Home – Home – Jornals – Combustion, Explosion and Shock Waves 2014 number 2

Analytical dependence for the diffusion flame lift-off length with fuel jet discharge into the atmosphere is obtained in criterial form. The dependence can be used to determine the limiting parameters at which blow-off occurs and describe the lift-off length variation under pre-blow-off conditions, and it correlates well with earlier generalizations of experimental data. The experiments on the combustion of propane-butane and natural gas confirm the correctness of the proposed form of the analytical generalization, show the marked influence of the external shape of the gas nozzle on the lift-off parameters, and indicate that the blow-off of the turbulent diffusion flame is slower than the characteristic combustion time.

A skeletal mechanism of inhibition and quenching of methane flames by addition of trimethyl phosphate was developed. It includes a mechanism of methane oxidation consisting of 19 elementary steps involving 15 species (including N₂), and four elementary reactions involving three phosphorus-containing species (PO₂, HOPO, and HOPO₂). The developed skeletal mechanism adequately predicts the burning velocity of flames with added inhibitor over a range of equivalence ratio of 0.7–1.4 and can be used to model fire suppression.

Additional data on parameters of detonation of monofuel components that ensure positive energy release during decomposition are reported. Being added to a combustible mixture, such substances can initiate the development of instabilities directly on the detonation wave front. In this case, energy release in the mixture proceeds in two stages. Both the basic and bifurcation structures arise in a jump-like manner (spontaneously), which disagrees with the hypothesis of gradual enhancement of weak perturbations usually used in numerical simulations of cellular structures.

This paper gives the results of a comparative analysis of methods of simulating airflow for specified flight conditions at the inlet of a high-speed scramjet engine using test facilities with combustion heating of the working fluid. Conditions of the adequacy of the compared heating methods were determined from the obtained values of the engine thrust.

Experiments with combustion of diluted liquefied petroleum gases used as a fuel premixed with air are performed. From the experiment results, one can see that low-heat-value gases are capable of stable burning in a porous media burner. Distributions of species concentrations and flame temperature are measured. Based on these data, the flame is found to be most stable if the equivalence ratio is equal to 0.8. To improve the burner performance, experiments with different characteristics of porous media are performed. Optimal parameters of porous media are confirmed by subsequent numerical simulations. It is demonstrated that the properties of combustion in the porous media burner are superior as compared to those in the free flame burner.

Combustion of mixtures of a narrow fraction of ammonium perchlorate (AP) with hydrocarbon binders and combustion catalysts diethylferrocene and 1,1'-bis(dimethyloctyloxysilyl)ferrocene, as well as nano-sized Fe₂O₃ is studied. It is shown that the efficiency of ferrocene compounds from the viewpoint of increasing the burning rate depends on the oxidizer/fuel ratio in the propellant and on the place of the leading reaction of combustion. In composites with a high oxidizer/fuel ratio whose combustion follows the gas-phase model, the catalyst efficiency is rather low. In systems with a low oxidizer/fuel ratio where the contribution of condensed-phase reactions to the burning rate of the system is rather large, the catalyst efficiency is noticeably greater, and it is directly related to the possibility of formation of a soot skeleton during combustion. The close values of the catalytic activity of ferrocenes and Fe₂O₃ in the case of their small concentrations in such compositions testify that the main contribution to the increase in the propellant burning rate is made by Fe₂O₃ formed due to rapid oxidation of ferrocene on the AP surface and accumulated on the soot skeleton. Thermocouple measurements of propellants with a low oxidizer/fuel ratio are performed, and it is shown that the temperature of their surface is determined by plasticizer evaporation. A phenomenological model of combustion of the examined propellants is proposed.

Theoretical basis and practical ways to improve the accuracy of measuring the burning rate of energetic condensed systems by a microwave technique are considered. An analysis is made of the factors that reduce the measurement accuracy, in particular, edge effects, the non-evanescence of the waveguide, change in the effective diameter of the waveguide due to incomplete burning of the reinforcing composition, intrachamber pressure, curvature (roughness) of the burning surface. The components of the systematic measurement error are determined, the measurement error is calculated, and a numerical correction method for reducing the error to 4.5% is proposed.

Optical methods in infrared (IR) and visible (VIS) ranges for detection of combustion zone propagation in solid high-energy materials, such as pyrotechnic compositions inserted into pyrolytic graphite (pyrographite) tubes and ignited at one end by a CO₂ laser beam are presented. The pyrographite tube is used as a thermal management transducer enabling detection of combustion zone movement because of unique thermal conductivity anisotropy of pyrographite resulting in low thermal conductivity of the tube along its axis and high thermal conductivity along the tube radius. In the first method, an IR thermal camera is applied for detection of heat zone movement induced on the external side surface of the pyrographite tube by the combustion zone travelling inside the tube. According to the second method, a VIS camera and a thermochromic layer covering the external side surface of the pyrographite tube are used for visualization of heat zone movement registered as the color change boundary traveling along the thermochromic layer. The change in color of this layer is caused by its thermochromic substance response to heat delivered by the heat zone. As thermochromic substances, leuko dyes or chiral-nematic liquid crystals are used. These methods seem to be particularly promising for continuous measurements of burning rates of solid high-energy materials, such as solid rocket propellants and pyrotechnic compositions.

Interaction of heterogeneous cellular detonation propagating in a plane channel with a cloud of inert particles is numerically studied. It is demonstrated that the presence of inert particles alters the detonation wave structure and its velocity.

A method of preparing nanosized titanium dioxide by gaseous detonation by using ethanol, hydrogen, and oxygen as an explosion source and titanium tetrachloride as a precursor is described. The results indicate that the rutile phase content of the obtained products is high, and the pure rutile phase can be produced by adding a small amount of hydrogen. Most of the particles obtained under the two different synthesis conditions are spherical (or sphere-like) with small and even particle sizes, generally about 30 nm, and has a very good size dispersion.

A model of a reacting two-phase medium consisting of a gas and incompressible particles, which takes into account the collisional dynamics of random motion of particles, is presented. Molecular-kinetic approaches of the theory of granular media are applied. Shock wave patterns are analyzed, and conditions on strong discontinuities in the two-phase mixture are obtained. Two types of discontinuities are identified: without and with generation of the random energy on the shock wave. For shock waves of the second type, the amplitude of the particle concentration is independent of the wave propagation velocity. The model is verified against the results on the velocity of sound measured in mixtures ejected from a high-pressure chamber of a shock tube.

Based on numerical methods and theoretical analysis, the influence of the tunnel wall surface conditions on the methane–air explosion is evaluated. A rough tunnel wall causes stronger turbulence in the methane–air explosion. In a straight tunnel where some part of the space is filled with the methane–air mixture, the turbulence intensity varies with distance along the tunnel axis: it is higher in the methane–air premixing region and also in the far region of air shock wave propagation; between these regions, the turbulence intensity is lower. In the methane–air premixing region, the effect of turbulence is manifested as a significant increase in the explosion pressure. In the far region of air shock wave propagation, turbulence makes the shock wave strength decrease, but its effect is indistinctive among others. In the original methane–air premixing region, the explosion pressure of the methane–air mixture in a tunnel with rough walls is higher than that in a tunnel with smooth walls. However, the air shock wave beyond the premixing region in a tunnel with rough walls is weaker than that in a tunnel with smooth walls.

Initiation of detonation in a fuel–air mixture flow formed in an annular cylindrical combustor 306 mm in diameter is studied. The source of detonation initiation is the detonation wave entering the annular channel from a plane–radial vortex chamber, a jet of products, or a low-power heat pulse. It is demonstrated that continuous spin detonation (CSD) can be ensured by all these methods. Its formation is accompanied by a transitional process with a duration up to 10 ms, which is associated with violation of injection of the species (initiation by the detonation wave) or with the time of evolution of tangential instability in CSD (jet or spark initiation). Transfer of detonation to a flow of fuel–air mixtures with low chemical activity (propane–air, methane–air, kerosene–air, and gasoline–air mixtures) by the initiating detonation wave formed within fractions of a millisecond by a low-energy pulse or as a result of self-ignition of the hydrogen–air mixture in the plane–radial vortex chamber is realized. It is found that organization of CSD in these mixtures requires combustors with greater (than 306 mm) diameters. A possibility of CSD in kerosene–air and gasoline–air mixtures with low chemical activity by means of air enrichment by oxygen ahead of the combustor entrance is demonstrated.

The explosive properties of a mixture of benzotrifuroxane and anthracene in the form of a molecular complex (mixing of the components at the molecular level) and a mechanical mixture with the same molar ratio of the components are investigated. It is found that the molecular complex is an individual explosive with modified properties.

In order to study the phase transformations of ytterbium under shock compression, the electrical resistance of ytterbium at the initial temperatures of 77 and 290 K and a shock pressure of p ≤ 20 GPa is measured. The dependence of ytterbium resistance on pressure is nonmonotonic and indicates three successive phase transitions. At p ≈ 2 GPa, ytterbium enters a state with a high electrical resistance of the semiconductor type. The ytterbium bandgap at p ≈ 1.8 GPa is estimated as ≈ 0.02 eV. At p ≈ 3 GPa, the electrical resistance of ytterbium decreases due to a polymorphic phase transition The electrical resistance grows with further increase in pressure, and at p > 11 GPa, it does not change. The nature of the third transition is determined by calculating the temperature of the sample under shock compression. Analysis of the dependence of sample temperature on shock pressure, together with the phase diagram of ytterbium, suggests that the third transition is caused by ytterbium melting.

The trajectory of motion of a copper shell, the velocity of its flight, and the pressure of the explosion products on the inside of the copper shell were simulated. It was found that there is a significant difference in pressure behavior between experiments and calculations performed using the standard equations of state of explosives and explosion products. Most likely, there is a significant contribution of kinetic processes to the energy release behind the Jouguet point. In this case, the conversion of explosives to explosion products apparently include not only exothermic reactions but also endothermic processes.

The possibility of low-temperature oxidation of a solid carbonized coal residue in a mixture of NH₄NO₃ and supercritical water (723 K and 30 MPa) is shown for the first time and its mechanism is described. Conjugate processes of oxidation of the carbonized residue and formation of combustible gases H₂ and CH₄ caused by the participation of H₂O in redox reactions was found. It was established that the ash residue has a high porosity and consists of agglomerated nanoparticles of silicon and metal oxides.