Self-ignition and ignition of aluminum powders in shock waves

Shock Waves (2002) 11: 289–295

Self-ignition and ignition of aluminum powders in shock waves V.M. Boiko, S.V. Poplavski Institute of Theoretical and Applied Mechanics, Russian Academy of Sciences, Institutskaya 4/1, 630090 Novosibirsk, Russia Received 4 December 2000 / Accepted 30 May 2001 Abstract. Ignition of fine aluminum powders in reflected shock waves has been studied. Two ignition regimes are found: self-ignition observed at temperatures higher than 1800 K and “low-temperature” ignition at temperatures of 1000–1800 K. The possibility of initiating the ignition of aluminum powders in air using combustible liquids has been studied too. Key words: Aluminum particle-gas mixture, Reflected shock wave, Ignition

1 Introduction Ignition of aluminum powders includes a wide spectrum of problems which have been attracting the attention of researchers for a long time both at the level of aluminum oxidation kinetics and from the viewpoint of external characteristics of the process such as critical conditions, ignition delays, and effective energies of activation. Many of these questions have been studied quite extensively, but there are also many unsolved problems. In connection with the expanding use of aluminum powders, this topic has not lost its importance up to the present time. Namely, aluminum is used as an admixture for rocket propellants and in technological processes and as a reagent in high-temperature synthesis of new materials. In addition to individual features of aluminum dust, there is a global problem connected with the explosion hazard of gas-dust mixtures of metal and organic powder materials in general. In this aspect, the safety of aluminum-based powder technologies demands the research of external characteristics and ignition mechanisms under conditions typical of explosion processes. The peculiarities of high-temperature oxidation of Al are caused by the presence of an oxide shell Al2 O3 whose diffusional resistance is several orders higher than that of the gas. It is the initial characteristics of Al2 O3 and their change in the process of particle heating that determine the period of induction and the critical ignition temperature. Large periods of ignition induction are typical of Al powders, which does not allow full realization of their energetical potential. Great attention is paid to intensification of the aluminum ignition processes. For example, it is material pulverization to fine and ultra-fine fractions. Various methods of influence on powders or the ambient gas medium are also used. The first method is the use of different coatCorrespondence to: V.M. Boiko (e-mail: [email protected])

ings, alloying, or amalgamation to decrease or eliminate protective features of the oxide shell (Zolotko et al. 1980). The second approach implies thermal influence on the gas medium with a suspension of powder hard to ignite. This may be reached using mixtures of powders of hard-toignite and combustible metals (Yagodnikov and Voronetski 1997) or mixtures including combustible liquids (CL) and metal powders (Boiko and Poplavski 1998). The most part of data on Al ignition delays tign is obtained under steady conditions - in heated quartz tubes, in burners, etc. These researches allow one to reveal the main kinetic regularities of oxidation and to find the critical temperatures of ignition. The most reliable data on the critical temperature T ∗ are derived for large, easily controlled single particles. In the work of Derevyaga et al. 1977, upon inductive heating of particles with d = 3–4 mm and the heating rate of 380–700 K/sec, it is established that T = 2050 K. In the work of Ermacov et al. 1982, upon heating of spherical particles with d = 0.4–1.2 mm in a CO2 -laser beam with the heating rate ∼ 104 K/sec, T = 2070 ± 50 K was obtained. The microfilming shows that ignition is related not to melting of the oxide shell (2318 K) but to the rupture of its integrity due to thermalmechanical tensions during heating. Concerning the small particles, a detailed control of their initial state is difficult; the sample is characterized statistically with a number of undetermined parameters. In this connection, a wide scatter of the studied parameters is observed. Thus, in the experiments with spherical particles (d = 15–65 µm) carried out on an installation with a gas burner (Friedman and Macek 1962), it was found that T ∗ is close to the melting point of the Al2 O3 oxide and practically independent of d. According to the data Gurevich et al. 1970 and Gurevich et al. 1978 obtained for particles with d ≤ 50 µm on an argon – arc burner, T depends on the particle size and has a minimum ∼ 1000–1300 K at d ∼ 6 µm, which is considerably lower than the melting point of Al2 O3 . It is the opinion

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V.M. Boiko, S.V. Poplavski: Self-ignition and ignition of aluminum powders in shock waves

of Gurevich et al. 1970 that the determining role in this process belongs to crystal-structure changes in the oxide coating. As a result, the concentration of defects in the protective layer and its diffusion penetrability increase. In two-phase systems like suspensions of fine particles in a gaseous oxidizer, the ignition processes behind the shock wave (SW) are more complicated because, along with chemical reactions of oxidation, there is a complex of physical-mechanical processes responsible for preparation of a combustible mixture to ignition and supporting subsequent burning. Relatively few works deal with the study of ignition characteristics of Al dust-gas mixtures in SW. For instance, it is established that T ∗ does not exceed 1300 K for Al particles (d = 15–20 µm) in oxygen behind a reflected SW (Borisov et al. 1984), which does not contradict the data under steady conditions (Gurevich et al. 1970) but does not agree with the data for large particles. This is a typical example of studying the external characteristics of the process without controlling the state and behavior of separate particles. The main purpose is to establish a tendency of heterogeneous mixtures to detonation. It is determined from a comparison of the ignition delay tign in the latter with tign for gas mixtures taken as reference data. Though such tests are important for explosion safety of powder technologies, they permit to study neither the determining parameters, nor the mechanism of low-temperature ignition of small Al particles. In the present work, results of an experimental research of ignition and combustion of Al powders behind a reflected SW at the temperature T = 1000–2000 K and pressure P = 1–3 MPa obtained with the help of highly informative methods of photographic record are described.

2 Experimental facility The experiments were carried out in a shock tube equipped with devices for the optical visualization of the shock wave processes in two-phase media, for measuring the pressure profiles and the shock wave velocity. Driver and driven sections were 2.4 and 5 m long, respectively; the channel cross section was 52 × 52 mm2 . The pressure of the driver gas (helium) and the driven gas (oxygen, air) were, respectively, 2.5–10 MPa and 0.01–0.1 MPa. The SW Mach range was M = 2.3–4.5. The gas parameters behind the SW were determined through the measured Mach number M taking into account the temperature dependence of the specific heat ratio (Lapworth 1970). A manufactured aluminum powder with the form of particles close to spherical was used. The powder composition was 99.2% Al, 0.2% Fe, 0.2% Si, and 0.02% H2 O. The initial powder was divided into narrow fractions. The boundaries of separation were determined through particle sizes with equal probability of their location both in large and small fractions. The fractions with particle sizes of 3–5, 10–14 and 14–20 µm were used. The samples of powders with the mass m varied from 0.2 to 20 mg were introduced into the channel either with the help of an electromagnetic striker placed at a distance of 10 mm from the reflecting wall, or through placing the

powder on a thin substrate at a level of the channel axis 10–40 mm from the reflecting wall. The powder spraying and gas-dust mixture formation occurred behind the incident SW; the ignition took place behind the reflected SW. The mixture ignition was registered with the help of a high-speed camera in the regime of multi-frame shadow laser visualization and in the regime of photochronograph of flame radiation (streak camera). It allowed registration of both time and space characteristics of the process. The radiation of burning particles was registered through a glass window 4 × 100 mm. A laser stroboscope was used for synchronization of the process with the moment of SW reflection from the wall and also for introduction of the time scale. The images of light marks are seen in the upper part of the pictures made with the photochronograph (Fig. 1 is a typical streak picture). The intervals between the light pulses were ∆t = 100 ± 0.1 µs. The first pulse was formed 100 µs after SW reflection from the wall. The ignition delay was determined as the residence time the ignition site in the high-temperature area behind the reflected SW before luminescence appearance.

3 Self-ignition of aluminum dust-gas mixtures in shock waves Self-ignition is understood as regimes studied previously by other authors under static conditions but reproduced by us in shock waves for simulation of explosion processes. Figure 1 shows streak pictures of Al powder ignition with different masses of the sample. In the late stage of burning, separate tracks of large particles are clearly seen. In any case, the volume glow without visible tracks corresponds to ignition beginning. This means that the self-ignition of a sprayed powder begins with the smallest particles but even for a comparatively narrow fraction composition of the sample, large particles are identified in the streak picture and considerably affect the total burning period. When m rises from 0.25 to 1 mg, the total burning period increases significantly, which is associated with ignition of progressively large particles of this fraction. The latter is obviously caused by the igniting affect of small particles. The further increase of m from 1 to 5 mg does not change the type of glow intensity distribution in time. These results show also that the ignition delay of a polydispersional sample does not depend on its mass. The data for self-ignition delays of Al powders versus the oxygen temperature behind the reflected SW are given in Fig. 2 (1). Curve (3) is calculated as the period of particle heating (the average diameter of the particle is d = 4 µm) up to some critical temperature T ∗ ∼ 1800 K (Friedman and Macek 1962): T − T0 ρd2 H c ln tign = , + 12λ T − T∗ T − Tm where ρ = 2.7 × 103 kg/m3 is the particles density, λ = λ0 (T /T0 )0.75 is the coefficient of thermal conductivity of the gas, λ0 = 2.4 × 10−2 J/(m·sec·K), T0 = 290 K is the


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Fig. 1a–c. Ignition and combustion of Al powder in oxygen for different sample masses. T = 1900 K, P = 1.1 MPa; a – 5 mg, b – 1 mg, c – 0.25 mg

4 Ignition of aluminum powders in the SW Low-temperature ignition means a regime that do not agree with the data of other authors obtained under static conditions but occur under conditions of shock-wave experiments. The point is that the effect of ignition of single Al particles at medium temperatures considerably lower than the critical T ∗ ∼ 1800 K was found in the described experiments (see Fig. 2, points 2). Figure 3 shows typical streak pictures of this process. Let us note its characteristic features: – a small part of ignited particles from the total sample mass, and from experiment to experiment this part is not equal (for example, from the sample 5–7 mg (Fig. 3a) only a few dozen of particles are ignited and their estimated mass is ∼ 10−5 mg); – simultaneous ignition of particles of various diameters including relatively large ones; – poor recurrence of the results, namely, a wide scatter of ignition delays under identical experimental conditions (0.4–2 msec); – separate ignited particles may result in the ignition of the whole gas-dust cloud (Fig. 3b).

Fig. 2. Ignition delays of Al powders in oxygen versus the oxygen temperature: (1) – self-ignition; (2) – ignition; (3) – calculation

initial particle temperature, Tm = 933 K is the melting point, c = 1.01 kJ/(kg·K) is the average heat capacity in the temperature range from T0 to T ∗ , and ∆H = 4 × 105 J/kg is the melting heat. The data presented above for self-ignition of fine particles in the SW are in excellent agreement with the results of static tests for large particles.

It is found that one of the conditions of the igniting effect of abnormal particles on the entire sample is a high concentration of the gas-dust suspension, but the nature of abnormal particles is not known. Moreover, a few mechanisms of abnormal ignition can exist. In this case, not the averaged parameters of the system but attendant facts may become important, such as defects of the particle surface and shape, the presence of a small share of alien particles, local overheating of the medium, etc. A comparison of data for Al ignition delays obtained by Borisov et al. 1984 with the results presented here shows that they also may be explained through the regime of “abnormal” ignition. In our opinion, this is the main rea-

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Fig. 3a,b. Streak pictures of low temperature ignition of Al powder for different sample masses: T = 1370 K; a – m = 7 mg, b – m = 10 mg

son for the difference between the critical ignition temperatures of large particles (Derevyaga et al. 1977; Ermacov et al. 1982) and fine fractions (Gurevich et al. 1970; Borisov et al. 1984). The effect of abnormal “low-temperature” ignition of separate particles might be interesting from the academic point of view if, under certain conditions, it did not initiate ignition of the whole sample. This circumstance simply expands the temperature range of aluminum dust ignition; as for danger, it levels off the phenomena of abnormal ignition with regular regimes.

Fig. 4a,b. Streak pictures of ignition of the Al + CL mixture; T = 1200 K; a – in oxygen, b – in air

5 Intensification of the processes of aluminum powder ignition The possibility of initiating “low-temperature” ignition of aluminum powders by small additives of combustible liquids (CL) is studied as one of the ignition peculiarities in the SW. It is possible to assume that, due to wide diversity, combustible liquids will allow one to affect the characteristics of aluminum powders ignition in wider limits. Tridecane (n − C13 H28 ), isopropylnitrate (C3 H7 NO2 IPN), and a mixture of diesel fuel (DF) with promoting admixtures (Boiko and Poplavski 1999) were used in the present work. The influence of CL on pre-ignition heating of metal particles in the reflected SW was studied stage by stage beginning from concentrations typical for suspensions with a further decrease in the liquid share in the system down to ϕ < 10−2 when only a small part of the sample is wetted with the CL. A drop of the suspension with a constant CL mass of ∼ 2 mg was put in a cup fixed on a fine tungsten wire at the level of the axis of the shock tube channel at a distance of 70 mm from the reflecting wall. Mixture spraying occurred in the incident SW, as it happened in the case with pure powders, and ignition was observed in the reflected SW. Figure 4 shows typical streak pictures of this process. The experiments have shown the following: – with CL present, Al is ignited at lower temperatures than the critical one (1800 K) both in oxygen (Fig. 4a) and in air (Fig. 4b);

Fig. 5a–c. Streak pictures of ignition of an Al powder layer (fraction 10–14 µm) behind the reflected SW in oxygen; P = 3.4 MPa, T = 1100 K; a – pure Al powder; b, c – Al + isopropyl-nitrate for the distance between the drop and the reflecting wall L = 5 mm (b) and L = 50 mm (c)

– the ignition delays of the suspensions are determined by the liquid, and the metal particles are ignited later; – the duration of suspension burning is longer than the corresponding period for a drop of a pure liquid and are determined by dispersion and amount of powder in the drop. A more detailed description of the ignition processes of metal powders, pure CL and promoting admixtures, as well as hybrid fuels can be found in the papers Boiko et al. 1989, Boiko et al. 1991, Boiko and Poplavski 1998, Boiko and Poplavski 1999. To summarize the facts presented, let us note the following:


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Fig. 6a–c. Streak pictures of ignition and combustion of Al powder (3–5 µm) in a mixture with CL for various ratios of the solid and liquid components ϕ = mAl /ml ; ml = 7 mg. T = 1500 K; a – ϕ = 0; b – ϕ = 1.5 for mAl = 10 mg; c – ϕ = 10 for mAl = 100 mg

– in spite of the fact that the masses of the liquid and metal powders are comparable, the influence of metals on CL ignition delays is not observed; – upon transition from oxygen to air, the suspension ignition delay increases (by ∼ 3 times) and the type of burning changes from explosion (in oxygen) to deflagration (in air); the same behavior was noted previously for pure CL (Boiko et al. 1991). The insensitivity of CL ignition parameters to the presence of a considerable mass of the solid component is likely to be connected to the peculiarity of suspension drop destruction in the incident SW. In fact, a three-phase fourcomponent system is formed at the moment of CL ignition. The burning in it is developed as follows: – CL burning noticeably increases the gas temperature, which favors the heating of solid particles; – after CL ignition, the temperature of some share of metal particles (the smallest ones) rises up to the critical value T ∗ and their ignition occurs; – due to burning of small particles, the medium temperature increases, which results in further heating of the total mass of metal particles (up to the same T ∗ ) and their ignition. The CL insensitivity to the presence of the solid component in the course of suspension ignition means that the mixture formation (drop destruction, micro-spray evaporation) and also the ignition of the suspension micro-spray follows the same mechanism as pure liquids. The above scheme of the process implies that CL is present in the

micro-spray both as drops of a pure liquid and as a film on particles, but the latter does not affect liquid phase ignition. After CL ignition, particle heating follows the heat transfer mechanism, which is limited by the thermal conductivity of combustion products. The correctness of this approach is confirmed by special experiments on ignition of solid and liquid components, which were not mixed beforehand (Boiko and Poplavski 1998). In this case, an Al powder sample was located as described before, and there was a drop of an initiating liquid 5 mm upstream. Other methods of separate disposition of the components were also used. These experiments show that the wetting of the powder with a liquid fuel as well as the method of separate disposition of these components does not influence considerably the ignition delay and the burning type in the system. In the present work, a series of experiments on ignition of low-temperature Al powders was also performed with powders arranged as a thin layer on the bottom wall of the channel. A small share of CL (∼ 1 mg) was put on the powder layer at various distances L from the reflecting end. The liquid wetted a small share of the dust sample, and the rest sample remained dry. Figure 5 shows typical streak pictures of ignition of the Al powder layer (fraction 10–14 µm) in oxygen behind the reflected SW for M = 2.6, P = 3.4 MPa, and T = 1100 K. In Fig. 5a, ignition of pure Al powder is registered; Fig. 5b,c illustrates the process initiated by IPN at different location of the drop. Apart from the fact of Al ignition and stable burning, these streak pictures allow us

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to state that the point of ignition is in the initial location of the initiating admixture. As was shown above, the method of disposition of the components (different location of the drop and the powder, the powder/liquid mixture put on a flat substrate or in a cup) does not affect noticeably the ignition delays and burning type in the system. An exception could be the case of the powder/liquid mixture with a great excess of the solid phase (Al particles wetted by CL). With a small diameter of the particles and a small thickness of the liquid film, such a complex may happen to be very stable to breakage. The mechanisms of liquid fuel vapors entering the gas phase will be limited by evaporation from some fixed surface without drop destruction with its typical increase in the effective liquid surface. A quantitative estimation of the ratio of the masses of the solid and liquid components of a hybrid fuel is presented for the initial state of the mixture when the powder has a packet density and the CL fills the pores between the particles. Thus, we consider that the liquid excess may occur behind the SW as a microspray, if the liquid volume exceeds the total volume of the pores. It is known that the relative volume of the pore space is ∼ 0.3 for the most dense packing of a monodisperse powder; the polydispersity may both increase and decrease this value. The average mass concentration of the liquid in the pores is assumed to be ∼ 0.3(ρAl /ρl ). Since we have ρAl /ρl ∼ 3 for Al and liquid hydrocarbons, the mass concentration of the liquid is ∼ 0.1 and free liquid will be obviously present in the microspray if the components have comparable masses. It is clear from the aforesaid why hybrid mixtures are studied mainly with comparable masses of the components though some experiments were carried out with powder excess. According to the results of liquid fuel researches (Boiko and Poplavski 1999), the following mixture was chosen for low-temperature ignition of Al powders in air: 70% of diesel fuel + 30% of alcohol nitrate. Ignition delays of this mixture are well approximated by the dependence tign = (6 × 10−6 ) exp(3.6 × 104 /RT )[sec]. Figure 6 shows streak pictures of ignition and burning of Al powder (3–5 µm) with this CL mixture for various ratios of the solid and liquid components ϕ = mAl /ml . In these experiments, we had ml = 7 mg. A considerable increase in the burning duration testifies to Al ignition (see Fig. 6b). The streak picture in Fig. 6c was obtained for ϕ = 14, which correlates with the “film-like” presence of the CL. Figure 7 shows the ignition delays of Al powder/CL mixtures as functions of the air temperature behind the reflected SW for various ratios of the masses of the solid and liquid components. It is seen that points 1 and 2 are grouped around line 5 corresponding to the equation tign = (1.4 × 105 ) exp(3.6 × 104 /RT ) [sec]. For these mixtures, we have Ea = 36 kJ/mole, which is close to the same value for the liquid component (line 6). This suggests that the conditions of generation of a combustible mixture behind the incident SW are practically identical for hybrid and pure liquid fuels, at least up to ϕ ≤ 1.5.

Fig. 7. Ignition delays of Al powder mixtures with CL versus the air temperature for various ratios of the masses of the solid and liquid components. ϕ = 0.3 for mAl = 10 mg (points 1); ϕ = 1.5 for mAl = 10 mg (points 2); ϕ = 10 for mAl = 100 mg (points 3); curves 4 and 5 are approximation for the data of (3) and (1,2), respectively; curve 6 refers to pure CL (70% diesel fuel +30% Cn H2n+1 ONO2 : n = 6–11)

With an increase in the solid component concentration in a hybrid fuel, physical conditions of formation of a combustible mixture change and the energy Ea increases. Thus, for ϕ = 10 the ignition delays are approximated by the equation tign = (1.2 × 10−9 ) exp(17.3 × 104 /RT ) [sec]. The value of Ea approaches the value typical for hydrocarbon fuel vapors such as a mixture of methane and oxygen diluted by nitrogen (187 kJ/mole) (Zellner et al. 1983).

6 Conclusion Ignition of fine aluminum powders in reflected shock waves has been studied. Two ignition regimes are found: selfignition observed at temperatures higher than 1800 K and “low-temperature” ignition at temperatures of 1000– 1800 K. A high concentration of the gas-dust mixture is one of the conditions of “low-temperature” regime. The possibility of initiating the ignition of aluminum powders in air using combustible liquids with promoting admixtures has been studied. The mixture of diesel fuel and alcohol nitrate is chosen with the indices “less than the least” either in the effective activation energy or in ignition delays in the range T = 1000–2000 K. It is shown that, if the share of powder is increased up to comparable


masses of the solid and liquid components, the ignition delays are determined by the liquid fraction with the effective activation energy typical of a pure gas-drop mixture behind the SW, and the burning period is determined mainly by the disperse composition of the solid phase. Acknowledgements. The work was supported by the Russian Foundation for Basic Research (Grant No. 99-01-00587) and by INTAS (Grant No. 97-2027).

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