Convective burning of fine ammonium nitrate–aluminum mixtures in a closed volume bomb

It is commonly assumed that the burning of ammonium nitrate–aluminum mixtures is much less prone to undergo a transition to explosion and detonation than similar mixtures based on ammonium perchlorate. However, this conclusion has been made for mixtures based on commercial-grade ammonium nitrate with large particles. In this study, the combustion of fine loose-packed mixtures of ammonium nitrate and aluminum in a closed-volume bomb has been examined. It has been shown that fine mixtures (ammonium nitrate with a particle size of less than 40 µm and an ASD-4 aluminum powder with spherical particles with a size of about 4 µm) undergo high-intensity combustion; in experiments with a stoichiometric mixture, explosions are observed. The explosions occur in the initial phase of convective combustion and lead to abrupt pressure pulsations with an amplitude of a few kilobars and to the destruction of the cup in which the sample is placed. The dynamics of development of the explosion has been analyzed in detail using numerical simulation. According to the results of experiments with varied parameters—the degree of dispersion of the ammonium nitrate powders, the aluminum content in the mixture, the length and diameter of the charge, and the level of pressure generated by the combustion of the igniter,—threshold conditions have been determined to separate the following modes: the absence of ignition, layer-by-layer combustion, or convective combustion with a transition into an explosion in experiments with a stoichiometric mixture.     

Convective burning of an aluminum-water mixture

The burning of a stoichiometric mixture of aluminum (PAP-2 powder) with water in a constant-volume bomb is studied. It is shown that, depending on the charge diameter and igniter-generated pressure, three situations can arise: the mixture does not burn, burns slowly (in the layer-by-layer mode), or burns rapidly in the convective mode. The characteristics of the rapid burning, such as the effect of the igniter-generated burning, charge length, and initial charge density, are in general similar to those of the convective burning of mixtures of aluminum powder with an oxidizing agent (AP or PA), described in the literature. The difference lies in the fact that, due to a relatively low water activity as an oxidant, the convective burning of aluminum-water mixtures is harder to initiate, and it proceeds at a much lower velocity.     

Onset and development of convective burning in highly porous charges of ammonium perchlorate and its mixtures with aluminum

The onset and development of convective burning in the charges with a high porosity prepared from the finely dispersed ammonium perchlorate and its mixtures with aluminum ASD-4 is studied. The experiments were carried out in a constant volume bomb with the record of the pressure-time history and in the confinement with a slit, which makes it possible to perform simultaneously the photographic and piezometric recording of the process. Special attention is given to the mixtures with the increased aluminum content. The minimum lengths of samples are determined at which convective burning or explosion occur. The dependence of these lengths on the aluminum concentration in the mixture is determined. The possibility of convective burning and low-velocity detonation in ammonium perchlorate without the combustible additive is shown. It is established that the introduction of aluminum causes the ignition of the dispersed suspension behind the front of convective burning with the formation of the brightly glowing high-pressure zone (the secondary wave), which intensively expands in both sides from the place of origin. When the secondary wave overtakes the front of convective burning, the low-velocity detonation appears. The obtained results are of interest for explosion safety of the mixtures of ammonium perchlorate with aluminum and for designing generators of high-temperature suspensions with aluminum particles.     

Burning of aluminum particles in water

Experiments in a constant-volume reactor and in a model rocket combustor demonstrated the possibility of convective burning of mixtures of water with PAP-2 flaky aluminum powder with the formation of alumina and hydrogen. It was shown that the porosity of the mixture is an important factor determining the mode of its burning. The burning occurred at a pressure of several hundred atmospheres in the convective mode.     

Burning velocity of methane-hydrogen mixtures at elevated pressures and temperatures

The effect of the initial pressure, temperature, and equivalence ratio on a number of combustion characteristics of methane-air mixtures with hydrogen additives in a closed vessel is experimentally studied. Experiments are conducted at 1, 5, and 10 atm and temperatures from 22 to 300°C. The hydrogen content in the fuel is 0, 10, and 20 vol %. The fuel equivalence ratio varies from 0.6 to 1.0. The limitations imposed by buoyancy on measurements of the laminar burning velocity by the constant-volume bomb method with recording of pressure-time histories are analyzed. It is shown that the laminar burning velocity can be appreciably increased by adding no less than 20 vol % of hydrogen to the fuel.     

Convective burning of pressed aluminum-ammonium perchlorate charges

The convective burning of pressed aluminum-ammonium perchlorate (AP) charges with a porosity of 7 to 18% was studied. The experiments were performed at pressures of up to 300 MPa in a constant volume bomb provided with means for recording pressure-time diagrams, and in a nozzle setup equipped with a streak photocamera and piezoelectric pressure gauges. In contrast to loose-packed-density charges, which are highly explosive, the burning of pressed aluminum-AP charges propagates without marked acceleration, with a moderate velocity and a relatively slow rise in pressure in the bomb. The basic regularities were studied, and the key factors that determine the characteristics of convective burning, such as the aluminum particle shape (when a finely dispersed spherical-particle powder was replaced by a flaky aluminum powder with the same speciic surface area, the convective burning velocity decreased by more than an order of magnitude), ratio of mixture components, and charge porosity, were identified. The effects of the ammonium perchlorate particle size, an organic additive, and the ignitor mass were also studied. The experimental data were analyzed by invoking numerical modeling. The calculations were performed using a program developed earlier based on a model of the convective burning of aluminum-AP mixtures. The calculation results, which were in qualitative agreement with the available experimental data, made it possible to explain the main experimentally observed regularities. The compositions tested and the results obtained are of considerable interest for designing convective-burning charges for multipurpose pulse engines and thermo-and gas generators with operation durations from a few milliseconds to several tens of milliseconds.     

Coupled combustion of mixture in hydrogen generation

The specifics of the combustion of micron-sized flaky aluminum powder in mixtures with crystal-line hydrates over a wide pressure range at various mass ratios between aluminum and crystalline hydrate are studied. Limiting conditions of combustion of mixtures regarding the composition and pressure are determined. With use of a stoichiometric mixture of aluminum with sodium sulfate crystalline hydrate, the coupled combustion composite samples with additional oxidants, ammonium nitrate and potassium nitrate, and samples with spread oxidant is examined. It is established that the velocity of coupled combustion can significantly exceed the velocity of combustion of the base mixture. Limiting conditions for combustion of mixtures are determined.     

Flame propagation of nano/micron-sized aluminum particles and ice (ALICE) mixtures

Flame propagation of aluminum–ice (ALICE) mixtures is studied theoretically and experimentally. Both a mono distribution of nano aluminum particles and a bimodal distribution of nano- and micron-sized aluminum particles are considered over a pressure range of 1–10 MPa. A multi-zone theoretical framework is established to predict the burning rate and temperature distribution by solving the energy equation in each zone and matching the temperature and heat flux at the interfacial boundaries. The burning rates are measured experimentally by burning aluminum–ice strands in a constant-volume vessel. For stoichiometric ALICE mixtures with 80 nm particles, the burning rate shows a pressure dependence of r_b = aPⁿ, with an exponent of 0.33. If a portion of 80 nm particles is replaced with 5 and 20 μm particles, the burning rate is not significantly affected for a loading density up to 15–25% and decreases significantly beyond this value. The flame thickness of a bimodal-particle mixture is greater than its counterpart of a mono-dispersed particle mixture. The theoretical and experimental results support the hypothesis that the combustion of aluminum–ice mixtures is controlled by diffusion processes across the oxide layers of particles.     

Specifics of the combustion of aluminum-water mixtures

Experimental studies of the combustion of mixtures of micron-sized flaky aluminum powder with unthickened water in different conditions at atmospheric and high pressure in nitrogen and argon are performed. The density and composition of the mixture are varied. The regularities of the combustion have been established. A filtration wave of hot hydrogen ahead of the combustion front in samples with high porosity has been revealed. For the combustion under a nitrogen atmosphere, the pressure exponent in the burning rate law is close to 0.47 in a wide range of pressures. For the combustion under an argon atmosphere at pressures above 50 atm, the pressure exponent becomes zero or negative. Aluminum powder is demonstrated to be able to burn under conditions of a separated charge, where the fuel (aluminum) and oxidizer (water) are separated by a thin partition or brought in direct contact. The fast convective burning of aluminum-water mixtures in a semiclosed volume is discovered.     

Deflagration-to-detonation transition in mixtures of finely divided ammonium perchlorate with submicron aluminum powder

Deflagration-to-detonation transition in binary mixtures of fine ammonium perchlorate (20-μm grains) with submicron ALEX-L aluminum powder (0.2-μm particles) is studied using high-speed photography and pressure recording with quartz crystal sensors. The test mixtures were loaded in thin-walled quartz tubes of inner diameter 10 mm. The charges had a porosity of ~50%. It has been shown that, even under very mild conditions (low-strength shell and a weak source of initiation), the deflagration mode of mixture combustion easily transforms into the detonation mode. The shortest length of the region of transition from deflagration to normal detonation (not more than 30 mm) was observed for a lean mixture, with an aluminum content of ~5%. The mechanism of transition to detonation involves the stage of convective combustion, resulting in the formation of a brightly luminescent crescent-shaped area behind the primary flame front, which, in turn, generates a forward (in the direction of propagation) and a backward wave. The forward wave gives rise to low-speed detonation, which later transforms into normal detonation. The pressure profile within the region of low speed detonation is measured. A comparison with similar experiments in which ALEX-L alu- minum powder was replaced by ASD-4 aluminum (4 μm particles) shows that ALEX-L sensitizes the mixture, resulting in a dramatic reduction of the length of the transition region, making it possible to produce normal detonation in low-strength shells.     

Multipoint radiation induced ignition of dust explosions: turbulent clustering of particles and increased transparency

Understanding the causes and mechanisms of large explosions, especially dust explosions, is essential for minimising devastating hazards in many industrial processes. It is known that unconfined dust explosions begin as primary (turbulent) deflagrations followed by a devastating secondary explosion. The secondary explosion may propagate with a speed of up to 1000 m/s producing overpressures of over 8–10 atm, which is comparable with overpressures produced in detonation. Since detonation is the only established theory that allows rapid burning producing a high pressure that can be sustained in open areas, the generally accepted view was that the mechanism explaining the high rate of combustion in dust explosions is deflagration-to-detonation transition. In the present work we propose a theoretical substantiation of an alternative mechanism explaining the origin of the secondary explosion producing high speeds of combustion and high overpressures in unconfined dust explosions. We show that the clustering of dust particles in a turbulent flow ahead of the advancing flame front gives rise to a significant increase of the thermal radiation absorption length. This effect ensures that clusters of dust particles are exposed to and heated by radiation from hot combustion products of dust explosions for a sufficiently long time to become multi-point ignition kernels in a large volume ahead of the advancing flame. The ignition times of a fuel–air mixture caused by radiatively heated clusters of particles is considerably reduced compared with the ignition time caused by an isolated particle. Radiation-induced multipoint ignitions of a large volume of fuel–air ahead of the primary flame efficiently increase the total flame area, giving rise to the secondary explosion, which results in the high rates of combustion and overpressures required to account for the observed level of overpressures and damage in unconfined dust explosions, such as for example the 2005 Buncefield explosion and several vapour cloud explosions of severity similar to that of the Buncefield incident.     

Effect of nitride additives (AlN and Si3N4) on the combustion of Fe2O3/Al mixtures and the formation of the chemical composition of the combustion products

The method of self-propagating high-temperature synthesis (SHS) is applied to prepare cast oxynitride ceramics using a Fe₂O₃ + 4Al thermite mixture. The nitrogen-containing components of the mixture were AlN and Si₃N₄ additives. The synthesis was performed in a reactor at an initial nitrogen pressure of 8MPa. The nitrogen-containing additives are demonstrated to influence the burning rate and combustion limits of the mixtures, as well as the yield and chemical composition of the cast ceramics.     

The composition of combustion products formed from gasoline-hydrogen-air mixtures in a constant-volume spherical chamber

The experimental unit, procedure for testing, and the results and reliability of the determination of the composition of combustion products formed from gasoline-hydrogen-air (containing 0–100% hydrogen), gasoline-air, and isooctane-hydrogen-air mixtures in a constant-volume chamber are described. Studies were performed at initial mixture temperatures of 20–70°C and a 0.1 MPa pressure.     

Characteristics of the underwater explosion of a nonideally detonating aluminum-rich energetic material

An experimental study of the characteristics of the explosion of mixtures of ammonium perchlorate, aluminum, and nitromethane with a large excess of aluminum (1.45 to 1.66 g/cm³ in density) confined in plastic enclosures and immersed in small elastic-wall reservoirs with water is conducted. It is shown that composite charges, 20 mm in diameter, surrounded by a water layer of thickness 20–30 cm and detonate in a nonideal detonation mode. High-speed cinematography records show the possibility of the intense mixing of the detonation products with the surrounding water and of the burning of excess aluminum particles in a heterogeneous cloud. The time scales of the development of secondary energy release by burning of aluminum particles in water are estimated. The possibility of controlling the characteristics of the pressure waves generated by the explosion, for example, by means of a preliminary bubbling of the water with air near the charge, is demonstrated.     

Effect of condensed products of silane combustion on a low-pressure oxyhydrogen flame

The kinetics of branching-chain oxidation of monosilane and hydrogen at and near the first explosion limit at temperatures within 630–850 K and pressures below 1 torr was studied. Atomic hydrogen and oxygen were observed in the silane-oxygen reaction zone. The effects of the reactor walls treated with silane-lean (1: 9: 10) and silane-rich (1: 1: 3) SiH₄-O ₂-He mixtures on the parameters of oxyhydrogen ignition were different. This modification of the surface was of long-term character. Water vapor was absorbed on the hot reactor walls treated with explosions of the silane-rich mixture. Such a surface was demonstrated to affect the combustion of oxyhydrogen, inducing pulsating combustion.     

Influence of inert and active additives on the initiation and propagation of laminar spherical flames in stoichiometric mixtures of methane,pentane, and hydrogen with air

The propagation of a laminar spherical flame in stoichiometric mixtures of methane, and pentane with air in the presence of argon and carbon dioxide and in hydrogen-air-propylene mixtures at atmospheric pressure in a constant-volume bomb is investigated using high-speed color cinematography. It is shown that, under the experimental conditions employed (at T ₀ = 298 K and a spark energy of E ₀ = 0.91 J), dilution of the combustible mixtures with these additives can cause a more than 10-fold increase in the time of formation of a steady flame front, with the inhibiting effect of carbon dioxide being stronger than that of argon. Small additives of propylene, a chemically active inhibitor, are demonstrated to substantially increase the time it takes to form a steady flame front and reduce the flame propagation velocity.     

天然气在不同初始温度和压力下的燃烧特性研究

利用定容燃烧弹研究了不同初始温度和初始压力下的天然气燃烧特性,分析了初始温度、初始压力和当量比对其燃烧过程的影响.研究结果表明:随着初始温度的升高(300～450 K),天然气质量燃烧速率明显增加,燃烧持续期和火焰发展期显著缩短.随着初始压力的升高(0.1～0.75 MPa),天然气质量燃烧速率明显减慢,燃烧持续期和火焰发展期显著增长.且稀混合气和浓混合气条件下初始温度和初始压力的变化对燃烧持续期和火焰发展期的影响更明显.     

The slowly reacting mode of combustion of gaseous mixtures in spherical vessels. Part 1: Transient analysis and explosion limits

Frank-Kamenetskii's analysis of thermal explosions is revisited, using also a single-reaction model with an Arrhenius rate having a large activation energy, to describe the transient combustion of initially cold gaseous mixtures enclosed in a spherical vessel with a constant wall temperature. The analysis shows two modes of combustion. There is a flameless slowly reacting mode for low wall temperatures or small vessel sizes, when the temperature rise resulting from the heat released by the reaction is kept small by the heat-conduction losses to the wall, so as not to change significantly the order of magnitude of the reaction rate. In the other mode, the slow reaction rates occur only in an initial ignition stage, which ends abruptly when very large reaction rates cause a temperature runaway, or thermal explosion, at a well-defined ignition time and location, thereby triggering a flame that propagates across the vessel to consume the reactant rapidly. Explosion limits are defined, in agreement with Frank-Kamenetskii's analysis, by the limiting conditions for existence of the slowly reacting mode of combustion. In this mode, a quasi-steady temperature distribution is established after a transient reaction stage with small reactant consumption. Most of the reactant is burnt, with nearly uniform mass fraction, in a subsequent long stage during which the temperature follows a quasi-steady balance between the rates of heat conduction to the wall and of chemical heat release. The changes in the explosion limits caused by the enhanced heat-transfer rates associated with buoyant motion are described in an accompanying paper.     

Nonideal regimes of deflagration and detonation of black powder

The explosive and deflagration properties of black powder differ significantly from those of modern propellants and compositions based on ammonium nitrate or ammonium perchlorate. Possessing a high combustibility, black powder is capable of maintaining stable combustion at high velocities in various shells, be it steel shells or thin-walled plastic tubes, without experiencing deflagration-to-detonation transition. It is extremely difficult to detonate black powder, even using a powerful booster detonator. The results of numerical simulations of a number of key experiments on the convective combustion and shock initiation of black powder described in the literature are presented. The calculations were performed within the framework of a model developed previously for describing the convective combustion of granulated pyroxylin powders, with small modifications being introduced to allow for the specific properties of black powder. The thermophysical properties of the products of combustion and detonation and the parameters of the equation of state of black powder were determined from thermodynamic calculations. The calculation results were found to be in close agreement with the experimental data. The simulation results were used to analyze the regularities of the wave processes in the system and their relation to the properties of black powder and the experimental conditions. It was demonstrated that the effects observed could be explained by a weak dependence of the burning rate of black powder on the pressure.     

On laminar premixed flame propagating into autoigniting mixtures under engine-relevant conditions

Usually premixed flame propagation and laminar burning velocity are studied for mixtures at normal or elevated temperatures and pressures, under which the ignition delay time of the premixture is much larger than the flame resistance time. However, in spark-ignition engines and spark-assisted compression ignition engines, the end-gas in the front of premixed flame is at the state that autoignition might happen before the mixture is consumed by the premixed flame. In this study, laminar premixed flames propagating into an autoigniting dimethyl ether/air mixture are simulated considering detailed chemistry and transport. The emphasis is on the laminar burning velocity of autoigniting mixtures under engine-relevant conditions. Two types of premixed flames are considered: one is the premixed planar flame propagating into an autoigniting DME/air without confinement; and the other is premixed spherical flame propagating inside a closed chamber, for which four stages are identified. Due to the confinement, the unburned mixture is compressed to high temperature and pressure close to or under engine-relevant conditions. The laminar burning velocity is determined from the constant-volume propagating spherical flame method as well as PREMIX. The laminar burning velocities of autoigniting DME/air mixture at different temperatures, pressures, and autoignition progresses are obtained. It is shown that the first-stage and second-stage autoignition can significantly accelerate the flame propagation and thereby greatly increase the laminar burning velocity. When the first-stage autoignition occurs in the unburned mixture, the isentropic compression assumption does not hold and thereby the traditional method cannot be used to calculate the laminar burning velocity. A modified method without using the isentropic compression assumption is proposed. It is shown to work well for autoigniting mixtures. Besides, a power law correlation is obtained based on all the laminar burning velocity data. It works well for mixtures before autoignition while improvement is still needed for mixtures after autoignition.