Home – Home – Jornals – Combustion, Explosion and Shock Waves 2021 number 2

Ignition and combustion of a homogeneous stoichiometric methane-air mixture under simultaneous local thermal and photochemical actions, resulting in the formation of either O₂(a¹\Δ_g ) molecules or O atoms, are numerically simulated. A two-dimensional unsteady multicomponent approach with the use of the known detailed kinetic mechanism of methane oxidation, which takes into account reactions with participation of electronically excited oxygen molecules O₂(a¹\Δ_g ) and O₂(b¹\ _{), is applied. It is shown that an additional photochemical action ensures ignition of the mixture in situations where the thermal action alone is insufficient. Moreover, for identical energy inputs, a higher burning rate at the initial stage is observed in the case of generation of oxygen atoms. This method of the photochemical action seems to be more effective from the viewpoint of combustion initiation. The computed results on propagation of turbulent combustion are in reasonable agreement with available experimental data.}

The ignition of a methane-air mixture with additives of ClF₅, ClF₃, OF₂, and H₂O₂ (additive content in the mixture ≤ 1%) is studied by numerical simulation. The ignition temperature of the mixture is determined as a function of the initial pressure and additive content. It is shown that the introduction of additives into the mixture leads to a significant decrease in the ignition temperature due to the acceleration of the formation of active particles and intensification of the chain mechanism of the process. Of the additives considered, chlorine pentafluoride is the most effective additive, and hydrogen peroxide is the least effective one.

A comparison of two approaches to describing a stretching spherical flame is made. Processing of a large amount of experimental data shows that the approach in which a parameter proportional to the strain rate is used as a small parameter gives a lower accuracy of approximation than the approach based on the use of the ratio of the flame thickness to the radius of curvature.

Methyl methacrylate (MMA) is the main pyrolysis product of the widely used polymer polymethyl methacrylate; therefore, the compact mechanism of MMA oxidation is of interest for CFD modeling of the spread of a fire over this polymer. A reduced mechanism for MMA combustion consisting of 263 elementary reactions involving 66 species was developed for a detailed chemical-kinetic mechanism of MMA oxidation in flames using Chemical Workbench software. The developed mechanism was validated against experimental data on the speed of premixed MMA/air flame at an equivalence ratio 0.9 < f < 1.3 and against literature data on the structure of an MMA/O₂/Ar (f = 1) flame stabilized on a flat burner at a pressure of 1 atm. The proposed reduced kinetic model for MMA describes the experimental data with satisfactory accuracy, and simulation results for the full and reduced mechanisms MMA oxidation are in good agreement with each other in terms of the concentrations of the main species of the flame and the concentration of most combustion intermediates, including hydrogen, methane, ethylene, acetylene, propane, acetaldehyde, methyl acrylate, etc.

A general model of combustion of mixed gases is proposed, taking into account the chemical work, in which one chemical reaction is assumed. An energy equation is written for an arbitrary mixture of fuel/oxidizer/combustion product. The combustion of a H₂/O₂ mixture is considered on the basis of the model. Modeling is carried out for two options: with and without account for a temperature dependence between the heat capacity of the components of the mixture H₂, O₂, and H₂O. The chemical work has a positive sign, and its relative contribution to the main characteristics of the process (temperature and burning rate) is estimated at 3.9 and 19 %.

A computational and experimental study of the passage of a gasless combustion wave through a wedge-shaped inert barrier was performed. The stability of the transient combustion process to a change in the thermophysical, geometric parameters of the barrier and the energy-kinetic parameters of chemically active layers was studied. By the region of stable transient combustion process is meant the range of parameters in which the passage of the combustion wave through the contact boundary ends with the establishment of a steady-state mode of its propagation along the ignited composition. The dynamics of transient combustion under near-critical conditions of its existence was investigated.

Effect of K₂CO₃ and KCl additives on the process of high-temperature oxidation and ignition of conglomerates of powdered boron and aluminum particles and on the critical conditions for the ignition of air-gas mixtures of these metals is studied experimentally. It is revealed that the mixtures of boron and aluminum particles covered (encapsulated) with salts ignite at temperatures lower than the ignition temperatures of mechanical mixtures of metals with these salts in appropriate proportions and significantly lower ignition temperatures of pure boron and aluminum. This indicates the active role of salts in the process of heterogeneous ignition. It is established that, for boron conglomerates, the accelerating effect of salt additives on the oxidation of conglomerates is noticeable at all investigated concentrations and compositions starting from 1 % (by weight). For aluminum, an increase in the reaction rate is noticeable only for KCl additives at concentrations of 3 % and above.

A Ni-Al-C based composite material is synthesized by the method of electric thermal explosion under a pressure of 96 MPa. During an electrothermal explosion in a powder reaction medium (Ni + Al + C), a Ni and Al based melt is formed, in which carbon dissolves. It is shown that in the process of crystallization of the final product, carbon, due to low solubility in NiAl, is located on the surface of intermetallic grains of NiAl in the form of multilayer graphite nanofilms with a thickness of 50 ÷ 80 nm, filling the intergranular space. The microhardness of the synthesized material is 3.084 MPa.

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 loadings, 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 µ m for non-catalyzed propellants and from 28 to 136 µ 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.

The dependence of the diameter of a porous carbon particle on stationary temperature at various pressures of the gas mixture is analyzed. Cases of self-ignition of a particle in a heated nitrogen-oxygen mixture and forced ignition in a cold nitrogen-oxygen mixture leading to quasi-stationary combustion, followed by spontaneous extinction, are considered. It is shown that the combustion temperature of small particles with a diameter d < 200  m, which are characterized by a transient regime of chemical reactions, increases with increasing pressure of the gas mixture. An analytical dependence that qualitatively describes this dynamics is obtained. The greatest growth falls on a pressure interval of 0.1 ÷ 0.3 MPa. A similar analysis for particles of various coals is presented. The dependence of the critical oxygen concentrations corresponding to ignition and extinction on the diameter of a carbon particle was obtained analytically taking into account the internal reaction. It is shown that an increase in the pressure in the mixture leads to a decrease in the critical concentrations of oxygen.

In this paper, confined explosions of HMX-based aluminized explosives in a spherical chamber are studied. The effects of aluminum particles on the afterburning reaction and explosive performance are obtained by changing the size of the particles and the gas environment. The results show that the concentration of oxygen in air is not sufficient to support complete combustion of aluminum particles. The estimated oxidation rate of aluminum particles is 87 ÷ 93 %, and it tends to decrease with increasing particle size. Part of aluminum particles oxidize with detonation products, and the reaction can last for hundreds of microseconds. However, the degree of oxidation between the large-sized aluminum particles and detonation products is small. A new method is used to estimate the initial energy of detonation by observing the time difference between sensing the initial light and the pressure wave. This method leads to a conclusion that some of the aluminum particles are oxidized during detonation and provide additional energy to the primary blast wave. Small micron-sized aluminum particles in the range of 48.9 nm ÷ 46.7 m extend the duration of the fireball.

A method of obtaining detonation nanodiamonds from tetryl by means of blasting the latter in a water shell with the explosive/water mass ratio of 1/(10 ÷ 14) is developed. The proposed process ensures the yield of the target product in the amount of 6 ÷ 7 wt. % of the initial mass of the reagent with the content of nanodiamonds in the resultant batch mixture equal to 57 ± 6 %. It is demonstrated that the use of individual explosives offers some advantages over the known multicomponent mixtures in terms of cost efficiency and safety of the charge preparation process.

It is revealed by scanning electron microscopy that the formation of filamentous structures up to 10 μm in length long can be induced in spheroplastics based on organosilicon elastomer by a shock-wave pulse of microsecond duration. Threaded structures are formed on the surface of fractured microspheres. Their formation is facilitated by the metallization of the surface of glass spheres. The results of an experimental study of changes in the dielectric and mechanical characteristics of metallized spheroplastics, caused by impact-wave action, are presented. Possible reasons for the formation of threaded structures upon a shock-wave impact on spheroplasics are discussed.

Natural fragmentation of steel cylinders with different charge sizes is investigated, and the correlation of different cylinders is proposed. The Mott fragment distribution has some obvious shortcomings, including the difficulty of accurately determining the number of fragments. Besides, there is no unified and convenient method to describe the fragmentation behavior of shells with different structures. The results show that the fragmentation behavior of the cylinder is self-similar statistically and can be characterized by a new integrated linear formula C_L = a + b (C / M). Due to the existence of the end face and the charge clearance, the fragmentation performance of the cylinder is reduced to a constant value, and the effect of the wall thickness is small. The influence of the end face and charge clearance on fragmentation is investigated by 3D simulations, and the numerical results ensure good validation of the experimental data.