Home – Home – Jornals – Combustion, Explosion and Shock Waves 2014 number 4

Results of an experimental study of the effect of addition of CO₂ to methane and to a propane-butane mixture on conditions of stabilization of the diffusion flame in air are reported. It is shown that the flame lift-off length substantially increases with increasing fraction of CO₂ in the fuel; the main reason is the changes in the velocity and concentration in the region of the “ignition” points; a secondary factor is an increase in the characteristic combustion time. Data on the influence of the CO₂ admixture in the fuel in amounts greater than the fuel flow rate up to »1.5 times on the characteristic combustion time are obtained.

The paper focuses on the application of mathematical modeling of methane turbulent combustion in a complex geometry and on the choice of parameters of one-step and two-step chemical kinetics models. Parameters of chemical kinetics have a profound influence on the correct implementation of the combustion mathematical model used in the CFD (computational fluid dynamics) simulation of the methane-air mixture explosion in a family house. Results are compared with experimental measurements.

The possibility of flame spread over the surface of a liquid fuel (n-butanol) in two-phase flow with a gaseous oxidizer in a narrow rectangular channel is demonstrated. The results of a detailed experimental study of combustion in this system are given. Dependences of the flame propagation speed on the initial temperature and oxidizer and fuel flow rates are obtained.

The formation of a composite material during combustion of a two-layer sample in a centrifugal device was studied by mathematical modeling. The liquid combustion products of the upper layer flow into the lower porous layer under the action of the centrifugal force and initiate its combustion to form a composite material. The critical conditions for the passage of the combustion wave from the upper to the lower layer of the sample are determined depending on the gravitational overload and initial porosity. Dependences of the burning velocity and the impregnation depth of the lower layer on its porosity and overload are obtained.

Burning rates were measured for samples of the starting mixture of Ni + Al and the same mixture subjected to mechanical treatment in argon and then in water. The dependences of the burning rate on the diameter of the samples of the original and mechanically activated mixtures are similar. The burning rate passes through a maximum as the diameter increases from 8 to 12 mm. It is found that the burning rate of 270–360 m m thick films obtained by rolling the starting mixture of Ni + Al and the mixture mechanically activated and then activated in water (dispersed) is 4–20 times the burning rate of cylindrical samples 8–12 mm in diameter, pressed from the same powders. The data obtained in this study were explained using a convective-conductive model of combustion-wave propagation.

The combustion products of mixtures of aluminum nanopowder with titanium and zirconium dioxides in air were studied. The products were found to contain stabilized crystalline phases of TiN and ZrN. Maximum weight content of TiN (29.4%) was observed in the combustion products of a starting mixture containing 52% aluminum nanopowder, and the maximum content of ZrN (28.6%) was observed in the combustion products of a starting mixture with 35 % aluminum nanopowder.

It is shown that critical conditions (temperature and pressure) in a fuel cell (metal tube piece) of a model nuclear reactor can be produced by burning energetic thermite SHS systems. The heating temperature of the zirconium or steel tube can be controlled by varying the composition of the thermite mixture, the gap between the thermite pellet and the tube wall, and the design of the thermite pellet. The results of the studies show that a WO₃—2Al based thermite mixture can be used to model an emergency situation (without the use of radioactive fuel) in the reactor fuel cell.

The problem of thermal explosion in a semi-batch reactor is solved. The critical conditions for the thermal explosion are studied. It is shown that the ignition resulting from the chemical reaction is possible both during the stage of feeding of the second reactant solution and after its completion. The dynamic behavior of the reactor depending on the temperature of the heat exchanger and the volumetric feed rate of the second reactant is analyzed. It is shown that ignition at the feeding stage occurs at high temperature of the heat exchanger and involves two macroscopic stages. In the first stage, the temperature increases with increasing rate (thermal explosion regime), and in the second stage, its further increase occurs in the regime of combustion of the second reactant. The critical ignition conditions are found to be independent on the feed rate of the second reactant in a particular range of its values.

The influence of phase interaction on the spatial structure of the detonation wave in dusty media is discussed. It is shown that dissipation of the energy of low-frequency acoustic perturbations in the gas phase may be drastically enhanced by the presence of minor fractions of small-size chemically inert solid particles and affect stability of perturbations with a scale comparable with the cell size of the gas detonation wave. A hypothesis is proposed that there exists an additional (dissipative) mechanism assisting in detonation suppression in the gas suspension owing to intense momentum transfer between the phases in an unsteady acoustic field in the reaction zone. Under certain conditions, this mechanism can substantially change the spectrum of unstable perturbations and attenuate spatial nonhomogeneity of the detonation wave at small energy expenses for acceleration and heating of particles. Owing to this fact, perturbation energy dissipation can affect the parameters and conditions of existence of the self-sustained detonation regime in the gas suspension.

The explosion wave overpressure and the flame propagation velocity of the gas explosion in a bifurcating duct are experimentally measured and theoretically analyzed. The results show that the effect of the bifurcation on the flame velocity and overpressure is obvious. Especially, the explosion wave overpressure and the flame propagation velocity increase sharply at the bifurcation point, and the surface at the bifurcation location is destroyed seriously. The gas explosion propagation is verified to suffer a dual effect of sudden expansion of the area and obstruction induction.

The effect of air pressure (0.01–0.3 MPa) on the detonatability of an aerosuspension of secondary explosive particles at a low mean volume density of the explosive (0.14–1.28 mg/cm³) is experimentally studied. The structure and basic parameters of detonation depending on the explosive density and initial gas pressure are determined, and the mechanism of low-velocity detonation propagation in explosive aerosuspensions is revealed. The lower concentration limits of detonation at different initial gas pressures are found.

A series of s-triazine substituted aminofurazan and aminofuroxan derivatives are investigated theoretically as potential high energy density materials (HEDMs). The crystal density, condensed heat of formation (HOF), detonation performance, acceleration ability, and critical pressure of initiation are estimated using empirical relationships. It is shown that the title compounds are characterized by high HOFs (817–4067 kJ/kg) and crystal density values (1.76–1.95 g/cm³). The calculated detonation performance shows that derivatives substituted with –N₃ and –C(NO₂)₃ are potential candidates for future HEDMs. Moreover, the values of the critical pressure of initiation indicate that furoxan derivatives are more sensitive to shock and impact compared to their furazan counterparts and that, for the substitutes –NH₂ or nitrogen–containing heterocycles, a similar or lower impact sensitivity than that of TNT (2,4,6–trinitrotoluene) can be obtained.

A thermodynamic analysis of the energy characteristics and combustion products of model mixtures based on polyethylene, tetranitromethane, and tetra(difluoroamino) methane is performed. Optimal ratios of the components are determined, and the maximum specific impulse was found for combustion of compositions in which the gram-atomic concentrations of hydrogen and fluorine are equal and, simultaneously, those of carbon and oxygen are equal. It is studied how the composition of the combustion products changes with a change in the ratio of the fuel components and how this is reflected in the value of the specific impulse.

The kinetics of devolatilization of some foodstuff materials like white wheat flour, sugar, and cocoa powders are studied by using thermogravimetric analysis, in order to measure their pyrolysis rate. The mean pyrolysis rate of these materials is used as a criterion to predict their explosivity. A comparison of the mean pyrolysis rates shows that the sugar powder is the most explosive material among others. Wheat flour explosivity is very close to sugar, and cocoa powder has the least tendency to explode. Our results are completely compatible with National Fire Protection Association reports.

Experimental data on the properties of explosion products in overdriven detonation for 50/50 TNT/RDX and PBX-9502 explosives indicate conditions characterized by a negative Grüneisen parameter. Given a possible relationship of this anomaly with the properties of explosion product components, available experimental data for nitrogen, carbon monoxide, carbon dioxide, and water are analyzed. The first three of these (carbon dioxide specifically) are capable of manifesting a negative Grüneisen parameter at pressures and temperatures characteristic of the anomaly of overdriven explosion products.

This paper presents an experimental study of the shock compression of an emulsion matrix based on an aqueous solution of ammonium and sodium nitrates at pressures up to 37 GPa, which is significantly higher than the calculated detonation pressure. The data obtained were used to determine the parameters of the Hayes equation of state and calculate the shock heating temperature of the matrix. At a pressure of more than 17 GPa, the input pressure profiles shows a rise associated with the chemical transformation of the emulsion.

In this paper, the application of emulsion explosives in synthesizing nanostructured ceria is introduced. The reaction mechanism to prepare nanostructured ceria with emulsion explosives by the detonation synthesis method is discussed. Nanostructured ceria powder is prepared by initiating special emulsion explosives in which Ce(NO₃)₃ · 6H₂O is regarded as the main oxidant in an explosion containment vessel. The phase composition, crystallite form, morphology, and microstructure of the as-synthesized products are characterized by x-ray diffraction analysis and transmission electron microscopy. The Brunauer-Emmett-Teller method is used to measure the specific surface areas of the powders. Nanostructured ceria belongs to the cubic phase. The morphology of as-synthesized ceria is nearly spherical, and the mean size of ceria particles is 55 nm. It is a mixture of monocrystal and polycrystal grains.

CO₂-laser welding of 12Kh18N10T stainless steel and a VT1-0 titanium alloy with the use of a composite insert obtained by means of explosive welding of four sheets (VT1-0 titanium, high-purity tantalum, M1 copper, and 12Kh18N10T steel) is experimentally studied. After mechanical processing, the insert is placed between the welded steel and titanium sheets. Butt-end welding of the steel and titanium sheets with the steel and titanium sides of the insert, respectively, is performed by a laser beam. Metallographic and spectrographic investigations are performed; mechanical properties of the resultant composite are tested. After thermal processing, the composite is found to break over the copper layer, which has a strength of 417 MPa.

A new aluminized explosive is proposed, and the approach is to replace the aluminum powder in the traditional aluminized explosive with an aluminum film. The purpose is not only to improve mechanical properties and lower the impact sensitivity of traditional aluminized explosives, but also to reduce environmental pollution in the aluminum particle production process. The pressure-time curves of the aluminum film explosive and RDX are measured in underwater explosion experiments. The peak pressure, impulse, shock wave energy, and bubble energy are obtained by analyzing the curves. The results of the study indicate that the peak pressure of the aluminum film explosive is lower than that of RDX. However, the aluminum film explosive maintains a high pressure for a longer period of time. The large amount of energy is found to liberate by subsequent reactions of the Al film with the primary detonation products. The increase in the explosion energy of the aluminum film explosive is based mainly on the increase in the bubble energy.

Results of interaction of a high-velocity annular steel impactor with a flat steel plate are reported. An annular crater is formed on the frontal surface of the plate. On the backward surface of the plate, the material is spalled in the form of a solid quasi-disk whose diameter is almost twice the outer diameter of the annular impactor. This observations shows that annular impactors are more hazardous than solid impactors (with an identical velocity and outer diameter).

Results of studying the normal impact of small-diameter steel spheres on the surface of semi-infinite targets made of porous copper are reported. Characteristics of craters being formed are compared with results of other investigations of the impact on high-porosity targets. New experimental data form the basis for continuing these studies and can be incorporated into models that describe a high-velocity impact on porous media.