Experimental study of stages in aluminium particle combustion in air

Experimental Study of Stages in Aluminum Particle Combustion in Air EDWARD L. DREIZIN AeroChem Research Laboratories, Inc., P.O. Box 12, Princeton, NJ 08542-0012 An experimental study of AI particle combustion in air is presented. Uniform AI particles were formed and ignited in air using a pulsed micro-arc discharge. Burning particle color temperatures were measured using a three-wavelength pyrometer, partially burned particles were quenched and cross-sectioned. Particle internal compositions were studied using a scanning electron microscope equipped with an x-ray energy dispersive spectroscopy detector and wavelength dispersive spectroscopy scan. Temporal variations of the particle diameter and the shape and size of the smoke cloud surrounding a burning particle were determined. The effect of an external electric field on AI particle combustion was also tested. Three distinct stages were identified in AI particle combustion, which correspond to different temperatures, internal particle compositions, and flame shapes. The transitions between the stages were shown to correlate with the internal phase transformations occurring in the burning AI droplets. Growing on spinning A1 particles "oxide caps" were shown to cause rapid changes of trajectories of the burning particles. The temperature histories of burning AI particles were affected by the electric field, and a reduction in the total combustion time due to external electric fields was observed.

INTRODUCTION Aluminum is widely used in solid propellants and explosives, and thus AI particle combustion is of great practical interest and has been extensively studied since the early 1960s [1-6]. Combustion times for different diameter particles have been reported [5-7] and temperatures have been measured during the combustion of relatively large A1 particles using thermocouples and brightness pyrometry [8, 9]. Nonsymmetric combustion, brightness oscillations, and micro-explosions were observed by many researchers beginning with the earliest AI particle combustion studies [1-6]. These phenomena are extremely important for practical applications, which explains why they have been addressed in many experimental and theoretical works. Attempts have been made to relate AI nonsymmetric, disruptive burning to nitrogen [6, 10] and/or hydrogen [4, 11] participation in the combustion since these phenomena were not observed during AI particle buming in A r - O 2 mixtures [6, 7]. However, the conditions at which the shift to the nonsymmetric combustion occurs and the causes of particle disruptions are still unclear. Similar phenomena (brightness and temperature jumps, micro-explosions) have recently been observed in the combustion of hot metal drops of Cu, Ta, W, Mo, and steel, produced COMBUSTION AND FLAME 105:541-556 (1996) Copyright © 1996 by The Combustion Institute Published by Elsevier Science Inc.

and ignited in air using a novel micro-arc device called GEnerator of Monodisperse MEtal Droplets (GEMMED) [12-14]. In that work it was suggested that such processes are caused by oxygen penetration into the burning metal particle and its subsequent reaction with metal, a process which is initiated when the metaloxygen system reaches its typical temperature of nonvariant transformation determined from the binary phase diagram. An important distinction for AI, compared with the metals studied in Refs. 12-14, is that AI is a vapor-phase burner [15]. The conventional metal vapor-phase combustion mechanism [15] assumes that no reaction occurs on the metal surface, and thus no oxygen is expected to penetrate into the burning particle. However, initial experiments on A1 combustion using GEMMED [16, 17] have shown evidence of oxygen buildup in burning AI particles. Thus, corrections of the mechanism of vapor-phase AI combustion need to be made, and transport processes resulting in heterogeneous AI-O 2 reactions should be addressed. Relevant processes of oxide accumulation on the metal droplet surface have been theoretically discussed [18, 19]; however, no adequate agreement with A1 combustion experimental data has been demonstrated. It was indicated in Refs. 20 and 21 that thermionic emission resuits in considerable flame ionization for a Mg 0010-2180/96/$15.00 SSD10010-2180(95)00224-3

542 particle, another metal burning in vapor phase. The ionization in a Mg particle flame was shown to strongly affect the transport processes and flame structure. The AI boiling point (2520°C) is noticeably higher than that of Mg (1090°C) [22] and thus AI is expected to burn at a higher temperature, which makes the consideration of ionization processes even more significant. The objective of this research is to experimentally characterize the entire process of AI particle combustion in air. It includes the characterization of the flame shape and measurements of the diameter, internal composition, and temperature histories of freely falling, burning AI particles, which should be considered in conjunction with the AI-O phase diagram in order to determine possible relevant AI-O heterogeneous reactions. The effect of an external electric field on AI particle combustion was examined in order to evaluate the importance of ionization and smoke particle charges on the flame shape and history. EXPERIMENTAL AI particle combustion experiments were conducted using the GEMMED described elsewhere [23, 24]. The GEMMED uses a pulsed micro-arc discharge to melt the tip of a consumable wire electrode so that molten droplets .separate from the wire. The droplet initial temperatures can be adjusted within a wide range above the metal melting point, which provides a means to ignite droplets instantly upon their formation in an oxygen containing environment. Uniform, free falling A1 droplets of 85, 120, 165, and 190 /zm diameter were produced, ignited, and burned in air at atmospheric pressure. Particle Temperature The color temperature of the burning droplets was monitored using a three-wavelength pyrometer. The pyrometer included an iris, a fibcroptics trifurcated bundle, three interference filters, and three HC120-01 Hamamatsu photo-sensor modules. The wavelengths of the interference filters used (520, 580, and 458 nm) were chosen so that no bands observed in the

E . L . DREIZIN AI-O spectrum [25] could contribute to the measured signals. The pyrometer was calibrated using a tungsten strip lamp, providing a maximum blackbody temperature of 2380°C. The calibration showed that two intensity ratios (rx = 1(580 nm)/I(520 nm) and r 2 = 1(580 nm)/I(458 nm)) could be used for temperature measurements with an acceptable error (less than __80°C) in the temperature range of 1630°-2380°C. The high sensitivity of the- pyrometer allowed it to be positioned far from the burning particle, so that the entire particle trajectory was in the pyrometer field of view. Three brightness signals were recorded simultaneously for each particle combustion event and used to compute two-color temperature histories (as well as indicate the entire particle combustion times). Temperatures above 2380°C were computed using an extrapolation of the calibration curve. The two-color temperatures inferred for the A1 combustion experiments agreed within _+50°C in the range of 16303000°C, although the difference increased to + 150°C (r 2 became higher) at higher temperatures. A1 is known to burn in the vapor-phase, and, therefore, continuum radiation from the combustion product submicrone particles produced in the reaction zone surrounding the burning AI droplet was expected to contribute significantly to the measured radiation. Such a contribution from many different surfaces with vastly different temperatures (from AI combustion temperature to room temperature) could distort the measured radiation spectrum, so that inferred color temperatures would not be valid. Actually, an inconsistency between the inferred temperatures has appeared in similar color temperature measurements for some burning metal particles which were surrounded by bright luminous clouds, (e.g., Mo and W particles burned in air) [26]. The inconsistency in that work was detected because the spectrum was distorted and the color temperatures did not agree, i.e., could not be measured correctly. However, the two temperatures inferred in this AI particle combustion research from the measurements of the three radiation intensities were quite consistent. The measured part of the radiation spectrum produced

ALUMINUM PARTICLE COMBUSTION by both burning A1 particle and surrounding cloud must have been similar to that of a mono-temperature gray-body radiator. Therefore, we concluded that the measured temperatures were meaningful. As will be discussed later, the properties of the radiating system of AI particle/surrounding cloud changed during combustion. Cloud density changed and the initially spherically symmetric cloud became conspicuously nonsymmetric. These changes will be considered to interpret the color temperatures measured during the AI particle combustion. Luminous Zone Structure

Particles were quenched on glass slides at different combustion times (distances from the GEMMED), and the shape of smoke traces surrounding the quenched particles were examined. The luminous flame shape was visualized by a video-recording of the combustion events using a free running camera at a fast shutter speed (0.5 ms exposure time). The signals measured by the separate channels of the optical pyrometer (consisting of an interference filter and a photomultiplier tube (PMT) based photo-sensor) indicated intensities of the flame radiation at the selected wavelengths. The spatial resolution of the optical signals produced by the particle and the flame zone was achieved using a screen positioned in front of the burning particle trajectory with an array of equally spaced, 0.2 mm wide slots perpendicular to the particle velocity vector. The radiation of burning particles moving behind the slots was registered as a series of pulses. The period of the pulses was used to determine the particle velocity, v. The duration tp of each pulse corresponded to the time needed to cross a slot. Particle velocity changes were negligible during the time for the particle to cross a slot. The size of the luminous domain was estimated as V . t p - w , where w is the slot width. Internal Particle Composition

Particles were quenched at different combustion times, embedded into epoxy and crosssectioned for microscopic inspection. Particle

543 quenching using inert gas separated from air by a soap-bubble film [14, 27] was found to be unacceptably slow, resulting in the particle blowing up and the formation of large internal voids, as will be discussed further in the Resuits section. Thus, more rapid quenching was used to observe particle internal compositions corresponding to those existing during combustion. The fastest cooling rate of a metal droplet, which can be provided by its impingement onto a cool metal surface, was utilized in this work. Burning AI droplets were impinged onto room temperature AI plates which resulted in quenching times less than 300 /xs for droplets of initial 165/zm diameter. An electron probe microanalyzer "Cameca SX50" was used for the cross-section examination, and an energy-dispersive spectroscopy (EDS) detector (Princeton Gamma Tech.) as well as a wavelength dispersive spectroscopy (WDS) scan were utilized to determine the particle internal compositions. Electric Field Effect

Two vertical capacitor plates were positioned below the GEMMED so that free falling particles entered a uniform electric field formed between the capacitor plates. The temperature histories of the particles burning in DC and AC (4-6 kHz) electric fields ranging from 500 to 2000 k V / c m were recorded. Particles were quenched in the electric field using glass slides positioned between the capacitor plates. Smoke traces surrounding the quenched particles indicated flame deformation caused by the electric field. RESULTS Combustion Time

The combustion times measured in this work for the AI particles produced and ignited using GEMMED correlate well with previously reported AI particle combustion times [5-7]. The combustion times measured are presented in Fig. 1 as a function of AI particle diameter along with related experimental results of J. Prentice [6] and the D-squared interpolation suggested in Ref. 7 to correlate all of the

544

E . L . DREIZIN During the initial stage (approximately, the first 20 ms for particles of 165 /zm diameter), the radiation intensity is fairly stable. The second stage (see Fig. 2, ca. 22-46 ms) is characterized by the remarkably increased intensity of the measured particle radiation, as well as by the initiation of strong intensity oscillations. The oscillations originate at the beginning of this stage, and their frequency depends on particle initial diameter and combustion time. In other words, for a given initial particle diameter, the oscillation period is a repeatable function of the combustion time (or, particle diameter, which continuously decreases during combustion). Detailed comparisons of the output signals from the two photo-sensors positioned on opposite sides of the burning particle have shown that the brightness oscillations observed are associated with the particle spinning. Finally, in the third combustion stage (46-90 /zs in Fig. 2) the intensity of the measured particle radiation rapidly decreases and then remains nearly constant except for the superposition of periodic oscillations. Burning particle radiation was also visualized on the streaks recorded using video and still cameras. The streaks were similar to those reported in previous AI combustion studies [1, 5, 6] and our preliminary experiments [16, 17]. The periodic brightness modulation observed on the streaks correlates exactly with the brightness oscillations measured using photoelectrical transducers. Systematic analysis of the streaks showed that during the first stage the direction of the particle motion does not change; a smooth

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experimental data reported to date. The consistency of the combustion times and general combustion behavior of A1 particles observed in this work with earlier results suggests that the method of particle production and ignition using a pulsed micro-arc [12-14] employed in this research does not affect the AI combustion scenario. Particle Radiation

AI particles burning in air studied in this research consistently displayed similar radiation signal histories in which three distinct combustion stages were observed. An example of the radiation signal recorded by one of the photomultipliers used in the three-wavelength pyrometer is shown in Fig. 2. The inferred color temperature history of a burning AI particle, which is also shown in Fig. 2, will be discussed below.

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ALUMINUM PARTICLE COMBUSTION

545

deviation of the direction is observed during the second stage; and several rapid directional changes occur during the third stage of AI particle combustion. It is interesting to note that some small satellite particles are formed during the second combustion stage, but these did not cause noticeable velocity variations of the parent particle. However, satellite formation was not observed during the third stage, where rapid variations of particle velocity and direction were seen. Data similar to that in Fig. 2 describing three stages of combustion were observed for AI particles of different diameter used in this research. While the total combustion times were different (see Fig. 1), the relative duration of the stages did not change dramatically. Figure 3 presents the relative stage durations for A1 particles of different initial diameters and indicates a slight increase of the first stage duration for the smaller particles.

Luminous Zone Shape The smoke traces observed around the particles quenched on glass slides were remarkably different for the three stages described above. l-xamples of typical traces are presented in Fig 4. Spherically symmetric vapor phase combustion is inferred from the shape of the cloud surrounding the AI particles (165 /xm diameter) quenched during the first stage (Fig. 4a). A loss of spherical symmetry, an increase of the cloud size and density, are observed during the .second stage (Fig. 4b). Some density decrease along with continuing non-symmetry in the l

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cloud shape are observed during the third stage (Fig. 4c). For both the second and third stages the smoke appears closer to the particle surface, although the cloud does not encircle the particle uniformly. The size and shape evolutions of the smoke traces around particles quenched on glass slides are consistent with the data inferred from the analysis of the spatially resolved radiation measurements for the particles crossing the slot array. A pulse sequence measured for a particle crossing the slot array provided the flame shape temporal evolution as well as the particle velocity variation. Examples of the measured signals indicating symmetric and nonsymmetric combustion regimes are presented in Fig. 5. Single peak sequences were observed when the particles crossed the slot array during the first combustion stage. The parameters of the signal presented in Fig. 5a suggest that particle velocity is ca. 1.8 m / s and the diameter of the luminous zone is in the range of 0.4-0.5 mm. Double peaks (cf. Fig. 5b) were often observed when a particle crossed

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Fig. 5. Output signal of a photo-sensor monitoring AI particle crossing slots positioned normally,to the particle trajectory. (a) Stage I, (b) Stage II; Slot width =0.205 mm; distance between slots = 3 mm. a slot during the second and third combustion stages. The particle velocity inferred from the signal presented in Fig. 5b, is 1.35 m / s , and the entire size of the luminous zone is close to I mm. A typical curve of the temporal variation of velocity during the combustion of a 165/zm initial diameter AI particle is shown in Fig. 6.

Combustion Temperature An example of the measured color temperature history of a burning 165 /xm diameter AI particle is shown in Fig. 2 (along with one of the radiation signals for the same particle dis3.0

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cussed above). The three stages in AI particle combustion which were observed in particle radiation histories, and were also associated with significant changes in the shape of the surrounding smoke clouds, can be easily distinguished in the particle temperature history. The measured temperature, similar to the radiation, is nearly constant during the first stage, and it is close to 2650°-2700°C. As indicated by the observation of the smoke cloud shape, a spherically symmetric luminous zone seems to surround a burning AI particle during the first stage of combustion. The burning particle could be seen through the cloud on photo and video-images, and thus it produced a stronger radiation than external luminous zone. This suggests that during the first stage, radiation from the particle surface was measured superimposed on the background of the radiation produced by the luminous zone. If a considerable difference between the particle surface temperature and external luminous zone temperature existed, the resultant spectrum would be different from that of a gray-body radiator. This should have caused an inconsistency between the two temperatures inferred from the radiation measurements because the pyrometer was calibrated using a gray-body radiator (a tungsten filament strip-lamp). However, while the intensities of the particle and

ALUMINUM PARTICLE COMBUSTION external luminous zone radiations were different, the two-color temperatures inferred from the measured radiation signals were consistent. This shows that the measured temperature was close to both the particle and external luminous zone temperatures and that these two temperatures were also fairly close. The experimental temperature range at which the first stage of AI combustion occurred was (2650-2700)+ 50°C, and it lies between the boiling points of A1 (2520°C) and AI20 3 (2980°C) [28]. While it is normally unexpected that the burning particle temperature can be higher than the metal boiling point, a significant superheating has been reported to occur in Mg particle combustion [29]. Apparently, such a superheating can occur if the stand-off flame temperature rapidly exceeds the particle boiling point a n d / o r evaporation from the particle surface is restrained. Regardless of the mechanism of the possible superheating, the conclusion which can be drown from the temperature measurements during the first stage of AI particle combustion is that the burning particle boiled, and a possibility exists that it was slightly superheated compared with A1 boiling temperature. The particle boiling is consistent with the prediction of the classic theory [15] for the AI vapor phase combustion. The changes occurring during the second stage of combustion in the shape of the luminous zone and the smoke cloud surrounding a burning AI particle affect the temperature measurements. The cloud becomes nonsymmetric and it spins around the burning particle (or, possibly, the entire particle/cloud system spins). Therefore, part of the time the radiation which is being measured is produced by both the particle and the cloud, and part of the time most of the radiation from the cloud is blocked by the particle itself, and thus, the measured radiation is produced mainly by the particle surface. The changes in the measured radiation can be clearly seen from the shape of the radiation pulses measured using a slot array (cf. Fig. 5b). The pulses have double peaks of different amplitude, produced by the radiations from the particle surface and from the cloud. Sometimes, when the particle shields the cloud, or the cloud shields the particle, the two peaks merge in one. If a difference exists

547 between the particle and the cloud temperatures, the measured temperatures are expected to oscillate with the frequency equal to that of the particle spinning. Such oscillations with the amplitude less than + 100°C are actually observed in the experimental temperature curves during the second stage of combustion, as shown in Fig. 2. On the background of the oscillations, the temperature is stable and is in the 2400°-2500°C range during the second combustion stage. This is approximately 200°C lower than the temperatures measured during the first stage of AI particle combustion and slightly lower than the AI boiling point. Because the vapor-phase oxidation provides most of the combustion heat release, it is reasonable to assume that the particle temperature corresponds to the minima of the oscillating temperature curves. The measured temperature, similar to the radiation signal, is observed to decrease during the third combustion stage, but this decrease starts after the radiation intensity has decreased and then continues on while the radiation intensity does not change appreciably. The amplitude of temperature oscillations increases to ± 150°C. When the average temperature decreases to approximately 2000°C, which is close to the AI20 3 melting point (2050°C), the periodic oscillations cease, and combustion terminates shortly thereafter, often with a micro-explosion, observable as a sharp peak on the intensity-versus-time curve, but hardly noticeable on the particle temperature history. Particle Diameter and Internal Composition

The diameter and internal structure of the quenched particles depended strongly upon the quenching method used. Quenching in inert gas, separated from air using a soap-bubble film resulted in particle expansion and formation of large internal voids (apparently filled with a gas). The process of particle expansion during the quenching was confirmed by monitoring the particle radiation using the three wavelength pyrometer. Consistent with the experimental data [16] reported earlier, some increase in the particle radiation was observed after its immersion into an inert gas (argon, helium, and nitrogen were used in this work).

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However, the color temperatures inferred from the intensity ratios were not observed to increase at that time. Thus, the radiation increase could have been associated with particle expansion. In fact, large hollow spheres were found in the quenching cell, similar to those observed in early AI combustion studies [4]. It is important to note that there were no hollow spheres or internal voids observed when the particles were quenched at similar combustion times by impinging them onto an AI plate. The particle size evolution, gathered from the size measurements of the particles quenched by impingement onto metal plates during the combustion, is presented in Fig. 7 and exhibits a monotonic size decrease with time, as would be expected for AI combustion. Because the particle expansion during the quenching clearly involves some gaseous species release and, thus, modification of the particle internal composition, the particle impingement onto a metal plate was used in this work to quench particles for internal composition analysis. Visual examination of the particle cross-sections did not show significantly different (in color or micro-structure) phases in particles quenched during the first and second combustion stages. Distinct "oxide caps" were seen in the cross-sections of particles quenched during the third stage. The details of the structure observed at that time are discussed below. The results of the analyses of the internal compositions of quenched A1 particles (165 /~m initial diameter) are presented in Fig. 8. There is a clear correlation of the internal composition

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changes with the stages noted above in temperature, radiation and flame shape histories of the burning particles. The measurements show only the background oxygen level during the first 25 ms of combustion. The same oxygen level was measured in a test sample made of the raw AI (aluminum 1100 wire used as a consumable electrode in the particle formation process). It apparently is associated with an oxide film which always forms on a clean metal surface exposed to room air. A higher oxygen content is measured in quenched particles at longer combustion times (corresponding to the second stage of combustion). Atomic oxygen concentration varies between 4% and 14% throughout a particle's cross-section and can change quite rapidly when the electron beam probe position is shifted slightly. Average oxygen content in the particles was quite uniform and was consistently higher than the background level. No

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ALUMINUM PARTICLE COMBUSTION

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oxide film or inclusions were seen in the particle interiors. Considerable changes in the structure of the quenched particle cross-sections were observed at the transition to the third combustion stage. Voids were observed in the particles quenched after approximately 45-50 ms of combustion (stage 2 to stage 3 transition period), and "oxide caps," similar to those described in Ref. 6, appear on the particles quenched at longer times. The size of the "caps" grows with the combustion time, which is illustrated in Fig. 9 showing cross-sections of the particles quenched at different times (all within the third stage of the combustion). It should be noted that no voids were observed in the particles quenched either before or after the 45-50 ms interval. The measured oxygen content in "'oxide caps" is shown as solid triangles in Fig. 8. It is slightly lower than the stoichiometric oxygen content in A120 3, but increases at longer combustion times. The composition of the "metallic" regions adjacent to the "oxide caps" did not change compared to that observed during the second stage. A Cameca SX50 WDS Scan of the x-ray spectrum excited by the electron beam was conducted to search for elements other than AI and O in the particle interior. In particular, the nitrogen content was of interest, because a mechanism of AI combustion has been suggested [5, 6, 10] involving nitrogen penetration in burning metal. No nitrogen traces were observed in cross-sections of particles quenched

during the first and the second stages. There were, however, traces of nitrogen detected in "oxide caps" originating during the third combustion stage, and the nitrogen concentrations there were approximately 3 % - 5 % of the oxygen content. The WDS scan reveals one more interesting detail in the particle compositions. There was an iron impurity in the raw AI wire used to produce droplets. Iron content was less than 0.5% (atomic), and such (minor) iron concentrations were consistently detected in particles quenched during the first and the second combustion stages. However, iron was detected only in the "metallic" part (adjacent to the "oxide caps") of the cross-sections of the particles quenched during the third stage. Therefore, the presence of nitrogen traces and the absence of iron in the "oxide caps" may suggest that the "caps" were formed through material deposition on the particle surface from the flame zone rather than through an internal phase transition. Electric Field Effect

Experiments conducted with a DC electric field have shown that the particles are attracted to the negative plate of the capacitor, and the combustion terminates because the particles hit the electrode. This shows that burning AI particles are positively charged, but the effect of the electric field on the combustion time was obscured.

COMBUSTION TIME 48 ms

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Fig. 9. SEM-produced images of the cross-sections of A1 particles with "oxide caps" quenched during the third stage of combustion.

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E . L . DREIZIN experimental data. The data set collected in this research made it possible to evaluate the temporal variation of the heat released in the AI particle during its combustion and the corresponding mass of AI required to react to provide such a heat release. The basic approach consists of the comparison of the change of the particle enthalpy caused by its contiguous mass, composition, and temperature changes in time (directly derived from the experimental data) with the heat lost from the particle by conduction, convection, radiation, and evaporation during the same (short) time step. The experimental data used are the measured histories of particle temperature T(t), radius r(t), velocity v(t), and internal composition, resulting in the variation of particle density p(t). Particle diameter and density histories were used to compute the particle mass history rn(t) and evaporation rate. The enthalpy for a (liquid) particle, Qp, was determined at each instant as:

AC electric fields resulted in a general decrease of the entire particle combustion time. A noticeable change in the temperature history, as well as a slight decrease of the frequency of the brightness oscillations, typical of the second stage of A1 combustion, were ob.served in the presence of an external AC field, Figure 10 presents the temperature history curves with and without an electric field for burning A1 particles of the same initial diameter, temperature and velocity. Smoke traces surrounding particles quenched on glass slides in electric fields (Fig. 11) show strong distortion of the cloud, indicating the presence of a great quantity of charged smoke particles. DATA EVALUATION The ultimate objective of this AI particle combustion research is to develop a comprehensive model which would adequately combine all the processes affecting the combustion scenario. It is clear that many parts of such a theory need to be developed (or incorporated from existing models), including external heat and mass transport affected by forced convection in strong temperature gradients, heterogeneous processes, chemical kinetics, etc. The probability is high that the combustion mechanism will not remain the same throughout a particle combustion event. Thus to provide a valid basis h~r modeling, a comprehensive experimental description of the combustion event is used to deduce the relevant parameters from direct

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(1) where m is particle mass, computed from the measured particle diameter and composition, T is particle temperature, T,, is the AI melting point, TO is the ambient temperature, q is the latent heat of fusion for A1, and CL and C s are the liquid and solid AI specific heat capacities, respectively. The change dQp in the particle

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enthalpy due to changes in its mass and temperature during a short time step At is:

for a spherical particle in accordance with Ref. 31:

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where F is the net heat flux to produce the particle enthalpy change. This heat flux is equal to F = F + + F where F + is the total heat flux to the particle (a part of the entire combustion heat release), and F - is the heat lost from the particle via conduction, convection, radiation, and evaporation: / " - = 4 7 r r 2 [ h N u ( T - To)/(2r) +o-e(T 4 - Ton)] + L d m / d t ,

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where A is gas heat conductivity, or is the Stefan-Boltzmann constant, e is emissivity (taken to be equal to 0.5 within the experimental temperature range [30]), L is the heat of vaporization for AI, dm/dt is the mass change per unit time derived from the m(t) mass history, and Nu is Nusselt number computed

(5)

where Re = 2rvps/ix, pg is gas density, Cp is gas heat capacity at 1 atm, and /z is gas viscosity. Preliminary estimates showed that all the terms in Eq. 4 are comparable and thus neither one can be neglected. It was also found that Nusselt number changed from 2.7 to 2.1 during combustion, which suggested that while particle motion in a gas was quite laminar, it affected noticeably particle heat balance. Thus, equating 2 and 3 and computing F using Eqs. 4 and 5 , we found the unknown heat release F ÷ caused by combustion at a time dependent reaction rate. Since combustion occurred in room temperature air, all of the gas (air) parameters were used at "film temperature" Tf = (T + T0)/2. The temperature dependence of the heat of vaporization for AI suggested in Ref. 32 was used. The time step At was 0.1 ms for a 165-/xm initial diameter AI particle burning for 90 ms. A further decrease of At did not affect the computed results. The results of the calculations are presented in Fig. 12. The heat release F + varies during the combustion time for each of the stages noted in the AI particle combustion history. There is a maximum value of the heat release during the first (spherically symmetric) stage,

552

E. L. DREIZIN 2,0 S T A G E II

S T A G E Ill

1.5

+

1.0

M.

0.5 i I

I

20

40

0.0

0

=

i

60

80

100

TIMp {ms) Fig. 12. Rate of heat release at the particle surface during A1 particle combustion estimated using experimental data.

and its value smoothly decreases and stabilizes at the beginning of the second stage. A steady fall of the heat release is observed during the third stage of combustion. The trends in heat release variation suggest that different reaction rate limiting processes may become important during different stages of AI particle combustion. Assuming condensed AI203 to be the only reaction product, an estimate shows that approximately half of the heat released during the combustion of the AI that was experimentally observed to evaporate, must be received by the particle (an amount equal to F ÷) in order to account for the experimental particle temperature history. This result is not surprising since part of the combustion heat is lost by the cloud's external radiation. The temporal variations of F ÷ will be used as a basis for the further development of a detailed A1 particle combustion model. DISCUSSION Temperature measurements indicate that AI metal boils during the first two combustion stages. The processes occurring during the first stage apparently can be well described by the conventional model of vapor-phase metal combustion [15]. However, an important additional detail is that oxygen penetrates inside an AI droplet, which results in some finite oxygen content being measured in particle interiors at the beginning of the second combustion stage. An assumption to consider is that suboxides (AIO and AlOz) form in the gaseous reaction

zone, diffuse to the particle, and a fraction of these gaseous suboxides dissolve in the liquid AI. Experimentally determined presence of oxygen inside AI particles suggests that internal processes in the binary AI-O system should be considered in order to understand AI particle combustion behavior. Traditional analysis of the evolution of condensed binary systems makes use of concentration-versus-temperature phase diagrams, thus the equilibrium A1-O binary phase diagram [33] will be employed as a guideline to the discussion of the experimental data. A portion of this phase diagram plotted for the temperature and concentration ranges of interest is shown in Fig. 13. Starting from the pure boiling AI (point a on the diagram), a quasi-steady system acquiring oxygen would follow the lines ab (boiling liquid) and ac (gas). The line ac indicates the minimal oxygen concentration in a gas phase, or, on the other hand, the limit oxygen concentration can reach in a liquid when the dissolution occurs rapidly (not quasisteadily). Thus, the oxygen content in an actual condensed AI-O system can exceed the equilibrium solubility limit denoted by the point b (which is less than 1%), but should always be lower than the concentration corresponding to the point c, or, approximately 14%, for any kind of unsteady (i.e., occurring rapidly) dissolution. When dissolved oxygen concentration increases, the boiling point of AI decreases somewhat (ca. 220°C) and, ultimately, after a three-phase (liquid AI-O solution, liquid A1203, and gas) equilibrium temperature (approximately, 2240 °C, line bce in Fig. 13) is attained, liquid AI203 starts to form. Thereafter (or, at higher dissolved oxygen concentrations in the condensed phase) the burning droplet consists of two liquids: an essentially pure AI (point b) and AI203 (point e). The droplet temperature is thus limited by the level of equilibrium bc, or, approximately 2240°C. The temperature can be higher above the particle surface and the corresponding vapor phase similarly has two components, reflected in the diagram by the ac and dc lines. Indeed, the new vapor-phase component (dc) is a vaporizing A1203, which is normally assumed to consist of molecular suboxide species AIO and AI20 [32, 33]. Their partial

ALUMINUM PARTICLE COMBUSTION

553

Al(liquid) + Gas 2500

Gas

I

/

d

2400

Gas + Al=O3(liquid)

Al~O3(li~ uid)

,\

u~

+ Gas

2300 v

< r~ uJ LU

2200

b

2100

solution

c

e Al(liquid) + AIzO3(liquid)

AI-O saturated s t o i c h i o m e t r i c AI =0=

k-

AIzOB(SOlid) 2000

Al(liquid) + AI203(solid)

1900 0

+ Gas [

i

i

i

i

i

10

20

30

40

50

60

70

OXYGEN C O N C E N T R A T I O N , %

Fig. 13. Equilibrium binary AI-O phase diagram at 105 Pa [30]. Dashed arrow shows a possible system evolution during the first combustion stage.

vapor pressure rapidly increases from 0.01 to 0.15 atm as the temperature increases from 2350° to 2650°C [28]. Considering these changes inferred from the AI-O phase diagram, it is suggested that the transition from the first to the second combustion stage occurs when the atomic concentration of oxygen dissolved in the AI particle reaches the solubility limit (restrained by the maximum of 14% for rapid dissolution) and the droplet temperature attains the three-phase equilibrium of 2240°C, as shown by the arrow in Fig. 13. The droplet composition transforms from an AI-O solution to the mixture of two liquids: almost pure AI (most of the droplet) and A120 3. The decrease of the boiling temperature (from 2460° to 2240°C) results at that time in the decrease of the measured droplet temperature. The suboxides AIO and A120 appear in the composition of the vapor phase formed above the droplet surface during the second combustion stage. This promotes the production of A120 3 in a wider vapor-phase reaction zone, which explains the observed expansion of the smoke cloud surrounding quenched particles and an increasing intensity of the burning particle radiation. On the other hand, as the droplet surface becomes a source for suboxide vapor production, the gaseous flux of suboxides from the vapor-phase flame zone to the droplet surface decreases (being bal-

anced by the vapor flux from the surface), which explains the stabilization of the droplet composition during the second combustion stage. When the droplet temperature decreases below the bce line level (at the beginning of the third combustion stage), boiling stops and the flux of gaseous oxygen-containing species to the droplet surface can be efficiently accommodated while liquid AI203 forms. This correlates well with the oxide cap growth observed during the third stage of combustion. This does not, however, provide clues as to why droplets start to spin during the second combustion stage, which results in the observed periodic brightness oscillations. The droplet spinning itself explains the change from a smooth trajectory during the second combustion stage due to the Magnus effect. "Oxide caps" observed to grow on the burning particles during the third combustion stage result in strong particle asymmetry and cause rapid trajectory changes of spinning particles, but the mechanism of the non-symmetric oxide cap growth remains to be explained. Therefore, although particle composition and structure changes explain some peculiarities in AI combustion, such as the sudden change in the burning regime and rapid trajectory changes, they still do not provide an explanation for the transition to the non-symmetric

554 combustion mode and initiation of burning particle spinning. Pronounced changes observed in the shapes of the quenched particle smoke traces at the transition to non-symmetric combustion (cf. Fig. 4) may provide some clues to a better understanding of the nature of that transition. Such changes have been observed by many investigators and were usually treated as the appearance of "mini-jets" from the surface of burning particles, which destroy the spherically symmetric flame. Note, however, that it is easy to postulate a configuration of electric charges located at the particle surface and in the flame zone in such a way that the smoke patterns will follow the field potential lines. This suggests that electric fields may form and affect the flame structure. The importance of such electrically driven effects was demonstrated in the present experiments on A1 particle combustion in AC electric fields. Distortion of the smoke cloud in the external electric field (Fig. 11) shows that the cloud structure is strongly affected by the electro-static interaction of the burning particle and spatial charges distributed in the flame zone. The change of the particle temperature history and the decrease of the entire combustion time in the presence of the electric field suggest that the transport processes crucial for the combustion rate were affected. For example, our experiments in DC electric fields showed that burning A1 particles were positively charged, apparently due to thermionic emission. Florko and co-workers [21, 22] demonstrated that ultrafine smoke particles are strongly affected by electric fields applied to Mg particle flames. Electrical fields formed in the particle combustion zone could also affect the "oxide cap" growth observed during the third stage of combustion. That charged species are expected to exist at the high temperatures of AI combustion can be demonstrated by a local thermodynamic equilibrium calculation. Computations conducted using the ISP Thermodynamic Equilibrium Code [34] for a stoichiometric Al-air system (neglecting thermionic emission) in the temperature range covering the experimental data, show that reasonably high concentrations (order of 109-1012 per cm 3) of A1 +, A I O - ions

E . L . DREIZIN 1014

J

10 la

Z I.-tv" <1""

S

1012 10 z~

~

10 '0

,,"

10 g

/-

Z

o

u

10 e

© AI +

/;/

•

~

10 7 10 e 1500

e

,~ AIOK 2000

, 2500

3000

3500

TEMPERATURE,°C Fig. ]4. Thermodynamic equilibrium ion and electron concentrations in Al-air flame.

and electrons will form at the experimental temperatures (see Fig. 14). These simple estimates illustrate the importance of the spatial charges and electric fields appearing in the A1 particle flames which certainly affect the transport processes governing the combustion dynamics and deserve more attention. Thus, as one can see, many of the essentially important attributes of AI particle combustion might be described taking into consideration ionization processes and electric fields forming in the flame zone. We note, finally, that the method of metal particle formation and ignition, which might result in some initial particle charging, would not be relevant, because at the high electron and ion concentrations existing at metal combustion temperatures, the thermodynamically driven electrostatic equilibrium is established quickly and defines the ensuing system evolution. This highlights the need to specifically address ionization and electro-static driven processes in studying metal particle combustion. CONCLUSIONS An experimental study of the combustion events of uniform AI particles in air has revealed three distinct stages in their combustion histories. Spherically symmetric vapor phase combustion, consistent with the conventional metal vapor-phase burning model, occurs during the first stage. The second stage of AI particle combustion is associated with an in-

ALUMINUM PARTICLE COMBUSTION crease of the size and density of the smoke cloud surrounding the particle, a shift to a nonsymmetric combustion regime, and initiation of particle spinning. The finite content of oxygen builds up in burning particles at this time. The particle temperature is close to the AI boiling point during the first two combuslion stages. In the third combustion stage an "'oxide cap" forms and grows on the burning particle, which continues to spin and bum nonsymmetrically. The particle temperature decreases continuously and the combustion terminates after the experimentally measured tcmperature reaches the AlzO 3 melting point. The transition from the first to the second combustion stage appears to occur when the dissolved oxygen content in the AI droplet reaches the limit needed for liquid AI203 formation (maximum of approximately 14% at.) and droplet temperature attains the threephase (liquid AI-O solution, liquid A1203, and gas) equilibrium point of 2240°C. Rapid changes of the burning particle trajectories observed during the third stage of the combustion are explained by the nonsymmetric growth of "oxide caps" on spinning particles. An external electric field was shown to reduce the total combustion time of AI particles and affect their temperature histories. This work has been managed by the NASA Lewis Research Center under Contract No. NAS3-27259. The support and encouragement of Mr. R. Friedman, the Contract Technical Monitor, is greatly appreciated. The author wish to thank the AeroChem research staff, and, in partitular, Drs. tl. F. Calcote, C. H. Berman, and D. G. Keil, for many helpful discussions and advice. The help of Mr. O. P. Andersen in sample preparation and Dr. E. Vicenzi of Princeton Materials Institute in SEM sample analysis is also appreciated. REFERENCES I. Friedman, R., and Macek, A., Combust. Flame 6:9-19 (1962). 2. Brzustowski, T. A., Combust. Flame 8:339-340 (1964). 3. Bartlett, R. W., Combust. Flame 8:341-342 (1964). 4. Drew, C. M., Combust. Flame 9:205-208 (1965).

555 5. Prentice, J. L., (Ed.), Metal Panicle Combustion Progress Report, NWC TP 4435, Naval Weapons Center, China Lake, CA, 1968. 6. Prentice, J. L., (Ed.), Combustion of Pulse-Heated Single Particles of Aluminum and Beryllium, NWC TP 5162, Naval Weapons Center, China Lake, CA, 1971. 7. Wilson, R. P., and Williams, F. A., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1971, pp. 833-845. 8. Derevjaga, E. M., Stesik, L. I., and Fedorin, E. A., Fizi. Gore. Vzryva 16:6-10 (1977) (in Russian). 9. Ermakov, B. A., Razdobreev, A. A., Skorik, A. I., Pozdeev, V. V., and Smolyakov, S. S., Fizi. Gore. Vzryva 18:141-143 (1982) (in Russian). 10. Boborykin, B. M., Gremyachkin, V. M., Istratov, A. G., Kolesnikov-Svinarjov, B. I., Kuznetsov, G. P., Leypunsky, O. I., and Puchkov, V. M., Fizi. Gore. Vzryva 19:22-30 (1983) (in Russian). 11. Drew, C. M., Gordon, A. S., and Knipe, R. H., Heterogeneous Combustion (H. G. Wolfgard, I. Glassman, and L. Green, Eds.) 15:17-40 (1964). 12. Suslov, A. V., Dreizin, E. L., and Trunov, M. A., Combustion Explosion Shock Waves 26:394-396 (1991). 13. Dreizin, E. L., Suslov, A. V., and Trunov, M. A., Combust. Sci. Technol. 87:45-48 (1992). 14. Dreizin, E. L., Suslov, A. V., and Trunov, M. A., Combust. Sci. Technol. 90:79-99 (1993). 15. Brzustowski, T. A., and Glassman, I., Heterogeneous Combustion (H. G. Wolfhard, I. Glassman, and L. Green, Eds.) 15:75-116 (1964). 16. Dreizin, E. L., and Trunov, M. A., Combust. Flame 101:378-382 (1995). 17. Dreizin, E. L., in Chemical and Physical Processes in Combustion, 1994 Fall Technical Meeting, The Eastern States Section of The Combustion Institute, Clearwater Beach, FL, 1994, pp. 51-54. 18. Law, C. K., Combust. Sci. Technol. 7:197-212 (1973). 19. Law, C. K., Combust. Sci. Technol. 12:113-124 (1976), 20. Florko, A. V., Kozitskii, S. V., Zolotko, A. N., and Golovko, V. V., Fiz. Gore. Vzryva 19:24-29 (1983) (in Russian). 21. Golovko, V. V., Kozitskii, S. V., and Florko, A. V., Fiz. Gore. Vzryva 21:27-32 (1985) (in Russian). 22. Smithells, C. J. Smithells Metals Reference Book, 7th ed. (E. A. Brandes and G. B. Brook, Eds.), Butterworth-Heinemann, 1992, p. 290. 23. Suslov, A. V., and Dreizin, E. L., Soviet Powder Metallur. Metal Ceramics 29:939-943 (1990). 24. Suslov, A. V., Dreizin, E. L., and Trunov, M. A., Powder Technol. 74:23-30 (1993). 25. Pearse, R. W. B., and Gaydon, A. G., The Identification of Molecular Spectra, Halsted, New York, 1976, p. 41. 26, Dreizin, E. L., and Andersen, O. P., Droplet Welding, A New Technique For Welding Electronic Contacts, AeroChem TP-530, AeroChem, Princeton, NJ, 1994. 27. Suslov, A. V., Dreizin, E. L., and Trunov, M. A., Fizi. Gore. Vzryva 4:138-140 (1991) (in Russian).

556 28. Lide, D. R. (Ed.), CRC Handbook of Chemistry and Physics, 71st ed., CRC Press, Boca Raton, FL, 1991. 29. Shafirovich, E. Ya., and Goldshleger, U. I., Combust. Flame, 88:425-432 (1992). 30. Goldsmith, A., Hirschhorn, H. J., and Waterman, T. E., Thermophysical Properties of Solids, WADC Technical Report 58-76. Vol. III, US Air Force Wright-Patterson AF Base, OH (1960). 31. Ranz, W. E., and Marshall, W. R., Chem. Eng. Prog. 48:141-152 (1951).

E.L.

DREIZIN

32. JANAF Thermoehemical Tables. J. Phys. Chem. Ref. Data. 14(SI):65 (1985). 33. Levinskiy, Y. V., P-T-XBinary Phase Diagrams of Metal Systems, Moscow, Metallurgia, 1990 (in Russian). 34. Seiph., C., and Hall, R., ISP Thermodynamic Equilibrium Code, Air Force Astronautics Laboratory, Edwards AFB, CA (continuously updated).

Received 11 May 1995; revised 3 October 1995