. Jiang, M.S. Bi, B. Li, D.W. Zhang, W. Gao, Inhibition evaluation of ABC powder in aluminium dust explosion. J. Hazard. Mater. 361 (2019) 273– 282.
*
19
Journal Pre-proof
[14] P.R. Amyotte, Solid inertants and their use in dust explosion prevention and mitigatio. J. Loss Prev. Process Ind. 19 (2006) 161– 173. [15] P.R. Amyotte, M.J. Pegg, F.I. Khan, Application of inherent safety principles to dust explosion prevention and mitigation. Process Saf. Environ. Prot. 87 (2009) 35– 39. [16] P.R. Amyotte, R.K. Eckhoff, Dust explosion causation, prevention and mitigation: an overview, J. Chem. Health
f
Saf. 17 (2010) 15– 28.
oo
[17] C. Proust, Turbulent flame propagation in large dust clouds, J. Loss Prev. Process Ind. 49 (2017) 859 – 869.
pr
[18] R.B. Moussa, C. Proust, M. Guessasma, K. Saleh, J. Fortin, Physical mechanisms involved into the flame
e-
propagation process through aluminium dust-air clouds: A review, J. Loss Prev. Process Ind. 45 (2016) 9 – 28.
Pr
[19] D.M. Solberg, J.A Pappas, E. Skramstad, Observations of flame instabilities in large scale vented gas explos ions, Symp. Combust. 18 (1981) 1607– 1614.
al
[20] D. Bradley, A. Mitcheson, The venting of gaseous explosions in spherical vessels. I-Theory, Combust. Flame 32
rn
(1978) 221– 236.
Jo u
[21] X.Q. Yan, J.L. Yu, W. Gao, Duct-venting of dust explosions in a 20 L sphere at elevated static activation overpressure. J. Loss Prev. Process Ind. 32 (2014) 63– 69. [22] X.Q. Yan, J.L. Yu, Dust explosion venting of small vessels at the elevated static activation overpressure. Powder Technol. 261 (2014) 250– 256. [23] O.J. Ugarte, V. Akkerman, A.S. Rangwala, A computational platform for gas explosion venting, Process Saf. Environ. Prog. 99 (2016) 167– 174. [24] H. Sezer, F. Kronz, V. Akkerman, A.S. Rangwala, Methane-induced explosions in vented enclosures, J. Loss Prevent. Proc. 48 (2017) 199– 206. [25] V.D. Sarli, A.D. Benedetto, E.J. Long, Time-resolved particle image velocimetry of dynamic interactions
20
Journal Pre-proof
between hydrogen-enriched methane/air premixed flames and toroidal vortex structures, Int. J. Hydrogen Energy 37 (2012) 16201– 16213. [26] V.D. Sarli, A.D. Benedetto, Effects of non-equidiffusion on unsteady propagation of hydrogen-enriched methane/air premixed flames. Int. J. Hydrogen Energy 38 (2013) 7510– 7518. [27] A.D. Benedetto, P. Russo, Thermo-kinetic modelling of dust explosions, J. Loss Prev. Process Ind. 20 (2007)
f
303– 309.
oo
[28] M. Bidabadi, A. Haghiri, A. Rahbari, The effect of Lewis and Damköhler numbers on the flame propagation
pr
through micro-organic dust particles, Int. J. Therm. Sci. 49 (2010) 534– 542.
e-
[29] S.K. Chow, R.P. Cleaver, M. Fairweather, D.G. Walker, An experimental study of vented explosions in a 3:1
Pr
aspect ratio cylindrical vessel, Process Saf. Environ. Prot. 78 (2000) 425 – 433. [30] Y. Cao, J. Guo, K.L. Hu, L.F. Xie, B. Li, Effect of ignition location on external explosion in hydrogen–air
al
explosion venting, Int. J. Hydrogen Energy 42 (2017) 10547– 10554.
Jo u
vessel, Energy 151 (2018) 26– 32.
rn
[31] Y. Cao, B. Li, K. H. Gao, Pressure characteristics during vented explosion of ethylene-air mixtures in a square
[32] W.G. Cao, Q.F. Qin, W. Cao, Y.H. Lan, T. Chen, S. Xu, X. Cao. Experimental and numerical studies on the explosion severities of coal dust/air mixtures in a 20-L spherical vessel, Powder Technol. 310 (2017) 17– 23. [33] Z. Salamonowicz, M. Kotowski, M. Półka, W. Barnat, Numerical simulation of dust explosion in the spherical 20L vessel, Bull. Pol. Acad. Sci. Tech. Sci. 63 (2015) 289– 293. [34] C. Murillo, O. Dufaud, N. Bardin-Monnier, O. López, F. Munoz, L. Perrin, Dust explosions: CFD modeling as a tool to characterize the relevant parameters of the dust dispersion, Chem. Eng. Sci. 18 (2013) 103–116. [35] S.A. Orszag, V. Yakhot, W.S. Flannery, F. Boysan, D. Choudhury, J. Maruzewski, B. Patel, Renormalization group modeling and turbulence simulations. In International Conference on Near-Wall Turbulent Flows, Tempe, *
21
Journal Pre-proof
Arizona. 1993. [36] S.A. Morsi, A.J. Alexander, An investigation of particle Trajectories in two-phase flow systems, J. Fluid Mech. 55 (1972) 198– 208. [37] T.L. Shi, Y.X. Tan, Y.Y. Li, C.J. Yu, Y.M. Ye, W.G. Cao, Effect of oxidation rate of aluminium on dust explosion properties, Fire Sci. Tech. 36 (2017) 1194– 1196.
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[38] NFPA 68. Standard on explosion protection by deflagration venting. Quincy, MA, USA: National Fire Protection
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Pr
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pr
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Association. 2013.
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H IG H LI GH T S
Severity parameters were evaluated for the aluminum powder explosion venting.
Single and secondary/multiple venting flame types were discussed.
Transient explosion venting parameters were revealed by simulation results.
Under-expansion jet phenomenon was formed in the venting process .
Formula calculation of venting area was used to verify the experimental results .
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*
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Table caption
Table 1 Test results of cracking pressure (P cra) and maximum reduced pressure (P red ). Table 1 Test results of cracking pressure (P cra) and maximum reduced pressure (P red ).
D=50 mm
P red / MPa P cra / MPa D=70 mm
P red / MPa
0.
40 2
4
6
0.3 4
0.4
0.5
2
3
0.
0.
20 0.
30
0.
0.
36
44
0.6
Pr al rn Jo u 24
0.7
0.7
40
0.
50
55
0.8
0.7 6
0.8 5
6
0.
70
0. 62
0.9
0.
60
0.
0.9 6
0.
50
04
4
0.
1.
92
2
0.
e-
Note: D is the vent diameter.
0
1
0.
10 29
8
04 0.
82
0.6
1.
91 0.
78
0.4
0.
78 0.
72
0.3
0.
65 0.
61
0.2
0.
52 0.
51
0.1
0.
39
f
0.
P red / MPa
0.
26
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D=30 mm
P cra / MPa
0.
13
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0.
P cra / MPa
80
0. 71
0. 80
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Figure captions Fig. 1. Scanning electron microscope (SEM) photo of aluminum powders. Fig. 2. Diameter distribution of aluminum powders. Fig. 3. Modified 20 L sphere test system. Fig. 4. Hybrid grids of the computational domain. Fig. 5. Typical pressure history of aluminum powder/air fuels (APAF) explosion.
oo
f
Fig. 6. Pressure rising of aluminum powder/air fuels (APAF) explosion.
pr
Fig. 7. Relationship between maximum reduced explosion pressure (Pred) and cracking pressure (Pcra).
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Fig. 8. High speed photographs of Flame venting. (a) Single flame type, (b) Secondary/multiple flame type.
Pr
Fig. 9. Relationship between maximum flame front distance and cracking pressure (Pcra).
(c) Vent diameter: 30 mm.
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Fig. 10. Flame type with different concentration. (a) Vent diameter: 70 mm, (b) Vent diameter: 50 mm,
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Fig.11. Spatial distribution of explosion venting characteristics. (a) Pressure (MPa) field in the process
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of explosion, (b) Gas temperature (K) field in the process of explosion, (c) Powders temperature (K) field in the process of explosion, (d) Gas-flow velocity (m/s) field in the process of explosion, (e) Turbulent kinetic energy (m2 /s2 ) field in the process of explosion. Fig. 12. Ratio of standard calculated area to experimental area (As /Ae) with different cracking pressures (Pcra)
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Fig. 1. Scanning electron microscope (SEM) photo of aluminum powders.
26
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Pr
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Fig. 2. Diameter distribution of aluminum powders.
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Fig. 3. Modified 20 L sphere test system.
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Fig. 4. Hybrid grids of the computational domain.
*
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Fig. 5. Typical pressure history of aluminum powder/air fuels (APAF) explosion.
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Fig. 6. Pressure rising of aluminum powder/air fuels (APAF) explosion.
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Fig. 7. Relationship between maximum reduced explosion pressure (Pred) and cracking pressure (Pcra).
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(a) Single flame type
(b) Secondary / multiple flame type
Fig. 8. High speed photographs of Flame venting.
*
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Fig. 9. Relationship between maximum flame front distance and cracking pressure (Pcra).
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(a) Vent diameter: 70 mm
(b) Vent diameter: 50 mm
(c) Vent diameter: 30 mm Fig. 10. Flame type with different concentration. *
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(a) Pressure (MPa) field in the process of explosion
(b) Gas temperature (K) field in the process of explosion 36
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(c) Powders temperature (K) field in the process of explosion
(d) Gas-flow velocity (m/s) field in the process of explosion *
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(e) Turbulent kinetic energy (m2 /s2 ) field in the process of explosion
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Fig.11. Spatial distribution of explosion venting characteristics.
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Fig. 12. Ratio of standard calculated area to experimental area (As /Ae) with different cracking pressures
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Pr
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(Pcra)
*
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Declaration of interest statement The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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