Ignition Of Aluminum Particles Research Articles

Investigation of Al-Li particle ignition dynamics with different Li content

Promoting the ignition of aluminum powder is considered an effective method to inhibit the agglomeration of aluminum powder in propellants and enhance combustion efficiency. This study utilizes laser ignition technology and high-speed photography to investigate the ignition and combustion processes of single aluminum particles and aluminum-lithium alloy particles. The focus is on comparing the effects of the diameter of micron-sized metal particles and the lithium content in aluminum-lithium alloy particles on the ignition and combustion processes of metal particles. The results show that the ignition delay time is directly proportional to the diameter of the metal particles and inversely proportional to the lithium content. For the aluminum-lithium alloy particle with a lithium content of 3.5 %, even if the diameter is close to 300 μm, the ignition delay time is only 125.5 ms, which is much smaller than that of the pure aluminum particle with a diameter of 208 μm. Compared to aluminum particles and aluminum-lithium alloy particles, there is basically no difference between the two during the combustion stage. However, in the ignition stage, aluminum-lithium alloy particles sequentially exhibit a red gas-phase flame corresponding to lithium and a yellow gas-phase flame corresponding to aluminum. This indicates that during the ignition process of aluminum-lithium alloy particles, lithium first reacts with the oxidative atmosphere and releases heat, providing a heat source for the subsequent ignition of aluminum particles. This also explains why the ignition delay time of metal particles is inversely proportional to the lithium content. An ignition model for aluminum particles in a multi-component atmosphere is established, which further considers the chemical reactions between lithium and oxidative gases, making the model applicable to aluminum-lithium alloy particles. This ignition model effectively describes the impact of particle diameter and lithium content on the ignition process of metal particles. The model is further verified, and the results show that the calculated ignition delay is in good agreement with the experimental data. Overall, this study provides deeper experimental and theoretical insights into the ignition and combustion processes of aluminum-lithium alloys, and the findings can guide the application of aluminum-lithium alloys in propellants.

Comprehensive modeling of ignition and combustion of multiscale aluminum particles under various pressure conditions

The ignition and combustion of aluminum particles are crucial to achieve optimal energy release in propulsion and power systems within a limited residence time. This study seeks to develop theoretical ignition and combustion models for aluminum particles ranging from 10 nm to 1000 μm under wide pressure ranges of normal to beyond 10 MPa. Firstly, a parametric analysis illustrates that the convective heat transfer and heterogeneous surface reaction are strongly influenced by pressure, which directly affects the ignition process. Accordingly, the ignition delay time can be correlated with pressure through the pb relationship, with b increasing from –1 to –0.1 as the system transitions from the free molecular regime to the continuum regime. Then, the circuit comparison analysis method was used to interpret an empirical formula capable of predicting the ignition delay time of aluminum particles over a wide range of pressures in N2, O2, H2O, and CO2 atmospheres. Secondly, an analysis of experimental data indicates that the exponents of pressure dependence in the combustion time of large micron-sized particles and nanoparticles are –0.15 and –0.65, respectively. Further, the dominant combustion mechanism of multiscale aluminum particles was quantitatively demonstrated through the Damköhler number (Da) concept. Results have shown that aluminum combustion is mainly controlled by diffusion as Da > 10, by chemical kinetics when Da ≤ 0.1, and codetermined by both diffusion and chemical kinetics when 0.1 < Da ≤ 10. Finally, an empirical formula was proposed to predict the combustion time of multiscale aluminum particles under high pressure, which showed good agreement with available experimental data.

Study on laser ignition and combustion characteristics of micron-sized aluminum and Al-Mg alloys particles

In response to the problems of easy sintering and long ignition delay time of micron aluminum in the combustion of aluminum-containing propellants, choose the way to add magnesium to metal aluminum to construct an alloy system, through boiling, micro-explosions are generated during the ignition and combustion process to weaken the sintering behavior and shorten the ignition delay time of aluminum. Selecting aluminum and Al-Mg alloy powder fuel with a particle diameter of about 10 μm as the research object, a set of individual-particle fuel laser ignition and microscopic high-speed imaging experimental devices was built that can observe the whole process of ignition and combustion of micron-sized fuel. Thermal analysis was used to detect and characterize the thermal decomposition process of micron-sized aluminum and Al-Mg alloy powders; combined with the results of scanning electron microscopy, the difference in ignition performance of micron-sized individual particle aluminum and Al-Mg alloys was studied. Experiments have found that, compared with aluminum, the initial oxidation temperature of Al-Mg alloys is lower and the combustion is more complete. However, the effect of adding magnesium to aluminum is only reflected before 900 °C. The ignition and combustion images and flame propagation laws of micron-sized single-particle aluminum and Al-Mg alloys were obtained. It was found that adding magnesium shortened the ignition delay time, and the combustion produced less residual.

Investigation of aluminum particle ignition dynamics in various propellant environments

The ignition delay of aluminum particles can be profoundly influenced by the agglomeration and combustion characteristics of solid propellants. In this experimental study, the ignition of individual aluminum particles in nitrate ester plasticized polyether (NEPE) and hydroxyl‑terminated polybutadiene (HTPB) propellant atmospheres was examined using laser ignition tests and high-speed photography. Focus was directed at the effects of the atmospheric composition, pressure, and aluminum particle size (500, 800 and 1000 µm). It was observed that the non-homogeneous reaction heat generation and convective heat exchange rates during particle ignition could be increased by increasing the CO2 content or ambient temperature, resulting in a shorter ignition delay time. This explains why aluminum particles tended to burn more efficiently in nitrate ester plasticized polyether than in hydroxyl‑terminated polybutadiene. In addition, it was found that the ignition delay time decreased with increasing pressure and was inversely proportional to the particle size squared. An ignition model for aluminum particle ignition in a multi-component atmosphere was developed and validated to describe the temperature change and energy transfer during ignition. The calculated ignition delay times were found to be within 5% of the experimental values. The model also showed that increasing the ambient temperature was more effective than increasing the CO2 content at enhancing ignition, and that CO2 was more effective than O2, which was itself more effective than H2O. Overall, this study provides useful insight into the ignition characteristics of aluminum particles in different propellant environments. The results could be used to develop more effective propellant formulations, optimize combustion processes, and improve safety in the handling and use of aluminum-based propellants.

Predictive model for the ignition and combustion behaviors of micron-sized aluminum particles in a water vapor environment

Predictive model for the ignition and combustion behaviors of micron-sized aluminum particles in a water vapor environment

AP and nFe2O3 synergistically improve the ignition and combustion performance of aluminum particles

AP and nFe2O3 synergistically improve the ignition and combustion performance of aluminum particles

Modeling of aluminum agglomeration at solid propellant burning surface by combining aluminum ignition with LDP-SC cluster analysis

Aluminum particles at burning surface may agglomerate during the combustion process of composite solid propellant, involving the aggregation and ignition of aluminum particles as well as the secondary coalescence of agglomerates. So far it is still a huge challenge to capture the agglomeration process and obtain the size distribution of agglomerates accurately. Modeling of aluminum agglomeration was carried out by constructing several submodels, including propellant combustion, cluster analysis of aluminum particles, aluminum ignition and coalescence based on the packed three-dimensional propellant microstructure unit. Dynamic combustion process of propellant microstructure unit was simulated firstly, which characterizes the temperature distribution above the burning surface. Next, aluminum clusters were grouped using a new local density peaks-spectral clustering (LDP-SC) algorithm for initial aluminum particles in microstructure unit. The ignition of aluminum particles in clusters exposed from burning surface was then examined by considering their distributions in the flame of propellant. These exposed aluminum particles in a cluster yield an agglomerate once an aluminum particle is successfully ignited. Secondary mergence of multiple agglomerates was also considered when the distance between two particles in different agglomerates was less than a critical separation distance. With these submodels, the agglomeration process of a typical propellant and properties of aluminum clusters and agglomerates were estimated. The predicted size distribution of agglomerates shows reasonable agreement with experimental data, indicating that the present model is capable of predicting aluminum particle agglomeration during composite propellant combustion.

Aluminum Particle Ignition Studies with Focus on Effect of Oxide Barrier

Aluminum particle ignition behavior in open atmosphere rocket propellants fires is of particular interest for preventing accidents for rockets carrying high-value payloads. For nominal motor pressures, aluminum particles oxidize to aluminum oxide in the gas phase and release significant combustion energy while minimizing motor instability. During rocket abort or launch pad malfunction which occur under atmospheric or low pressure, behavior of aluminum particle combustion becomes complex and aluminum appears to melt, agglomerate or form a skeletal structure. Furthermore, an oxide shell of alumina instantly forms on any fresh aluminum surface which is exposed to an oxidizing environment. Aluminum combustion then strongly depends on the oxide layer growth, which is influenced by causative factors, including particle size, environmental gas composition, and heating rate. This work focuses on the effect of the oxide barrier which forms on the surface of aluminum that is recognized to impede combustion of aluminum in solid rocket propellants. Understanding the mechanism for breach of this barrier is deemed to be an important consideration in the overall process. In this discussion, results of various experiments will be discussed which have a bearing on this process. Basically, a recognized criterion is the melting of the oxide layer at 2350 K is sufficient. However, in other situations, depending on the mechanism of oxide formation, there will occur defects in the oxide shell which provide for aluminum ignition at lower temperatures. For slow heating in an oxidizing environment, where the oxide layer can grow thick, then ignition is more difficult. Because there is no uniform model to establish an ignition criterion due to the unknown history of an aluminum particle, this paper reports experimental findings involving oxyacetylene torch, thermogravimetric analysis with differential scanning calorimeter, aluminum particle heating, electric ignition and aluminum powder heating, to address the influence of the oxide layer on the aluminum particle ignition.

Investigation on the thermal decomposition of the elastomer containing fluoroolefin segment by DSC‐TG‐MS‐FTIR

AbstractBased on the cross‐linking reaction of bisphenol A epoxy resin and liquid carboxy‐terminated poly(tetrafluoroethylene‐hexafluoropropylene‐vinylidene fluoride) copolymer (CTTHV), the elastomer containing fluoroolefin segment was prepared. The bis(2,2‐dinitropropyl)‐acetal/formal (BDNPA/F) was used as an energetic plasticizer for this fluoroolefin segment‐containing elastomer (FO elastomer). The compatibility of CTTHV and BDNPA/F was evaluated by the combination of differential scanning calorimetry (DSC) and derivative thermogravimetry (DTG) methods. Thermal decomposition of the FO elastomers was investigated by TG‐DSC. The kinetic parameters of the FO elastomers were calculated by Kissinger‐Akahira‐Sunose method. DSC‐TG‐MS‐Fourier transform infrared spectroscopy analysis was used to detect the thermal decomposition products. The thermal decomposition mechanisms of the FO and FO/BDNPA/F elastomers were discussed. The results show that CTTHV has a good compatibility with BDNPA/F, and the prepared FO/BDNPA/F elastomers have good thermal stability. BDNPA/F can promote the thermal decomposition of the FO elastomer and bring more heat for the binder system according to the thermal decomposition behaviors and kinetic calculation results of FO/BDNPA/F elastomers. The FO elastomer preferentially undergoes depolymerization, HF elimination with the formation of polyene sequences, and subsequent main‐chain cleavage, polyaromatization. The H ion fragments of the decomposition of BDNPA/F can be an important factor in the improvement of HF concentration at the initial thermal decomposition of FO elastomer. An abundant HF released during all decomposition of FO/BDNPA/F elastomers may be beneficial to the ignition and combustion of aluminum particles in future applications.

Temperature measurements and high-speed photography of micron-sized aluminum particles burning in methane flat-flame exhaust

This work seeks to investigate the ignition and burning of micron-sized aluminum particles using nonintrusive temperature measurements and high-speed photography. First, the detected emission spectra were used to deduce the particle temperature according to Planck’s laws, and the emissivity dependence on the wavelength was determined. Second, the ratio pyrometer method using a Nikon digital camera was developed to specify the two-dimensional temperature profile. These two methods were preliminarily validated through a B-type thermocouple of known temperature. Further, aluminum particles with controlled sizes of approximately 45 and 125 µm were injected as a well-dispersed stream or discrete single particles, respectively, into the hydrocarbon combustion exhaust with controlled gas temperature and compositions. The temperature evolutions of 45 µm aluminum particles derived from the camera pyrometer are in reasonable agreement with the numerical results predicted by a previously developed model. Regarding 125 µm particles, the temperature contour shows a sharp temperature increase and flame expansion during ignition. It can be attributed to the transition from surface reaction to more vigorous combustion in the vapor phase, which was visualized by a high-speed camera equipped with a long-distance microscopic lens. In this work, the camera pyrometer has demonstrated the capability of measuring the average projected particle temperature and characterizing the combustion process of micron-sized aluminum particles, with promising simplicity and economy.

Laser-Induced Ignition and Combustion of Individual Aluminum Particles Below 10 μm by Microscopic High-Speed Cinematography

Metal aluminum has been widely used as an ingredient in propellant, gunpowder and thermite, but there is less understanding of the combustion mechanism of aluminum particles from submicron to several microns in diameter. This paper proposes to experimentally investigate the ignition and combustion characteristics of individual aluminum particles below 10 μm. A specific in situ diagnostic experimental apparatus was first designed for directly observing the ignition and combustion behaviors of individual aluminum particles, with a submicrometer spatial resolution and a temporal resolution of tens of microseconds. Direct observation through microscopic high-speed cinematography demonstrated that, when heated by a continuous laser, individual aluminum particles thermally expanded, followed by shell rupture; the molten aluminum core overflowed and evaporated, leading to ignition and combustion. Further results showed that, when the laser power densities were gradually increased (5.88, 7.56 and 8.81 × 105 W/cm2), the durations of thermal expansion, melting and evaporation were shortened. The required time for the aluminum particles to expand to 150% of their initial diameter was shortened (34 s, 0.34 s and 0.0125 s, respectively). This study will be beneficial to further extend the investigation of other individual metal particles and reveal their combustion mechanism by direct observation.

Potassium fluoride improving the ignition and combustion performance of micron-sized aluminum particles in high temperature water vapor

ABSTRACT To improve the ignition and combustion characteristics and combustion efficiency of micron-sized aluminum particle in high temperature water vapor atmosphere, a self-designed tube furnace setup was used to study the effects of potassium fluoride (KF) on the ignition and combustion characteristics of micron-sized aluminum particle in high temperature water vapor. The ignition and combustion processes, components and morphologies of combustion products, and combustion efficiency were recorded and analyzed by a high-speed photographic system, X-ray diffraction, scanning electron microscopy with energy-dispersive spectrometer, and chemical analysis method, respectively. Results show that the addition of KF to 5 μm aluminum powder significantly decreases its ignition delay time because the hydrolysis of KF generates KOH in water vapor, and KOH corrodes the oxide shell on the aluminum surface to promote the reaction of aluminum with water, which can be proved by the morphologies of the products. The hydrolysis product of KF can destroy the oxidation shell on the aluminum powder surface, but it does not cause the aluminum particles to undergo shell-breaking combustion. As the KF content increases, the oxide shell is dramatically destroyed, so the ignition delay time of aluminum powder in high temperature water vapor decreases. Moreover, the combustion efficiency of aluminum powder with addition of KF increases considerably with increasing of the KF content.

Investigation of the ignition and combustion of compressed aluminum/polytetrafluoroethylene bulk composites

The thermal pyrolysis process of mold-compressed Al/PTFE composites has been investigated. TG–DSC analysis of the Al/PTFE composites revealed a slight exothermic pre-reaction in the range 500–600 °C. Based on XRD and XPS analyses of Al/PTFE composites subjected to heat treatment at 500–600 °C, this pre-reaction is believed to be fluoridation of the alumina passivating layer on the surface of Al particles. This fluoridation has been confirmed to be a crucial factor in triggering the ignition of aluminum particles in an O2 flow at a temperature much lower than the melting point of aluminum (660 °C). The energetic properties of the Al/PTFE composites have been studied, and the compositions of their combustion residues have been analyzed by XRD. The results indicated that the energy release from the composites and the compositions of their solid combustion residues were determined by their PTFE contents. Composition analysis of these combustion residues has revealed the formation of AlF3, Al2O3, Al4C3, and amorphous carbon.

Single Particle Combustion of Pre-Stressed Aluminum.

An approach for optimizing fuel particle reactivity involves the metallurgical process of pre-stressing. This study examined the effects of pre-stressing on aluminum (Al) particle ignition delay and burn times upon thermal ignition by laser heating. Pre-stressing was by annealing Al powder at 573 K and quenching ranged from slow (i.e., 200 K/min) identified as pre-stressed (PS) Al to fast (i.e., 900 K/min) identified as super quenched (SQ) Al. Synchrotron X-ray Diffraction (XRD) analysis quantified an order of magnitude which increased dilatational strain that resulted from PS Al and SQ Al compared to untreated (UN) Al powder. The results show PS Al particles exhibit reduced ignition delay times resulting from elevated strain that relaxes upon laser heating. SQ Al particles exhibit faster burn times resulting from delamination at the particle core-shell interface that reduces dilatational strain and promotes accelerated diffusion reactions. These results link the mechanical property of strain to reaction mechanisms associated with shell mechanics that explain ignition and burning behavior, and show pre-stressing has the potential to improve particle reactivity.

Ignition and combustion of nano-sized aluminum particles: A reactive molecular dynamics study

Combustion mechanism of nano Al particles remains controversies for years. In this paper, we use ReaxFF molecular dynamics simulations to study thermodynamic and structure properties of Al particles from ignition to combustion. Two typical core–shell nano Al models with different oxide shell thickness are studied. Effect of radiative heat transfer is also considered in the combustion stage. Our results show that no shell cracks are observed during both heating and burning periods. The oxide shell hinders the thermal diffusion of core Al atoms and causes the abnormal melting rule. The stress variations at the core–shell interface are the most dramatic during the heating period. The combustion processes are controlled by diffusion behaviors of components in particles and external oxygen molecules. Detailed structural evolution processes are studied by displacement magnitude analysis. Particle with thinner shell has a shorter ignition delay and a higher combustion temperature which directly affect the radiative heat transfer rate during the combustion period.

Impact ignition and combustion of micron-scale aluminum particles pre-stressed with different quenching rates

Pre-stressing aluminum (Al) particles by annealing and quenching alters dilatational strain and is linked to increased particle reactivity. The quenching rate associated with pre-stressing is a key parameter affecting the final stress state within the Al particle, with faster quenching rates theoretically favoring a higher, more desirable stress state. Micron scale Al particles are annealed to 573 K, then quenched at different rates (i.e., 200 and 900 K/min), mixed with bismuth oxide (Bi2O3), and the Al + Bi2O3 mixtures are examined under low-velocity, drop-weight impact conditions. Both quenching rates showed increased impact ignition sensitivity (i.e., between 83% and 89% decrease in ignition energy). However, the slower quenching rate showed a 100% increase in pressurization rate compared to untreated particles, while the faster quenching rate showed a 97% increase in peak pressure, indicating that these two quenching rates affect Al particles differently. Surprisingly, synchrotron X-ray diffraction data show that the 200 K/min quenched particles have a higher dilatational strain than the untreated particles or the 900 K/min quenched particles. Results are rationalized with the help of a simple mechanical model that takes into account elastic stresses, creep in the alumina shell, and delamination of shell from the core. The model predicts that Al powder quenched at 200 K/min did not experience delamination. In contrast, Al quenched at 900 K/min did not have creep but does have delamination, and under impact, delamination led to major fracture, greater oxygen access to the core, and significant promotion of reaction. Thus, the increase in quenching rate and shell-core delamination are more important for the increase in Al reactivity than pre-stressing alone.

Thermal theory of aluminum particle ignition in continuum, free-molecular, and transition heat transfer regimes

Most studies on nano- and micro- sized aluminum particle ignition have been focused on the processes occurring inside particles. In the current paper, thermal ignition of an aluminum particle in the air is simulated with different heat transfer models: continuum, free-molecular, and Fuchs model. A single parabolic oxidation law is assumed in the particle size range from nano- to millimeter diameters. A particle is considered ignited when it reaches the oxide melting point. The criterion defining the limits of validity for each model is the ratio of continuum and free-molecular heat transfer rates. The dependence of ignition temperature Ti on the particle size is in qualitative agreement with the experimental trends: Ti can have values in the range of 700–1500 K for nanoparticles due to the dominating contribution of a free-molecular heat transfer, and sharp growth of Ti with the particle size in the range of 1–100 μm diameter is due to the transitional character of heat transfer. For small values of the accommodation coefficient, ignition may occur in the critical ignition mode with the thermal runaway. The results suggest the importance of non-continuous heat transfer and, in particular, energy accommodation in ignition of nano- and micro- sized particles.

The formation of (Al2O3)n clusters as a probable mechanism of aluminum oxide nucleation during the combustion of aluminized fuels: Numerical analysis

The model of formation and growth of stoichiometric (Al2O3)n clusters during the combustion of aluminized fuels has been developed. In this model, the thermodynamic properties of large clusters (n = 5–75) have been determined by matching the thermodynamic properties of small clusters n = 2–4, calculated earlier by quantum-chemical methods, with similar characteristics of liquid droplets. The developed model was used for a numerical simulation of the formation of (Al2O3)n clusters during the combustion a single aluminum particle with a diameter of 200 μm in O2/Ar atmosphere. It has been shown that, during the first 12 ms after the aluminum particle ignition, the rapid growth of clusters occurs. The mass of clusters in the combustion zone is comparable to the mass of aluminum oxide. The modeling results indicate that the growth of (Al2O3)n clusters can be the most probable mechanism of the formation of condensation nuclei of alumina.

Effect of particle size and oxygen content on ignition and combustion of aluminum particles

Particle size and oxygen content are two of the key factors that affect the ignition and combustion properties of aluminum particles. In this study, a laser ignition experimental system and flame test system were built to analyze the ignition and combustion characteristics and the flame morphology of aluminum particles. A thermobalance system was used to analyze the thermal oxidation characteristics. In addition, the microstructure of aluminum was analyzed by scanning electron microscopy. It was found that the oxidized products were some of the gas phase products agglomerated. Smaller particle size samples showed better combustion characteristics. The combustion intensity, self-sustaining combustion time and the burn-off rate showed a rising trend with the decrease in the particle size. Increasing the oxygen content in the atmosphere could improve the ignition and combustion characteristics of the samples. Four distinct stages were observed in the process of ignition and combustion. Small particle size samples had a larger flame height and luminance, and the self-sustaining combustion time was much longer. Three distinct stages were observed during the thermal oxidation process. The degree of oxidation for small-sized samples was significantly higher than that for the larger particle size samples. Moreover, it was observed that the higher the oxygen content, the higher the degree of oxidation was.

Micro and Nano Scale Phenomena of Aluminum Agglomeration During Solid Propellant Combustion

Combustion of aluminized solid propellants exhibits phenomena associated with accumulation, agglomeration, ignition, and combustion of micro and nano-size aluminum particles. In general, agglomeration is an undesirable phenomenon, as it turns small particles into relatively large agglomerates, each containing many original particles, resulting in long combustion times which may lead to incomplete reaction, reduced jet momentum, and enhanced slag formation which adds parasite mass and may damage the motor insulation. This article presents a physical mechanism explaining the agglomeration process, revealing that small particles tend to agglomerate more than large particles. In addition, it suggests ways to reduce agglomeration of the aluminum particles via nano-coatings generating reactive heating and promoting ignition.