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Experimental investigation on the combustion characteristics of aluminum in air
Acta Astronautica 129 (2016) 1–7
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Acta Astronautica journal homepage: www.elsevier.com/locate/aa
Experimental investigation on the combustion characteristics of aluminum in air Yunchao Feng, Zhixun Xia, Liya Huang n, Xiaoting Yan College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Hunan 410073, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2016 Accepted 28 June 2016 Available online 29 June 2016
With the aim of revealing the detailed process of aluminum combustion in air, this paper reports an experimental study on the combustion of aluminum droplets. In this work, the aluminum wires were exposed and heated by a CO2 laser to produce aluminum droplets, and then these droplets were ignited and burnt in air. The changing processes of aluminum wires, droplets and flames were directly recorded by a high-speed camera, which was equipped with a high magnification zoom lens. Meanwhile, the spectrum distribution of the flame was also registered by an optical spectrometer. Besides, burning residuals were collected and analyzed by the methods of Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS). Experimental results show that, during combustion, the aluminum droplet is covered by a spherical vapor-phase flame, and the diameter of this flame is about 1.4 times of the droplet diameter, statistically. In the later stages of combustion, the molten aluminum and condensed oxide products can react to generate gaseous Al and Al2O spontaneously. Little holes are found on the surface of residuals, which are the transport channels of gaseous products, namely the gaseous Al and Al2O. The combustion residuals are consisted by lots of aluminum oxide particles with diameters less than 1 μm. & 2016 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Aluminum droplet Combustion Oxide cap Oxide bubble CO2 laser Ducted rocket
1. Introduction Compared with other additives [1,2], the advantages of aluminum are its high volumetric combustion enthalpy, high combustion temperature, non-toxicity and relatively inexpensive. Therefore, aluminum is an important energetic ingredient of many solid propellants [3]. In ducted rocket, the reaction of aluminum with the air breathed from inlets in second combustion chamber is the main energy release process. However, aluminum particles containing in the solid propellants ignite at fairly high temperature and they usually get melted before ignition. Because of fusion and agglomeration, the diameters of liquid aluminum droplets can actually reach several hundred microns [4] and burn slowly and incompletely. Thus, the research on the combustion characteristics of aluminum has been an ongoing effort. Although aluminum combustion has been studied for several decades, several questions about the detailed processes remain to be answered, such as oxide cap formation and consumption, asymmetric combustion, oxide bubble formation and fragmentation. In Ref. [5], the combustion characteristics of aluminum n
Corresponding author. E-mail addresses: [email protected] (Y. Feng), [email protected] (Z. Xia), [email protected] (L. Huang), [email protected] (X. Yan). http://dx.doi.org/10.1016/j.actaastro.2016.06.049 0094-5765/& 2016 IAA. Published by Elsevier Ltd. All rights reserved.
particles have been reviewed with focus on the burning time of individual particles. As for the particle combustion details, a pulsed micro-arc discharge was used in Refs. [6,7] to form and ignite aluminum particle in different atmospheres. In the experiments, partially burned particles were rapidly quenched, and then, their surfaces and interiors were examined by electron microscopy. The reasons causing oscillations in the particle radiation, irregularities in the particle trajectories, and variation in the relative size of the oxide caps as compared to the particle size are analyzed. The effect of gravity on the aluminum particles burning in air was studied in Refs. [8,9] experimentally. Experimental results indicated that the asymmetric flame structure and brightness oscillations were intrinsic features of aluminum particle burning rather than the result of forced or natural convection flows caused by gravity [8]. However, the gravity has a significant influence on the dynamics of aluminum droplet combustion and morphology of combustion products [9]. In Ref. [10], the aluminum particles were ignited in a high-pressure chamber with the help of an electrodynamic levitator and a CO2 laser. During combustion, the thermal emission light of the particle and the surrounding flame emission were collected by a photomultiplier. Brightness oscillations were explained by the oscillations of burning rate, as well as the presence of an oxide cap and occurrence of micro flames on the particle surface. Furthermore, in Ref. [11], the authors used the electrostatic particulate method to aerosolize the micron-sized
2
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aluminum powders. The aerosol was carried away by a laminar gas jet, and then the single particle was ignited by a CO2 laser one by one. In their experiments, the particle sizes, optical signatures and combustion times were measured in real time. In other experiments [12,13], an electrically heated aluminum wire was placed in a flow of an oxidizing gas. The ignition temperature of aluminum wire was detected optically [12], or from the measured changes in the wire electric resistance [13]. In fact, the combustion of a large aluminum particle is generally similar to the diffusion combustion of a fuel droplet [14], except for the effect of the aluminum oxide. Therefore, the condensed phase oxide products of burning aluminum droplet was studied in Refs. [5,15] and several typical combustion models [16,17] have been proposed based on the diffusion combustion model and emphasized on the oxide accumulation on the surface of aluminum droplet. Because of experimental limitations, the details of aluminum droplet, especially for those smaller than 1.0 mm, can hardly be recorded during combustion in real time. The interaction mechanism between aluminum droplet and oxide deposits is still uncertain. The objectives of the present work are to determine the ignition and combustion characteristics of aluminum in air and to provide new experimental dataset on the combustion of single aluminum droplet. In this work, aluminum droplets are produced by partially molten aluminum wires, and then ignited by CO2 laser. Based on the experimental results, the interaction mechanisms between oxide cap and aluminum droplet are also discussed.
2. Experimental methods 2.1. Experimental setup A metal droplet combustion system was designed and used to conduct current experiments. This system mainly consists of CO2 laser, high-speed camera, high magnification zoom lens, optical spectrometer and three-dimensional platform. The CO2 laser, highspeed camera and optical spectrometer are all controlled by the computer. A schematic diagram illustrating such experiments is shown in Fig. 1. In this experimental system, the three-dimensional platform is used to fix and adjust the position of the aluminum wire. To reduce the amount of heat loss caused by conduction, two ceramic tubes are installed to fix the wire, of which center holes are about 400 μm in diameter. The CO2 laser (CWQ2000) is the heating source and its nominal maximum output power is 200 W. The CO2 laser beam is focused by a convex ZnSe lens (focal length ¼100 mm) to a 500 μm focal spot. The high-speed
Fig. 1. Experimental setup for metal droplet combustion.
camera (PCO. dimax S4) is equipped with a complementary metal oxide semiconductor (CMOS) image sensor and its maximum frame rate is 1102 fps at 2016n2016 pixels2. In this study, to provide sufficient spatial resolution, the high-speed camera is equipped with a high magnification zoom lens, which consists of a zoom system (NAVITAR, 0.58-7X) and an adapter (NAVITAR, 2X). The working distance of this lens is 86 mm and its depth of the focal field ranges from 1.39mm for the lowest magnification (1.16 ) to 0.05 mm for the highest magnification (14 ). The optical spectrometer (HR2000 þ ES by Ocean Optics, Inc.) and the probe (74-UV by Ocean Optics, Inc.) are connected by an optical fiber. The spectrometer has an optical resolution of 0.48 nm and its integration time is from 1 ms to 20 s. Besides, the aluminum wires used in these experiments are 99% pure aluminum with nominal diameters of 100 μm, 200 μm and 300 μm by Beijing Zhongnuo Material Technology, Inc. All the experiments were conducted indoor, where the relative air humidity was controlled between 50% and 60% and the room temperature was 296 300 K. 2.2. Experimental procedure With the help of the experimental system introduced in Section of 2.1, a series of experiments on the ignition and combustion characteristics of aluminum droplets in air were conducted. Firstly, the focal plane of high-speed camera and the spatial location of aluminum wires needed to be adjusted to be overlapped at the focus of the CO2 laser. Before the laser started to work, the highspeed camera and optical spectrometer had begun to record the experimental phenomena. Second, CO2 laser begun to work and aluminum wire was heated and ignited. And then, CO2 laser would continue to work until the end of the experiments. The detailed phenomena of experiment was recorded by the high-speed camera directly, and the spectral information was also recorded by the optical spectrometer at the same time. Finally, the burning residuals were collected and analyzed by the methods of SEM and EDS.
3. Experimental results and discussion 3.1. Combustion phenomena 3.1.1. Combustion processes To obtain a general knowledge of the combustion characteristics of the aluminum droplets in air, Fig. 2 shows the sequential images captured by high-speed camera. The initial diameter of aluminum wire was 200 μm and the heating power of the CO2 laser was 200 W. Firstly, the aluminum wire turned dull-red and started to emit light near the laser focus as soon as the CO2 laser worked (Fig. 2 (a)). Few milliseconds later, the initial oxide film rupture appeared on the focus of CO2 laser, which was caused by the difference of thermal expansion coefficients between metal and initial oxide film, as well as the melting of oxide film. As soon as the oxide film had been removed, the uncoated aluminum wire evaporated rapidly and the aluminum vapor reacted with air to form the gasphase flame (Fig. 2(c)). When the partial wire was heated into molten state, the wire would fuse and the molten aluminum droplet would adhere to one of the two ends randomly (Fig. 2(e)). Because of the surface tension, the shape of aluminum droplet was near spherical. The aluminum droplet was covered by a flamesheet at a certain distance from the droplet surface. The gas-phase flame would maintain few tens milliseconds. However, there were several “oxide islets” forming on the surface of droplet, which would move together to accumulate larger “oxide islets”. It should be noted here that the “oxide islets” are both derived from the
Y. Feng et al. / Acta Astronautica 129 (2016) 1–7
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Fig. 2. Images of 200 μm aluminum wire ignition and combustion obtained by high-speed camera. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. Oxide cap and asymmetric flame.
initial oxide film on the aluminum wire and the deposition of gaseous oxide products. As more and more “islets” moved together, the cover area of oxide became larger and the oxide cap formed. Because of the oxide cap, the gas-phase flame became asymmetric and weaker, as shown in Fig. 2(i). Finally, the oxide deposition covered the droplet completely and the flame extinguished.
Fig. 4. Measured spectral emissivities before droplet ignition and during droplet combustion.
During the burning of aluminum droplet in air, lots of gaseous and condensed oxides were produced. Some of those products diffused to the outer of the flame, and others diffused to the surface of the droplet. Compared with the temperature of flame
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Fig. 5. Measurement method of the diameters of droplet and flame.
3.1.2. Flame spectra Typical spectral emissivities before droplet ignition and during droplet combustion are presented in Fig. 4. Before ignition, there was no characteristic wave peak in the spectra distribution line. During combustion, three typical wave peaks (396.1 nm, 484.2 nm, 510.2 nm) could be distinguished clearly. Based on those three wave peaks, the main gaseous species, Al (396.1 nm) and AlO (484.2 and 510.2 nm), occurred during combustion can be inferred. The gaseous species presented during combustion are the reactants or intermediate products, which means the aluminum burning in air is mainly gas-phase combustion. 3.2. Flame sizes
Fig. 6. Flame sizes in different droplet diameters.
(above 3000 K usually), the temperature of the droplet is relatively low, and it is generally considered to be the boiling point of liquid aluminum in local pressure [18]. Fig. 3 shows the oxide caps accumulated on the surface of droplet. By comparing the melting and boiling point of aluminum with those of aluminum oxide, it can be inferred that the oxide cap should be in the liquid phase. Because the evaporation and diffusion of aluminum vapor is blocked by the oxide cap, the gas-phase flame gets to be eccentric and asymmetric. Therefore, it should be the oxide cap that results in the asymmetric combustion and this observation is consistent with the inference in Ref. [7].
It has been mentioned that the molten aluminum would adhere to one of the broken ends randomly. In those experiments, an image of the unburned sample was captured prior to ignition. Thus was to provide a post-experiment reference for computing the diameters of droplet and flame. For convenience, the diameters of the droplet and flame were processed by converting the original images to gray images and extracting the gray levels of line 1 (see Fig. 5). Here, the distance between the two brightness peaks of the curves is defined as the diameter of flame, and the distance between the two valleys is defined as the diameter of droplet. These structure measurements will aid the development of detailed numerical models. Using the measurement method mentioned above, a series of droplet and flame diameters couples are presented on Fig. 6. Those data are processed by linear fitting method with the intercept of the line equaling to zero. The relationship between droplet size and flame size can be expressed as
Df = 1.4Dd
Fig. 7. Oxide bubbles before flame extinction.
(1)
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where, Dd is the diameter of droplet, μm; Df is the diameter of flame, μm. It suggests that the diameter of flame is about 1.4 times of that of droplet during steady combustion. However, the concentration peak of intermediate product, AlO, was detected at the location of 2.8 times particle diameter by the method of PLIF [19]. The flame diameters measured in this study are smaller than that of isolated aluminum particle in Ref. [19], and that should be caused by the heat conduction between the droplet and wire. 3.3. Oxide bubble In the later stages of combustion, the aluminum droplet was covered by a film of combustion products (see Fig. 7(a)). Although the fierce gas-phase flame around the droplet had disappeared, the heterogeneous chemical reactions (HCRs) still ongoing at the interface between the oxide film and droplet. It was the HCRs that resulted in the emergence of oxide bubble (see Fig. 7(b and c)). In general, there were numerous of little oxide bubbles forming and fracturing on the surface of deposition. The big oxide bubble presented on Fig. 7 only emerged under some suitable conditions. Also, the big oxide bubble should be the results of the mergence of large amounts of little oxide bubbles. Furthermore, in fact, the deposition, namely the Al2O3, could react with the molten aluminum at high temperature. The reaction between molten
5
aluminum and aluminum oxide was regarded as a problem of assigned temperature and pressure and simulated by the NASA Chemical Equilibrium with Application (CEA) code [20]. In this problem, the reaction temperature was set as the boiling point of aluminum, 2790 K. The relationship between the mole fraction of combustion products and the oxide (Al2O3) to fuel (Al) weight ratio (O/F) is presented on Fig. 8. The simulation results show that the main combustion products are gaseous Al2O and Al. Compared with other products, the mole fraction of AlO(g) is very small (no more than 0.1%) and can be neglected. At the point of O/F ¼0.8, the mole fraction of Al2O reaches its maximum. In this case, the reaction of Al and Al2O3 can be expressed as 4.7Al(c) þAl2O3(c)-0.829Al(g) þ 2.935Al2O(g)
(R1)
Therefore, it can be inferred that the gaseous products should be Al(g) and Al2O(g) and result in the formation of oxide bubbles in the later stages of combustion (see Fig. 8). Moreover, the enthalpy of reaction and the Gibbs free energy change are also presented on Fig. 9. At the point of O/F ¼0.8, the enthalpy of reaction is greater than zero, which means this is endothermic reaction. The Gibbs free energy change is negative on a large scale of O/F, which means this reaction might occur spontaneously.
Fig. 8. The simulation results of Al reacting with Al2O3 in different O/F.
Fig. 9. The simulation results of the enthalpy of reaction and the Gibbs free energy change.
Fig. 10. Apparent structure of combustion residuals of 200 μm and 300 μm aluminum wire.
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combustion residuals are a kind of white porous material, which consist of lots of little oxide particles ranging on the order a few tens to hundreds of a micrometer in diameter. The 200 μm residuals is different from that of 300 μm, and the major differences between the two kinds of residuals are their porousness and overall diameter. One of the two reasons caused the differences is the 200 μm wire is thinner and the heat loss due to conduction to the wire is relatively small. Therefore, as shown in Fig. 11, the burning rate of 200 μm, 1.60 μm/ms, is faster than that of 300 μm, 0.73 μm/ms. Another reason is that the 200 μm aluminum wire has a larger specific surface area, so it can react with air more sufficiently. 3.4.2. Micro structure The combustion residuals were also characterized by method of SEM, as shown in Fig. 12. On the surface of the little oxide particles shown in Fig. 10, there are several gas holes with diameters of a few microns or smaller. Those holes should be the transport channels of gaseous products in R1. The Fig. 12(b) shows that the porous combustion residual is mainly consisted by oxide particles with diameters less that 1 μm. 3.4.3. Components of residuals The contents of different elements in the combustion residuals are measured by EDS, as shown in Fig. 13. The two main elements in the residuals are Al and O, which means the residual predominantly consists of aluminum and its oxides. As the increasing of initial diameter of aluminum wire, the content of Al is also increased while the content of O is decreased. Thus, it can be concluded that the combustion gets much more incomplete as the diameter k of aluminum wire increases. Besides, in the combustion residuals, there are little other elements, such as C and N. The presence of C and N may be caused by the CO2 and N2 in air, which have also taken part in the reaction. However, the mole fraction of element N is less than 2% in the residuals of three diameters wires. Even though the effects of pure nitrogen on the aluminum droplet combustion under forced convection conditions are significant [21], the aluminum can hardly react with the N2 in air.
4. Conclusions A metal droplet combustion system was designed and built. The characteristics of aluminum ignition and combustion were revealed with help of the CO2 laser, high-speed camera, the high magnification zoom lens and the optical spectrometer. Based on the observation and measurements, the followings are found:
Fig. 11. Measurement method of the burnt amount and burning rate.
3.4. Residuals analysis 3.4.1. Apparent structure Fig. 10 shows the typical apparent structure of the 200 μm and 300 μm aluminum wires combustion residuals. Apparently, the
(1) The ignition and combustion process of aluminum wire in air consists of initial oxide film rupture, locally gas-phase combustion, melting and fusing of wire, fiercely gas-phase combustion and the oxides deposition. (2) During steady combustion, the aluminum droplet is surrounded with gas-phase flame, of which diameter is statistically about 1.4 times of that of the droplet. (3) In the later stages of combustion, the molten aluminum and aluminum oxide can react to form gaseous Al and Al2O spontaneously; (4) Little holes are found on the surface of residuals, which are the transport channels of gaseous products, namely the gaseous Al and Al2O. The combustion residuals are consisted by lots of oxide particles with a diameter less than 1 μm.
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Fig. 12. SEM photographs of combustion residual.
[4]
[5] [6] [7] [8] [9]
[10] [11] [12] [13] Fig. 13. EDS result of combustion residuals.
[14] [15]
Acknowledgments The authors would like to express their thanks for the support from the National Natural Science Foundation of China (No. 51406231). They would also like to thank the anonymous reviewers for their critical and constructive recommendations on this manuscript.
[16] [17]
[18] [19]
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B. d'Andrea, Solid propulsion for space applications: an updated roadmap, Acta Astronaut. 66 (2010) 201–219. V.V. Karasev, A.A. Onischuk, O.G. Glotov, A.M. Baklanov, A.G. Maryasov, V. E. Zarko, V.N. Panfilov, A.I. Levykin, K.K. Sabelfeld, Formation of charged aggregates of Al2O3 nanoparticles by combustion of aluminum droplets in air, Combust. Flame 138 (2004) 40–54. M. Beckstead, A summary of aluminum combustion, DTIC Document, 2004. E.L. Dreizin, Experimental study of stages in aluminum particle combustion in air, Combust. Flame 105 (1996) 541–556. E.L. Dreizin, On the mechanism of asymmetric aluminum particle combustion, Combust. Flame 117 (1999) 841–850. E.L. Dreizin, Experimental study of aluminum particle flame evolution in normal and micro-gravity, Combust. Flame 116 (1999) 323–333. I.G. Assovskii, O.M. Zhigalina, N.A. Kiselev, G.P. Kuznetsov, V. I. Kolesnikovsvinarev, The effect of gravity on aluminum droplet combustion, Dokl. Phys. 44 (1999) 409–412. M. Marion, C. Chauveau, I. GöKalp, Studies on the ignition and burning of levitated aluminum particles, Combust. Sci. Technol. 115 (1996) 369–390. S. Mohan, M.A. Trunov, E.L. Dreizin, Heating and ignition of metallic particles by a CO2 laser, J. Propuls. Power 24 (2008) 199–205. D.K. Kuehl, Ignition and combustion of aluminum and beryllium, AIAA J. 3 (1965) 2239–2247. A.G. Merzhanov, Y.M. Grigorjev, Y.A. Gal’chenko, Aluminum ignition, Combust. Flame 29 (1977) 1–14. V.V. Tyurenkova, Non-equilibrium diffusion combustion of a fuel droplet, Acta Astronaut. 75 (2012) 78–84. I.G. Assovskii, A.N. Streletskii, V.I. Kolesnikov-Svinarev, Mechanism of formation of the condensed phase in aluminum combustion in carbon dioxide, Dokl. Phys. Chem. 405 (2005) 235–239. K.P. Brooks, M.W. Beckstead, Dynamics of aluminum combustion, J. Propuls. Power 11 (1995) 769–780. Y. Heesung, Y. Woongsup, Modeling of aluminum particle combustion with emphasis on the oxide effects and variable transport properties, J. Mech. Sci. Technol. 24 (2010) 909–921. E.L. Dreizin, Effect of surface tension on the temperature of burning metal droplets, Combust. Flame 161 (2014) 3263–3266. P. Bucher, R. Yetter, F. Dryer, T. Parr, D. Hanson-Parr, E. Viceni, Flames structure measurement of single, isolated aluminum particles burning in air, in: Proceedings of the 26th Symposium (International) on Combustion, Elsevier, 1996, pp. 1899–1908. S. Gordon, B.J. McBride, Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications, National Aeronautics and Space Administration, Office of Management, Scientific and Technical Information Program, 1996. V. Sarou-Kanian, J.C. Rifflet, F. Millot, G. Matzen, I. Gökalp, Influence of nitrogen in aluminum droplet combustion, Proc. Combust. Inst. 30 (2005) 2063–2070.
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/aa
Experimental investigation on the combustion characteristics of aluminum in air Yunchao Feng, Zhixun Xia, Liya Huang n, Xiaoting Yan College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Hunan 410073, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2016 Accepted 28 June 2016 Available online 29 June 2016
With the aim of revealing the detailed process of aluminum combustion in air, this paper reports an experimental study on the combustion of aluminum droplets. In this work, the aluminum wires were exposed and heated by a CO2 laser to produce aluminum droplets, and then these droplets were ignited and burnt in air. The changing processes of aluminum wires, droplets and flames were directly recorded by a high-speed camera, which was equipped with a high magnification zoom lens. Meanwhile, the spectrum distribution of the flame was also registered by an optical spectrometer. Besides, burning residuals were collected and analyzed by the methods of Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS). Experimental results show that, during combustion, the aluminum droplet is covered by a spherical vapor-phase flame, and the diameter of this flame is about 1.4 times of the droplet diameter, statistically. In the later stages of combustion, the molten aluminum and condensed oxide products can react to generate gaseous Al and Al2O spontaneously. Little holes are found on the surface of residuals, which are the transport channels of gaseous products, namely the gaseous Al and Al2O. The combustion residuals are consisted by lots of aluminum oxide particles with diameters less than 1 μm. & 2016 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Aluminum droplet Combustion Oxide cap Oxide bubble CO2 laser Ducted rocket
1. Introduction Compared with other additives [1,2], the advantages of aluminum are its high volumetric combustion enthalpy, high combustion temperature, non-toxicity and relatively inexpensive. Therefore, aluminum is an important energetic ingredient of many solid propellants [3]. In ducted rocket, the reaction of aluminum with the air breathed from inlets in second combustion chamber is the main energy release process. However, aluminum particles containing in the solid propellants ignite at fairly high temperature and they usually get melted before ignition. Because of fusion and agglomeration, the diameters of liquid aluminum droplets can actually reach several hundred microns [4] and burn slowly and incompletely. Thus, the research on the combustion characteristics of aluminum has been an ongoing effort. Although aluminum combustion has been studied for several decades, several questions about the detailed processes remain to be answered, such as oxide cap formation and consumption, asymmetric combustion, oxide bubble formation and fragmentation. In Ref. [5], the combustion characteristics of aluminum n
Corresponding author. E-mail addresses: [email protected] (Y. Feng), [email protected] (Z. Xia), [email protected] (L. Huang), [email protected] (X. Yan). http://dx.doi.org/10.1016/j.actaastro.2016.06.049 0094-5765/& 2016 IAA. Published by Elsevier Ltd. All rights reserved.
particles have been reviewed with focus on the burning time of individual particles. As for the particle combustion details, a pulsed micro-arc discharge was used in Refs. [6,7] to form and ignite aluminum particle in different atmospheres. In the experiments, partially burned particles were rapidly quenched, and then, their surfaces and interiors were examined by electron microscopy. The reasons causing oscillations in the particle radiation, irregularities in the particle trajectories, and variation in the relative size of the oxide caps as compared to the particle size are analyzed. The effect of gravity on the aluminum particles burning in air was studied in Refs. [8,9] experimentally. Experimental results indicated that the asymmetric flame structure and brightness oscillations were intrinsic features of aluminum particle burning rather than the result of forced or natural convection flows caused by gravity [8]. However, the gravity has a significant influence on the dynamics of aluminum droplet combustion and morphology of combustion products [9]. In Ref. [10], the aluminum particles were ignited in a high-pressure chamber with the help of an electrodynamic levitator and a CO2 laser. During combustion, the thermal emission light of the particle and the surrounding flame emission were collected by a photomultiplier. Brightness oscillations were explained by the oscillations of burning rate, as well as the presence of an oxide cap and occurrence of micro flames on the particle surface. Furthermore, in Ref. [11], the authors used the electrostatic particulate method to aerosolize the micron-sized
2
Y. Feng et al. / Acta Astronautica 129 (2016) 1–7
aluminum powders. The aerosol was carried away by a laminar gas jet, and then the single particle was ignited by a CO2 laser one by one. In their experiments, the particle sizes, optical signatures and combustion times were measured in real time. In other experiments [12,13], an electrically heated aluminum wire was placed in a flow of an oxidizing gas. The ignition temperature of aluminum wire was detected optically [12], or from the measured changes in the wire electric resistance [13]. In fact, the combustion of a large aluminum particle is generally similar to the diffusion combustion of a fuel droplet [14], except for the effect of the aluminum oxide. Therefore, the condensed phase oxide products of burning aluminum droplet was studied in Refs. [5,15] and several typical combustion models [16,17] have been proposed based on the diffusion combustion model and emphasized on the oxide accumulation on the surface of aluminum droplet. Because of experimental limitations, the details of aluminum droplet, especially for those smaller than 1.0 mm, can hardly be recorded during combustion in real time. The interaction mechanism between aluminum droplet and oxide deposits is still uncertain. The objectives of the present work are to determine the ignition and combustion characteristics of aluminum in air and to provide new experimental dataset on the combustion of single aluminum droplet. In this work, aluminum droplets are produced by partially molten aluminum wires, and then ignited by CO2 laser. Based on the experimental results, the interaction mechanisms between oxide cap and aluminum droplet are also discussed.
2. Experimental methods 2.1. Experimental setup A metal droplet combustion system was designed and used to conduct current experiments. This system mainly consists of CO2 laser, high-speed camera, high magnification zoom lens, optical spectrometer and three-dimensional platform. The CO2 laser, highspeed camera and optical spectrometer are all controlled by the computer. A schematic diagram illustrating such experiments is shown in Fig. 1. In this experimental system, the three-dimensional platform is used to fix and adjust the position of the aluminum wire. To reduce the amount of heat loss caused by conduction, two ceramic tubes are installed to fix the wire, of which center holes are about 400 μm in diameter. The CO2 laser (CWQ2000) is the heating source and its nominal maximum output power is 200 W. The CO2 laser beam is focused by a convex ZnSe lens (focal length ¼100 mm) to a 500 μm focal spot. The high-speed
Fig. 1. Experimental setup for metal droplet combustion.
camera (PCO. dimax S4) is equipped with a complementary metal oxide semiconductor (CMOS) image sensor and its maximum frame rate is 1102 fps at 2016n2016 pixels2. In this study, to provide sufficient spatial resolution, the high-speed camera is equipped with a high magnification zoom lens, which consists of a zoom system (NAVITAR, 0.58-7X) and an adapter (NAVITAR, 2X). The working distance of this lens is 86 mm and its depth of the focal field ranges from 1.39mm for the lowest magnification (1.16 ) to 0.05 mm for the highest magnification (14 ). The optical spectrometer (HR2000 þ ES by Ocean Optics, Inc.) and the probe (74-UV by Ocean Optics, Inc.) are connected by an optical fiber. The spectrometer has an optical resolution of 0.48 nm and its integration time is from 1 ms to 20 s. Besides, the aluminum wires used in these experiments are 99% pure aluminum with nominal diameters of 100 μm, 200 μm and 300 μm by Beijing Zhongnuo Material Technology, Inc. All the experiments were conducted indoor, where the relative air humidity was controlled between 50% and 60% and the room temperature was 296 300 K. 2.2. Experimental procedure With the help of the experimental system introduced in Section of 2.1, a series of experiments on the ignition and combustion characteristics of aluminum droplets in air were conducted. Firstly, the focal plane of high-speed camera and the spatial location of aluminum wires needed to be adjusted to be overlapped at the focus of the CO2 laser. Before the laser started to work, the highspeed camera and optical spectrometer had begun to record the experimental phenomena. Second, CO2 laser begun to work and aluminum wire was heated and ignited. And then, CO2 laser would continue to work until the end of the experiments. The detailed phenomena of experiment was recorded by the high-speed camera directly, and the spectral information was also recorded by the optical spectrometer at the same time. Finally, the burning residuals were collected and analyzed by the methods of SEM and EDS.
3. Experimental results and discussion 3.1. Combustion phenomena 3.1.1. Combustion processes To obtain a general knowledge of the combustion characteristics of the aluminum droplets in air, Fig. 2 shows the sequential images captured by high-speed camera. The initial diameter of aluminum wire was 200 μm and the heating power of the CO2 laser was 200 W. Firstly, the aluminum wire turned dull-red and started to emit light near the laser focus as soon as the CO2 laser worked (Fig. 2 (a)). Few milliseconds later, the initial oxide film rupture appeared on the focus of CO2 laser, which was caused by the difference of thermal expansion coefficients between metal and initial oxide film, as well as the melting of oxide film. As soon as the oxide film had been removed, the uncoated aluminum wire evaporated rapidly and the aluminum vapor reacted with air to form the gasphase flame (Fig. 2(c)). When the partial wire was heated into molten state, the wire would fuse and the molten aluminum droplet would adhere to one of the two ends randomly (Fig. 2(e)). Because of the surface tension, the shape of aluminum droplet was near spherical. The aluminum droplet was covered by a flamesheet at a certain distance from the droplet surface. The gas-phase flame would maintain few tens milliseconds. However, there were several “oxide islets” forming on the surface of droplet, which would move together to accumulate larger “oxide islets”. It should be noted here that the “oxide islets” are both derived from the
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Fig. 2. Images of 200 μm aluminum wire ignition and combustion obtained by high-speed camera. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. Oxide cap and asymmetric flame.
initial oxide film on the aluminum wire and the deposition of gaseous oxide products. As more and more “islets” moved together, the cover area of oxide became larger and the oxide cap formed. Because of the oxide cap, the gas-phase flame became asymmetric and weaker, as shown in Fig. 2(i). Finally, the oxide deposition covered the droplet completely and the flame extinguished.
Fig. 4. Measured spectral emissivities before droplet ignition and during droplet combustion.
During the burning of aluminum droplet in air, lots of gaseous and condensed oxides were produced. Some of those products diffused to the outer of the flame, and others diffused to the surface of the droplet. Compared with the temperature of flame
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Fig. 5. Measurement method of the diameters of droplet and flame.
3.1.2. Flame spectra Typical spectral emissivities before droplet ignition and during droplet combustion are presented in Fig. 4. Before ignition, there was no characteristic wave peak in the spectra distribution line. During combustion, three typical wave peaks (396.1 nm, 484.2 nm, 510.2 nm) could be distinguished clearly. Based on those three wave peaks, the main gaseous species, Al (396.1 nm) and AlO (484.2 and 510.2 nm), occurred during combustion can be inferred. The gaseous species presented during combustion are the reactants or intermediate products, which means the aluminum burning in air is mainly gas-phase combustion. 3.2. Flame sizes
Fig. 6. Flame sizes in different droplet diameters.
(above 3000 K usually), the temperature of the droplet is relatively low, and it is generally considered to be the boiling point of liquid aluminum in local pressure [18]. Fig. 3 shows the oxide caps accumulated on the surface of droplet. By comparing the melting and boiling point of aluminum with those of aluminum oxide, it can be inferred that the oxide cap should be in the liquid phase. Because the evaporation and diffusion of aluminum vapor is blocked by the oxide cap, the gas-phase flame gets to be eccentric and asymmetric. Therefore, it should be the oxide cap that results in the asymmetric combustion and this observation is consistent with the inference in Ref. [7].
It has been mentioned that the molten aluminum would adhere to one of the broken ends randomly. In those experiments, an image of the unburned sample was captured prior to ignition. Thus was to provide a post-experiment reference for computing the diameters of droplet and flame. For convenience, the diameters of the droplet and flame were processed by converting the original images to gray images and extracting the gray levels of line 1 (see Fig. 5). Here, the distance between the two brightness peaks of the curves is defined as the diameter of flame, and the distance between the two valleys is defined as the diameter of droplet. These structure measurements will aid the development of detailed numerical models. Using the measurement method mentioned above, a series of droplet and flame diameters couples are presented on Fig. 6. Those data are processed by linear fitting method with the intercept of the line equaling to zero. The relationship between droplet size and flame size can be expressed as
Df = 1.4Dd
Fig. 7. Oxide bubbles before flame extinction.
(1)
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where, Dd is the diameter of droplet, μm; Df is the diameter of flame, μm. It suggests that the diameter of flame is about 1.4 times of that of droplet during steady combustion. However, the concentration peak of intermediate product, AlO, was detected at the location of 2.8 times particle diameter by the method of PLIF [19]. The flame diameters measured in this study are smaller than that of isolated aluminum particle in Ref. [19], and that should be caused by the heat conduction between the droplet and wire. 3.3. Oxide bubble In the later stages of combustion, the aluminum droplet was covered by a film of combustion products (see Fig. 7(a)). Although the fierce gas-phase flame around the droplet had disappeared, the heterogeneous chemical reactions (HCRs) still ongoing at the interface between the oxide film and droplet. It was the HCRs that resulted in the emergence of oxide bubble (see Fig. 7(b and c)). In general, there were numerous of little oxide bubbles forming and fracturing on the surface of deposition. The big oxide bubble presented on Fig. 7 only emerged under some suitable conditions. Also, the big oxide bubble should be the results of the mergence of large amounts of little oxide bubbles. Furthermore, in fact, the deposition, namely the Al2O3, could react with the molten aluminum at high temperature. The reaction between molten
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aluminum and aluminum oxide was regarded as a problem of assigned temperature and pressure and simulated by the NASA Chemical Equilibrium with Application (CEA) code [20]. In this problem, the reaction temperature was set as the boiling point of aluminum, 2790 K. The relationship between the mole fraction of combustion products and the oxide (Al2O3) to fuel (Al) weight ratio (O/F) is presented on Fig. 8. The simulation results show that the main combustion products are gaseous Al2O and Al. Compared with other products, the mole fraction of AlO(g) is very small (no more than 0.1%) and can be neglected. At the point of O/F ¼0.8, the mole fraction of Al2O reaches its maximum. In this case, the reaction of Al and Al2O3 can be expressed as 4.7Al(c) þAl2O3(c)-0.829Al(g) þ 2.935Al2O(g)
(R1)
Therefore, it can be inferred that the gaseous products should be Al(g) and Al2O(g) and result in the formation of oxide bubbles in the later stages of combustion (see Fig. 8). Moreover, the enthalpy of reaction and the Gibbs free energy change are also presented on Fig. 9. At the point of O/F ¼0.8, the enthalpy of reaction is greater than zero, which means this is endothermic reaction. The Gibbs free energy change is negative on a large scale of O/F, which means this reaction might occur spontaneously.
Fig. 8. The simulation results of Al reacting with Al2O3 in different O/F.
Fig. 9. The simulation results of the enthalpy of reaction and the Gibbs free energy change.
Fig. 10. Apparent structure of combustion residuals of 200 μm and 300 μm aluminum wire.
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combustion residuals are a kind of white porous material, which consist of lots of little oxide particles ranging on the order a few tens to hundreds of a micrometer in diameter. The 200 μm residuals is different from that of 300 μm, and the major differences between the two kinds of residuals are their porousness and overall diameter. One of the two reasons caused the differences is the 200 μm wire is thinner and the heat loss due to conduction to the wire is relatively small. Therefore, as shown in Fig. 11, the burning rate of 200 μm, 1.60 μm/ms, is faster than that of 300 μm, 0.73 μm/ms. Another reason is that the 200 μm aluminum wire has a larger specific surface area, so it can react with air more sufficiently. 3.4.2. Micro structure The combustion residuals were also characterized by method of SEM, as shown in Fig. 12. On the surface of the little oxide particles shown in Fig. 10, there are several gas holes with diameters of a few microns or smaller. Those holes should be the transport channels of gaseous products in R1. The Fig. 12(b) shows that the porous combustion residual is mainly consisted by oxide particles with diameters less that 1 μm. 3.4.3. Components of residuals The contents of different elements in the combustion residuals are measured by EDS, as shown in Fig. 13. The two main elements in the residuals are Al and O, which means the residual predominantly consists of aluminum and its oxides. As the increasing of initial diameter of aluminum wire, the content of Al is also increased while the content of O is decreased. Thus, it can be concluded that the combustion gets much more incomplete as the diameter k of aluminum wire increases. Besides, in the combustion residuals, there are little other elements, such as C and N. The presence of C and N may be caused by the CO2 and N2 in air, which have also taken part in the reaction. However, the mole fraction of element N is less than 2% in the residuals of three diameters wires. Even though the effects of pure nitrogen on the aluminum droplet combustion under forced convection conditions are significant [21], the aluminum can hardly react with the N2 in air.
4. Conclusions A metal droplet combustion system was designed and built. The characteristics of aluminum ignition and combustion were revealed with help of the CO2 laser, high-speed camera, the high magnification zoom lens and the optical spectrometer. Based on the observation and measurements, the followings are found:
Fig. 11. Measurement method of the burnt amount and burning rate.
3.4. Residuals analysis 3.4.1. Apparent structure Fig. 10 shows the typical apparent structure of the 200 μm and 300 μm aluminum wires combustion residuals. Apparently, the
(1) The ignition and combustion process of aluminum wire in air consists of initial oxide film rupture, locally gas-phase combustion, melting and fusing of wire, fiercely gas-phase combustion and the oxides deposition. (2) During steady combustion, the aluminum droplet is surrounded with gas-phase flame, of which diameter is statistically about 1.4 times of that of the droplet. (3) In the later stages of combustion, the molten aluminum and aluminum oxide can react to form gaseous Al and Al2O spontaneously; (4) Little holes are found on the surface of residuals, which are the transport channels of gaseous products, namely the gaseous Al and Al2O. The combustion residuals are consisted by lots of oxide particles with a diameter less than 1 μm.
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Fig. 12. SEM photographs of combustion residual.
[4]
[5] [6] [7] [8] [9]
[10] [11] [12] [13] Fig. 13. EDS result of combustion residuals.
[14] [15]
Acknowledgments The authors would like to express their thanks for the support from the National Natural Science Foundation of China (No. 51406231). They would also like to thank the anonymous reviewers for their critical and constructive recommendations on this manuscript.
[16] [17]
[18] [19]
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