Aluminum Agglomeration Research Articles

An aluminum agglomeration model based on realistic pocket distribution via microcomputed tomography

The condensed combustion products (CCPs) resulted from aluminum agglomeration significantly affect the operation and safety of solid rocket motor, so it is essential to create a precise agglomeration model to predict the size of the CCPs of aluminized solid propellant. To accomplish this, high-precision microcomputed tomography (micro-CT) scanning was employed to obtain the quasi-three-dimensional structure of the propellant. The distribution of the ammonium perchlorate (AP) pockets was recognized via artificial intelligence (AI) method. The results revealed that the pocket size was mainly influenced by the AP grade. As the fraction of the coarse AP fraction declined from 43.1 % to 23.1 %, the average pocket diameter reduced from 388.91 to 68.23 μm. Size distribution predictions were subsequently performed for the agglomerations based on the pocket model theory. The equation relationship between agglomeration coefficient and propellant formulation was presented and corrected through mathematical fitting. The proposed agglomeration model was validated using the CCPs collection experiments of six aluminized propellants at 7 MPa. The agglomeration model produced an average particle size prediction error of <9.8 %, and the goodness of fit of the agglomerate distribution was >0.85. Subsequent analysis indicated that the agglomerate size mainly depended on the percentage of coarse AP, the burn rate, and the size distribution of raw aluminum particles. The present model is expected to offer a new way to achieve the accurate prediction of solid propellant agglomeration behaviors.

Numerical study of aluminum combustion with agglomerate size distribution in solid rocket motor

The aluminum agglomerate size distribution plays an important role in influencing the particle distributed combustion in motor, which subsequently affects the performance of solid rocket motor significantly. In this work, a three-phase model to describe distributed combustion of aluminum agglomerates is established based on the Eulerian-Lagrangian method. Then, the agglomerate size, including mono size and distributed size, is studied to reveal its effect on aluminum combustion and motor flow field. The simulated results indicate that the increasing agglomerate mono size observes the obvious decrease of the average temperature inside the motor combustion chamber, implying the low combustion efficiency of large agglomerate size. When considering the agglomerate size distribution, the size distribution mode and the mean size D43 determine the combustion efficiency together. In particular, even the mean size is similar, with different distribution mode, like skewed distribution, bimodal or trimodal distribution, the combustion efficiency and flow field parameters are nonnegligible different. However, when the size distribution mode is the same and the peak range is similar, the mean size D43 becomes the only and predominant factor as the mono size.

Effect of aluminum and ammonium perchlorate particle sizes on the condensed combustion products characteristics of aluminized NEPE propellants

Aluminum (Al) is usually added to solid propellants to improve the combustion performance, however the condensed combustion products (CCPs) especially the large agglomerates generated from aluminum combustion can reduce the specific impulse of the engine, and result in two-phase loss, residue accumulation and throat liner ablation. Al and ammonium perchlorate (AP), as important components of NEPE propellants, can affect the formation process of the CCPs of aluminized NEPE propellants. To clarify the effect of Al and AP particle sizes on the properties of the CCPs of aluminized NEPE propellants, a constant-pressure quench vessel was adopted to collect the combustion products of four different formulations of NEPE propellants. It was found that the condensed combustion products are mainly divided into aluminum agglomerates and oxide particles, the diameter of the aluminum agglomerates of these four different formulations of NEPE propellants at 7 MPa was smaller than that in 3 MPa, and the shells of the aluminum agglomerates were smoother and the spherical shape was more perfect. X-ray diffraction analysis of the CCPs of the four NEPE propellants under 3 MPa revealed the presence of both Al and Al2O3. With the increase of the particle size of Al and AP, the oxidation degree of aluminum particles decreases. The particle size of the CCPs of the four different formulations of NEPE propellants under 1 and 3 MPa was analyzed by using a laser particle size analyzer, it is found that the increase of AP particle size is helpful to reduce the size of condensate combustion products. Based on the classical pocket theory, establishing a new agglomeration size prediction model, which can be used to predict the agglomeration size on the burning surface. Compared with the empirical model, the new agglomeration size prediction model is in good agreement with the experimental results.

In situ preparation of aluminum-copper pentafluorobenzoate energy microspheres: Enhancing aluminum combustion properties in composite propellants

Aluminum powder is the main metal fuel in composite propellants, and upgrading the proportion of aluminum powder loaded in solid propellants is currently one of the most effective means of increasing propellant energy. Nonetheless, the substantial disparity in boiling points between the core aluminum particles and the alumina layer enveloping their surfaces often results in reduced reactivity, inadequate combustion, and combustion agglomeration during the combustion process of the aluminum powder. Herein, copper pentafluorobenzoate (CuPF) interfacial layer was established on the surface of aluminum powder through the self-assembly coordination reaction between pentafluorobenzoic acid and copper ions, and Al-CuPF energy microspheres were successfully prepared. A comprehensive examination was carried out to scrutinize the surface morphology, elemental composition, and distribution of the Al-CuPF energy microspheres. Furthermore, meticulous studies were conducted on the ignition and combustion properties of these Al-CuPF energy microspheres under various atmospheric conditions. It was found that the Al-CuPF interfacial layer within the Al-CuPF energy microspheres plays a pivotal role in enhancing ignition initiation and acting as a catalyst to facilitate the more efficient combustion of aluminum powder. Moreover, the combustion behavior of Al-CuPF energy microspheres when integrated into composite propellants has been exhaustively researched. Notably, composite propellants incorporating Al-CuPF energy microspheres exhibit heightened burning temperatures and an increased burning rate. Additionally, the combustion and agglomeration dynamics of Al-CuPF energy microspheres on the propellant burning surface were meticulously documented using a high-speed camera. The findings revealed that Al-CuPF energy microspheres spend a significantly shorter duration on the propellant burning surface and notably mitigate the occurrence of aluminum agglomeration issues thereon. This research contributes fresh insights into the application of innovative aluminum-based fuels within the realm of composite propellants.

Thermal shock triggers microexplosion combustion in graded fuel and oxidizer encapsulation microspheres with improved combustion efficiency

The combustion agglomeration of aluminum (Al) and the long heat and mass transfer distance between Al and oxidizer cause serious two-phase flow loss and slag accumulation in the engine of solid propellants. Here, we present an effective strategy by in-situ encapsulating graded nano-aluminum (nAl) and micron-aluminum (μAl) in ammonium perchlorate (AP) simultaneously by using hydroxymethyl cellulose (HMC) as adhesive and constructing a compact coupling fuel oxidizer complex high-energy microsphere (μAl/nAl@AP/HMC) to enhance the energy release efficiency and combustion efficiency. Compared with physical mixture (PM), the ignition delay time and combustion time of μAl/nAl@AP/HMC microspheres are reduced by 69.58 % and 69.87 %, owing to the decomposition process control of AP through the adsorption and spatial constraint effect on the interface between metal fuels and oxidizer. Furthermore, the nAl embedded in the AP surface forms a hot spot around the μAl to promote the combustion reaction, thereby shortening the cavitation threshold and tensile limit time of μAl and accelerating the occurrence of microexplosion. Microexplosion effect in μAl/nAl@AP/HMC microsphere significantly weakens combustion agglomeration and improves combustion efficiency and energy release efficiency by sharp shrinkage of combustion condensation products, which addresses the challenges on energy output and combustion efficiency in high-density solid propellants.

Combustion performance modulation and agglomeration inhibition of HTPB propellants by fine Al/oxidizers integration

Modulation of burning rates and inhibition of aluminum agglomeration in solid propellants have generated considerable research interest in the field of energetic materials. In this paper, integrated Al/oxidizer composites have been prepared by a spray-drying technique, which offer intimate contact between the fuel and oxidizers. The morphologies, thermal stability, density, heats of explosion and combustion performance of the resulted solid propellants containing these Al-based composites were systematically investigated, where the condensed combustion products (CCPs) analysis and the agglomeration behavior of Al particles have been focused. After well control of the Al/oxidizers structure, 9 % higher explosion heat, 58 % higher pressurization rate and 65 % stronger flame radiation intensity were achieved in comparison to the conventional solid propellants with the same formulation. The burn rate of the propellants can also be well controlled by using different type of Al/oxidizers composites, in the range of 3.6–5.9 mm s−1 under 0.5 MPa and 9.2–10.9 mm s−1 under 15 MPa. More importantly, very low burn rate pressure exponent of 0.1 within 0.5–15 MPa was obtained. Inspired by decreasing size of CCPs, the evolution of Al on the burning surface was studied and the agglomeration inhibition was observed, whose potential mechanism was then proposed and elucidated.

Effect of Fluoroalcohol Chain Extension Modified HTPB Binder on the Combustion Performance of Aluminized Propellants

To enhance both the mechanical properties of hydroxyl-terminated polybutadiene (HTPB) binder and the combustion efficiency of aluminized propellants, 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (OFHD) was employed as a chain extender to impart mechanical regulation to the HTPB binder. Mechanical testing showed that the mechanical properties of fluoride-modified HTPB polyurethane (FPU) were significantly improved: the peak tensile strength of the optimized samples reached 1.99 MPa, and the elongation at break attained 486%. The structural characterization of the FPUs was conducted using Fourier transform infrared (FTIR) spectroscopy. Thermogravimetry-mass spectrometer (TG-MS) analysis revealed that the initial thermal decomposition temperature of the FPU shifted from 170 °C to 162 °C, accompanied by the release of fluorine-containing fragments during decomposition. Analysis of the combustion residue indicated that the addition of OFHD can reduce the agglomeration of aluminum (Al) powder in aluminized propellants. Dynamic pressure characteristics results showed an augmented pressurization rate under argon and oxygen atmospheres, increased by 18.67% and 37.29%, respectively. Heat release tests indicated that the aluminized propellants with the addition of OFHD had a higher combustion heat, being increased by 6.57%. The binder system is expected to be applied in aluminized propellants to improve the mechanical properties and combustion efficiency of Al powder.

Experimental and numerical investigation on agglomeration of aluminum during combustion process of aluminized composite propellant

The addition of aluminum particles can effectively improve the energy characteristics of solid composite propellants in propulsion systems. However, these aluminum particles affecting propellant mixture viscosity, ignition delay time, burning time and specific impulse could also lead to undesirable effects. Therefore, it is a general practice that in the early stage of propellant formulation design that the particle size of condensed phase products is optimized/minimized through formulation adjustment to reduce its negative impact on the engine. In practice, it means that a costly manpower and materials are needed to obtain the size distribution of condensed phase products during the combustion process of propellants via experimental tests. Thus, there is a strong industrial need for a reasonable and feasible simulation method of agglomeration process during the combustion process of propellants. In this work, we propose and examine the implementation of molecular dynamics and a random packing algorithm on creating numerically models of aluminized composite propellants. Considering the pyrolysis of ammonium perchlorate (AP) and the combustion process of aluminum, we develop a more accurate agglomeration model basing on the classical Jackson model. Our proposed methods and simulations models are validated by our experimental results. It confirms that the present work provides a prediction tool for simulating and improving the agglomeration process, thus aiming at optimizing solid propellant formula.

A new fluorocarbon adhesive: Inhibiting agglomeration during combustion of propellant via efficient F–Al2O3 preignition reaction

AbstractInhibiting the agglomeration of molten aluminum particles packed in the binder network is a promising scheme to achieve efficient combustion of solid propellants. In this investigation, the hydroxyl‐terminated structured fluorinated alcohol compound (PFD) was introduced to modify the traditional polyethylene glycol/polytetrahydrofuran block copolymerization (HTPE) binder; that is, a unique fluorinated polyether (FTPE) binder was synthesized by embedding fluorinated organic segments into the HTPE binder via crosslinking curing. The FTPE was applied in aluminum‐based propellants for the first time. Due to the complete release of fluorinated organic active segments in the range of 300°C to 400°C, the burning rate of FTPE‐based propellant increased from 4.07 (0% PFD) to 6.36 mm/s (5% PFD), increased by 56.27% under 1 MPa. The reaction heat of FTPE propellants increased from 5.95 (0% PFD) to 7.18 MJ/kg (5% PFD) under 3.0 MPa, indicating that HTPE binder modified with PFD would be conducive to inhibiting the D90 of condensed combustion products (CCPs) dropped by 81.84% from 75.46 (0% PFD) to 13.71 μm (5% PFD) under 3.0 MPa, in consistent with the significant reduction of aluminum agglomerates observed on the quenched burning surface of the propellants. Those results demonstrated that a novel FTPE binder with PFD can release fluorinated organic active segments, which motivate preignition reaction with the alumina shell in the early stage of aluminum combustion, and then enhance the melting diffusion effect of aluminum to inhibit the agglomeration.

Combustion Enhancement of Gel Propellant Containing High Concentration Aluminum Particles Based on Carbon Synergistic Effect.

The addition of aluminum particles to gel propellants can improve combustion performance. However, the agglomeration of aluminum during the combustion process can result in a series of negative effects. In this paper, the aluminum agglomeration inhibition method of gel propellant based on carbon synergistic effect is proposed. Carbon particles exhibit excellent combustion properties, and the gaseous product CO2 generated during combustion can mitigate the agglomeration of aluminum. The research demonstrates that incorporating carbon particles into aluminum-containing gel effectively reduces the incomplete combustion of aluminum particles and increases the volumetric calorific value of the gel. When the mass fraction of carbon is 5 wt%, the volume calorific value of the gel reaches the highest. Meanwhile, the rheological experiments show that the addition of carbon particles can improve the shear-thinning properties of the gel, which is beneficial to the atomization and combustion processes of the gel.

Reduction of agglomeration effect by aluminum trihydride in solid propellant combustion

AbstractAluminum trihydride (AlH3) has gained considerable attention as a substitute fuel for aluminum in solid propellants. In this study, we conducted a systematic investigation to evaluate the effects of AlH3 content, ranging from 0 % to 18 %, on propellant ignition, combustion, and agglomeration. Experimental methods such as thermogravimetry−differential scanning calorimetry (TG‐DSC), laser ignition, high‐speed photography, and collecting condensed combustion products (CCPs) were employed. The results indicate that AlH3 significantly promotes the high‐temperature decomposition of ammonium perchlorate (AP). Meanwhile, the addition of cyclotetramethylene tetranitramine (HMX) in propellant does not affect the hydrogen release reaction of AlH3. As the AlH3 content increases from 0 % to 18 %, the spectral emission intensity of the propellants decreases, and the ignition delay time initially increases from 253 ms to 321 ms, and then decreases to 258 ms. Furthermore, the burning rate increases with increasing the AlH3 content, while the pressure exponent is reduced from 0.551 to 0.460. The inclusion of AlH3 in propellants significantly inhibits aluminum agglomeration near the burning surface. Additionally, as the AlH3 content increases, the mean particle size D43 of the CCPs decreases from 50.95 μm to 8.28 μm at 1 MPa. The agglomeration degree of aluminum is very weak at 7 MPa, especially when the AlH3 content exceeds 9 %. The conclusions drawn from this study can serve as valuable guidance for optimizing propellant formulations.

Pocket Model with a Tetrahedral Cell for Aluminum Agglomeration in Composite Propellants

Pocket Model with a Tetrahedral Cell for Aluminum Agglomeration in Composite Propellants

Experimental Investigation on Aluminum-Based Water Ramjet for Propelling High-Speed Underwater Vehicles

Major challenges in developing and realizing a novel aluminum–water reaction-based water ramjet propulsion system for high-speed underwater vehicles and demonstration of a water-breathing jet propulsion test facility are investigated. Two stages of combustion, propellant grain combustion and subsequent water combustion, with primary combustion products are adopted. High-pressure-molded propellant grains up to 45% of micro–nano () aluminum were prepared and combusted in the primary chamber, which exhibits mild ignition delay, and a residue of 4–6% was retained. Once water is injected into the secondary chamber, the net thrust generation is increased more than twice from the exhaust jet and improves the specific impulse by 40%. The lean fuel conditions in the secondary chamber lead to reduction in combustion propensity, which causes drop in efficiency. The ultrafine iron-oxide-catalyzed micro–nano blended propellants marginally improved the propulsive performance than the uncatalyzed compositions. The efficiency of the catalyzed propellants was enhanced up to 38.6%. Aluminum agglomeration in primary combustion considerably occurred; apparently, only a fraction of aluminum particles or agglomerates are completely burnt within the secondary chamber, and the remaining aluminum particles are either partially burnt or go unreacted.

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.

Research on aluminum agglomeration characteristics of Al/NEPE propellant

Adding aluminum particles to solid propellants is used to improve the energy density of the propellant and the specific impulse of the engine. However, the aluminum particles will agglomerate to form large-sized condensed combustion products, which can cause nozzle erosion and two-phase flow loss. Therefore, it is necessary to study the aluminum agglomeration characteristics of solid propellants. In this paper, the combustion process of the Al/NEPE propellant was recorded by a high speed camera and a fiber optic spectrometer, and the condensed combustion products of the propellant were collected. The microstructure of the condensed combustion products was analyzed by a scanning electron microscope coupled with energy dispersive (SEM-EDS). These results show that the agglomeration process of aluminum particles may go through three stages: accumulation, aggregation and agglomeration. The condensed combustion products are mainly divided into aluminum agglomerates and smoke oxide particles. Generally, the size of agglomerates exceeds 50 μm, while the size of smoke oxide particles is less than 1 μm.

Agglomerate Size Evolution in Solid Propellant Combustion under High Pressure

Solid propellant combustion and flow are significantly affected by condensed combustion products (CCPs) in solid rocket motors. A new aluminum agglomeration model is established using the discrete element method, considering the burning rate and formulation of the propellant. Combining the aluminum combustion and alumina deposition model, an analytical model of the evolution of CCPs is proposed, capable of predicting the particle-size distribution of completely burned CCPs. The CCPs near and away from the propellant burning surface are collected by a special quench vessel under 6~10 MPa, to verify the applicability of the CCP evolution model. Experimental results show that the predicted error of the proposed CCP evolution model is less than 8.5%. Results are expected to help develop better analytical tools for the combustion of solid propellants and solid rocket motors.

Using microfluidic technology to prepare octogen high-energy microspheres containing copper–aluminum composite particles with enhanced combustion performance

Aluminized explosives are widely used in military applications. However, the easy agglomeration of aluminum (Al) powder and its high ignition temperature greatly hinder the reaction of nano aluminum (n-Al) in the process of explosion, thus affecting the combustion and explosion of aluminum-containing explosives. In the present study, copper (Cu) coated n-Al composite particles were introduced into aluminized explosives, and Cu coated n-Al composite particles and ball-milled octogen (HMX) energetic microspheres were prepared via droplet microfluidic technology using binder. The morphology, crystal form, thermal properties, combustion, and mechanical sensitivity of the microspheres were studied using a variety of techniques. The prepared microspheres had regular spherical morphology and uniform particle size, with an even distribution of particles. Using microfluidic technology will not destroy the crystal form of each particle. The introduction of Cu promoted the thermal decomposition of HMX. The mechanical sensitivity of the prepared Cu/Al microspheres was lower than that of the microspheres only containing n-Al. The Cu/Al microsphere samples exhibited excellent and stable combustion performance, and that the addition of Cu increased the combustion speed compared to the microspheres only containing n-Al. This technology represents an inexpensive and simple way to improve on existing aluminized explosives.

Comparative study on aluminum agglomeration characteristics in HTPB and NEPE propellants: The critical effect of accumulation

Comparative study on aluminum agglomeration characteristics in HTPB and NEPE propellants: The critical effect of accumulation

Investigation on burning behaviors of aluminum agglomerates in solid rocket motor with detailed combustion model

Solid rocket motor has been widely used in space exploration because of its advantages such as high energy density, compact structure and fast response etc., in which aluminum powder is commonly employed as a fuel additive to facilitate density and specific impulse. Evolution process of aluminum particles in solid rocket motor is significantly complicated. To characterize the detailed burning behaviors of aluminum agglomerates in solid rocket motor, this study develops a computational framework, in which propellant combustion model, agglomeration and burning models of aluminum particles are integrated into multiphase flow approach. The model was gradually validated by experimental results. States of single particle and particle clouds in solid rocket motor were analyzed, and the influences of agglomerate size, coarse-to-fine ratio of ammonium perchlorate and pressure on combustion properties of aluminum particles and specific impulse were examined. Results show that the highest and average temperatures in combustion chamber are 3300 and 3100 K, respectively. The agglomerates with diameter less 60 μm can achieve complete combustion at 10 mm away from burning surface, whereas agglomerates with larger size injected from the latter half chamber account for greater proportion in incompletely consumed aluminum particles at nozzle outlet. Larger agglomerate size at burning surface leads to greater particle size distribution in the entire space of solid rocket motor. The combustion efficiency and specific impulse increase with reducing size of agglomerates, and coarse-to-fine ratio of ammonium perchlorate away from critical value, and increase of pressure.

Numerical simulation of aluminum particle agglomeration near the burning surface of solid propellants

Aluminum is an important additive in solid propellants used to improve energy efficiency, but agglomeration of aluminum particles can reduce the specific impulse of the engine, increase two-phase flow losses and cause ablation of the adiabatic layer. Therefore, it is important to understand the mechanism of formation, mode of movement and particle size characteristics of aluminum agglomerates in solid propellants to study the inhibition of agglomeration. In this paper, a three-dimensional numerical simulation of aluminum particle agglomeration near the burning surface of solid propellants was conducted using the discrete unit method based on direct tracking calculations of the detailed motion of individual particles. The model considered physical processes such as burning surface motion, aluminum particle precipitation, turbulent pulsation and the heating and melting of aluminum particle and used a grid-based contact detection method to accelerate the computational process. The agglomeration process of a solid propellant with specific formulation was experimentally investigated. Aluminum particle aggregates underwent splitting, and the final aluminum particle agglomerates were spherical droplets with typical structure. The agglomeration process of this propellant was investigated by means of a simulation. The accuracy of the agglomeration model was verified by comparing the results of the model for the agglomeration process and the shape and size distribution of agglomerates with available experimental data. Compared with the experimental results, the deviations of the equivalent particle sizes of agglomerates D10, D50, D90, D4,3, and D3,2 obtained by using the simulation results were 6.3%, 6.3%, 9.4%, 7.5% and 6.7%, respectively. The aluminum particle agglomeration model established in this paper can predict the particle size distribution of aluminum agglomerates near the burning surface of solid propellants. The model was also used to further investigate the effects of aluminum particle content, ammonium perchlorate (AP) particle size and environmental pressure on aluminum particle agglomeration. Increasing the aluminum content or AP particle size aggravated aluminum particle agglomeration, whereas increasing pressure weakened it.