Home – Home – Jornals – Combustion, Explosion and Shock Waves 2002 number 1

By the example of natural flames with energy excess, it is shown that these flames can exist in various systems and combustion regimes. The mere existence of some flames, such as cellular and spin flames, is caused by the excess energy. Mechanisms of energy concentration are also numerous. In addition to heat-transfer processes (conduction, convection, and radiation), energy concentration may be caused by mass-transfer processes, phase transitions, filtration, gas compressibility, etc. Examples of flames with artificial conditions for energy concentration demonstrate a wide range of possible applications of this phenomenon.  P.1-8

Self-drifting combustion spots or flame balls localized in space that have been recently found by numerical simulation of combustion of near<!dash!>limit premixed gases with low Lewis numbers are considered as bifurcations of stationary spherical flame balls.P. 9-18

A two-dimensional steady&-state structure of a film flow of a combustible liquid on a heat-conducting substrate in the case of combustion-wave propagation is considered in the hydrodynamic formulation. The physical mechanism of formation of this structure is analyzed. It is shown that the thermocapillary effect plays an important role. The conclusion is justified that the existence of a two<!dash!>dimensional regime is possible only for rather low values of the temperature gradient on the film surface. A critical condition is obtained, which determines the transition to a three<!dash!>dimensional regime. This condition implies that the flow velocity is equal to the velocity induced by the thermocapillary force. If the temperature gradient is higher than a certain critical value, a zone with a reverse flow should appear, in accordance with the two<!dash!>dimensional model. It is assumed that such a regime cannot exist due to its instability to three<!dash!>dimensional perturbations. Experiments with a liquid film flowing down due to gravity in the presence of an immovable heat source (without the combustion wave) support the conclusion of the transition to a three<!dash!>dimensional regular flow structure if the temperature gradient is rather high. The first part of the paper deals with simulation of the film structure for the critical condition satisfied. The second part deals with generalization of the problem to the case of a movable heat source moving with a constant velocity. This formulation of the problem includes the situation with combustion<!dash!>wave propagation. The mathematical formulation of this problem allows us to assume that the existence of a two<!dash!>dimensional steady<!dash!>state regime in this case is limited by the same critical condition. If the temperature gradient on the film surface is higher than the critical value, the two<!dash!>dimensional steady<!dash!>state solution does not exist. This concept justified in the present work offers a generic explanation of phenomena observed in liquid films in the presence of local heat sources of various natures. P. 19-23

The paper reports results of experimental ignition of litter layers consisting of needles of cedar, pine, and firtree, birch leaves, lichen Cladonia, and moss Pleurozium shreberi. It is established that the moss is ignited faster than the other combustible forest materials. It is shown that with equal moisture contents, the ignition times of needle litter from different trees are identical within the experimental error, and for litter of birch leaves, the ignition time is shorter than that for litter of coniferous trees. This difference is found to be due to differences in the interaction of the radiant flux with litter layers of needles and leaves. Minimum values of the ignition heat pulses for needle and leaf litter layers are estimated for various heatflux densities. These values tend to a minimum for a heatflux density of 0.50.8 MWm². P. 24-29

Parametric analysis of mathematical models of amyl nitration in a continuous stirred tank reactor and a tube reactor was erformed. Curves of steady states versus control parameters, multiplicity and neutrality curves of the steady states, and parametric and phase portraits of the system were plotted. The critical values of the input concentration and the heat<!dash!>transfer coefficient leading to disturbance of the steady<!dash!>state nitration of amyl were obtained for the tube reactor. P.30-36

When specimens pressed from a chromium powder react with nitrogen at pressures of 1<!ndash!>8 MPa and relative densities of 0.47<!ndash!>0.55, unsteady combustion of a nonthermal nature, which was not known previously, is observed. It is shown that the unsteadiness is due to cracks formed ahead of the combustion front. The specimens fail as a result of an increase in the condensed-phase volume upon formation of nitrides. P. 37-40

A three-dimensional mathematical model is constructed for the gasless combustion of a solid circular cylindrical specimen. The steady-stated spin regimes obtained were studied by numerical methods. The structure and mechanism of spin combustion are illustrated and discussed. It is shown how the space-time pattern of spin-wave propagation is complicated as the radius of the cylinder increases. Spin propagation of the front can proceed in a regime in which the structure of the front does not change (for small radii of the sample) or in an unsteady regime, in which the structure of the front undergoes numerous changes over a period. In the second case, a synchronous or alternate <!ldquo!>flicker<!rdquo!> of the sites is observed on the surface of the cylinder. The nonuniqueness of the combustion regimes is detected. It is shown that the average velocity of propagation of the spin-combustion front is of the order of the velocity of steady front propagation under adiabatic conditions. P. 41-48

The mechanism of combustion in a multilayer system of disks pressed from a 5Ti+3Si mixture under compressive stresses was studied experimentally. In the examined range of process parameters, the existence of only the relay-race combustion regime is revealed. The times of combustion transfer between the disks are determined. P. 49-53

The paper deals with the combustion of a SiO₂-Al stoichiometric mixture after mechanochemical treatment in activator mills of two types: mills using predominantly shear loading and those using sheardynamic compression. It is shown that treatment in different loading regimes, leading to different energy states of the material, and the use of modifying organic additives change significantly the ignition and combustion temperature of the mixture. Modification of the surface of the quartz particle by mechanochemical treatment in the presence of butanol or polystyrene and aluminum activates the combustion process, thus ensuring greater completeness of the oxidation-reduction reaction. P. 54-59

The combustion of a mixture of molybdenum and carbon activated by a high-energy magnesium-fluoroplastic mixture was studied. The adiabatic combustion temperature and the equilibrium compositions of the combustion products were calculated and, experimental dependences of the combustion temperature and rate on the main parameters of the process are obtained. It is shown that the activation of the formation of molybdenum carbides is of a thermal nature. The two observed mechanisms of formation of molybdenum carbides are found to differ in combustion temperature: at T= 2470 K, formation of carbides proceeds by a diffusion mechanism, and at T = 2470 K it proceeds by crystallization from a melt. The possibility of synthesis of single-phase Mo₂C in the combustion regime is shown. P. 60-64

A new method for an experimental study of burning processes in condensed substances is suggested, based on the statement and solution of inverse problems. An inverse problem of reconstructing the erosive burning rate of solid propellants from experimental data is formulated. The choice of an approach to solving the problem by the joint application of well<!dash!>known methods for inverse problem solution and specific features of experimental studies of burning processes, in particular, erosive burning, has been justified. The problem solution is illustrated by a numerical example. The testing involves a comparative analysis of two optimization methods: although both methods are characterized by an identical accuracy, the steepest descent method has a higher rate of convergence for this class of problems than the conjugate gradient method. P. 65-70

The paper presents estimates of the dynamic characteristics of unsteady combustion of a solid rocket propellant in a combustion chamber with in pressure is varied automatically by a specified program. The estimates are obtained from experimental data by mathematical modeling with the use of a simplified model of unsteady processes in the combustion chamber. A linear approximation is obtained for the transfer function of the unsteady-combustion rate, and its coefficients are expressed analytically in terms of the parameters of a simplified model of unsteady processes. Results of identification of the model for several fragments of experimental processes are given. Families of approximate frequency characteristics of the unsteady-combustion rate are constructed, analyzed, and compared with available results. P. 71-80

The paper describes a new probe method for determining the quantitative composition of solid-propellant combustion products at temperatures of 25003200 K and pressures of 48 MPa under conditions typical of rocket motor conditions. A twostep probe is described, which allows a sample to be frozen without passing through the main shocks inside the sampler. The gas dynamics and the kinetics of chemical reactions were simulated to asses the correctness of sampling. It is shown that during sampling from a flame, the relative change in concentrations for most of the stable components does not exceed 3, and for H₂ and O₂, it does not exceed 12. The method permits additional operations with a sample, in particular, separation of CO and N₂ with subsequent analysis on a timeofflight mass spectrometer. The CO and CO₂ concentrations in the combustion products of the model composite solid propellant ammonium dinitramide ADN with polycaprolactone pCLN were determined at a pressure of 4 MPa. P. 81-91

The effect of gaseous combustion environment on particle size distribution and chemical compositions of condensed combustion products of a model propellant containing ammonium perchlorate, binder, and 23.4% aluminum was studied. Experiments were conducted at pressures of 0.6, 4.0, and 7.5MPa. Oxide particles with sizes of 1.2<!ndash!>60 <!mgr!>m and agglomerates with sizes from 60 <!mgr!>m to maximum were investigated. In experiments with nitrogen and helium, the difference in the mean sizes of the sampled agglomerates does not exceed the experimental error. The difference in the amount of unreacted <!lpar!>metallic<!rpar!> aluminum in the agglomerates sampled in nitrogen and helium is also negligible. Replacement of nitrogen by helium affects the size distribution of the oxide particles by increasing the mass fraction of particles with sizes of 1.2<!ndash!>10 <!mgr!>m, and this effect is enhanced with pressure. P. 92-100

Based on the mathematical model of a reacting two-phase medium in the two-velocity, two-emperature approximation, the process of planar shock wave entering a cloud of aluminum particles is numerically studied. The incident shock wave may have either a rectangular or a triangular profile, i.e., it may be accompanied by a rarefaction wave. An analysis of numerical data allowed us to determine conditions of possible establishment of a steady detonation regime in the cloud. Scenarios of initiation and types of detonation flows in the cloud are determined as functions of the amplitude of the incident shock wave and initiation energy. Criteria of detonation initiation for various fractions of particles are obtained, which express the dependence of the energy stored in the shock wave on its Mach number. P.101-108

The formation dynamics and structural features of the two-dimensional reaction zone of a detonation wave propagating in a two-layer bubbly medium is numerically studied within the framework of the Iordanskii-Kogarko one-velocity model. P.109-113

It is shown that chemical interaction between a carbon-containing shaped-charge flow and titanium target is possible. The upper layers of the targets tested are examined by x<!dash!>ray diffractometry. The dependence of the phase composition on the experimental conditions and composition of the initial porous liner is studied. P. 114-118

Plate acceleration by a gliding detonation wave is studied experimentally for detonation rates of <!ap!> 950<!ndash!>2000 m<!sol!>sec. It is found that regular elastic waves occur at the surface of the projectile plate made of a rather strong material, which disappear upon attainment of the upper limit of the detonation rate. P. 119-122

The paper studies properties of an ultrafine aluminum powder produced by electric explosions of conductors, whose particles are stabilized by coating with aluminum diboride immediately during the synthesis of the powder. The ultrafine aluminum powder stabilized in such a manner has special properties<!colon!> narrow particle size distribution, increased dispersity, and higher resistance to oxidation upon heating. P. 123-139