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Stabilized flames in hybrid aluminum-methane-air mixtures
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Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 34 (2013) 2213–2220
www.elsevier.com/locate/proci
Stabilized flames in hybrid aluminum-methane-air mixtures Michael Soo 1, Philippe Julien, Samuel Goroshin ⇑, Jeffrey M. Bergthorson, David L. Frost Department of Mechanical Engineering, McGill University, Montreal, Canada Available online 15 June 2012
Abstract A premixed methane–air bunsen-type flame is seeded with micron-sized (d32 = 5.6 lm) atomized aluminum powder over a wide range of solid fuel concentrations. The burning velocities of the resulting twophase hybrid flame are determined using the total surface area of the inner flame cone and the known volumetric flow rate, and spatially resolved flame spectra are obtained with a spectral scanning system. Flame temperatures are derived through polychromatic fitting of Planck’s law to the continuous part of the spectrum. It is found that an increase in the solid fuel concentration changes the aluminum combustion regime from low temperature oxidation to full-fledged flame front propagation. For stoichiometric methane–air mixtures, the transition occurs in the aluminum concentration range of 140–220 g/m3 and is manifested by the appearance of AlO sub-oxide bands and an increase in the flame temperature to 2500 K. The flame burning velocity is found to decrease only slightly with an increase in aluminum concentration, in contrast to the rapid decrease in flame speed, followed by quenching, that is observed for flames seeded with inert SiC particles. The observed behavior of the burning velocity and flame temperature leads to the conclusion that intense aluminum combustion in a hybrid flame only occurs when the flame front propagating through the aluminum suspension is coupled to the methane–air flame. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Flame; Premixed; Hybrid; Aluminum; Methane
1. Introduction Two-phase hybrid combustible mixtures of a solid and gaseous fuel are encountered in numerous practical applications, such as coal combus⇑ Corresponding author. Address: Department of Mechanical Engineering, McGill University, 817 Sherbrooke Street West, Quebec, Canada H3A 2K6. Fax: +1 (514) 398 7365. E-mail address: [email protected] (S. Goroshin). 1 Presenting author.
tion and combustion material synthesis, as well as in accidental explosions in industry. The gaseous and solid combustible components can be either premixed in a cold state as, for example, during coal–methane explosions in mines, or combustible gases can evolve from the solid or liquid state directly in a flame, such as coal volatiles or fuel vapors in the combustion of liquid hydrocarbon-metal slurries. The reaction of particulate suspensions of light metals (aluminum in particular) with the products of hydrocarbon flames is one of the critical stages in the combustion of solid and metalized gelled
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.05.044
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propellants, pyrotechnics, and metalized explosives. The majority of experimental work in this area is focused on the combustion of large (tens of microns) individual particles and agglomerates [1–4], or on the combustion of metal particulates in specifically formulated solid propellant compositions with very low metal particle loading [5]. It is often implicitly assumed that the results can be extrapolated to smaller particles and dense suspensions in real systems. Recent work has already shown that extrapolating particle combustion models based on results derived from experiments with large-sized particles to micron and submicron sizes may be misleading due to a possible change from the diffusion-limited to the kinetically-limited regime of particle combustion [6]. In addition, little attention has been paid to the differences in combustion physics for a single particle and dilute particle suspensions as compared to that of dense suspensions. Most previous attempts to experimentally study combustion of particle suspensions and gaseous fuels were restricted to standard safety tests in constant pressure bombs that provided little information on hybrid flame features and structure [7,8]. Other investigations of stabilized hybrid flames were performed mostly on complex coal–methane mixtures [9], with the notable exception of the work by Bryant and Sippe [10] who studied the combustion of amorphous boron suspensions in propane– oxygen mixtures. The present study investigates stabilized flames in aluminum-seeded methane–air hybrid mixtures for a range of aluminum particle concentrations. Optical measurements are used to determine the burning velocity and nature of the combustion as a function of aluminum particle concentration and gas stoichiometry. The results are compared with similar tests using methane–air mixtures seeded with inert silicon carbide particles. Spectroscopic measurements are used to estimate the flame temperature and detect the presence of aluminum combustion products. The new experimental evidence unambiguously shows that the aluminum concentration has a strong influence on the combustion behavior of the hybrid mixture. The aluminum particles influence the combustion parameters, such as the flame temperature and propagation rate, and the results also indicate that aluminum combustion in a hybrid solid–gas mixture must be treated as a frontal flame propagation phenomenon. 2. Experimental apparatus 2.1. Hybrid flame burner and optical diagnostics A schematic of the newly constructed hybrid fuel burner is shown in Fig. 1. Its design implements a number of technical solutions developed
previously in our study of dust flames [11,12]. In particular, it uses a similar dust dispersion system comprised of a piston dust feeder and a “flow knife” disperser [12]. The thin, high-velocity jet (“flow knife”) is formed by flowing a premixed methane–air mixture through a 40 lm wide circular slot concentric with the top surface of the dust column. The jet removes and disperses the dust pushed by the advancing piston, layer by layer in a continuous fashion. Before going through the nozzle, the initially turbulent flow is laminarized in a narrow angle conical diffuser followed by a smooth 60 cm long supply tube. The dust concentration in the flow is regulated by changing the speed of the dust feeder piston. The flow rate through the disperser remains constant to maintain a stable dispersion regime. To regulate the flow rate through the nozzle, a small volume of the main flow is ejected from the main dispersion line into the ejection line (see Fig. 1), where it is exhausted without burning. The flow rate of the ejected mixture is determined by using pure nitrogen as the ejector driver gas and measuring the oxygen concentration at the exit of the ejection line with an oxygen analyzer. Alternatively, the ejector can be calibrated using the surface area of a stabilized stoichiometric pure methane flame as an indicator of the flow rate through the nozzle. The dust concentration in the flow is monitored with a laser light attenuation probe consisting of a diode laser (k = 632 nm), cylindrical lenses forming a rectangular 6 2 mm beam, and a diode sensor with a narrow bandpass filter and focusing lenses. The laser beam is transmitted across the dusty flow through a rectangular slot cut in the walls of the conical nozzle and protected by a transparent, heat-resistant Teflon film. The output of the diode sensor is continuously recorded by a data acquisition system. The dust concentration probe is calibrated by the complete aspiration of dust from the flow through a set of fine multilayered filters by a vacuum pump for a determined period of time. The average dust concentration is then determined by dividing the total mass of the aspirated dust by the volume of gas that flowed through the dispersing system during that time interval. A high-resolution digital camera with a set of different density gray filters is used to record flame images. The signal marks generated by the camera at the moment of shutter release are used to synchronize the flame images with the dust concentration monitoring signal. A flame spectral scanning system, similar to one described in our previous work [13], is used to acquire spatially resolved flame spectra across the flame cone. Two new spectrometers are used: Ocean Optics HR 4000 CG-UV-NIR and USB 4000. The first spectrometer, equipped with a 5 lm wide entrance slit and coupled to a 0.6 mm fiber, is used to acquire spectra in the 250–1000 nm spectral
M. Soo et al. / Proceedings of the Combustion Institute 34 (2013) 2213–2220
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Fig. 1. Schematic of the hybrid flame burner and composite photograph of the optical diagnostic elements.
range with a resolution of about 0.75 nm. The second USB 4000 spectrometer uses a 0.1 mm fiber without an entrance slit and records spectra in the 350–900 nm range with a spectral resolution of about 2.5 nm. With the second spectrometer, the system has a spatial resolution of about 0.1 mm with scanning steps of 0.2 or 0.4 mm whereas the larger fiber of the first spectrometer limits the spatial resolution to about 0.6 mm. The first spectrometer is used to acquire higher quality spectra with resolved signature AlO molecular bands and Al atomic lines whereas the second, more light-sensitive, low-resolution spectrometer is primarily used to acquire continuous spectra used for flame temperature measurements. 2.2. Aluminum powder In order to compare parameters of the hybrid flames with benchmark data of pure aluminum dust flames, the same atomized aluminum powder (Ampal 637, Ampal NJ) is used as in our previous experiments with stabilized dust flames and aluminum flames in tubes [11,12]. As shown in Fig. 2, the aluminum particles are of spheroidal or nodular shape. The Sauter mean diameter, d32, is derived from the particle distribution obtained from SEM images of the powder (see Fig. 2) and is shown to be about 5.6 lm, compared with a value of about 6.9 lm from the distribution obtained by the light scattering technique with a Mastersizer 2000 Malvern particle sizer instrument.
Fig. 2. Scanning Electron Microscope (SEM) photograph and particle size distribution in atomized Ampal637 aluminum powder.
3. Experimental results 3.1. Flame appearance, spectra and temperature at different aluminum concentrations Aluminum–methane–air hybrid flames are studied at two different methane/air equivalence ratios: / = 1 (stoichiometric) and / = 0.8 (fuel lean). First, a Bunsen-type conical flame is established at the exit of the nozzle without activating the piston in the dust dispersion system. Even without moving the piston, the mass concentration of aluminum in the mixture never falls below 20 g/m3 due to the surface erosion of the
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compacted powder column by the dispersing gas. Thus, the dust dispersion system must be completely emptied of powder in order to verify flame speeds in pure methane–air mixtures. Upon activation of the dispersion system, the aluminum concentration in the flow is slowly increased until reaching a plateau at the end of the dispersion cycle. Depending on the piston speed, the total dispersion time varies from 3.5 to 6 min and the corresponding maximum aluminum concentration at the plateau varies in the range 300–450 g/ m3. As can be seen from the flame photographs in Fig. 3, the color, brightness, and structure of the flames are strikingly different at low and high aluminum concentrations. At aluminum concentrations below 50 g/m3, the flame cone has a sharp inner boundary and an ill-defined outer boundary (A-type flame). The flame is yellow in color and individual particle streaks are clearly visible under magnification. At high aluminum concentrations the flame is bright-white in color and is extremely luminous (C-type flame). The thin inner flame cone has a well defined inner and outer boundary. It is enveloped by a much larger flame with a well defined outer boundary that has the appearance of a gaseous diffusion flame. The base of the outer diffusion flame is usually lifted from the inner cone base by about 2–3 mm. Preliminary analysis of the combustion products collected with copper plates passed through the tip of the flame cone also shows a noticeable difference between A-type and C-type flames. At low metal fuel concentrations, the products are gray in color and are primarily comprised of aluminum droplets encapsulated in a thick oxide shell. At high aluminum concentrations, the products are bright white and consist primarily of a mixture of submicron aluminum oxide particle agglomerates and larger
(a)
(b)
(c)
Fig. 4. Flame spectra in the 3 different concentration ranges: (A) 20–50 g/m3, (B) 140–220 g/m3, and (C) 250– 400 g/m3.
alumina shells. The transition from A-type to Ctype flames occurs in a relatively narrow region of dust concentration of about 140–220 g/m3 for stoichiometric mixtures and 100–140 g/m3 for lean methane–air flames. The transition usually starts from the tip of the flame with the appearance of a white diffuse glow attached to the inner flame cone (B-type flame in Fig. 3). With an increase of aluminum concentration, the glowing region then slowly spreads down to the flame base. Simultaneously, the outer diffusion flame cone is formed.
Fig. 3. Flame images at three different aluminum concentrations: (A) 20–50 g/m3, (B) 140–200 g/m3, and (C) 250–400 g/ m3.
M. Soo et al. / Proceedings of the Combustion Institute 34 (2013) 2213–2220
The flame spectral signatures associated with low aluminum concentration (A), high concentration (C) and transition type (B) flames are shown in Fig. 4. At low aluminum concentrations, the Atype flame spectra are comprised mostly of continuous blackbody radiation with a lone sodium line. The characteristic AlO signature bands only start to appear in the transitional B-type flames. They are, however, underdeveloped and only clearly show two of the three characteristic AlO band sequences, in contrast with the well-pronounced AlO bands in high aluminum concentration Ctype flames or in the pure Al dust-air flames studied previously [13]. The spectra of transitional Btype flames also show lithium and potassium atomic lines that have higher ionization potentials than sodium, indicating an increase in flame temperature. The high aluminum concentration Ctype flames also demonstrate well-pronounced 2 0 2 P – S aluminum atomic lines (394.4 and 396.1 nm), although there is no evidence of the self-absorption observed in aluminum dust flames [13]. Flame temperatures are derived using a polychromatic fitting procedure of the low-resolution continuum spectra in the 600–850 nm spectral range to Planck’s law [13].An emissivity dependence on the inverse wavelength squared demonstrates excellent fitting and reasonable temperatures below the thermodynamic predictions in accordance with [13]. This wavelength dependence was corroborated by [14,15]. About
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25 spectra were acquired during each flame scan from the center of the flame to the periphery. Temperatures were derived only from the spectra taken within the flame front, i.e. from spectra with the maximum intensity for a given scan. The acquisition time for each spatial point was about 2 s for low luminosity A-type flames, about 0.5– 0.7 s for B-type flames in the transition regime, and only about 30–50 ms for high aluminum concentration C-type flames. The total flame scan took about 50, 15 and 1.2 s for 25 points for A, B, and C type flames, respectively. Due to the relatively long scan times and drift of the dust concentration, the temperature data can only be associated with a dust concentration range and not with a particular concentration value. The measured flame temperatures are presented in Fig. 5 in three dust concentration intervals related to the three particular flame types: low and high aluminum concentration flames (A and C) and transitional flame (B). As seen from the temperature data, the flame temperature is very stable at low aluminum concentrations, oscillating around a value of about 1950 K. The temperature also stays within a relatively narrow interval of about 2750–2900 K at concentrations above 250 g/m3. In contrast, wider fluctuations of the temperature, between 2200 and 2500 K, for type B flames indicate a stronger dependence of the flame temperature on aluminum concentration in this transitional concentration range. The change in flame appearance, spectral signature, and temperature with dust concentration in fuel-lean methane–air mixtures (/ = 0.8) is similar to stoichiometric mixtures. However, the appearance of the transitional B-type flames is observed at considerably lower values of dust concentrations, about 100–140 g/m3, indicating that intensive aluminum combustion can be initiated at lower aluminum concentrations when excess oxygen is present. The temperatures of methanelean hybrid flames are also approximately 100 K higher than the temperatures of stoichiometric methane–air mixtures across the entire range of dust concentrations. 3.2. Burning velocities of aluminum–methane–air hybrid mixtures
Fig. 5. Flame temperature in three different ranges of aluminum concentration.
The flame burning velocities in aluminum– methane–air mixtures are determined using the total flame surface area method, i.e. by dividing the known volumetric flow rate through the nozzle by the total surface area of the inner flame cone [16]. Each flame image is associated with a particular value of dust concentration using timing marks produced by the camera output on the data acquisition system record. The boundary of the inner flame cones are manually traced using magnified flame images on a drawing tablet
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monitor. The coordinates of the cone are then fitted to a polynomial and the resulting curves on the left and the right sides of the vertex are separately revolved about the cone axis. The surface areas of the two obtained figures are then averaged to give the surface area of the flame cone. The method was tested by measuring burning velocities in pure methane–air and aluminum–air flames and comparing the resulting flame speeds to literature data. The burning velocity of the stoichiometric methane–air mixture determined in this manner is found to be about 36 ± 2 cm/s, in good agreement with literature data obtained by the same method [17]. The burning velocity in a pure aluminum flame is found to be about 20 ± 3 cm/s with a dust concentration range of 400–450 g/m3, in agreement with our previous measurements performed with a different set-up, but for the same aluminum powder [11]. Burning velocities in stoichiometric and lean methane–air hybrid mixtures measured at different aluminum concentrations are shown in Fig. 6. As can be seen from the data presented in Fig. 6, the flame burning velocity decreases only slightly with an increase in dust concentration in both stoichiometric and lean methane–air mixtures, suggesting that the heat released by the reacting aluminum contributes to the flame propagation. In order to make this conclusion more definitive, additional experiments were performed with dusty methane–air stoichiometric mixtures, where the reactive aluminum was replaced by an inert silicon carbide powder with a similar particle size (d32 = 6 lm). The comparison of the influence of the SiC and Al powders on the flame is presented in Fig. 7. The comparison shows that, in spite of the lower value of specific heat of SiC in comparison to aluminum (0.75 versus 0.90 J/g K) and the additional latent heat required to melt aluminum
Fig. 6. Burning velocities at different aluminum concentrations in stoichiometric (/ = 1) and lean (/ = 0.8) methane–air mixtures.
Fig. 7. Comparison of burning velocities in methane–air mixtures seeded with aluminum (Al) and silicon carbide (SiC) particles.
in the flame, the decrease in flame burning velocity with an increase of SiC concentration is considerably sharper than that with aluminum at the same dust mass concentrations. At SiC concentrations higher than 200 g/m3, the tip of the flame opens and a part of the mixture escapes without reacting. At concentrations above 300 g/ m3, the flame covers only the surface area adjacent to the nozzle walls and the majority of the fuel mixture escapes unburned. 4. Discussion The experimental evidence presented in this work clearly demonstrates that the mechanism of aluminum combustion in a hot oxidizing flow strongly depends on the mass concentration of the solid fuel in suspension. The observed transition from low-intensity oxidation to fast, fullyestablished combustion reflects the ability of the reacting aluminum suspension to modify the combustion properties, i.e. increase the flame temperature, which in turn affects the aluminum combustion mode. A thermodynamic equilibrium analysis of constant-pressure aluminum–methane–air flames at different aluminum concentrations is presented in Fig. 8 and provides the boundary for such influence in the absence of any limitations by kinetic rates. The comparison of the calculated equilibrium flame temperatures and species compositions with the experimental data shows that the measured flame temperatures and flame spectra are, in general, in good agreement with the thermodynamic predictions in the high fuel concentration range. Indeed, the maximum measured flame temperature values in the dust concentration ranges characteristic of B-type and C-type flames are only about 150–200 K lower than the thermodynamic predictions. Additionally,
M. Soo et al. / Proceedings of the Combustion Institute 34 (2013) 2213–2220
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Fig. 8. Results of the constant pressure thermodynamic calculations of flame temperature and gaseous species composition as a function of aluminum content in stoichiometric, / ¼ 1 (on the left), and lean, / ¼ 0:8 (on the right) methane–air mixtures seeded with aluminum.
the experimental results also agree with the thermodynamic predictions regarding the appearance of AlO suboxides at concentrations above 150 g/m3. In contrast, the flame temperatures at low aluminum concentrations, of about 50 g/m3, fall below the thermodynamic predictions by about 400–500 K and are even below the methane–air flame temperature. Clearly, the influence of the aluminum reaction on the flame is practically negligible in this concentration range. The apparent explanation of this qualitative difference in flame behavior from low to high seeding densities is the relatively rapid coupling of the aluminum combustion front to the hydrocarbon flame that is observed over some critical concentration range, and the absence of any coupling for lower concentrations. This relatively rapid transition from uncoupled to coupled aluminum–hydrocarbon flame combustion gives an appearance of a sharp, “ignition-like” transition from A-type to C-type flames. The very possibility of such a transition indicates that aluminum combustion in hybrid mixtures needs to be treated in future modeling efforts as a frontal flame propagation phenomenon
with the corresponding ability to transfer heat and active species downstream of the combustion front. The coupled, frontal nature of aluminum combustion in hybrid flames is also evident from the fact that the flame speed of the hybrid mixture is practically unchanged with an increase of the solid fuel concentration. In contrast, the addition of inert silicon carbide solid particles leads to a sharp decrease in burning velocity followed by flame quenching. It is likely that the same quenching behavior would occur for any solid phase fuel suspension whose combustion cannot couple to the hydrocarbon flame front. The observed transition from slow to fast aluminum burning should not be confused with the ignition phenomenon associated with a single aluminum particle. A thermal explosion, runaway reaction leading to a sharp separation of particle and gas temperatures, as well as the subsequent transition to a particle combustion regime limited by the diffusion of oxidizer(s), was not observed in the present experiments. In contrast, the particle temperature increases only gradually with aluminum concentration, never exceeding values
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stipulated by equilibrium thermodynamics at each particular fuel concentration,2 thus indicating evidence of kinetically-limited particle combustion. The possibility that small particle combustion in the products of hydrocarbon flames would be kinetically-limited was previously indicated by Glumac et al. [18] and is not surprising considering the small particle sizes and the high diffusivity associated with the high-temperature, hydrogenenriched products encountered in the present experiments. 5. Conclusion The combustion behavior of micron-sized aluminum particles in the combustion products of methane strongly depends on the aluminum mass concentration. In the case of stoichiometric methane–air mixtures, the transition from slow oxidation of the aluminum particles to rapid burning occurs in the particle concentration range of 140–220 g/m3. The transitional concentration is shown to be lower in the case of lean methane– air mixtures. Above this concentration range, the flame has a significant increase in temperature to 2500 K and has clear AlO sub-oxide bands in the spectra. Increasing the aluminum concentration causes the flame speed to decrease slowly; in contrast, an increase in inert particle concentration causes a more rapid decrease in the flame burning velocity that eventually leads to flame quenching. These observations suggest that the combustion of aluminum particles at high concentrations is a frontal phenomenon and that the hydrocarbon and metallic flames are coupled. Furthermore, the measurements indicate that the combustion of an aluminum suspension in an aluminum–methane–air hybrid mixture is in the kinetic or close to kinetic diffusion transitional combustion regime [19].
Acknowledgements Support for this work was provided by the Natural Sciences and Engineering Research Council of Canada and Martec, Ltd., under a Collaborative Research and Development Grant, and
2
Note that in the diffusive combustion regime, the particle temperature would be close to the maximum possible adiabatic flame temperature, about 3000 K, at any fuel concentration.
the Defense Threat Reduction Agency under contract HDTRA1-11-1-0014 (program manager Suhithi Peiris). We also thank Fan Zhang of Defense Research and Development Canada–Suffield for his useful input to the work.
References [1] R. Friedman, A. Macˇek, Combust. Flame 6 (1962) 9–19. [2] M. Jackson, M.L. Pantoya, W. Gill, Combust. Flame 153 (2008) 58–70. [3] M.W. Beckstead, Combust. Explo. Shock 41 (5) (2005) 533–546. [4] E.L. Dreizin, Combust. Flame 105 (4) (1996) 541– 556. [5] P.F. Pokhil, A.F. Belyayev, Y.V. Frolov, V.S Logachev, A. I. Korotkov, Combustion of Powdered Metals in Active Media, Foreign Technology Div., FTD-MT-24-551-73, Wright-Patterson Air Force Base, OH, 1973. [6] T. Bazyn, H. Krier, N. Glumac, Proc. Combust. Inst. 31 (2007) 2021–2028. [7] P.R. Amyotte, K.J. Mintz, M.J. Pegg, Y. Sun, Fuel 72 (5) (1993) 671–679. [8] Y. Liu, J. Sun, D. Chen, J. Loss, Prevent. Proc. 20 (4-6) (2007) 691–697. [9] M.D. Horton, F.P. Goodman, L.D. Smoot, Combust. Flame 28 (1977) 187–195. [10] J.T. Bryant, N.J. Sippe, Combust. Sci. Technol. 3 (3) (1971) 145–152. [11] S. Goroshin, I. Fomenko, J.H.S. Lee, Proc. Combust. Inst. 26 (2) (1996) 1961–1967. [12] S. Goroshin, M. BidabadI, J.H.S. Lee, Combust. Flame 105 (1996) 147–160. [13] S. Goroshin, J. Mamen, A. Higgins, T. Bazyn, N. Glumac, H. Krier, Proc. Combust. Inst. 31 (2007) 2011–2019. [14] D.L. Parry, M.Q. Brewster, J. Thermophys. Heat Transf. 5 (2) (1990) 142–149. [15] P.T. Lynch, H. Krier, N.G. Glumac, J. Thermophys. Heat Transf. 24 (2010) 301–308. [16] B. Lewis, G. von Elbe, Combustion Flames and Explosions of Gas, Academic Press, New York, 1961. [17] G.E. Andrews, D. Bradley, Combust. Flame 18 (1972) 133–153. [18] T. Bazyn, H. Krier, N. Glumac, J. Propul. Power 21 (4) (2005) 577–582. [19] T. Bazyn, H. Krier, N.G. Glumac, Proc. Combust. Inst. 31 (2007) 2021–2028.
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 34 (2013) 2213–2220
www.elsevier.com/locate/proci
Stabilized flames in hybrid aluminum-methane-air mixtures Michael Soo 1, Philippe Julien, Samuel Goroshin ⇑, Jeffrey M. Bergthorson, David L. Frost Department of Mechanical Engineering, McGill University, Montreal, Canada Available online 15 June 2012
Abstract A premixed methane–air bunsen-type flame is seeded with micron-sized (d32 = 5.6 lm) atomized aluminum powder over a wide range of solid fuel concentrations. The burning velocities of the resulting twophase hybrid flame are determined using the total surface area of the inner flame cone and the known volumetric flow rate, and spatially resolved flame spectra are obtained with a spectral scanning system. Flame temperatures are derived through polychromatic fitting of Planck’s law to the continuous part of the spectrum. It is found that an increase in the solid fuel concentration changes the aluminum combustion regime from low temperature oxidation to full-fledged flame front propagation. For stoichiometric methane–air mixtures, the transition occurs in the aluminum concentration range of 140–220 g/m3 and is manifested by the appearance of AlO sub-oxide bands and an increase in the flame temperature to 2500 K. The flame burning velocity is found to decrease only slightly with an increase in aluminum concentration, in contrast to the rapid decrease in flame speed, followed by quenching, that is observed for flames seeded with inert SiC particles. The observed behavior of the burning velocity and flame temperature leads to the conclusion that intense aluminum combustion in a hybrid flame only occurs when the flame front propagating through the aluminum suspension is coupled to the methane–air flame. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Flame; Premixed; Hybrid; Aluminum; Methane
1. Introduction Two-phase hybrid combustible mixtures of a solid and gaseous fuel are encountered in numerous practical applications, such as coal combus⇑ Corresponding author. Address: Department of Mechanical Engineering, McGill University, 817 Sherbrooke Street West, Quebec, Canada H3A 2K6. Fax: +1 (514) 398 7365. E-mail address: [email protected] (S. Goroshin). 1 Presenting author.
tion and combustion material synthesis, as well as in accidental explosions in industry. The gaseous and solid combustible components can be either premixed in a cold state as, for example, during coal–methane explosions in mines, or combustible gases can evolve from the solid or liquid state directly in a flame, such as coal volatiles or fuel vapors in the combustion of liquid hydrocarbon-metal slurries. The reaction of particulate suspensions of light metals (aluminum in particular) with the products of hydrocarbon flames is one of the critical stages in the combustion of solid and metalized gelled
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.05.044
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propellants, pyrotechnics, and metalized explosives. The majority of experimental work in this area is focused on the combustion of large (tens of microns) individual particles and agglomerates [1–4], or on the combustion of metal particulates in specifically formulated solid propellant compositions with very low metal particle loading [5]. It is often implicitly assumed that the results can be extrapolated to smaller particles and dense suspensions in real systems. Recent work has already shown that extrapolating particle combustion models based on results derived from experiments with large-sized particles to micron and submicron sizes may be misleading due to a possible change from the diffusion-limited to the kinetically-limited regime of particle combustion [6]. In addition, little attention has been paid to the differences in combustion physics for a single particle and dilute particle suspensions as compared to that of dense suspensions. Most previous attempts to experimentally study combustion of particle suspensions and gaseous fuels were restricted to standard safety tests in constant pressure bombs that provided little information on hybrid flame features and structure [7,8]. Other investigations of stabilized hybrid flames were performed mostly on complex coal–methane mixtures [9], with the notable exception of the work by Bryant and Sippe [10] who studied the combustion of amorphous boron suspensions in propane– oxygen mixtures. The present study investigates stabilized flames in aluminum-seeded methane–air hybrid mixtures for a range of aluminum particle concentrations. Optical measurements are used to determine the burning velocity and nature of the combustion as a function of aluminum particle concentration and gas stoichiometry. The results are compared with similar tests using methane–air mixtures seeded with inert silicon carbide particles. Spectroscopic measurements are used to estimate the flame temperature and detect the presence of aluminum combustion products. The new experimental evidence unambiguously shows that the aluminum concentration has a strong influence on the combustion behavior of the hybrid mixture. The aluminum particles influence the combustion parameters, such as the flame temperature and propagation rate, and the results also indicate that aluminum combustion in a hybrid solid–gas mixture must be treated as a frontal flame propagation phenomenon. 2. Experimental apparatus 2.1. Hybrid flame burner and optical diagnostics A schematic of the newly constructed hybrid fuel burner is shown in Fig. 1. Its design implements a number of technical solutions developed
previously in our study of dust flames [11,12]. In particular, it uses a similar dust dispersion system comprised of a piston dust feeder and a “flow knife” disperser [12]. The thin, high-velocity jet (“flow knife”) is formed by flowing a premixed methane–air mixture through a 40 lm wide circular slot concentric with the top surface of the dust column. The jet removes and disperses the dust pushed by the advancing piston, layer by layer in a continuous fashion. Before going through the nozzle, the initially turbulent flow is laminarized in a narrow angle conical diffuser followed by a smooth 60 cm long supply tube. The dust concentration in the flow is regulated by changing the speed of the dust feeder piston. The flow rate through the disperser remains constant to maintain a stable dispersion regime. To regulate the flow rate through the nozzle, a small volume of the main flow is ejected from the main dispersion line into the ejection line (see Fig. 1), where it is exhausted without burning. The flow rate of the ejected mixture is determined by using pure nitrogen as the ejector driver gas and measuring the oxygen concentration at the exit of the ejection line with an oxygen analyzer. Alternatively, the ejector can be calibrated using the surface area of a stabilized stoichiometric pure methane flame as an indicator of the flow rate through the nozzle. The dust concentration in the flow is monitored with a laser light attenuation probe consisting of a diode laser (k = 632 nm), cylindrical lenses forming a rectangular 6 2 mm beam, and a diode sensor with a narrow bandpass filter and focusing lenses. The laser beam is transmitted across the dusty flow through a rectangular slot cut in the walls of the conical nozzle and protected by a transparent, heat-resistant Teflon film. The output of the diode sensor is continuously recorded by a data acquisition system. The dust concentration probe is calibrated by the complete aspiration of dust from the flow through a set of fine multilayered filters by a vacuum pump for a determined period of time. The average dust concentration is then determined by dividing the total mass of the aspirated dust by the volume of gas that flowed through the dispersing system during that time interval. A high-resolution digital camera with a set of different density gray filters is used to record flame images. The signal marks generated by the camera at the moment of shutter release are used to synchronize the flame images with the dust concentration monitoring signal. A flame spectral scanning system, similar to one described in our previous work [13], is used to acquire spatially resolved flame spectra across the flame cone. Two new spectrometers are used: Ocean Optics HR 4000 CG-UV-NIR and USB 4000. The first spectrometer, equipped with a 5 lm wide entrance slit and coupled to a 0.6 mm fiber, is used to acquire spectra in the 250–1000 nm spectral
M. Soo et al. / Proceedings of the Combustion Institute 34 (2013) 2213–2220
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Fig. 1. Schematic of the hybrid flame burner and composite photograph of the optical diagnostic elements.
range with a resolution of about 0.75 nm. The second USB 4000 spectrometer uses a 0.1 mm fiber without an entrance slit and records spectra in the 350–900 nm range with a spectral resolution of about 2.5 nm. With the second spectrometer, the system has a spatial resolution of about 0.1 mm with scanning steps of 0.2 or 0.4 mm whereas the larger fiber of the first spectrometer limits the spatial resolution to about 0.6 mm. The first spectrometer is used to acquire higher quality spectra with resolved signature AlO molecular bands and Al atomic lines whereas the second, more light-sensitive, low-resolution spectrometer is primarily used to acquire continuous spectra used for flame temperature measurements. 2.2. Aluminum powder In order to compare parameters of the hybrid flames with benchmark data of pure aluminum dust flames, the same atomized aluminum powder (Ampal 637, Ampal NJ) is used as in our previous experiments with stabilized dust flames and aluminum flames in tubes [11,12]. As shown in Fig. 2, the aluminum particles are of spheroidal or nodular shape. The Sauter mean diameter, d32, is derived from the particle distribution obtained from SEM images of the powder (see Fig. 2) and is shown to be about 5.6 lm, compared with a value of about 6.9 lm from the distribution obtained by the light scattering technique with a Mastersizer 2000 Malvern particle sizer instrument.
Fig. 2. Scanning Electron Microscope (SEM) photograph and particle size distribution in atomized Ampal637 aluminum powder.
3. Experimental results 3.1. Flame appearance, spectra and temperature at different aluminum concentrations Aluminum–methane–air hybrid flames are studied at two different methane/air equivalence ratios: / = 1 (stoichiometric) and / = 0.8 (fuel lean). First, a Bunsen-type conical flame is established at the exit of the nozzle without activating the piston in the dust dispersion system. Even without moving the piston, the mass concentration of aluminum in the mixture never falls below 20 g/m3 due to the surface erosion of the
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compacted powder column by the dispersing gas. Thus, the dust dispersion system must be completely emptied of powder in order to verify flame speeds in pure methane–air mixtures. Upon activation of the dispersion system, the aluminum concentration in the flow is slowly increased until reaching a plateau at the end of the dispersion cycle. Depending on the piston speed, the total dispersion time varies from 3.5 to 6 min and the corresponding maximum aluminum concentration at the plateau varies in the range 300–450 g/ m3. As can be seen from the flame photographs in Fig. 3, the color, brightness, and structure of the flames are strikingly different at low and high aluminum concentrations. At aluminum concentrations below 50 g/m3, the flame cone has a sharp inner boundary and an ill-defined outer boundary (A-type flame). The flame is yellow in color and individual particle streaks are clearly visible under magnification. At high aluminum concentrations the flame is bright-white in color and is extremely luminous (C-type flame). The thin inner flame cone has a well defined inner and outer boundary. It is enveloped by a much larger flame with a well defined outer boundary that has the appearance of a gaseous diffusion flame. The base of the outer diffusion flame is usually lifted from the inner cone base by about 2–3 mm. Preliminary analysis of the combustion products collected with copper plates passed through the tip of the flame cone also shows a noticeable difference between A-type and C-type flames. At low metal fuel concentrations, the products are gray in color and are primarily comprised of aluminum droplets encapsulated in a thick oxide shell. At high aluminum concentrations, the products are bright white and consist primarily of a mixture of submicron aluminum oxide particle agglomerates and larger
(a)
(b)
(c)
Fig. 4. Flame spectra in the 3 different concentration ranges: (A) 20–50 g/m3, (B) 140–220 g/m3, and (C) 250– 400 g/m3.
alumina shells. The transition from A-type to Ctype flames occurs in a relatively narrow region of dust concentration of about 140–220 g/m3 for stoichiometric mixtures and 100–140 g/m3 for lean methane–air flames. The transition usually starts from the tip of the flame with the appearance of a white diffuse glow attached to the inner flame cone (B-type flame in Fig. 3). With an increase of aluminum concentration, the glowing region then slowly spreads down to the flame base. Simultaneously, the outer diffusion flame cone is formed.
Fig. 3. Flame images at three different aluminum concentrations: (A) 20–50 g/m3, (B) 140–200 g/m3, and (C) 250–400 g/ m3.
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The flame spectral signatures associated with low aluminum concentration (A), high concentration (C) and transition type (B) flames are shown in Fig. 4. At low aluminum concentrations, the Atype flame spectra are comprised mostly of continuous blackbody radiation with a lone sodium line. The characteristic AlO signature bands only start to appear in the transitional B-type flames. They are, however, underdeveloped and only clearly show two of the three characteristic AlO band sequences, in contrast with the well-pronounced AlO bands in high aluminum concentration Ctype flames or in the pure Al dust-air flames studied previously [13]. The spectra of transitional Btype flames also show lithium and potassium atomic lines that have higher ionization potentials than sodium, indicating an increase in flame temperature. The high aluminum concentration Ctype flames also demonstrate well-pronounced 2 0 2 P – S aluminum atomic lines (394.4 and 396.1 nm), although there is no evidence of the self-absorption observed in aluminum dust flames [13]. Flame temperatures are derived using a polychromatic fitting procedure of the low-resolution continuum spectra in the 600–850 nm spectral range to Planck’s law [13].An emissivity dependence on the inverse wavelength squared demonstrates excellent fitting and reasonable temperatures below the thermodynamic predictions in accordance with [13]. This wavelength dependence was corroborated by [14,15]. About
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25 spectra were acquired during each flame scan from the center of the flame to the periphery. Temperatures were derived only from the spectra taken within the flame front, i.e. from spectra with the maximum intensity for a given scan. The acquisition time for each spatial point was about 2 s for low luminosity A-type flames, about 0.5– 0.7 s for B-type flames in the transition regime, and only about 30–50 ms for high aluminum concentration C-type flames. The total flame scan took about 50, 15 and 1.2 s for 25 points for A, B, and C type flames, respectively. Due to the relatively long scan times and drift of the dust concentration, the temperature data can only be associated with a dust concentration range and not with a particular concentration value. The measured flame temperatures are presented in Fig. 5 in three dust concentration intervals related to the three particular flame types: low and high aluminum concentration flames (A and C) and transitional flame (B). As seen from the temperature data, the flame temperature is very stable at low aluminum concentrations, oscillating around a value of about 1950 K. The temperature also stays within a relatively narrow interval of about 2750–2900 K at concentrations above 250 g/m3. In contrast, wider fluctuations of the temperature, between 2200 and 2500 K, for type B flames indicate a stronger dependence of the flame temperature on aluminum concentration in this transitional concentration range. The change in flame appearance, spectral signature, and temperature with dust concentration in fuel-lean methane–air mixtures (/ = 0.8) is similar to stoichiometric mixtures. However, the appearance of the transitional B-type flames is observed at considerably lower values of dust concentrations, about 100–140 g/m3, indicating that intensive aluminum combustion can be initiated at lower aluminum concentrations when excess oxygen is present. The temperatures of methanelean hybrid flames are also approximately 100 K higher than the temperatures of stoichiometric methane–air mixtures across the entire range of dust concentrations. 3.2. Burning velocities of aluminum–methane–air hybrid mixtures
Fig. 5. Flame temperature in three different ranges of aluminum concentration.
The flame burning velocities in aluminum– methane–air mixtures are determined using the total flame surface area method, i.e. by dividing the known volumetric flow rate through the nozzle by the total surface area of the inner flame cone [16]. Each flame image is associated with a particular value of dust concentration using timing marks produced by the camera output on the data acquisition system record. The boundary of the inner flame cones are manually traced using magnified flame images on a drawing tablet
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monitor. The coordinates of the cone are then fitted to a polynomial and the resulting curves on the left and the right sides of the vertex are separately revolved about the cone axis. The surface areas of the two obtained figures are then averaged to give the surface area of the flame cone. The method was tested by measuring burning velocities in pure methane–air and aluminum–air flames and comparing the resulting flame speeds to literature data. The burning velocity of the stoichiometric methane–air mixture determined in this manner is found to be about 36 ± 2 cm/s, in good agreement with literature data obtained by the same method [17]. The burning velocity in a pure aluminum flame is found to be about 20 ± 3 cm/s with a dust concentration range of 400–450 g/m3, in agreement with our previous measurements performed with a different set-up, but for the same aluminum powder [11]. Burning velocities in stoichiometric and lean methane–air hybrid mixtures measured at different aluminum concentrations are shown in Fig. 6. As can be seen from the data presented in Fig. 6, the flame burning velocity decreases only slightly with an increase in dust concentration in both stoichiometric and lean methane–air mixtures, suggesting that the heat released by the reacting aluminum contributes to the flame propagation. In order to make this conclusion more definitive, additional experiments were performed with dusty methane–air stoichiometric mixtures, where the reactive aluminum was replaced by an inert silicon carbide powder with a similar particle size (d32 = 6 lm). The comparison of the influence of the SiC and Al powders on the flame is presented in Fig. 7. The comparison shows that, in spite of the lower value of specific heat of SiC in comparison to aluminum (0.75 versus 0.90 J/g K) and the additional latent heat required to melt aluminum
Fig. 6. Burning velocities at different aluminum concentrations in stoichiometric (/ = 1) and lean (/ = 0.8) methane–air mixtures.
Fig. 7. Comparison of burning velocities in methane–air mixtures seeded with aluminum (Al) and silicon carbide (SiC) particles.
in the flame, the decrease in flame burning velocity with an increase of SiC concentration is considerably sharper than that with aluminum at the same dust mass concentrations. At SiC concentrations higher than 200 g/m3, the tip of the flame opens and a part of the mixture escapes without reacting. At concentrations above 300 g/ m3, the flame covers only the surface area adjacent to the nozzle walls and the majority of the fuel mixture escapes unburned. 4. Discussion The experimental evidence presented in this work clearly demonstrates that the mechanism of aluminum combustion in a hot oxidizing flow strongly depends on the mass concentration of the solid fuel in suspension. The observed transition from low-intensity oxidation to fast, fullyestablished combustion reflects the ability of the reacting aluminum suspension to modify the combustion properties, i.e. increase the flame temperature, which in turn affects the aluminum combustion mode. A thermodynamic equilibrium analysis of constant-pressure aluminum–methane–air flames at different aluminum concentrations is presented in Fig. 8 and provides the boundary for such influence in the absence of any limitations by kinetic rates. The comparison of the calculated equilibrium flame temperatures and species compositions with the experimental data shows that the measured flame temperatures and flame spectra are, in general, in good agreement with the thermodynamic predictions in the high fuel concentration range. Indeed, the maximum measured flame temperature values in the dust concentration ranges characteristic of B-type and C-type flames are only about 150–200 K lower than the thermodynamic predictions. Additionally,
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Fig. 8. Results of the constant pressure thermodynamic calculations of flame temperature and gaseous species composition as a function of aluminum content in stoichiometric, / ¼ 1 (on the left), and lean, / ¼ 0:8 (on the right) methane–air mixtures seeded with aluminum.
the experimental results also agree with the thermodynamic predictions regarding the appearance of AlO suboxides at concentrations above 150 g/m3. In contrast, the flame temperatures at low aluminum concentrations, of about 50 g/m3, fall below the thermodynamic predictions by about 400–500 K and are even below the methane–air flame temperature. Clearly, the influence of the aluminum reaction on the flame is practically negligible in this concentration range. The apparent explanation of this qualitative difference in flame behavior from low to high seeding densities is the relatively rapid coupling of the aluminum combustion front to the hydrocarbon flame that is observed over some critical concentration range, and the absence of any coupling for lower concentrations. This relatively rapid transition from uncoupled to coupled aluminum–hydrocarbon flame combustion gives an appearance of a sharp, “ignition-like” transition from A-type to C-type flames. The very possibility of such a transition indicates that aluminum combustion in hybrid mixtures needs to be treated in future modeling efforts as a frontal flame propagation phenomenon
with the corresponding ability to transfer heat and active species downstream of the combustion front. The coupled, frontal nature of aluminum combustion in hybrid flames is also evident from the fact that the flame speed of the hybrid mixture is practically unchanged with an increase of the solid fuel concentration. In contrast, the addition of inert silicon carbide solid particles leads to a sharp decrease in burning velocity followed by flame quenching. It is likely that the same quenching behavior would occur for any solid phase fuel suspension whose combustion cannot couple to the hydrocarbon flame front. The observed transition from slow to fast aluminum burning should not be confused with the ignition phenomenon associated with a single aluminum particle. A thermal explosion, runaway reaction leading to a sharp separation of particle and gas temperatures, as well as the subsequent transition to a particle combustion regime limited by the diffusion of oxidizer(s), was not observed in the present experiments. In contrast, the particle temperature increases only gradually with aluminum concentration, never exceeding values
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stipulated by equilibrium thermodynamics at each particular fuel concentration,2 thus indicating evidence of kinetically-limited particle combustion. The possibility that small particle combustion in the products of hydrocarbon flames would be kinetically-limited was previously indicated by Glumac et al. [18] and is not surprising considering the small particle sizes and the high diffusivity associated with the high-temperature, hydrogenenriched products encountered in the present experiments. 5. Conclusion The combustion behavior of micron-sized aluminum particles in the combustion products of methane strongly depends on the aluminum mass concentration. In the case of stoichiometric methane–air mixtures, the transition from slow oxidation of the aluminum particles to rapid burning occurs in the particle concentration range of 140–220 g/m3. The transitional concentration is shown to be lower in the case of lean methane– air mixtures. Above this concentration range, the flame has a significant increase in temperature to 2500 K and has clear AlO sub-oxide bands in the spectra. Increasing the aluminum concentration causes the flame speed to decrease slowly; in contrast, an increase in inert particle concentration causes a more rapid decrease in the flame burning velocity that eventually leads to flame quenching. These observations suggest that the combustion of aluminum particles at high concentrations is a frontal phenomenon and that the hydrocarbon and metallic flames are coupled. Furthermore, the measurements indicate that the combustion of an aluminum suspension in an aluminum–methane–air hybrid mixture is in the kinetic or close to kinetic diffusion transitional combustion regime [19].
Acknowledgements Support for this work was provided by the Natural Sciences and Engineering Research Council of Canada and Martec, Ltd., under a Collaborative Research and Development Grant, and
2
Note that in the diffusive combustion regime, the particle temperature would be close to the maximum possible adiabatic flame temperature, about 3000 K, at any fuel concentration.
the Defense Threat Reduction Agency under contract HDTRA1-11-1-0014 (program manager Suhithi Peiris). We also thank Fan Zhang of Defense Research and Development Canada–Suffield for his useful input to the work.
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