Combustion Characteristics of Nanoaluminum Cloud in Different

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Combustion characteristics of nanoaluminum cloud in different atmospheres yunlan sun, Rong Sun, Baozhong Zhu, yuxin wu, qichang wang, and weikang han Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04021 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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Industrial & Engineering Chemistry Research

Combustion characteristics of nano-aluminum cloud in different atmospheres

Yunlan Sun, Rong Sun, Baozhong Zhu ∗, Yuxin Wu, Qichang Wang, Weikang Han School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui 243002, China

Abstract: The flame propagation of nano-aluminum cloud in different atmospheres such as CO2, air, and O2 was experimentally studied in a transparent quartz tube. Measurements of the flame length, height, propagation velocity, and temperature indicate that these parameters undergo regular changes in different atmospheres. The maximum measured temperature in CO2 was lower than that in air. Based on these results, the flame propagation mechanism of nano-aluminum cloud is discussed. Scanning electron microscope revealed that the combustion products consist of uniform spherical particles. On the other hand, the X-ray diffraction analysis indicated the presence of Al, γ-Al2O3, θ-Al2O3 and α-Al2O3 in the combustion products of nano-aluminum cloud in air and the existence of Al, θ-Al2O3, α-Al2O3 and Al2OC when the cloud burns in CO2.

∗

Corresponding author. Tel.: +86 555 2312885; fax: +86 555 2312885.

E-mail address: [email protected]

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Keywords:

Nano-aluminum

cloud;

Combustion

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atmospheres;

Flame

propagation

1 Introduction Metal powders with high energy density, such as aluminum powder, are widely used as propellants, explosives, and pyrotechnics. For example, Alex powder—an ultra-fine aluminum powder— has been found effective in enhancing the burning rate of solid propellants.

1, 2

In relation to this, Sippel et al.

3

studied the

agglomeration of aluminum powder in a composite propellant, consisting of aluminum and polytetrafluoroethylene (PTFE), and obtained results suggesting that the most significant reduction in aluminum agglomeration occurred when the proportion of Al to PTFE is 70/30 wt%. Vummidi et al. 4 investigated the thermal reaction of superfine aluminum powder coated with nickel and found that it ignites more readily and seemingly more violently than other combination. On the other hand, Trunov et al. 5 studied the oxidation and melting behavior of nano-aluminum powder using a thermal analyzer but found no correlation between them. Corcoran et al.

6

investigated the combustion characteristics of fine aluminum powder in

water vapor and discovered that the correlation between burn times (t) and particle diameter (D) can be approximately described as t~D0.64. In the aforementioned studies, the aluminum particles were at rest in a reactor, whereas in fact, aluminum particles are in motion inside a rocket engine. Accordingly, to understand the combustion of nano-scale metal particles that are in motion, various experiments have been conducted. Frost and Julien et al. researched on particle explosions and Sun et al.

11-13


7–10

studied the combustion

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characteristics and mechanisms of metal clouds in air using a high-speed video camera coupled with an optical amplifying device. Their results showed that iron particles burned in the condensed phase, whereas aluminum dust cloud burned in the gas phase. Ding et al.

14

investigated flame propagation through zirconium

particle clouds in a small-scale vertical rectangle chamber. Their results demonstrated that the flame propagation velocity and flame temperature of the zirconium cloud increases with increasing zirconium concentration, and there was a slight deflagration of zirconium particles. Bocanegra et al. 15 studied the ignition and combustion characteristics of nickel-coated aluminum dust cloud, and discovered that the ignition delay time of nickel-coated aluminum particles decreased, whereas combustion time increased compared with un-coated aluminum particles. In addition, Julien et al.16 studied the freely propagating flames in aluminum dust clouds inside transparent latex balloons and found that flame speed was strongly affected by the oxygen concentration in fuel-rich mixtures, whereas dependence on this concentration was less in fuel-lean mixtures. However, most of the previous research on metal particle ignition and combustion were carried out in air. Only a few studies were conducted to investigate the combustion of moving nano-scale metal particles in other atmospheres, especially in carbon dioxide. To take this further, it is well known that the Martian atmosphere consists of approximately 96% CO2, making the achievement of an in-situ resource utilization possible. 17 The Al/CO2 mixture is regarded as a promising fuel to realize a future exploration mission to Mars. Nevertheless, most of research was aimed at preventing powder explosions to



study the free propagation of metal dust clouds and there is less research on promoting metal combustion by suspending metal particles. In particular, aluminum powder is easy to agglomerate when it burns in a quiescent state. Therefore, in this study, the effects of CO2 on the combustion characteristics of aluminum clouds with average particle diameters of 50 and 100 nm were investigated. The flame propagation velocity, length, height, and combustion temperature of aluminum clouds were also measured. The combustion products were collected and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD).

2 Experiment 2.1 Experimental apparatus and materials The combustion apparatus used in this study is shown schematically in Fig. 1. It consists of a combustion chamber, an injection system of particles, and temperature measurement and product collection systems. The combustion chamber — having an inner diameter and height of 14.0 and 300.0 mm, respectively—consists of five quartz tubes, which are connected together and sealed at the junctions by PTFE tape. The bottom end of the combustion chamber is connected to a porous metal (nickel) plate and the top end is covered by a cellulose filter. There are two gas outlets on the side wall of the combustion chamber close to its top. The combustion chamber is equipped with an injection system of particles, which consists of a high limit relay, rotameter, pulsed gas controller, and an electrical valve. Gases, such as CO2, O2 and air flow into the combustion chamber via the high limit relay, rotameter, and electrical valve.


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Pressure in the gas transmission pipeline is controlled at 0.05 MPa by the high limit relay and the flow rate, 27 L/min, is measured by the rotameter. The aluminum particles are transported and dispersed in the combustion chamber by the pulsed gas flow. For the temperature measurement, the system consists of a data recorder (Agilent 34972A) and three thermocouples which are fixed at 120 (bottom), 180 (middle) and 240 mm (top) above the bottom of the combustion chamber along its centerline. The thermocouples are composed of 100 μm diameter Pt–Pt/Rh wire and the ignition voltage of the heating wire is 10 V. In addition, a high-speed video camera (Phantom V311, 3250 fps speed, 1280× 800 pixels maximum resolution) is used to record and measure the flame length, height, and flame propagation velocity. For the experiments, the aluminum powder—with average diameters of 50 and 100 nm, containing 79% and 82% active aluminum, respectively—was provided by the Nanometer Material Limited Company in Henan province. On the other hand, the experimental gases—CO2 (99.99% purity), O2 (99.99% purity), and air —were supplied by the Nanjing Special Gas Company in Jiangsu province. Finally, the combustion products were collected and analyzed by SEM (NOVA NANOSEM 430) and XRD (Bruker ADVANCE D8). 2.2 Experimental methods In the experiment, 0.03 g aluminum powder is placed on the porous nickel plate. Thereafter, an aluminum cloud is produced using an electrical valve that produces a pulsed gas flow to transport and disperse the particles inside the combustion chamber. The injection pressure and duration was controlled to obtain uniform distribution of particles inside the combustion chamber in a short time, so the



cloud concentration of approximately 650 g/m3 was calculated by having the aluminum mass divided by the inner volume of the combustion chamber. When the aluminum cloud is dispersed, it is ignited by a heating wire placed along the axis at 30.0 mm above the porous nickel plate. The combustion process is recorded by a high-speed video camera. Accordingly, the propagation velocity of the burning particle cloud can be obtained based on the displacement of the flame front at known time intervals. The flame temperature of the aluminum cloud is measured by the thermocouples and recorded by a data-acquisition instrument. Finally, the combustion products that fall on the porous nickel plate and heating wire are collected to study their detailed morphology and structure by SEM and XRD. To reduce experimental error and quantify the uncertainties of the experimental results, all experiments were repeated thrice.

3 Results and discussion 3.1 Flame propagation 3.1.1 Effects of oxidation atmosphere A series of typical high-speed video photographs were recorded at 100 fps, revealing the flame propagation through the 100-nm aluminum cloud in a CO2 atmosphere, shown in Fig. 2. The propagation is displayed from 0.00–0.24 s at a time interval of 0.01 s and the ignition moment of the aluminum cloud is set at 0.00 s. At the initial stage, the flame height defined as the distance between the heating wire and flame front—increases significantly with flame propagation. However, the rate of increase gradually reduces until the flame height stabilizes at one value. This process begins from 0.00 and lasts for approximately 0.19 s.


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Afterwards, the flame height decreases. Based on the above discussion, the changes in the flame height of the 100-nm aluminum cloud can be summarized as the following processes—the flame increases, reaches a stable value, and then decreases. Another custom parameter that was considered in evaluating flame characteristics is flame length, which is defined as the distance from the flame front to the flame base. As shown in Fig. 2, the flame length increases at the initial stage and then stabilizes from 0.15–0.20 s. Finally, it decreases as the aluminum particles are depleted. It is noteworthy that there is a distinct dark region observed between the times 0.19 and 0.24 s in the first two tubes. This may have been caused by either the complete combustion of the aluminum powder or driving up of the burning aluminum particles by the gas flow. A series of flame propagation photos of the 100-nm aluminum cloud in air is displayed in Fig. 3. Comparing the flame photos shown in Fig. 2 (in CO2) and Fig. 3 (in air), some similarities can be observed. At the initial stage, the flame height significantly increases with flame propagation. Thereafter, the increase in flame height becomes slow and gradually stabilizes. Finally, at 0.19 s, the flame height begins to drop. Nevertheless, despite these similarities, some differences also exist, as follows. The aluminum cloud burns more fiercely in air than in CO2. Furthermore, the flame length and height in air are also longer than those in CO2 because the flame front propagates faster in air than in CO2. Conduction and radiation heat transfer play an important role in the flame propagation in dust clouds.18 Particle temperature depends on energy transferred and heat capacity of the particle. The amounts of energy released for the combustion of aluminum dust cloud are different in various oxidizers. It can be found from Figs. 2 and 3 that the



flame luminosity in air is stronger in CO2, indicating that the flame temperature of the nano-Al particles in air is higher in CO2, so the flame length and height in air are longer than those in CO2. The flame propagation of the 100-nm aluminum cloud in O2 is shown in Fig. 4. The overall flame propagation process is similar to that in CO2 and air, i.e., the flame height first increases quickly, reaches a plateau, and then decreases. However, the aluminum cloud burns violently in O2 and emits dazzling white lights during the burning process, as also shown in Fig. 4. In addition, among the three oxidation atmospheres for the study, the flame length and height is largest for the combustion of the aluminum cloud in O2. During the period from 0.13–0.16 s, the flame of the aluminum cloud almost fills the entire combustion chamber. After 0.19 s, the flames in the middle and upper parts of the combustion chamber disappear quickly with some residue burning pockets, which are possibly falling combustion products. To compare the effects of oxidation atmospheres on the flame height of the 100nm aluminum cloud, the evolution of the flame heights versus time in the three atmospheres is illustrated in Fig. 5. The flame height-time curves (F-T curves) and flame propagation velocity curves (V-T curves) of the 100-nm aluminum cloud during 0–0.20 s in CO2, air, and O2 atmospheres are shown in this figure. As shown, the flame height of the 100-nm aluminum cloud is highest in O2 and lowest in CO2. In all three oxidation environments, the flame propagation velocity first exhibits an increase with increase in time and then, gradually decreases after reaching a maximum value. The flame propagation velocity is fastest in O2 and slowest in CO2. However, it is noteworthy that the flame propagation velocities are


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representative of the particle cloud burning under the current experimental conditions. Evidently, the present measurements do not represent the fundamental flame propagation velocities. Nonetheless, a preliminary understanding of the influence of the different factors on flame propagating can be obtained through the current experiment. The F-T curves shown in Fig. 5 are fitted by the sigmoidal fitting method. The presented data are collected from three sets of experiments so as to establish repeatability. The error bars, also shown in Fig. 5 were obtained, which was calculated via equation (1).

σ (χ ) =

1 N (χ i − χ )2 ∑ N − 1 i =1

(1)

where σ ( χ ) is the sample standard deviation, N is the number of tests and χ is the mean measured data for three repeatability tests. Among the variety of fitting functions in sigmoidal fitting, the logistics regression function was found to be most appropriate, having a high value of the correlation coefficient (R2). The fitting equations and corresponding correlation coefficients are listed in Table 1. In the fitting equations, variables t and h are the time (s) after ignition and flame height (m), respectively. According to these equations, the experimental flame height at a certain time can be well reproduced.



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Table 1 Parameters of the fitting F-T curves of the 100-nm aluminum cloud under different atmospheres Atmosphere CO2

Air

O2

R2 0.995

0.998

0.989

Equation ℎ = 23.65 − ℎ = 23.73 − ℎ = 28.31 −

23.96 𝑡𝑡

(1 +

)1.29

0.05

23.53

1+(

𝑡𝑡

1+(

𝑡𝑡

)1.68

0.03

28.2

)1.35

0.02

** h represents flame height, m; t represents time, s.

Apart from the flame height, the combustion temperature of the aluminum cloud is another important parameter to consider in evaluating the combustion characteristics. However, the combustion temperature of the aluminum powder in O2 is extremely high and beyond the maximum value that thermocouple can bear. Thus, only the combustion temperatures of the 100-nm aluminum cloud in air and CO2 are obtained. The maximum measured temperatures at three measurement locations far from the bottom of the combustion chamber are shown in Fig. 6. Based on the measured maximum temperatures in air and CO2 shown in Fig. 6, it is apparent that there is no change in the pattern with respect to temperature. In air, the measured maximum temperature increases with increasing the distance. However, when the aluminum cloud burns in CO2, the maximum temperature occurs in the middle thermocouple inside the chamber. Moreover, the measured maximum temperature of the aluminum cloud in CO2, being lower than that in air, suggests that the aluminum cloud burns more violently in air than in CO2. Nevertheless, it should be noted that the flame temperature is not the temperature


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of the burning particles but merely the temperature of the hot gas heated by the burning particles through thermal radiation. Additionally, the cooling effect of the apparatus on the wall may have a significant influence on the combustion temperature as well. Therefore, the measured flame temperature is lower than the adiabatic flame temperature. As discussed above, the flame propagation of the 100-nm aluminum cloud exhibits significant variations, which may result from the different oxidation capacity of these gases — in different oxidation atmospheres. The oxidation capacity of O2 is stronger than that of air, whereas the oxidation capacity of air is stronger than that of CO2. According to the chemical reaction principle, the stronger the oxidation capacity of an oxidizer is, the easier electrons can be obtained and the reactions run. Thus, in O2, the combustion of the aluminum cloud is most violent in O2 and emits dazzling white lights, indicating the release of a substantial amount of heat over a brief period. During this time, the emitted heat far exceeds the loss of heat, leading to a rapidly rising temperature inside the combustion chamber. As a result of the rapid temperature increase, the thermal motion of the molecules and heat convection coefficient increase. This process further accelerates reaction and may be referred to as self-accelerating combustion. It was also observed that the flame propagation velocity decreases as the aluminum powder is consumed. This explains why the velocity first increases and then decreases. Based on this velocity change rule, self-accelerating combustion is more prominent when the oxidation capacity of the oxidizer is stronger. Therefore, as shown in Fig. 5, the maximum flame propagation velocity of the aluminum cloud is largest in O2 and smallest in CO2. As for the change in flame height under



different oxidation environments, it can be considered that the stronger the oxidation capacity of the oxidizer is, the more prominent the self-accelerating combustion and the higher the flame height. 3.1.2 Effects of particle size The flame propagation of the 50-nm aluminum cloud in CO2—in which the mass of the aluminum powder is the same as that of the 100-nm aluminum cloud—is shown in Fig. 7. It can be observed that there are no distinct differences between the combustions of the 100- (shown in Fig. 2) and 50-nm (shown in Fig. 7) aluminum clouds at the initial stage. However, as the flame propagates, the flame luminosity of the 50-nm aluminum cloud is greater than that of the 100-nm cloud. In the 100-nm aluminum cloud, flame luminosity gradually decreases with flame propagation. At the above occurs, a few yellow glowing particles, representing the formation of combustion products, appear. The flame heights of the 50-and 100-nm aluminum clouds under the three different atmospheres are compared in the following. The changes in the flame height and propagation velocity of the aluminum powder in air and O2 are the same as those of the aluminum powder in CO2. Thus, only the curves of flame height (F-T) and propagation velocity (V-T) versus time in CO2 are shown in Fig. 8. It can be seen that the flame height of the 100-nm aluminum cloud is higher than that of the 50-nm aluminum cloud in CO2 at 0.02–0.18 s. As shown in Fig. 8, the global shapes of the V-T curves first increase and then decrease with time. The maximum flame propagation velocity of the 100-nm aluminum cloud is larger than that of the 50-nm cloud in CO2, which is possibly because of the high content of activated aluminum in the 100-nm aluminum


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particles. The larger the size of the aluminum powder, the less the proportion of aluminum oxide. 19 This indicates that the content of activated aluminum in the 100-nm aluminum particles is higher than that in the 50-nm particles. 3.1.3 Flame propagation mechanism of the nano-aluminum cloud Based on the flame propagation of the nano-aluminum cloud under different atmospheres, the mechanism of the flame propagation of this cloud is briefly described in Fig. 9. As shown in the figure, only a small part of the aluminum cloud can be ignited by the heating wire at the initial stage. Thereafter, the flame gradually spreads into the cloud, which releases energy and emits bright white lights. As the propagation gradually stabilizes, the occurrence of flames in the combustion chamber can be divided into four zones—reacted, post flame, flame front, and unreacted zones. These zones, including the changes in the aluminum particles are shown schematically in Fig. 10. In this figure, there is no reaction at the unreacted zone because the flame front has not reached this region. The region under the unreacted zone is the flame front zone, where the particles are preheated and ready to burn, i.e., the beginning of the flame spread in the downstream direction. The region under the flame front zone, called the post flame zone, is filled with stabilized and burning particles that emit bright flames. At this region, aluminum particles melt and form bigger particles. At the reacted zone, some hot pockets that contain combustion products, emit yellow lights. At this region, the aluminum particles are almost completely burned out. The three kinds of combusted products are shown in Fig. 10. One is a single product particle, produced by a burning aluminum particle with an enveloping flame. On the other hand, two other products are generated by clusters of burning particles. One of the



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two is the irregular slagging solid product composed of some nano-alumina particles, resulting from incomplete melting and sintering. Another product is a big alumina particle produced by the further melting and sintering of the burning particle cluster. According to previous studies,

20, 21

the flame propagation velocity can be

calculated by the following formula: VL= (α/τb) 1/2

(2)

where VL is the velocity of the flame propagating, cm/s; α means the temperature-dependent coefficient of thermal diffusivity of the gas mixture, cm2/s; τb is the combustion time of the nano-aluminum, s. In this study, the model described in the work of Huang et al.

20

is adopted. To

satisfy the model, it is necessary to make further assumptions on the basis of the original model.

20

The additional hypotheses are as follows: the effects of

gravitational and impulse airflows should be neglected although they may perform a function in flame propagation; the value of α (cm2/s) is assumed to be the thermal diffusivity of the oxidizer because the concentration of aluminum particles in the airflow is quite small. Accordingly, the formula to calculate the value of α (cm2/s) is as follows: α=λ/ (ρ·cp)

(3)

Here, λ is the coefficient of thermal conductivity of the oxidizer; ρ and cp are the density of the oxidizer and constant pressure heat capacity, respectively. Hence, the only missing information for Eq. (2) is the aluminum particle combustion time, τb (s), which is calculated by the d0.3 law, 22 as follows.


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τb=

d 0.3 X eff × C2 × e

− Eb RT

(4)

where C2=8.75×105; Eb=73.6 kJ/mol; d is the particle diameter, μm; Xeff is the effective oxidizer concentration, Xeff=CO2+0.6CH2O+0.22CCO2; R is the universal gas constant. Therefore, the values of VL in CO2 (VL1) and air (VL2) are as follows. VL1= (α1/τb1)1/2 ≈14.06 cm/s =0.14 m/s;

(5)

where λ1 of CO2 at 300 K is approximately equal to 0.0137 W/(m·K) (3.27×10-5 cal/(s·cm·K)) and the value of ρ1·cp1 ≈ 0.6859 cm2/s. VL2= (α2/τb2)1/2 ≈ 58.30 cm/s = 0.58 m/s;

(6)

where λ2 of air at 300 K is approximately equal to 0.0256 W/(m·K) (6.13×10-5 cal/(s·cm·K)) and the value of ρ2·cp2 ≈ 0.2224 cm2/s. The theoretical calculation results were obtained under an ideal condition, but the experimental values were affected by both gravity and impulse airflows. To compare the theoretical values with experimental ones, the influence of the jet velocity and gravity on the latter values needs to be eliminated. When the particle is not ignited, the propagation velocity can be considered as the airflow velocity, which is 2.92 m/s. However, when the particle ignites, the impulse airflow velocity is higher than that in the non-ignition condition because the heat from the aluminum accelerates airflow. Therefore, the airflow velocity needs to be subtracted from the velocity obtained by experiment. The maximum flame propagation velocities in CO2 and air are as follows. VLCO2= 3.2−2.92=0.28 m/s

(7)

VLAir= 4.2−2.92=1.28 m/s

(8)

These flame propagation velocities in CO2 and air (0.28 and 1.28 m/s,



respectively) are higher than the calculated theoretical values (0.14 and 0.58 m/s, respectively). The reason for this is, when nano-aluminum particles are ignited, the acceleration of the impulse airflow has a greater impact on the flame propagation velocity at high temperatures. It can be also seen that the flame propagation velocity in CO2 is slower than that in air. This result is consistent with the result of Bidabadi et al.18 3.2 Combustion product analyses 3.2.1 SEM analyses The typical SEM photographs of the combustion products of the 100-nm aluminum cloud in CO2 and air are presented in Fig. 11. As shown in the figure, the combustion products consist of spherical particles. Some are big particles, implying that a part of the aluminum particles melts and agglomerates into larger particles. This result is consistent with the findings reported in Ref. 23-26. However, some bigger particles are found in the products of Al/air than those of Al/CO2. A closer examination of the two images in Fig. 11 reveals that there is no distinct difference in morphology between the combustion products in air and CO2. 3.2.2 XRD analyses During the oxidation of the aluminum particles, the alumina evolves as follows: amorphous-Al2O3 → γ-Al2O3 → θ-Al2O3 (δ-Al2O3) → α-Al2O3. 27-30 Among the various kinds of alumina, α-Al2O3, with a density of 3.99 g/cm3, is the most stable form. 31 The XRD patterns of the combustion products of the 50- and 100-nm aluminum


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clouds in CO2 and air, respectively, are shown in Figs. 12 and 13. The results indicate the same polymorph features in the combustion products of the aluminum clouds of different particle sizes. The observed XRD peaks correspond to Al and different polymorphs of Al2O3, such as θ-Al2O3 and α-Al2O3. In addition, an oxycarbide, Al2OC, is also detected. As shown in Fig. 13, the combustion products of the aluminum clouds of different particle sizes in air also have the same polymorphs, such as Al, γ-Al2O3, θ-Al2O3, and α-Al2O3. Again, these results suggest that particle size has no significant influence on product polymorphs. However, according to the XRD patterns shown in Figs. 12 and 13, it can be observed that different polymorphs are formed under in different atmospheres. The first difference noted is the presence of intermediate Al2OC, which is produced in CO2 only and may be produced through the following reactions:

4 Al ( s ) + 3CO2 ( g ) = 2 Al2O3 ( s ) + 3C ( s )

Al2O3 ( s ) + 3C (= s ) Al2OC ( s ) + 2CO( g )

(9) (10)

In fact, whether in wet or dry CO2, carbon can be produced in the reaction between an aluminum particle and CO2.

32

In order to prove the existence of

carbon, the combustion products of the aluminum cloud and CO2 are analyzed by energy-dispersive X-ray spectroscopy. The results are shown and listed in Fig. 14 and Table 2, respectively.



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Table 2 Elemental content of the combustion products of the 100-nm aluminum cloud under different spectrum points Element

Spectrum 1

Spectrum 2

Spectrum 3

Weight, %

Atomic, %

Weight, %

Atomic, %

Weight, %

Atomic, %

C

31.28

46.08

41.35

55.78

34.17

47.91

O

19.63

21.72

21.81

22.09

25.65

27.01

Al

49.09

32.2

36.84

22.13

40.18

25.08

As listed in Table 2, carbon was detected at the three detection points, proving the existence of carbon in the combustion products of aluminum and CO2. Therefore, it is possible that Al2OC is produced by the reaction of carbon and alumina (shown in reaction (2)) and the remaining carbon still exists in the combustion products of the aluminum cloud. The second difference is, γ-Al2O3 was detected in the combustion product of aluminum cloud and air, but not in the combustion product of the aluminum cloud and CO2. The difference in the Al2O3 structure may have resulted from various reaction mechanisms. In the CO2 atmosphere, the presence of the carbon element increases the complexity of the reaction mechanism, producing the compounds of Al2OC. This may also lead to the difference between the formed Al2O3 phases. Moreover, the combustion temperatures of the nano-aluminum cloud in air and CO2 are different. Accordingly, the formed Al2O3 products undergo different temperatures, which affect their structure. Nevertheless, this conjecture deserves further investigation in the future.


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4 Conclusions This study revealed the flame propagation behaviors of the aluminum clouds of varying particle sizes in different oxidation atmospheres. The morphology and polymorph of the combustion products were also analyzed by SEM and XRD, respectively. The flame length and height exhibited a regular changes in initial increase, stability plateau stage, and final decrease in the different atmospheres. However, the flame height and length of the nano- aluminum clouds that burned in O2 are larger than those produced when the cloud is oxidized in CO2 and air. In addition, the maximum measured combustion temperature of the aluminum cloud in CO2 is lower than that in air. It was seen that the flame propagation velocity first increases and then decreases with flame propagation. It was also observed that the combustion products of the nano- aluminum cloud are made up of spherical particles. In the combustion of the nano-aluminum cloud in air, Al, γ-Al2O3, θ-Al2O3 and α-Al2O3 are detected in the combustion products. On the other hand, crystals of Al, θ-Al2O3, α-Al2O3 and Al2OC are detected when the aluminum cloud burns in CO2.

Acknowledgements We greatly appreciate the financial support provided by the National Natural Science Foundation of China (No. 51376007, 51676001 and 51206001) and the Anhui Provincial Natural Science Foundation (No. 1608085ME104).



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Figure captions Fig. 1 Experimental setup of metal particle cloud combustion. Fig. 2 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 100-nm aluminum cloud in CO2 between the moment of ignition (t = 0) and 0.24 s. Fig. 3 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 100-nm aluminum cloud in air between the moment of ignition (t = 0) and 0.24 s. Fig. 4 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 100-nm aluminum cloud in O2 between the moment of ignition (t = 0) and 0.24 s. Fig. 5 F-T and V-T curves of the flame propagation of the 100-nm aluminum cloud in different oxidation environments. Fig. 6 The measured maximum temperatures at the three thermocouple locations in air and CO2. Fig. 7 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 50-nm aluminum cloud in CO2 between the moment of ignition (t = 0) and 0.24 s. Fig. 8 F-T and V-T curves of flame propagation through the 50- and 100-nm aluminum clouds in CO2. Fig. 9 Flame propagation model of the aluminum cloud. Fig. 10 The changes of aluminum particles during combustion. Fig. 11 SEM photographs of the combustion products of the 100-nm aluminum particles in (a) CO2; (b) air.


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Fig. 12 XRD patterns of the combustion products of the nano-aluminum cloud in CO2 (a) 50nm; (b) 100nm. Fig. 13 XRD patterns of combustion products of the nano-aluminum cloud in air (a) 50 nm; (b) 100 nm. Fig. 14 Energy-dispersive X-ray spectroscopy of the products of the 100-nm nano-aluminum cloud in CO2.



Figures

Fig. 1 Experimental setup of metal particle cloud combustion.

Fig. 2 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 100-nm aluminum cloud in CO2 between the moment of ignition (t = 0) and 0.24 s.


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Fig. 3 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 100-nm aluminum cloud in air between the moment of ignition (t = 0) and 0.24 s.

Fig. 4 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 100-nm aluminum cloud in O2 between the moment of ignition (t = 0) and 0.24 s.



Fig. 5 F-T and V-T curves of the flame propagation of the 100-nm aluminum cloud in different oxidation environments.

Fig. 6 The measured maximum temperatures at the three thermocouple locations in air and CO2.


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Fig. 7 Photos captured by the high-speed video camera at an interval of 0.01 s showing the flame propagation of the 50-nm aluminum cloud in CO2 between the moment of ignition (t = 0) and 0.24 s.

Fig. 8 F-T and V-T curves of flame propagation through the 50- and 100-nm aluminum clouds in CO2.



Fig. 9 Flame propagation model of the aluminum cloud.

Fig. 10 The changes of aluminum particles during combustion.


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Fig. 11 SEM photographs of the combustion products of the 100-nm aluminum particles in (a) CO2; (b) air.



Fig. 12 XRD patterns of the combustion products of the nano-aluminum cloud in CO2 (a) 50nm; (b) 100nm.


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Fig. 13 XRD patterns of combustion products of the nano-aluminum cloud in air (a) 50 nm; (b) 100 nm.



Fig. 14 Energy-dispersive X-ray spectroscopy of the products of the 100-nm nano-aluminum cloud in CO2.


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