Aluminum Dust Cloud Research Articles

Investigation of niacin and aluminum dust cloud ignition characteristics in an explosion hazard testing device using high-speed imaging

Experimental investigations of dust explosions in standard industrial testing equipment such as the MIKE3 minimum ignition energy (MIE) testing device are promising ways of producing fundamental insights into dust cloud ignitability. However, advanced experimental methods must be developed to characterize the dust cloud ignition and combustion behavior of various types of dust. In this work, high-speed broadband and species-specific chemiluminescence imaging diagnostics are implemented to explore similarities and differences between organic and metal powder ignition kernel development. A selected set of 600-mg niacin (ϕ = 2.9) and aluminum (ϕ = 1.6) powder samples were dispersed in the air and ignited using a high-voltage spark inside a standard MIKE3 device. The resulting broadband flame emission was recorded at 4 kHz using a high-speed camera for the first 10 ms after the spark. For the niacin sample, species-specific chemiluminescence emissions from excited-state hydroxyl (OH*) and methylidyne (CH*) radicals were also recorded at 4 kHz and 1 kHz, respectively. The flame kernel in each image frame was detected using an intensity thresholding algorithm and tracked throughout the video sequence, yielding quantitative size, position, and velocity measurements of the evolving flame kernel during the first 10 ms after the ignition spark. For the niacin sample, a continuous and non-uniform reaction zone composed of burning particle clusters and excited-state radicals was observed. The niacin flame kernels grew from 5 mm to 17 mm with an initial velocity of 5 m/s. Conversely, an intensely bright reaction zone and discrete burning particles near the flame kernel boundary were observed in the aluminum sample. The aluminum flame kernels grew from 7 mm to 10 mm with an initial velocity of 3 m/s. The niacin and aluminum flame kernels traveled 7–11 mm away from the central spark gap and slowed down to 1 m/s by the end of the 10-ms period. This time-resolved imaging study, when coupled with previously reported three-dimensional particle and flow field data prior to the arrival of the ignition spark, sets the foundation for an improved multi-physics understanding of the initiation of dust cloud ignition and explosion processes.

Investigation of aluminum dust cloud dispersion characteristics in an explosion hazard testing device using laser-based particle and flow diagnostics

Monitoring the dust cloud dispersion process inside testing equipment like the MIKE3 minimum ignition energy device is useful for assessing the ignitability characteristics of different dust materials. However, it is difficult to comprehensively capture the relevant micro-scale particle properties and macro-scale flow behavior of a dust cloud dispersion using a single measurement method. Thus, the objective of this work is to combine two complementary laser diagnostic techniques to obtain quantitative particle and flow properties from dust clouds in the MIKE3 glass tube. For this experiment, thirty repeated runs for each method were conducted using small (15-mg) aluminum powder samples. No ignition electrodes are included in the test geometry to establish a baseline flow configuration for the glass tube, although measurements are still made at the characteristic time delay and location of ignition tests. The first technique, digital in-line holography (DIH) is used for three-dimensional micro-scale particle diagnostics, yielding particle number (n = 78 ± 20), size (D̅ = 18.9 ± 0.6 µm), and three-component velocity (v̅x = 0.05 ± 0.05 m/s, v̅y = 0.60 ± 0.09 m/s, v̅z = − 0.07 ± 0.16 m/s) measurements. The second laser diagnostic technique, particle image velocimetry (PIV), is used for macro-scale flow diagnostics, yielding two-dimensional flow velocity and vorticity vector field measurements. Particle sizes are observed to follow a lognormal distribution (μ = 2.89, σ = 0.31), and a discrepancy between in-situ and ex-situ sizing results is identified for particle sizes larger than ~55 µm. Two-dimensional particle velocities follow a Weibull distribution (θ = 0.86, β = 2.12), and reasonable agreement is found between the DIH and PIV velocity measurements.

A numerical investigation on heterogeneous combustion of aluminum nanoparticle clouds

This study investigated the heterogeneous combustion of nano-sized aluminum dust clouds. A numerical model was developed to capture the combustion properties of aluminum nanoparticle clouds by employing multiphase flow approach and combustion model of nano-aluminum particle. The simulation framework adopts an accurate modeling of heat convection in the transition and free-molecular regimes besides considering inter-particle radiation and surface reaction. The numerical model was validated by comparison against predicted/experimental results. The flame structure of nano-aluminum dust-air mixture has been demonstrated, whose flame thickness is thinner than their micron-sized counterparts. The ignition front propagation speed increases with raising equivalence ratio in the lean mixture and almost maintains a constant under fuel-rich conditions; and it raises with decreasing particle size and increasing oxygen concentration. The oxygen content plays a significant role on flame behaviors than the particle concentration and size. The flame temperature and feedback of heat from reaction zone to preheat zone plays a crucial role in ignition front propagating process.

An investigation on the dust explosion of micron and nano scale aluminium particles

A series of dust explosion were conducted to compare the flame structure between nano and micron aluminium dusts. Two-color pyrometer technique is applied to have qualitative observation of flame development. Measurement of temperature indicates that explosion in micron aluminium dust clouds start in a single spot at 3000 K, in contrast, explosion in nano aluminium dust clouds start when hot powder accumulated to a certain amount at lower temperature of 2600 K. For micron aluminium dust clouds, flame at leading edge has the highest temperature and propagates in all directions. On the other hand, flame in nano aluminium dust clouds propagate only upward with the hottest part left behind at the downside. As flame propagates, the temperature at top edge gradually decreases from 2600 K to finally 2000 K, but temperature at bottom edge maintains in 3000 K with no significant displacement. The unevenness of flame structure is considered as the consequence of different particle densities, which suggests that the reaction of nano aluminium particles stays in molten state, meanwhile, the high surface area also leads to unignorable heat loss.

Theoretical analysis of flame propagation velocity in aluminum dust clouds

Theoretical analysis of flame propagation velocity in aluminum dust clouds

Experiment on the flammability variation of micron-sized aluminum powder depending on combustion environment for burning of aluminum-diluted oxygen

Experimental research was conducted to study the combustion characteristics of clouds composed of micrometer-sized aluminum particles with an oxidizer blend of oxygen and superheated steam. Both ignition and combustion of an aluminum particle are difficult because it is enveloped by an oxide layer that has a high melting point (above 2300 K). To overcome the problem, we induced a particular shape of the combustor that can keep a stable flame of aluminum dust clouds at a lower feeding-rate. The combustion characteristics were examined using a system applied combustor involving a variety of parameters affecting the burning of the dust clouds. Based on these attempts, the aluminum dust clouds had an upper flammability limit above the equivalence ratio (Ф) 1.9 for oxygen diluted by air. Varying the lower limit of flammability, we found, crucially, that the feeding-rate and velocity of the particles are more effective than the equivalence ratio or combustor length. Furthermore, we could confirm the changing of the lower limit from 0.3 to 0.26 because the steam was additionally fed in the combustion of aluminum-oxygen.

Minimum ignition temperatures and explosion characteristics of micron-sized aluminium powder

To forestall, control, and mitigate the detrimental effects of aluminium dust, a 20-L near-spherical dust explosion experimental system and an HY16429 type dust-cloud ignition temperature test device were employed to explore the explosion characteristics of micron-sized aluminium powder under different ignition energies, dust particle sizes, and dust cloud concentration (Cdust) values; the minimum ignition temperature (MIT) values of aluminium powder under different dust particle sizes and Cdust were also examined. Flame images at different times were photographed by a high-speed camera. Results revealed that under similar dust-cloud concentrations and with dust particle size increasing from 42.89 to 141.70 μm, the MIT of aluminium powder increased. Under various Cdust values, the MIT of aluminium dust clouds attained peak value when concentrations enhanced. Furthermore, the increase of ignition energy contributed to the increase of the explosion pressure (Pex) and the rate of explosion pressure rise [(dP/dt)ex]. When dust particle size was augmented gradually, the Pex and (dP/dt)ex attenuated. Decreasing particle size lowered both the most violent explosion concentration and explosive limits.

Minimum ignition temperature of aluminium dust clouds via the Godbert–Greenwald furnace

Aluminium dust is an energetic material widely used in chemical production, industrial manufacturing, construction, the military, and other fields. It not only has dust characteristics but also is highly combustible. In this study, the minimum ignition temperature (MIT) of aluminium dust clouds was investigated using a Godbert–Greenwald (G–G) furnace. A theoretical model for analysing aluminium dust clouds in the hot zone of the G–G furnace was established based on two-phase flow, heat transfer, and chemical reaction kinetic. The kinetic parameters of Ea and A in model were calculated by FWO method. A series of ignition experiments were achieved by G–G furnace. The ignition phenomenon, which initially emitted dazzling white flame and burned fiercely, was recorded by a high-speed camera. According to the comparison between the experimental and theoretical results for the MIT under different conditions, the theoretical model could predict the MIT exceptionally well.

Effect of monoammonium phosphate particle size on flame propagation of aluminum dust cloud

The effect of monoammonium phosphate (NH4H2PO4) particles on 5 μm aluminum dust flames is investigated experimentally and computationally. NH4H2PO4 in three particle size is employed to determine the inhibition efficiency on aluminum flame propagation. Flame inhibition mechanism considering both gas and surface chemistry of aluminum particles is developed. Results show that the inhibition effectiveness monotonously increases as NH4H2PO4 particle size is reduced to 25 μm. Flame morphology and flame microstructure change with the addition of different particle size NH4H2PO4. Small NH4H2PO4 particles within the range studied have a greater reduction in average flame propagation compared to the coarser one. Meanwhile, the fine NH4H2PO4 particles almost decompose completely during the penetration of aluminum flame and then undergo a sufficient chemical interaction with the flame. The simulations indicate that the decomposition products of NH4H2PO4 particles obstruct the oxidation of aluminum particles through flame radical consumption. Additionally, the addition of NH4H2PO4 can reduce the vaporization rate and surface reaction rate of aluminum particles.

Flame suppression mechanism of aluminum dust cloud by melamine cyanurate and melamine polyphosphate.

The suppression effect of melamine cyanurate (MCA) and melamine polyphosphate (MPP) on the flame propagation of aluminum dust were studied experimentally. The results indicated that the concentration of MPP required to supress the aluminum dust explosion was lower than MCA. As the concentration of suppressant increased, the acceleration and the maximum flame speed extremely decreased, and the flame morphology became isolated spot flames. The MPP addition exerted a stronger suppression effect on the temperature of aluminum flame compared to MCA addition. Meanwhile, the mechanism of flame suppression was further investigated. Decomposition of MCA and MPP particles could absorb the heat released from the flame front. Scanning electron microscopy demonstrated that the gas phase reaction of aluminum particles was suppressed by MCA and MPP, resulting in a larger particle size of the explosion products. XRD results indicated that MCA and MPP did not react with aluminum. Gaseous products of suppressant decomposition altered flame chemistry by radically recombining O atom and reducing AlO, which resulted in less amount of heat release, lower flame speed and lower flame temperature.

Flame propagation of micron sized aluminum dust cloud in oxygenated media with different neutralize gas

In this research an analytical study has been conducted to determine flame propagation speed and quenching distance of aluminum dust particle in an oxygenated medium with different neutralized gas including nitrogen, argon, and helium which acts as the oxidizer carrier gas. Flame propagation speed as a function of aluminum dust cloud concentration has been studied based on a thermal diffusion model. Additionally quenching distance for different dust particle concentration in the intended neutralize gas is investigated. Reasonable agreement between the present analytical model and experimental results reported in literature has been observed in terms of flame propagation speed in different dust concentrations.

Flame propagation behaviours in nano-metal dust explosions

Flame propagation behaviours in 40nm titanium, aluminium and iron dust clouds were investigated by high-speed photography. 40nm titanium, aluminium and iron dust flames were all characterized by discrete single glowing burning particles with smoothly spherical front. “Micro explosions” caused by the fragment ejection occurred in both 40nm titanium and aluminium particles burning processes. At nano scales, the heat and mass transfer regimes were absolutely different with micro particles, which resulted in different propagation velocities. The average pulsating flame propagation velocities of 40nm titanium, aluminium and iron dust clouds were 0.565m/s, 0.189m/s and 0.035m/s, respectively. From SEM analysis, it was inferred that 40nm titanium, aluminium and iron particles were burnt in liquid, gas, and solid-phase, respectively. From XPS analysis, it was found that N2 played a significant role in titanium particles combustion. 40nm titanium combustion products contained 43% TiO2 (Ti4+), 27% Ti2O3 (Ti3+), 21% TiO (Ti2+) and 9% TiN (Ti3+). While 40nm aluminium combustion products just contained 100% Al2O3 (Al3+), and 40nm iron combustion products contained 49% Fe2O3 (Fe3+), 26% Fe3O4 (Fe3+/Fe2+), 15% FeO (Fe2+) and 10% iron nonoxides. Inerted by nitrogen for preventing nano‑titanium dust explosions was inadvisable.

A detailed flame structure and burning velocity analysis of aluminum dust cloud combustion using the Eulerian–Lagrangian method

Aluminum particle dust cloud combustion with oxygen is numerically simulated using an Eulerian–Lagrangian two-phase approach. A single aluminum combustion model containing detailed aluminum combustion phenomena (melting, the heterogeneous surface reaction, alumina shell growth, combustion heat distribution, etc.) is suggested and applied to a three-dimensional, compressible CFD in-house code. High order space discretization and field variable interpolation for particle technics are employed to reduce grid dependency. Flame propagation is simulated in a slip-wall bounded open duct with well distributed aluminum particles in an O2–N2 mixture. The flame structure shows a flame zone that is longer than the pre-heating zone. This is unlike a gas phase flame, and it is due to the long combustion time needed for solid particles. The temperature behaviors of the gas and particle phase are captured reverse as the temperature rises. The dust cloud combustion model is validated by comparing burning velocities with the Bunsen burner experimental data in various oxygen and dust concentrations. Additionally, flame structure analysis is accomplished for better understandings on the behaviors of burning velocity in different conditions.

Explosion severity of micro-sized aluminum dust and its flame propagation properties in 20 L spherical vessel

Using 20L explosion vessel, the present research work reports some experimental results which elucidate the effects of dust concentration, particle size and specific surface areas on aluminum dust explosion severity. Dust flame propagation property and explosion products were analyzed systemically. Six kinds of aluminum powders with different size distributions were selected and specially prepared for explosion tests. Results show that explosion parameters present an increasing and then decreasing trend with dust concentrations. The optimum explosion concentrations for all the selected aluminum dusts are similarly equal to 500g/m3. Maximum explosion pressures would be quadratic correlated with the decreasing of particle size. However, (dP/dt)max would be exponentially increasing with particle size decreasing. Aluminum dust flame propagation speed would be increased and the combustion mechanism would be transited from diffusion-controlled mode to kinetically controlled mode with particles diameter decreasing. For the finer dust cloud (dS less than 10μm), dust flame propagation speed would be exponential increased with dust concentration. However, there is a power function relation between flame propagation speeds and dust concentration for the coarse aluminum dust cloud (dS larger than 10μm). At the experimental condition, flame propagation speed presents an exponentially decreased trend with the increasing of particle size. Explosion products obtained at different conditions present different apparent morphology. The higher explosion pressure, the finer explosion fragments and nano-scale spherical structure would be produced. For coarse aluminum particles, explosion products present fluffy flocculent structure. Furthermore, alumina produced under lower explosion pressure conditions are mainly γ-Al2O3 components. Under high explosion pressure condition (such as 0.56MPa), there are notable bulges corresponding to α-Al2O3 are produced. Present research results may have important significance for understanding the explosion mechanism of aluminum-air dust/air cloud.

Modeling Counterflow Combustion of Dust Particle Cloud in Heterogeneous Media

AbstractThis study investigates the counterflow combustion of an aluminum dust cloud in a heterogeneous system of discrete heat sources. A numerical thermal model was developed to estimate the burning velocity at different concentrations of the dust cloud and oxidizer. The proposed model considers the effects of heat transfer mechanisms, and the calculation of the flame speed is based on a point-source approach. The model equations were derived and solved first to study the single-particle combustion. Later, to investigate the dust cloud combustion, the superposition principle was applied to include the effects of particles. The burning velocity and flame stabilization point were studied at different particle diameters, and the effect of counterflow strain rate was analyzed as well. Moreover, the minimum ignition energy was obtained as a function of dust cloud concentration. The predicted results for the burning velocity were in reasonable agreement with experimental data from the literature.

Reaction-diffusion fronts of aluminum dust cloud in a system of random discrete sources

Combustion of micron-sized aluminum dust particles was studied numerically in an environment with spatially discrete sources distributed in a random way. A thermal model was employed and developed to estimate flame propagation speed in a lean quiescent reaction medium. The model is based on conduction heat transfer mechanism. The random distribution of heat sources in a discrete environment is described by using the Gaussian random distribution function with different variance numbers. Flame propagation speed is obtained by using the one-dimensional diffusion-reaction equation which includes the source terms. Oxygen is considered as the main oxidizer and nitrogen is considered as the inert gas. Flame propagation speed, the lean flammability limit and the minimum activation energy were investigated as a function of dust concentration and particle diameter. The predicted results were compared with the experimental data and were found to be in a reasonable agreement. It can be concluded that the random model shows better predictions as compared to the uniform model.

A comparative study of different burning time models for the combustion of aluminum dust particles

In the present paper combustion of micron-sized aluminum dust cloud has been studied in a quiescent reaction medium with spatially discrete sources. A new thermal model is generated to estimate the flame front speed in a lean reaction environment in different oxidizer concentrations. Different burning time models for aluminum are utilized in the generated thermal model to compare their applicability by using the existing experimental data. The model is based on conduction and radiative heat transfer mechanisms using the heat point source method. The combustion of single-particle was first studied and the solution is presented. Then the dust combustion was investigated using the superposition principle to include the effects of inter-particles. The flame propagation speed as a function of aluminum dust concentration for various particle diameters is studied and the effects of radiation heat transfer are taken into account in the governing equations. In addition, flame speed in different types of oxidizers such as air, carbon dioxide, and water vapor is investigated and nitrogen is considered as the inert gas. A reasonable agreement between the results of the numerical solution of combustion of an aluminum dust cloud and the experimental data was obtained in terms of flame propagation speed. It is found that the value of flame speed of combustion of aluminum dust particles in carbon dioxide reaction medium in a considered dust concentration and particle size, is the lowest compared to the other oxidizers.

Laminar Flame Speed Prediction in Lean Mixture of Aluminum Dust Clouds by Considering the Effect of Random Distribution of Particles in Two-dimension

In the present study, the effect of random distribution of reactants and products on laminar, 2D and steady-state flame propagation in aluminium particles has been investigated. The equations are solved only for lean mixture. The flame structure is assumed to consist of a preheat zone, a reaction zone and a post flame zone. It is presumed that in the preheat zone particles are heated and reaction does not exists. Then energy conservation equations of laminar flame had been solved two-dimensionally for different zones and algebraic equations of flame speed are obtained then a code was developed for modeling random situation of the combustion. Finally gas temperature distribution in different flame zones in the channel and also flame speed changes in terms of particles diameter, equivalence ratio, and channel width in random media are presented and compared with previous experimental and theoretical results. The gained results show a reasonable compatibility with the existed theories and experimental data

Analysis of Aluminum Dust Cloud Combustion Using Flame Emission Spectroscopy.

In this study, aluminum flame analysis was researched in order to develop a measurement method for high-energy-density metal aluminum dust cloud combustion, and the flame temperature and UV-VIS-IR emission spectra were precisely measured using a spectrometer. Because the micron-sized aluminum flame temperature was higher than 2400 K, Flame temperature was measured by a non-contact optical technique, namely, a modified two-color method using 520 and 640 nm light, as well as by a polychromatic fitting method. These methods were applied experimentally after accurate calibration. The flame temperature was identified to be higher than 2400 K using both methods. By analyzing the emission spectra, we could identify AlO radicals, which occur dominantly in aluminum combustion. This study paves the way for realization of a measurement technique for aluminum dust cloud combustion flames, and it will be applied in the aluminum combustors that are in development for military purposes.

Freely-propagating flames in aluminum dust clouds

The free propagation of isobaric flames through aluminum dust clouds is investigated in an extensive series of experiments using two facilities with different scales. In small-scale laboratory experiments, spherical flame propagation occurs in aluminum dust clouds contained within 30-cm-diameter latex balloons, whereas in large-scale tests, flames propagate vertically through unconfined aluminum dust clouds with a vertical scale of about 4 m. The balloon experiments are performed with suspensions of aluminum powder in oxygen mixed with nitrogen, argon, or helium with various concentrations of oxygen and aluminum. It is found that stable flame propagation is only observed for aluminum concentrations near stoichiometric to rich conditions. Pulsating and spiral-like flames are discovered in fuel-lean mixtures, and flames with cellular patterns occur in very-fuel-rich suspensions. The burning velocities in the stable propagation regime derived from the balloon experiments correlate well with data previously obtained with stabilized Bunsen-type flames. The flame speed of stable flames is found to be a strong function of the heat conductivity of the gas mixture. In addition, the oxygen concentration has a strong influence on the flame speed for fuel-rich mixtures but dependence is reduced for fuel-lean mixtures. In the large-scale experiments, the burning velocity is estimated to be about two times larger than for the small-scale experiments. The increase in burning velocity is attributed to preheating of the unburned mixture by radiation from the condensed-phase combustion products. The degree of preheating, determined with an array of fine thermocouples, is found to be in the range of 150–200 K. The propagation of stable flames is discussed in light of existing qualitative dust flame models, whereas the pulsating flame propagation regime observed is interpreted in terms of the thermo-diffusive instability theory developed for high Lewis number flames in gases and solid reactive powder mixtures.