B. S. Ermolaev

Initial stage of the explosion of ammonium nitrate and its powder mixtures

There is an obvious contradiction between the statistics of the devastating explosions that take place with the participation of ammonium nitrate and explosive properties of this material determined in standard tests. Pure ammonium nitrate does not burn under normal conditions and has a very low sensitivity to conventional mechanical and thermal stimuli. So far, ammonium nitrate has been detonated only by using high explosives. Causes of accidental explosions involving large masses of ammonium nitrate are likely to be found in a nonconventional behavior of ammonium nitrate. These changes may arise due to different chemical or physical factors, such as those associated with the presence of active additives, crushing of particles, etc., and lead to acceleration of the process at the initial stage of explosion. This work is devoted studying the convective burning and the initial stage of deflagration-to-detonation transition in dry and wet mixtures of ammonium nitrate with various, largely combustible additives. Experiments were conducted on loose-packed charges in a constant-volume bomb and by using the method of the critical bed height with recording pressure-time diagrams by a piezoelectric sensor. Ammonium nitrate of two different types was used: granular and powdered. The fuel additives were charcoal and aluminum powder, whereas the additives inhibiting the combustion of ammonium nitrate were water and monosodium salt of phosphoric acid. In addition, finely dispersed mixture of four components (ammonium nitrate, aluminum, powdered sugar, and TNT in a proportion of 76: 8: 12: 4) was used. The experiments in the constant-volume bomb were supplemented by numerical simulations, which made it possible to obtain a better understanding of the convective burning of the test mixtures and to evaluate the possibility of using a constant-volume bomb to collect quantitative information on the intensity of the combustion of the mixture at the initial stage of the explosion.

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.

Combustion and detonation of mechanoactivated aluminum–potassium perchlorate mixtures

The properties of mechanoactivated energetic composites based on aluminum and potassium perchlorate with high rates of self-sustaining chemical reactions under conditions of combustion and detonation are examined. The results of experiments on studying the combustion, deflagration-to-detonation transition, and sensitivity to friction of these composites are reported. The activation duration and aluminum content in the mixture are varied. The experiments on the deflagration-to-detonation transition of mechanoactivated composites are supplemented by the results of numerical simulations. The calculations and experiments on the dynamics of development of a blast wave and on the steady detonation velocity are found to be in qualitative agreement. It is shown that the velocity of the observed process is significantly (by about 40%) lower than the normal detonation velocity obtained from thermodynamic calculations.

Numerical simulation of modes of combustion and detonation of hydrogen-air mixtures in porous medium in the framework of the mechanics of two-phase reaction mediums

Numerical simulation is carried out for combustion and detonation waves propagating through a motionless gas mixture in a porous inert charge. Computations are performed in a one-dimensional approximation by means of an EFAE computer program that was developed in the framework of the mechanics of multiphase reaction mediums. The chemical conversion of gas is modeled by a one-stage reaction of the Arrhenius type with constants selected based on existing experimental data on the ignition lags behind the reflected shock waves. Computations are performed for hydrogen-air mixtures with 35 and 15% hydrogen and compared with literature experimental data in which the initial pressure and the diameter of charged particles are varied. All three combustion modes (slow, fast, and supersonic) observed in the experiment and combustion failure under conditions lower than threshold are followed by numerical simulation. In addition, the computations qualitatively reproduced experimental data on the change of the combustion mode in the case of transfer from stoichiometric to a lean mixture and data on the combustion wave velocity and limiting conditions of combustion mode transition and failure of flame as a function of the initial pressure and the charged particle size. It is shown that supersonic waves propagating with a velocity of lower than 1100 m/s do not have a Chapman-Jouguet surface in the end of the reaction zone and it is evident that they can be related to detonation, as in the cited literature.

Convective burning of block charges prepared from seven-perforation propellant powder grains inhibited by polyvinyl butyral

The burning of block charges prepared from seven-perforation propellant powder grains inhibited with polyvinyl butyral is studied. The experiments are carried out in a constant-volume bomb, nozzle bomb with a post-combustion chamber, and a 23-mm laboratory gun device, setups that provide a wide range of combustion conditions. The progress of the combustion process and the motion of the projectile along the barrel are recorded using a set of piezoelectric sensors. The varied parameters are the amount of inhibitor, density of the monoblock (1.2–1.45 g/cm3), and the power of the igniter. The different combustion conditions achievable in the bomb and laboratory gun device enable to assess the impact of the pressure-rise rate on the rate of the convective burning of the block charge. Adjusting the properties of the latter makes it possible to vary the convective burning rate within 10–60 m/s, which is optimal for using the block charge for producing a shot. The results show that, when used in block charges, coarse seven-perforation propellant powders can provide, despite their own high burning progressivity, an equally high effect as the previously studied fine-grain propellant powders. At a given maximum pressure, the observed increase in the muzzle velocity for block charges exceed 12% as compared to a pour-density charge prepared from regular seven-perforation propellant powder. Based on numerical simulations, an analysis of the experimental data is performed and an explanation of the experimentally observed influence of the pressure rise rate on the convective burning rate of block charges is proposed.

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.

Theoretical issues of steady non-ideal detonation in the ternary nitromethane-ammonium perchlorate-aluminum system

A 1D model of steady non-ideal detonation in the ternary nitromethane-ammonium perchlorate-aluminum systems enriched with aluminum is developed. The model is confirmed by its comparison with the experimental data on detonation of two- and three-component mixtures. The presence of a significant unburnt fraction of the components at the Chapman-Jouguet plane is shown.

Convective burning and detonability of aluminum-rich ammonium perchlorate-aluminum-nitromethane mixtures: 1. Experiment

The initiation and propagation of low-velocity detonation in ammonium perchlorate (AP)-aluminum-nitromethane (NM) mixtures with Al: AP ratios of 1: 1 to 2.5: 1 and porosities from 40 (10 wt % HM) to 0% (40 wt % NM) in strong steel shells (in the air) and plastic shells submerged in water are experimentally studied. The optimum contents of the components that provide reliable initiation of steady detonation (at velocities from 2 to 5 km/s) by weak explosive sources in mixtures with an Al: AP ratio of 1: 1 and above are determined. The selected mixtures reproducibly detonate in plastic shells surrounded by a 30-cm-thick layer of water at velocities somewhat lower than in strong steel shells in the air.

Generation of blast waves in a channel by nonideal detonation of aluminum-enriched high-density compositions

The results of experimental studies of the nonideal detonation of high-density, high-energy aluminum-ammonium perchlorate-organic fuel-HE compositions and of the blast waves it generates in a channel filled with air are presented. Aluminum-enriched compositions have high densities (up to 2 g/cm3) and high heats of explosion, nearly twice that for TNT. The studies were performed to work out scientific fundamentals of controlling nonideal detonation and to explore the possibility of creating new high-energy high-density formulations with an enhanced fugacity effect. The factors that enable controlling the nonideal detonation of such charges were determined. It was demonstrated that, at RDX contents above 15%, the detonation velocity increases linearly with the charge density while the critical detonation diameter decreases. Adjusting the density, HE content, ratio of the components makes it possible to vary the detonation velocity in high-density charges over a wide range, from 4 to 7 km/s. The experimental data were compared to the thermodynamically calculated velocity of ideal detonation. For the compositions under study, the pressure- time histories of the blast wave generated in a cylindrical tube by the expanding detonation products at different distances from the charge were measured. The results were compared to analogous data obtained under the same conditions for the detonation of the same mass of TNT (100 g). The parameters of blast waves generated by the test compositions are markedly superior to those characteristic of TNT: the pressure at the leading front of the wave and pressure impulse at a given distance from the charge were found to be 1.5–2.0 (or even more) times those observed for TNT. The TNT equivalency at pressures 30–60 atm has similar values. The TNT equivalencies in pressure and pressure impulse depend nonmonotonically on the distance from the charge, so far unclear why. It was established that the interaction between excess fuel and air oxygen during the expansion of detonation products contributes little to supporting the blast wave.

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.