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Prediction of combustion characteristics of aluminum particle is of great significance for a variety of propulsion and power systems to achieve optimal energy release within a limited residence time. In this study, a diffusion-controlled combustion model for micro aluminum particle was developed and validated to predict the burning time and capture the evolution of particle size during combustion in environments with multiple oxidizers. Thereafter, the key factors influencing particle combustion were evaluated including particle size, ambient temperature, oxidizer concentration and characteristics of oxide cap. The burning time drops as the oxidizer concentration and the ambient temperature increase, and their influences on burning time become weakened gradually. The effect of oxidizer concentration presents more obvious than ambient temperature. As the number of oxide cap increases from 2 to 5, the burning time rises by 6.9%–27.4%. Then a non-linear empirical equation was proposed to describe the effective oxidizer, which is more accurate than the existing linear correlation, especially for H 2 O. Finally, a formula capable of predicting the burning time was proposed and validated, providing a convenient and accurate method for practical application. • A model is developed to simulate combustion of micro aluminum particle in multiple oxidizers. • Influences of environment and particle property on combustion behaviors are examined. • A non-linear correlation is proposed for the equation of effective oxidizer. • A more accurate prediction formula for burning time is proposed. [ABSTRACT FROM AUTHOR]

Aluminum (Al) particles are regarded as general addictive into solid propellants not only as an extra energy source but also as a metallic damping-carrier on combustion instability. It is possible that the stability in propellant can be improved through partial Al substitution. However, knowledge of unsteady combustion characteristics of Al particles has been insufficient. In order to explore the unsteady combustion and attenuation characteristics of Al particle, a methane micro-combustor is designed. The acoustic oscillation of combustor is found to be excited, regardless of the Al particles mass of diameter. Moreover, the effects of 1) particle size and 2) mass are examined in detail. The burning of micro-sized Al particles in present experiment is attributed to melt dispersion mechanism. The rupture time is in the range of 2.18 ms to 2.81 ms for Al particles with mean diameter of 10 μm, 20 μm and 30 μm. With the increasing of particle size, the sound level of high-frequency oscillation increases from 65 dB to 85 dB. However, the attenuation effect for high-frequency oscillation is weakened for larger particle size. Finally, it is found that the pressure growth rate can be dramatically increased from 1000 Pa/s to 11,000 Pa/s and the peak amplitude of high frequency oscillation is augmented, as the Al particles mass ratio is increased. The present research sheds lights on an effective evaluation of unsteady Al combustion by considering particles parameters. [Display omitted] • A methane micro-combustor is proposed to investigate combustion oscillation characteristics of Al particle. • The acoustic oscillation of combustor is excited by combustion of Al particles. • The effects of particle parametes on the unsteady combustion are discussed. • Stimulation and damping effect of particle parameters on the combustion oscillation are summarized. [ABSTRACT FROM AUTHOR]

A three-dimensional model has been developed to capture burning properties of ammonium perchlorate(AP)/hydroxyl–terminated polybutadiene(HTPB) composite propellant. Interfacial coupling between condensed domain and gas phase was conducted to obtain the surface temperature and burning rate. The condensed phase was governed by heat transfer equation, and reacting Navier-Stokes equations with pressure dependent 3-step and 12-species global kinetics were adopted. The current model was verified and validated by comparing with analytical and experimental results. Thereafter, the influences of ellipsoidal AP's orientation and aspect ratio on the burning rate were investigated. The flame structure of a typical propellant with ellipsoidal AP particle was examined, and the characteristic heights of multiple flames were determined. Besides, the flame properties of a typical propellant with spherical/ellipsoidal AP particle and different AP topologies were analyzed. Furthermore, the temperature sensitivity coefficients were calculated for composite propellant with different AP contents and sizes, matching reasonably with the experimental results. [Display omitted] • A 3-D model was developed to estimate the burning properties of composite propellant under different initial conditions. • Influences of the shape and orientation of ellipsoidal AP particles on burning rate were analyzed. • Temperature distribution characteristics above propellant unit with different AP particle topologies were elucidated. • Temperature sensitivity coefficient was determined for composite propellant with different AP particle contents and sizes. [ABSTRACT FROM AUTHOR]

A series of experiments was conducted to study the ignition and combustion characteristics of aluminum particle-laden flows in a propulsion system that used powder as the main fuel. In these experiments, a laminar flame produced by hydrocarbon fuel provided a high-temperature environment for the ignition and combustion of the aluminum particles. The effective oxidant content and flow velocity were adjusted by varying the mass flow rates of three gases (methane, air, and oxygen), and the ignition delay and burning time of the aluminum particles in the particle-laden flow were determined using a high-speed camera. The total time was the sum of the ignition delay time and burning time. The experimental results showed that the ignition delay time could be fitted as a function of the particle diameter, expressed as t i = a 0 + b 0 D ; the burning time and total time could also be fitted as functions of the particle diameter, expressed as t b = aD b . As the effective oxidant content increased, the burning time decreased significantly, and the total time decreased slightly. The ignition delay time, burning time, and total time were obviously decreased with an increase in the flow velocity of the hot gas. Compared with the effective oxidant content, the flow velocity of the hot gas played a greater role in the reduction of the total time for the aluminum particles in a low-oxidant environment. The agglomeration and separation processes for the burning particles in aluminum particle-laden flows were analyzed in detail. • A low-density powder feed device is developed. • The time of dynamic ignition delay and combustion is used to assess the combustion characteristics of aluminum particle. • The effects of oxidant concentration and velocity of hot gas on the total time are studied in low oxidant environment. • The agglomeration and separation processes of aluminum particle-laden flow are analyzed in detail. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel, inexpensive and controllable 'one-step' in-situ electroless plating strategy was proposed. • Three types of spherical core-shell structured energetic Al@Ni-P composites with a double shell were designed and fabricated. • Al@Ni-P composites showed enhanced thermal reactivity, shorter ignition delay time and higher rate of energy release. • The coating formation and combustion mechanisms of energetic Al@Ni-P composites were revealed. A novel 'one-step' in-situ electroless plating method capable of controlling phosphorus content was proposed for the fabrication of spherical core-shell energetic Al@Ni-P composites, aiming to prompt particle ignition by adding easy-to-ignite P element. In this experimental study, the preparation and characterization of Al@Ni-P composites, as well as the effect of coating an alloy shell on the Al particle surface were mainly addressed. The morphology and chemical composition heterogeneity demonstrated that the Al@Ni-P composites possessed comparatively dense double-shell structure. Thermal analysis showed that these composites could accelerate the rate of energy release by decreasing the apparent activation energy (E a) for particles oxidation. Laser ignition experiments were then conducted to examine the influence of Ni-P alloy on breaking through the barrier of ceramic phase Al 2 O 3 shell to improve the ignition and combustion performances. Moreover, the composites with surface P of 12.68 at.% exhibited extremely shorter laser ignition delay time of 86 ms under 1 atm O 2 atmosphere, reducing by 65.46% compared with that of raw Al particle (249 ms); and the pressurization rate reached 88.08 kPa/s under 10 atm O 2 atmosphere, which is approximately 11 times that of raw Al particle (8.09 kPa/s). These drastic improvements were attributed to the synergistic effects of double-shell structure and intermetallic compound exothermic reaction, which enhanced mass and heat transfer. These results demonstrate that Al@Ni-P composites have a promising future in solid composite propellants, which exhibit shorter ignition delay time and more vigorous combustion than raw Al. This strategy is expected to provide a new method for the construction of complex core-shell structural materials in other research fields. [ABSTRACT FROM AUTHOR]

• A 2D/3D model was developed to simulate composite propellant burning under wide pressure range. • Flame structure and standoff distance for multiple flame were examined in detail. • Heat flux determined by AP monopropellant, premixed binder and primary diffusion flames controls the burning rate. • Effect of primary diffusion flame on the burning rate alleviates with increasing AP particle size. • Relationship between cone-angle of final diffusion flame and flow velocity was elucidated. This study developed a combustion model for ammonium perchlorate (AP)/hydroxyl–terminated polybutadiene (HTPB) composite propellant to capture the flame structure and elucidate the mechanism governing burning rate at different pressures, AP sizes and surface morphologies. A numerical model was proposed to characterize gas domain by solving Navier-Stokes equations of reacting flow field with 3-step and 12-species global kinetics; and the condensed domain was coupled by considering interfacial energy balance. The present model was validated with experimental results of both pure AP and AP/HTPB combustion, in which the mean absolute percentage error of burning rate for all cases was less than 5%. The standoff distances of AP monopropellant, premixed binder and primary diffusion flames were determined as 14.2, 12.1 and 25.8 μm at 2 MPa, respectively, reducing with pressure. The front and height of final diffusion flame were determined by axially varied radial gradient of temperature and its height was obtained as 152 μm at 2 MPa, which rises as the pressure or AP particle size increases. The increase of burning rate with pressure is attributed to the raise of surface heat flux dominated by standoff distances of AP monopropellant, premixed binder and primary diffusion flames, in which the effect of primary diffusion flame enhances as the AP size decreases resulting from enlarged lateral diffusion of species. Finally, the effects of protrusive, recessive AP surfaces and their combination with planar AP surface on the flow and flame properties were examined. [ABSTRACT FROM AUTHOR]