Home – Home – Jornals – Combustion, Explosion and Shock Waves 2016 number 6

The theory of a strong explosion is used as a basis for the development of an experimental technique for determining the source energy that ensures initiation of the combustible mixture. The technique is tested in experiments aimed at determining the critical energies of spherical detonation initiation E ₃^* with the use of an electric discharge for a stoichiometric acetylene-oxygen mixture and also for two-fuel mixtures (acetylene-nitrous oxide-oxygen) possessing bifurcation properties of cellular structures. The critical energy E ₃^* for the stoichiometric two-fuel mixture in terms of both fuels with a bifurcation structure is several-fold lower than the value of E ₃^* for the monofuel mixture whose cell size at a given pressure is determined by the large scale of bifurcation cells. This result testifies that the value of E ₃^* decreases with increasing number of “hot points,” which are numerous regions of collisions of large-scale and small-scale transverse waves in the mixture with bifurcation properties.

The processes of ignition and combustion of i -C₈H₁₈-H₂ and n -C₁₀H₂₂-H₂ composite propellants in air are analyzed numerically. It is demonstrated that addition of hydrogen both to standard alkane ( n -C₁₀H₂₂) and to alkane with a branched structure ( i -C₈H₁₈) leads to an increase in the ignition delay time τ_ind if the initial temperature of the mixture T ₀ is lower than a certain value T _l and, vice versa, to a decrease in τ_ind at T ₀ > T _l. The greater than fraction of hydrogen in the mixture, the greater the change in τ_ind . At sufficiently high temperatures ( T ₀ > T_h ), addition of a small amount of alkane (»2-10%) to hydrogen reduces the ignition delay time. The value of T _l depends on the pressure of the fuel-air mixture and, to a smaller extent, on the n -alkane type. The value of T_h depends on the fraction of alkane in the composite propellant. If the initial pressure is sufficiently high (10 atm and more), addition of a small amount of i -C₈H₁₈ or n -C₁₀H₂₂ to the hydrogen-air mixture reduces the value of τ_ind for all values of T ₀. These features are caused by close interaction of the alkane and hydrogen oxidation kinetics. It is demonstrated that composite propellants consisting of hydrogen and n -C₁₀H₂₂ ( i -C₈H₁₈) have a higher velocity of the laminar flame and wider limits of stable combustion than the hydrocarbons themselves. Nevertheless, a noticeable increase in the laminar flame velocity is observed only for the molar fraction of hydrogen in the composite mixture greater than 50%. In this case, it becomes possible to ensure stable combustion with a smaller fraction of NO in combustion products.

The chemical conversion of SO₂ to elemental sulfur in the chain reaction of hydrogen oxidation in a low-temperature flame has been studied. The possible elementary reactions involving atoms and free radicals that may be responsible for the chemical conversion of SO₂ with the formation of sulfur in the conjugate radical chain process are discussed based on the thermodynamic and kinetic characteristics.

Kinetic analysis of the mechanism of chemical conversion of SO₂ in a coupled radical-chain process in low-temperature low-pressure oxyhydrogen flames has been performed. The set of possible elementary reactions is considered, and the main routes of SO₂ conversion to elemental sulfur are identified.

The features of the structure and phase formation in the Ti : Nb : 2Al, Ti : Nb : 2.5Al and Ti : Nb : 3Al systems in the self-propagating high-temperature synthesis in the thermal explosion mode. The morphology, phase composition, microstructure, and physical properties have been studied. It has been found that compounds with the highest content of aluminum have the most homogeneous composition and the lowest porosity. The main phase of the product synthesis is a phase based a solid solution of Nb in γ-TiAl.

This paper describes the experiments on the combustion of the powder and granular mixtures of Ti + 0.5C, Ti + 0.75C, and Ti + C. Despite the fact that there is no convective heat transfer and the contact area between the particles is small, the burning rate of granular compositions (both linear and mass) turns out to be several times greater than in the case of powder mixtures of the same composition. The experimental and computational values of the adiabatic combustion temperature were used to estimate the contribution of the radiant and conductive heat transfer in the combustion wave propagation along the granulated mixtures. The experiments with pressed samples showed that the high combustion rate of the granular mixtures is due to great velocity of the combustion wave propagation along the granule rather than the specific features of the original reagents.

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dusty flame of fine-grain metal particles with diameters d₁₀ < 15 μm and particle concentrations from ≈10¹⁰ to 10¹¹ m^{-3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d}₁₀ = 4 μm) in a laminar dusty flame is controlled by diffusion of the oxidizer toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d₁₀ = 5÷15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional regime.

This paper presents the new way of the occurrence of <natural> turbulence in the Gusachenko-Zarko mechanism of negative erosion effect during the of combustion fuel. It is shown that the gasification zone of a solid fuel can generate hydrodynamic instability if its burning rate at a constant temperature depends on the pressure. The hydrodynamic instability of the combustion of fuels that decompose according to the solid phase ® liquid phase ® gas and solid phase ® gas scheme occurs under quite different conditions. The gasification zone in fuels of the first type is more inclined to instability generation than that in fuels of the second type. The hydrodynamic instability occurs as the value of the Reynolds number is exceeded, which depends on the properties of the fuel and environmental conditions.

The solid-state ignition of a metallized composite propellant (ammonium perchlorate + 14% butyl rubber + 5% aluminum powder + 6% plasticizer) under local heating by several sources of limited power capacity (the dimensions of the hot particle x_p = 4 mm and y_p = 2 mm) was studied by mathematical modeling. For temperature of the heated steel particles and the distance between them varied in the ranges 700 < T_p < 1500 K and 0.1x _p < Δx <1.5x_p, respectively, the values of T_p and Δx were determined for which the ignition delay corresponds to the initiation of combustion of the composite propellant by a single particle, a plate at a constant temperature or several particles. In the region of low initial temperatures of local sources ( T_p < 1100 K), the limiting values Δx - 0.1 x_p and Δx > 1.5x_p were identified for which the characteristics and mechanism of ignition of the propellant by a group of heated particles can be studied using the plate-fuel-gas model and the single particle-fuel-gas model, respectively. Reducing the distance Δx at T_p < 1R 100 leads to a reduction in the induction period to 50% and a decrease in the minimum initial temperature of the source required for propellant combustion initiation from 830 to 700 K. At T _p > 1100 K, the ignition of the metallized composite solid propellant by a single or several particles can be studied using relatively simple one-dimensional models of condensed matter ignition by a plate at constant temperature. The variation in the ignition delay in this case is less than 5%.

Regimes of continuous spin detonation of anthracite and lignite particles in an air flow in a radial vortex combustor 500 mm in diameter with a constant (along the radius cross-sectional area are studied. Crushed coal with a particle size of 1-12 μm is used. For transporting coal into the combustor and promoting the chemical reaction on the surface of solid particles, hydrogen or syngas was added in the ratio CO/Н₂ = 1/1, 1/2, or 1/3. Continuous spin detonation of two-phase mixtures of fine anthracite and lignite particles and air with addition of hydrogen up to 4% of the coal consumption rate is obtained for the first time. The amount of syngas added to coal increases with decreasing fraction of hydrogen in the syngas: 14, 21, and 27% for anthracite and 11, 20, and 29% for lignite at СО/Н₂ = 1/3, 1/2, and 1/1, respectively. The detonation wave structure and the flow in their vicinity are not principally different from those observed previously for long-flame hard coal and charcoal. Higher detonation velocities are observed for more energy-intensive coal (anthracite). A higher pressure is obtained near the cylindrical wall of the combustor in cold runs as compared to detonation in the case with identical flow rates of the coal-air mixtures.

This paper describes the study on the dependence of the critical energy density of the explosive decomposition of PETN on the mass concentration of the inclusions of ultrafine Al particles (100÷120 nm) in the range of 0.025÷1% under the influence of the first and second harmonics of neodymium laser (12 ns, 1064 and 532 nm). It is shown that the critical energy of the explosion initiation by the first and second harmonics of laser radiation reaches a minimum value at the same absorption rates, but different concentrations of inclusions. The photoacoustic method is used to show that, as the laser energy radiation is absorbed, the pressure amplitude in the heated layer reaches a maximum value when the concentration of inclusions corresponds to the minimum value of the critical energy density.

In order to study the corner turning performance of detonation waves for TATB (1,3,5-triamino-2,4,6-trinitrobenzene) based and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) based polymer bonded explosives (PBXs with PBX-I, PBX-II, and PBX-III modifications), mushroom tests are used to obtain the first breakout angles, failure angles, and delay times with initiating diameters of 10 and 15 mm. The results show that these parameters of PBX-I increase with an increase in the initiating diameter. The first breakout angles and failure angles of PBX-II and PBX-III are 90^o for the initiating diameter of 10 mm, while these angles for PBX-I are 22.7 and 31.9^o for the same initiating diameter, which implies that CL-20 based explosives have excellent corner turning performance, even with 13.5 wt.% aluminum powders added to PBX-III. Then, two-dimensional numerical simulations of PBX-I are performed by using the Lee-Tarver ignition and growth model. The computed results agree well with the measured results for all cases studied.

This paper presents the result of a study of the chemical composition, physicotechnical properties and structure of various types of granulated ammonium nitrate (high-density, porous and) made in Russia and abroad. It is shown that heat treatment of high-density GOST 2-2013 ammonium nitrate granules leads to changes in the crystal structure (aeration) that increase the retention capacity relative to the fuel oil. The detonation velocity of ammonium nitrate/fuel oil compositions based on aerated ammonium nitrate was measured.