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Abstract 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.

Aluminum 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. [ABSTRACT FROM AUTHOR]

Aluminum (Al) particles are commonly added to energetic materials including propellants, explosives and pyrotechnics to increase the overall energy density of the composite, but aluminum agglomeration on the combustion surface may lower the combustion efficiency of propellants, resulting in a loss in two-phase flow. Therefore, it is necessary to understand the agglomeration mechanism of aluminum particles on the combustion surface. In this paper, a high-pressure sealed combustion chamber is constructed, and high-speed camera is used to capture the whole process of aluminum accumulation, aggregation and agglomeration on the combustion surface, and the secondary agglomeration process near the combustion surface. The microscopic morphology and chemical composition of the condensed combustion products (CCPs) are then studied by using scanning electron microscopy coupled with energy dispersive (SEM-EDS) method. Results show that there are three main types of condensed combustion products: small smoke oxide particles oxidized by aluminum vapor, usually less than 1 μm; typical agglomerates formed by the combustion of aluminum agglomerates; carbonized agglomerates that are widely distributed, usually formed by irregular movements of aluminum agglomerates. The particle size of condensed combustion products is measured by laser particle size meter. As the pressure increases from 0.5 MPa to 1.0 MPa in nitrogen, the mass average particle size of aluminum agglomerates decreases by 49.7%. As the ambient gas is changed from 0.5 MPa nitrogen to 0.5 MPa air, the mass average particle size of aluminum agglomerates decreases by 67.3%. Results show that as the ambient pressure increases, the higher oxygen content can improve combustion efficiency and reduce the average agglomeration size of aluminum particles.

A method is developed to produce large titanium monolithic burning particles with a diameter of 250–550 μm. The combustion of titanium particles in free fall in air is investigated. The characteristic times of the beginning of fragmentation, end of fragmentation, and end of burning as well as the laws of motion of particles (in particular, coordinate and velocity at the starting moment of fragmentation) are determined using video recording. The size of particles at which the fragmentation mode changes from "star" to "fir branch" is estimated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper describes a technique for the sampling of condensed combustion products (CCPs) of metalized propellants. Experiments were carried out with model propellants containing 15.7% of a metallic fuels, ammonium perchlorate, and an inert binder under pressure levels of about 2, 4, and 8 MPa. We used microsized powders of aluminum, boron, aluminum diboride and aluminum dodecaboride as metallic fuels. For the initial fuels, we defined the granulometric composition and determined the content of the active (non-oxidized) metal by the cerimetric method. The quenching of the dispersed phase particles was performed near the burning surface by a co-current argon flow. The paper provides the burning rate of the propellants and CCPs characteristics – the particle size and morphological composition, and the combustion incompleteness of the metal fuels. We have identified the morphology features of the agglomerated particles of aluminum and boron. The paper also contains calculated data on the heat release efficiency of the tested propellants. The obtained experimental data provide a basis for the development and verification of theoretical models on the combustion of metalized propellants. For the studied propellant formulations, we have not identified the increase in the boron combustion completeness due to the adding of aluminum into the propellant composition. • Propellant, boron, aluminum boride, combustion completeness, cerimetric analysis. [ABSTRACT FROM AUTHOR]

In order to solve the problem of weak injection of fuel-rich propellant and to enhance its burning rate, the combustion behaviors of Al/Mg-based fuel-rich HTPB propellants with 40wt% metal content were studied with and without the addition of fast burning compound hydrated tetra-(4-amino-1, 2, 4-triazole) copper perchlorate (ACP) and burning rate catalyst Pb5. The results show that the burning rates of the Al/Mg-based fuel-rich HTPB propellants with the addition of fast burning compound and catalyst increase significantly, and the pressure exponents decrease. With the increase of pressure, the fluctuation of burning interface of the propellants increases and the flame height decreases. Compared with the Mg-based propellants of high burning rate, the agglomeration of condensed combustion products of the Al-based propellants is much more serious; the combustion extent of aluminum in the Al-based propellants is obviously lower than that of magnesium in the Mg-based propellants. These results are very meaningful for the design of fuel-rich HTPB propellants with high performance.

Methods for the collection and analysis of condensed combustion products (CCPs) of large monolithic titanium particles with a diameter of 350–460 m in air at atmospheric pressure are described. Detailed data on the granulometric, morphological, and phase compositions of CCPs and the number of particles produced by a single burning mother particle are presented. The following morphological types of CCP particles were identified: compact spheres (combustion residues of mother particles and their fragments) and aerogel round and elongated comet-shaped objects (rarefied openwork particles consisting of chains of nanosized spherules). According to the O/Ti atom ratio, all types of CCP particles are oxide. The mass fraction of aerogel objects in CCPs is 0.52–0.98, and their physical density is about 0.8 g/cm3. The characteristic sizes of compact spheres are 2–410 m, those of aerogel round objects are 11–470 m, and the length of aerogel comet-shaped objects can reach 13 mm. The typical sizes of spherules are 25–100 nm. Large compact spheres 200–400 m in size typically contain a gaseous bubble and have a density of about 0.9 g/cm3. [ABSTRACT FROM AUTHOR]

The screening method has been used to investigate the effects of magnesium fluoride, polytetrafluoroethylene, and ammonium hexafluorotitanate additives in a composite propellant on the basis of boron or aluminum dodecaboride as a fuel (37%), ammonium perchlorate as an oxidizer and an active binder on the parameters of propellant combustion. At pressures of 1.2 and 2.4 MPa, propellant combustion rates were measured, samples of condensed combustion products were taken, and burning residue in the form of a carcass was investigated. Using the cerimetric method of chemical analysis of the selected combustion products, incompleteness of propellant combustion was indentified and the efficiency of its energy release was evaluated. It is shown that the addition of small quantities (1%) of the said substances to the propellant makes it possible to control the velocity of its combustion and, in some cases, to raise the efficiency of energy release. [ABSTRACT FROM AUTHOR]

The dispersed composition and mass concentration of the condensed phase formed during the combustion of high-energy materials, model fuels containing aluminum and boron, were studied at atmospheric pressure and at elevated pressures in a closed high-pressure vessel. Particle size distribution, as well as mass concentration of the suspended dispersed phase in a wide range of particle sizes and pressures were determined. [ABSTRACT FROM AUTHOR]

Energy density, which is an important indicator of the performance of solid propellants, is known to increase with the addition of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). However, it remains unclear how CL-20 affects the decomposition of ammonium perchlorate (AP) and energy release. Here, the effects of CL-20 on the combustion performance and agglomeration of propellants were investigated. The addition of CL-20 decreased AP decomposition temperature and the energy required for the transformation of AP crystals from orthorhombic to cubic. The burning rate and pressure exponent of the propellant with 42% CL-20 were significantly higher than those of the propellant containing 20% CL-20. Thus, adding CL-20 to the propellant improves the energy characteristics and burning rate and the pressure exponent increases. At low combustion chamber pressure, the agglomeration of the propellant containing a high content of CL-20 will be blown away from the combustion surface only after staying on that surface for a short time. In this process, the probability of volume growth of the agglomeration after merging with other agglomerations greatly decreases, thus reducing the overall agglomerate particle sizes; further, the addition of a small amount of CL-20 to the propellant may lead to a reduction in agglomerate particle sizes. AP with a smaller particle size weakens the agglomeration in the combustion process and decreases the number of agglomerates with large particle sizes. These findings lay the foundation for the development of novel high-energy propellants. [ABSTRACT FROM AUTHOR]

• The realistic distribution of propellant pockets is obtained through micro-CT imaging. • A new aluminum agglomeration model based on realistic pocket distribution is established. • The relationship between the agglomerate size and the melted Al within the pocket is modified. • The predicted D 4, 3 and goodness of fit for distribution are verified by experiment. 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. [ABSTRACT FROM AUTHOR]

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.

The effect of polymer binders on the thermal behavior, combustion, and composition of condensed gasification products of model boron-containing compositions was studied to determine the optimal fuel composition for the gas generator of a ducted rocket engine. The physicochemical transformations of boron particles during combustion wave propagation in a solid propellant and during the interaction with the decomposition products of oxidizer and binder were investigated. The binder components were oligodiene urethane and polyether urethane rubbers plasticized with various substances: phthalic acid dimethyl ester, transformer oil, and dioctyl sebacate. Samples of successively complicated composition with the same rubber-to-plasticizer ratio were studied. After thermal analysis of plasticizers and rubbers, plasticized binders and compositions of these binders with boron were investigated and the combustion parameters of model compositions obtained by adding a third component—ammonium perchlorate—were determined. It was found that the thermal decomposition of the more heat-resistant binder and the melt formation of boron oxide largely overlapped and proceeded simultaneously. Features of the combustion of compositions with various binders were identified by high-speed video recording. The composition and microstructure of condensed gasification products collected from the gas phase directly from the burning surface and those remaining in the form of a porous framework after combustion were studied in detail by electron microscopy and thermogravimetry. Based on a comparative analysis of experimental data, it is concluded that it is preferable to use plasticizers and rubbers with reduced thermal stability in inert binders for compositions with amorphous boron designed for gas generators of ducted rocket engines. The possibility of the formation of boron suboxide (B6O) crystals during combustion of boron-containing compositions is shown for the first time. [ABSTRACT FROM AUTHOR]

Condensed combustion products of model propellants based on aluminum boride in the combustion chamber of the gas generator of a ducted rocket engine have been studied. The mass fraction of the condensed combustion products, their chemical composition, and their particle size distribution have been determined. It has been demonstrated that the condensed combustion products mainly contain boron compounds with carbon, nitrogen, and oxygen, and it is in this form that boron enters the secondary combustion chamber of the ducted rocket engine. The mass fraction of elemental boron in the condensed combustion products does not exceed 15% and decreases with increasing pressure. It has been found that, regardless of the types and contents of the major components of the model propellants, there are no large (more than 100 μm in size) particle conglomerates at the outlet of the gas-generator nozzle. A comparison of the experimental data with the results of thermodynamic calculations indicates that the calculation methods should be corrected for the nonequilibrium of the combustion products. [ABSTRACT FROM AUTHOR]

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Aluminum(Al)/ammonium perchlorate(AP)/hydroxyl-terminated polybutadiene (HTPB) propellant is the most widely used propulsion system currently. Its ignition and combustion processes are fairly complex and need to be better understood. In this study, the thermal decomposition, ignition, and combustion properties of an Al/AP/HTPB propellant were investigated using a thermogravimetry–differential scanning calorimetry and a laser ignition testing system. The morphology and size distribution of the condensed combustion products (CCPs) were analyzed using a laser particle size analyzer and scanning electron microscopy. Results showed that the thermal decomposition process of the propellant consisted of three stages. The first stage (100–430 °C) was a major mass loss stage and exhibited typical features of AP decomposition. The second stage (430–630 °C) was mainly accompanied by the decomposition of remaining HTPB as well as slight oxidation of Al particles. In the third stage, further oxidation of Al particles resulted in a small mass increase. Due to the continuous emission problem, only a few combustion intermediates were identified in the combustion emission spectrum curves. The propellant combustion process could be roughly divided into three stages, and the flame development stage lasted longer than the flame decline stage. As the pressure increased, the propellant ignition delay time decreased and the burning rate increased significantly. The effect of pressure on ignition delay was more pronounced at low pressures. The CCPs consisted of three types. The oxidation of Al particles in the propellant followed the diffusion reaction mechanism. Agglomerates appeared in several different morphologies and had relatively low combustion efficiency under experimental conditions. A 2-D pocket-like structure was observed on the propellant surface. [ABSTRACT FROM AUTHOR]

Composite propellants are tested using the quench particle collection bomb (QPCB) for the pressure ranging from 2 to 8 MPa to estimate the particle size distribution of aluminum agglomerates from quenched combustion residues emerged out from the burning surface. The major ingredients included in the propellants are ammonium perchlorate (AP), aluminum (Al), hydroxyl-terminated polybutadiene (HTPB), and toluene diisocyanate (TDI). Five propellant compositions are considered in this study; two of them are mixed with catalysts. Propellant formulation variables like the coarse AP/fine AP ratio, total solid loading, catalyst percentage, and aluminum content are varied to assess their effects on the aluminum agglomeration process at different pressures. Unburnt aluminum in agglomerates is continuously getting combusted as they move out from the propellant burning surface. Large agglomerates comprise both Al2O3 and unburnt aluminum. The majority of agglomerates are spherical in shape, and the sizes vary from 31 to 115 m for non-catalyzed propellants and from 28 to 136 m for catalyzed propellants over the tested pressure conditions. These results can give further insight into the aluminum agglomeration process of catalyzed and non-catalyzed propellants and also affect the choice of the propellant ingredient percentage aimed at reducing aluminum agglomeration, which causes two-phase flow losses of thrust and slag accumulation in full-scale solid rocket motors. [ABSTRACT FROM AUTHOR]

Cyclotrimethylene trinitramine (RDX) is widely used in aluminized solid propellants to increase specific impulse. Experimental investigations on the effect of RDX content on the propellant combustion and agglomeration were thoroughly conducted based on Thermogravimetry-differential scanning calorimetry, laser ignition, combustion diagnosis and a new home-made condensed combustion products (CCPs) collection device. The results show RDX inclusion inhibits the decomposition of AP. The ignition delay and self-sustaining combustion time of aluminized propellant increase, while the combustion intensity decreases with increasing RDX content. RDX addition is found to decrease the burning rate within 6–10 MPa. Compared with the baseline propellant, RDX leads to obvious aggravation of aluminum agglomeration on the propellant burning surface. The 6 wt% and 12 wt% RDX-included propellants increase the average CCPs size from 46.3 μm to 86.7 μm and 96.6 μm, respectively. The combustion efficiency of aluminum in propellant is reduced by 15% when the RDX content is increased from 0 to 12%. Overall, RDX plays a comprehensive effect on the ignition, combustion and agglomeration characteristics of aluminized propellant. Results of this study can help guide the application and development of RDX in aluminized propellants for solid rocket motors. • A new device was developed to collect the CCPs of aluminized propellant. • The effects of RDX content on propellant agglomeration and combustion were studied. • 12% RDX addition leads to 109% increase in the CCPs size. • Detailed mechanism of RDX on propellant combustion and agglomeration was proposed. [ABSTRACT FROM AUTHOR]

This study investigated the behavior and temperature of condensed combustion products (CCPs) in a magnesium–water vapor flame. The experiments were conducted using magnesium plates with a surface area of 5 × 5 mm 2 and an oxidizer pressure in the range of 10–100 kPa, in consideration of potential applications in microspacecraft. Compared with the air and oxygen cases, a substantial quantity of white residue remained around the fuel when the oxidizer was water vapor. Scanning electron microscopy revealed that this was deposited by submicron CCPs formed in the flame. Additionally, the laser light extinction method revealed the spatial distribution of the CCP. In the case of water vapor, the point of maximum CCP density was closer to the fuel than to the oxygen, which caused a difference in residue formation. Next, the temperature of the CCP in the combustion flame was calculated using Planck's law with a spectrometer. This study utilized six types of emissivity indices, ranging from 0 to -4, for power fitting. A more plausible temperature range was determined using the coefficient of determination, leading to an uncertainty of 20% in the temperature calculation using Planck's law. The observed temperature was approximately 600 K lower for water vapor than for oxygen, a discrepancy exceeding the difference predicted by the adiabatic flame temperatures. The two-dimensional temperature distribution was measured with CCD cameras using the two-color pyrometer method, and the distance to the point of maximum reaction temperature was shorter for water vapor than for oxygen. This study concluded that the decrease in the reaction distance in the water vapor case owing to the lower temperature significantly influenced the spatial distribution of CCPs within the flame. Novelty and significance statement : The present study is novel in that the condensed combustion products in a magnesium flame were identified, and their temperature was measured distinctly from the fuel. These observations explain why the oxide residue accumulated only in water vapor cases. Our research stands out, recognizing the limited experimental forays into magnesium combustion with pure water vapor. The combustion research on these materials, which are safe and obtainable from space bodies, has profound implications for advancing space propulsion and energy. [ABSTRACT FROM AUTHOR]

The combustion of 81/19 Al/B agglomerates with a diameter of 320–780 μm in free fall in air was first studied using model monodisperse agglomerates. The dependence of the burning time on size was determined. Burning residue particles have been studied by morphological, chemical, mass, particle size, and elemental (EDS) analyses. It has been found that the essential distinctive features of the combustion mechanism of Al/B agglomerates compared to aluminum are a long combustion duration; a specific core-shell structure of the particles, with boron present in the core and absent in the shell; slight changes in particle mass and diameter during combustion. [ABSTRACT FROM AUTHOR]

This paper presents a review of studies of the combustion of composite propellants containing a combined fuel based on aluminum and boron. A method for studying the combustion of large particles of the combined Al + B fuel in air is described. Burning agglomerated Al/B particles 300–700 μm in diameter were obtained by ignition of miniature pieces of a composition containing 32% binder and 68% micron-sized aluminum and boron powders in the ratio Al/B = 81/19 placed in a burning metal-free sample. Agglomerates formed by the merger of many small particles burned in free fall in air. Procedures are described that were used to process video records of the combustion process and study condensed combustion products (combustion residues of agglomerates) in order to determine the burning time and analyze the transformation of the combined fuel into oxide. [ABSTRACT FROM AUTHOR]

Abstract Main positions of the mathematical model of smoke oxide particles (SOPs) formation at the burning surface of condensed system are developed. The model includes description of SOPs formation during burning of non-agglomerating metal in gas phase near surface of the burning condensed system, and during burning of metal of agglomerating particles on the surface of skeleton layer. Results and perspectives of using the model are considered for two different condensed systems (based on active binder and based on ammonium nitrate), which produce SOPs mostly during the development of one of the mentioned mechanisms. A comparison of numerical and experimental results has allowed to make a conclusion about their satisfactory agreement. An analysis of the model allows to create a base for its future improvement and application. Highlights • Smoke oxide particles (SOPs) modeling during solid propellants combustion. • Application of the model for propellants with active binders and NH 3 NO 3 -based. • SOPs modeling in gas phase near the burning surface for non-agglomeration of particles. • SOPs modeling on the burning surface for agglomerating particles. • Validation of the model with experimental results. [ABSTRACT FROM AUTHOR]

Abstract An intrusive technique for particles capturing in supersonic-high temperature flows to be used in solid rocket motors plume characterization is proposed. A supersonic probe for the collection of the condensed combustion products in the proximity of the rocket nozzle has been designed to handle a progressive deceleration and cooling down of the exhaust gas, aiming at preserving liquid particles from break-up. A quasy-1D gas dynamics software (POLIRocket-V2) based on the Shapiro method and normal shock wave theory, supported by a CFD investigation using the DLR TAU code, was employed for the feasibility and the design study. Preliminary cold flow tests have been performed in the VMK supersonic vertical wind tunnel at DLR in relevant flow conditions. A proofs of concept of the probe working principle and the collection methodology were studied. Highlights • A supersonic device and a scrubber merged for CCPs collection outside a rocket nozzle. • Shapiro approach as a valid alternative to CFD for the feasibility study. • Cold flow tests at M = 3: proof of concept of the working principle. • Seeded flow at M = 3: demonstration of the collection methodology. • Size distribution of original and collected particles: no break up and coalescence. [ABSTRACT FROM AUTHOR]

The combustion of monodisperse titanium particles with a characteristic size of 38 and 320 μm moving in air was studied. Pyrotechnic samples generating monodisperse particles were burned in a chamber with a nozzle to impart an initial velocity to the burning particles. The particles were accelerated by the jet of gaseous combustion products discharged from the nozzle. The maximum path-averaged particle velocity relative to the ambient air was 7.9 m/s. Combustion of moving particles was carried out in a quartz tube 2 m long. At the end of the combustion, combustion products (oxide aerosol) were sampled from the tube using a thermophoretic precipitator. Size distribution functions of nanometer-sized spherule particles were determined by processing electron micrographs of samples. The velocity and burning time of burning particles were determined by video recording at a speed of 300 fps. It was found that increasing the velocity of motion of agglomerate particles with a diameter of 320 μm relative to the gas from 0.9 to 7.9 m/s leads to a decrease in the size of the spherules from 28 to 19 nm and to a decrease in the burning time from 0.45 to 0.26 s. [ABSTRACT FROM AUTHOR]

Ignition and combustion characteristics of amorphous boron (B) have received much attention from researchers in recent decades. A pressurized concentrated ignition experimental system was designed to evaluate the ignition and combustion characteristics of molded B samples. The ignition experiments were carried out under different oxygen pressures (1–9 atm). The condensed combustion products were then analyzed using a scanning electron microscope, an X-ray energy dispersive spectrometer, and an X-ray diffractometer. Furthermore, the complete oxidation rates of the samples were detected by inductively coupled plasma chromatography. As the oxygen pressure increased, the combustion intensity of the samples steadily increased, and the ignition delay time and combustion time both decreased. Under the oxygen pressure of 9 atm, the average ignition delay time and combustion time were 2640 ms and 2596 ms, respectively, and the highest combustion temperature reached 1561.5 °C. The initial diffusion flame on the sample surface was green and the brightest, which was produced by an intermediate combustion product, BO 2 (corresponding molecular emission spectrum wavelength, 547.3 nm). Emission spectra of another intermediate product, BO (431.9 nm) was also detected. Two different types of structures were found in the condensed combustion products of the samples. The first type was the flaky B 2 O 3 structure, and the second type was the flocculent structure of incomplete combustion products. The B 2 O 3 content in the condensed combustion products increased with the oxygen pressure during combustion. The complete oxidation ratio of the samples also increased with the oxygen pressure, and reached the maximum value of 68.71% under 9 atm. Overall, the samples showed better ignition and combustion characteristics under higher oxygen pressure. [ABSTRACT FROM AUTHOR]

The paper presents the results of measurement of the burning rate of aluminized composite solid propellants (CSPs) and parameters of sampled condensed combustion products including their particle size distribution, chemical and phase composition. Effect of ultra-fine iron and amorphous boron additives in CSP formulations based on AP, butadiene rubber and Alex (2 wt. %) on the combustion characteristics was studied. The sampled particles of condensed combustion products (CCPs) were classified as oxide particles (less than 55 µm) and agglomerate particles (up to 500 µm). The CCPs particles were subjected to morphological, particle size, chemical and phase analyses. It was found that partial replacement of Alex by 2 wt. % of iron in CSP leads to 1.3-1.4 fold increase in the burning rate in the pressure range of 2.2-7.5 MPa. At the same time the agglomeration of metal fuel is slightly increased: the mean diameter of agglomerate particles in CCPs is increased up to 1.2 fold and their content is increased up to 1.4 fold. The content and mean diameter of oxide particles in CCPs are reduced by 16 % and 13 %, respectively. Upon partial replacement of Alex by 2 wt. % of boron the burning rate is practically unchanged as compared with that for basic propellant with Alex. However the agglomeration is significantly enhanced, which is manifested at the increase in the agglomerate particles content in CCPs by 1.8-2.2 times, increase by 1.6-1.7 times in the agglomerates mean diameter and increase in the unburned metal fraction in agglomerates by 1.6-1.9 times. The content and the mean diameter of the oxide particles are reduced more significantly than in the case of iron introduction, namely, by 20-30 % and 30-40 %, respectively. [ABSTRACT FROM AUTHOR]

Aluminum hydride (AlH 3 ) has attracted much attention due to its potential to replace aluminum (Al) as a novel energetic material in solid propellants. In this research, ammonium perchlorate (AP) and perfluoropolyether (PFPE) as functionalized coatings and a combination of acoustic resonance and spray drying technology have been employed to prepare AlH 3 @Al@AP (AHAPs) and AlH 3 @Al@AP@PFPE (AHAPs-F) energetic composite particles. The formulations of composite propellants and modified AlH 3 particles were designed and fabricated. Their thermal reactivity, reaction heat, density, vacuum stability, combustion performance, and condensed combustion products (CCPs) have been systematically investigated. The results show that the solid propellants containing AHAPs (SP13) and AHAPs-F (SP14) composites can significantly enhance the reactivity and energy output compared to conventional solid propellants with the mechanical mixture Al/AlH 3 (SP12). In particular, the total heat releases of SP13 and SP14 are almost 1.2 and 1.7 times higher than those of conventional ones (SP12, 1442 J g -1 ), respectively. Among the AlH 3 -based propellants, SP14 propellants exhibit the highest reaction heat of 5887 J g -1 , the most intensive flame radiation of 31.4 × 10 3 , and the highest combustion wave temperature of 2495 °C. Moreover, the particle size distribution of CCPs from SP14 propellants is much narrower and smaller than that of SP12, resulting in higher combustion efficiency.

Cyclotetramethylene tetranitramine (HMX) is usually added to solid propellants, but the inclusion of HMX significantly affects the propellant combustion characteristics, especially by yielding more condensed combustion products (CCPs). The effect of HMX size on aluminum agglomeration and CCPs of hydroxyl-terminated polybutadiene (HTPB)-based propellant is evaluated experimentally in this paper. Using a thermogravimetry–differential scanning calorimeter, laser ignition setup, and high-pressure combustion bomb, we have examined the propellants' thermal reactivity, ignition behavior, combustion characteristics, agglomeration, and CCPs with 10–200 μm virgin HMX particles. The results indicate that HMX in propellant suppresses the pyrolysis reaction of ammonium perchlorate. Meanwhile, the exothermic peak temperature of 200 μm HMX increases by 17.8 °C compared to 10 μm HMX. With the increase of HMX size from 10 μm to 200 μm, the combustion intensity of the propellant increases, and the ignition delay time of aluminum particles decreases from 580.6 ms to 498.3 ms. Increasing HMX size leads to an increased burning rate, while the pressure exponent decreases from 0.34 to 0.23. Including HMX in propellants also promotes aluminum agglomeration during propellant combustion. With HMX size expanding from 10 μm to 200 μm, the mean agglomerate size in the CCPs decreases from 221.9 μm to 124.5 μm, and the fraction of aluminum involved in agglomeration decreases from 0.206 to 0.104. The unburned aluminum content in agglomerated particles increases monotonically with increasing agglomerates size. Overall, HMX size has a significant impact on the ignition, combustion, and agglomeration characteristics of aluminized propellant. The conclusions of the paper can be used as a guidance for optimizing the propellant formulation [ABSTRACT FROM AUTHOR]

In solid propellants, aluminum is widely used to improve the performance, however the condensed combustion products especially the large agglomerates generated from aluminum combustion significantly affect the combustion and internal flow inside the solid rocket motor. To clarify the properties of the condensed combustion products of aluminized propellants, a constant-pressure quench vessel was adopted to collect the combustion products. The morphology and chemical compositions of the collected products, were then studied by using scanning electron microscopy coupled with energy dispersive (SEM-EDS) method. Various structures have been observed in the condensed combustion products. Apart from the typical agglomerates or smoke oxide particles observed before, new structures including the smoke oxide clusters, irregular agglomerates and carbon-inclusions are discovered and investigated. Smoke oxide particles have the highest amount in the products. The highly dispersed oxide particle is spherical with very smooth surface and is on the order of 1–2 µm, but due to the high temperature and long residence time, these small particles will aggregate into smoke oxide clusters which are much larger than the initial particles. Three types of spherical agglomerates have been found. As the ambient gas temperature is much higher than the boiling point of Al 2 O 3 , the condensation layer inside which the aluminum drop is burning would evaporate quickly, which result in the fact that few “hollow agglomerates” has been found compared to “cap agglomerates” and “solid agglomerates”. Irregular agglomerates usually larger than spherical agglomerates. The formation of irregular agglomerates likely happens by three stages: deformation of spherical aluminum drops; combination of particles with various shape; finally production of irregular agglomerates. EDS results show the ratio of O to Al on the surface of agglomerates is lower in comparison to smoke oxide particles. C and O account for most element compositions for all the carbon inclusions. The rough, spherical, strip shape and flake shape carbon-inclusions are believed to be derived from the degradation products of the binder or oxidizer, while the fiber silk is possibly the combustion product of fiber inside the heat insulation layer of the propellants. Images of products at different pressures reveal high pressure reduces the degree of agglomeration. The chemical compositions, size range and content of all the observed structures are given in this paper. Results of our study are expected to provide better insight in the working process of solid rocket motor. [ABSTRACT FROM AUTHOR]

In this study, we investigate the aluminum agglomeration characteristics in hydroxyl‑terminated polybutadiene (HTPB) and nitrate ester plasticized polyether (NEPE) propellants during combustion. Based on thermogravimetric-differential scanning calorimetry (TG-DSC), sample quenching, microscope diagnoses, and collection of condensed combustion product, the effects of oxidants and binders on the thermal decomposition and surface characteristics of quenched samples, and combustion and agglomeration characteristics of propellants were analyzed. The Al/AP/HTPB composite has a one-step exothermic process with a heat of 1723 J·g−1, which is divided into two exothermic reactions with the addition of hexogen (RDX), with the first heat release being only 102.8 J·g−1. The first heat release of NEPE propellant is 1059 J·g−1, while CL-20 reduces it to only 520.5 J·g−1. At 14 MPa, the agglomerated peak particle size of AP/HTPB propellant is the smallest (50 µm), and the agglomerated particle size of AP/RDX/HTPB is 63 µm. The aggregation peak particle size of HMX/AP/NEPE propellant is 56 µm at 14 MPa, and the aggregation peak particle size of CL-20/HMX/AP/NEPE propellant is the largest among all samples (79 µm). The agglomeration of the two propellants is all controlled by the characteristics of the melting layer on the burning surface. The existence of pockets composed by large particle ammonium perchlorate (AP) in HTPB propellant also plays an important role in the agglomeration process of aluminum particles, which is different from NEPE propellant. The decomposition of melted binder leads to the accumulation of unlighted aluminum particles, and the size of agglomerates is mainly affected by the accumulation process. The distributed heat of different kinds of oxidants during propellant combustion may lead to the rapid decomposition of the binder before aluminum ignition, leading to the aggravation of the accumulation process and the increase of the agglomerated particle size. [ABSTRACT FROM AUTHOR]

• Designed a combustion experiment platform that can simulate high-altitude environment. • Revealed the intrinsic effect of pressure on the fluorination and oxidation of Mg. • Provided further insight into micro-scale combustion process of Mg particles. • Heterogeneous condensation reactions are innovatively treated in combustion simulation. The exploration of condensed combustion products (CCPs) is essential to reveal the ignition, combustion, and infrared radiation mechanisms of Mg-based (Magnesium/polytetrafluoroethylene/Viton) solid fuels that have been considerably employed in the field of aerospace. Systematic experiments and numerical simulations are performed with the physicochemical properties (including morphology, element distribution, phase composition and particle size distribution) and flow field distribution characteristics of CCPs, respectively. The effects of formulation and pressure on the combustion behaviors of pyrotechnic compositions are thoroughly investigated based on thermal analysis measurements, laser ignition, optical diagnosis and CCPs collection. The results demonstrated that the major solid products for all cases are MgF 2 agglomerates and MgO smoke oxide particles (SOPs). Increasing the Mg content in the formulation is simultaneously beneficial for the reduction of large agglomerates and SOPs. As the pressure is decreased from 0.1 MPa to 0.02 MPa, the volume weighted mean diameter of CCPs increases significantly due to the decrease in burning rate. The numerical model established in this paper better approximates the realistic combustion process by coupling the heterogeneous condensation reactions, and is adopted to predict the distribution of CCPs in the combustion flow field under different experimental conditions. Numerical results reveal that the MgF 2 is mainly distributed in the anaerobic combustion core zone immediately above the burning surface, while MgO is located in the peripheral aerobic combustion diffusion zone. The negative pressure environment has more profound influence on the oxidation reaction of Mg than fluorination reaction. The scientific findings are expected to facilitate the development of more complete combustion and infrared radiation models, while theoretically providing in-depth guidance for the increasing military and civilian application of Mg-based solid fuels. [ABSTRACT FROM AUTHOR]

This article is devoted to a new block-module device used in the diagnostics of condensed combustion products of rocket engines during research and development with liquid-propellant rocket engines (Glushko NPO Energomash; engines RD-171, RD-180, and RD-191) and solid-propellant rocket motors. Soot samplings from the supersonic high-temperature jet of a high-power liquid-propellant rocket engine were taken by the given device for the first time in practice for closed-exhaust lines. A large quantity of significant results was also obtained during a combustion investigation of solid propellants within solid-propellant rocket motors. [ABSTRACT FROM AUTHOR]

This paper describes a method for studying the combustion of titanium particles with a diameter of 300-500 µm obtained by agglomeration of many small particles. Burning monodisperse particles of titanium were produced by ignition of miniature pieces of a pyrotechnic composition containing 69% titanium powder placed in the burning sample. The resultant agglomerated particles burned in free fall in air. Their motion and evolution, including fragmentation, were video-recorded. Condensed combustion products were quenched, sampled, and studied. [ABSTRACT FROM AUTHOR]

The properties of the surface layer of burning metallized solid propellants are analyzed on the basis of available experimental data. A general physical concept of processes in the surface layer is developed. The structure called a skeleton layer is shown to play a key role in combustion. The factors influencing the properties of the skeleton layer are determined, and the effect of these of properties on the properties of condensed combustion products and the propellant burning-rate law is studied. [ABSTRACT FROM AUTHOR]

The combustion characteristics of propellants containing AP, HMX, an energetic binder, and aluminum particles with various polymer coatings are studied at pressures of 0.15 and 4.6 MPa. It is found that the coatings influence the burning rate, the particle size distribution of condensed combustion products, and the completeness of aluminum combustion. It is shown that the agglomeration can be reduced by using aluminum with fluorine-containing coatings. The application of some coatings results in a reduction in the mass of the agglomerates with an insignificant increase in their size. The greatest effect was achieved when using aluminum coated with (CH2=CH-CH2-O)2Si[OCH2(CF2-CF2)2H]2 [bis(allyloxy)-bis(2,2,3,3,4,4,5,5-octafluoropentyloxy)silane]. For this coating, a size reduction is also observed for micron-size oxide particles. [ABSTRACT FROM AUTHOR]

Abstract: Nano-aluminized propellants are investigated and compared with corresponding micro-aluminized propellants in order to evaluate the actual pros and cons in the use of metal nano-powders for solid rocket applications. A detailed characterization of the original metal powder and condensed combustion products is performed and discussed. Under the explored operating conditions, the results confirm that nano-aluminized propellants show larger steady burning rate, without significant change in pressure sensitivity, and lower aggregation/agglomeration phenomena in combustion products. Combustion efficiency is in turn favored by those factors reducing the importance of aggregation/agglomeration phenomena in the combustion process. [Copyright &y& Elsevier]

The condensed combustion products of two model propellants consisting of ammonium perchlorate, aluminum, nitramine, and an energetic binder were studied by a sampling method. One of the propellants contained HMX with a particle size D 10 ≈ 490 µm, and the other RDX with a particle size D 10 ≈ 380 µm. The particle-size distribution and the content of metallic aluminum in particles of condensed combustion products with a particle size of 1.2 µm to the maximum particle size in the pressure range of 0.1–6.5 MPa were determined with variation in the particle quenching distance from the burning surface to 100 mm. For agglomerates, dependences of the incompleteness of aluminum combustion on the residence time in the propellant flame were obtained. The RDX-based propellant is characterized by more severe agglomeration than the HMX-based propellant — the agglomerate size and mass are larger and the aluminum burnout proceeds more slowly. The ratio of the mass of the oxide accumulated on the agglomerates to the total mass of the oxide formed is determined. The agglomerate size is shown to be the main physical factor that governs the accumulation of the oxide on the burning agglomerate. [ABSTRACT FROM AUTHOR]

Results of numerical simulations are reported for two‐phase flows with allowance for coagulation and fragmentation of particles of polydisperse condensate in the combustor duct of the solid rocket motor (SRM) with a sudden change in cross‐sectional area typical of solid‐propellant charges of a complex shape. It is shown that a sudden change in cross‐sectional area can lead to a significant (multiple) coagulation‐induced increase in the mean particle size. The calculation results are validated by a test series on a model SRM with the use of a laser system for diagnosing particle dispersion. [ABSTRACT FROM AUTHOR]

In work explored effect of mixed metallic fuel on the thermodynamic and ballistic characteristics of High-Energy Materials (HEMs). Efficiency of mixed metallic fuel estimated in relation to the characteristics of HEMs, containing conventional metallic fuel - aluminum. The main criteria of comparison are the specific impulse and the composition of the combustion products HEMs.

A micro-sized B-Al alloy has application potential to be an energetic material for metal fuel. To understand the combustion mechanism of micro-sized B-Al compound powder and explosives containing B-Al (30 wt.%), the ignition and combustion characteristics of B-Al compound powder without and with HMX (cyclotetramethylenetetranitramine) under different pressures (0.4, 0.8, 1.2, 1.5 and 1.8 MPa) and atmospheres (60% O + 2 40 % N 2 , 80% O + 2 20 % N 2 and 100% O 2) were studied via a laser ignition experimental system. The images and characteristic parameters of the ignition and combustion process with different conditions for the B-Al compound powder without and with HMX were obtained. The effects of atmosphere and pressure on the ignition and combustion characteristics of the B-Al compound powder without and with HMX were discussed through a comparative analysis. For the condensed combustion products (CCPs), the phase crystal structures were analysed with X-ray diffraction (XRD), the micro-morphology of the surface was observed with scanning electron microscopy (SEM), and the micro-area elementary composition was analysed with X-ray energy dispersive spectrum (EDS). The results show that whether the increase in pressure or oxygen concentration, the ignition delay time (t id) and combustion time (t ct) of samples without and with HMX decrease, while the maximum spectral intensity (I max) and the maximum combustion temperature (T max) increase. According to the flame morphology, the combustion of the B-Al compound powder is weaker, while that of the B-Al powder with HMX is more intense, and the flame area is larger, which produces a large amount of smoke and presents a deflagration phenomenon. Specially, for various B-Al powder (B/Al = 3/7, 1/1 and 7/3) with HMX under high pressures (1.2, 1.5 and 1.8 MPa), the higher ratio of B leads to shorter combustion time and stronger spectral intensity. Unreacted B, Al and their final oxides, B 2 O 3 and Al 2 O 3 , were detected in the CCPs of both samples without and with HMX. In addition, the CCPs like BN, AlN and Al 5 O 6 N were also detected. The experimental results can help us better understand the chemical reaction mechanism and energy release characteristics of HMX-based explosives containing B-Al. [ABSTRACT FROM AUTHOR]

Boron (B) has potential applications in energetic materials as additive fuel. In order to understand the combustion mechanism of B with various scales, the ignition and combustion characteristics of micro- and nano-sized B under different pressures (0.4 MPa, 0.8 MPa and 1.2 MPa) and atmospheres (60% O 2 + 40% N 2 , 80% O 2 + 20% N 2 and 100% O 2) were studied by a laser ignition and combustion experimental system. The evolution images and specialized parameters of ignition and combustion under different conditions were obtained, and the effects of atmosphere and pressure on the ignition and combustion characteristics of micro- and nano-sized B were discussed. Meanwhile, the crystal structure and morphology of condensed combustion products were observed by X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM), and then the compositions in micro-areas were analyzed by energy dispersive X-ray spectroscopy (EDX). The results show that, the burning of micro-sized B is relatively slow, while the burning of nano-sized B is intensified, and a deflagration-like phenomenon is observed for nano-sized B in pure oxygen with pressures of 0.8 and 1.2 MPa. The ignition delay time, combustion time and maximum flame temperature of nano-sized B are smaller than those of micro-sized B, while the maximum characteristic spectral intensity of nano-sized B is stronger. Besides, B 2 O 3 , B 6 O and BN exist in condensed combustion products of micro-sized B, but only B 2 O 3 is detected for nano-sized B, and products agglomerating is observed for nano-sized B. In general, nano-sized B has significant advantages than micro-sized B in the ignition and combustion characteristics, especially under conditions of high oxygen concentrations and high pressures. [ABSTRACT FROM AUTHOR]

The condensed combustion products (CCPs) of solid propellants significantly affect the combustion and internal flow inside the solid rocket motor. A constant-pressure quench vessel was used to collect the condensed combustion products of an aluminized glycidyl azide polymer (GAP)-based propellant. The effects of freezing medium, quench distance, chamber pressure (5∼9 MPa) and virgin aluminum size (13∼40 μm) on the physicochemical properties of the condensed combustion products were studied. The typical size distribution of the condensed combustion products is in three modes, 1∼2 μm, 20∼30 μm, ∼300 μm and their sizes vary from 0.3 to 600 μm, corresponding to smoke oxide particles and agglomerates. Freezing medium is found to have little impact on the particle size, while the agglomeration ratio in water is larger than nitrogen and argon. In contrast to the freezing medium, pressure does affect the size of CCPs, but has little effect on the agglomeration ratio. Quench distance has no significant effect on the particle size. Agglomeration is found to increase first and then decreases with the increase of virgin aluminum size. The influence mechanism of freezing medium and virgin aluminum size on CCPs is proposed. The combustion-agglomeration map is particularly obtained, which shows that freezing medium and virgin aluminum particle size have more profound influence on the agglomeration than quench distance and pressure. The low-pressure condition presents poorest performance with high fraction of agglomerates, large agglomerate size and low combustion efficiency. Results of this work are expected to provide better insight in the combustion of solid propellant and solid rocket motor. [ABSTRACT FROM AUTHOR]