Home – Home – Jornals – Combustion, Explosion and Shock Waves 2019 number 5

This paper presents a kinetic analysis of a chemical model for the oxidative conversion of methane which includes all possible elementary reactions that can occur in this complex radical chain process. It is found that the model fully reflects the kinetic features of degenerate-branched chain reactions. The key stages of the process responsible for the formation of the main reaction products and the development of the process as a whole are identified. The nonlinear reactions of free peroxide radicals play a decisive role in these stages. The results of the kinetic analysis of the model confirm the previously obtained direct experimental data on free radicals and their kinetic behavior during methane oxidation.

This paper describes the study of self-ignition delays of stoichiometric binary, triple, and multicomponent mixtures of methane with the addition of up to 10% of normal T₂-T₆ alkanes in air at temperatures of 523-1000 K and a pressure of 1 atm. It is determined that the additives of all T₂-T₆ alkanes can reduce the self-ignition delay of methane already at a level of 1%. With the addition of 10%, the ignition delay of this mixture virtually becomes equal to that of the added alkane itself. In binary mixtures with methane, ethane additives have an accelerating effect that is no lower than that of propane and butane additives. In triple and more complex mixtures, the self-ignition delay is determined by the total sum of added alkanes and is virtually independent within the framework of a determination error on their particular ratio.

The combustion of ammonium, hydrazine, hydroxylammonium, ethylenediamine, guanidine, aminoguanidine, diaminoguanidine, and triaminoguanidine salts of 5.5'-azotetrazole is under study. The dependences of the burning rate on pressure and temperature distribution in a combustion wave are determined. It is shown that the combustion of salts obeys a model with a leading reaction in the condensed phase. The surface temperature during the combustion of 5.5'-azotetrazole salts is controlled by a dissociative evaporation process. Based on the parameters of combustion and established combustion mechanism, the kinetics of the leading reaction of the combustion of 5.5'-azotetrazole salts, which is a decomposition reaction, is determined. The ambiguity of the influence of the base strength on the combustion process is shown. On the one hand, an increase in the base strength decreases the decomposition rate of a substance due to salt dissociation suppression. On the other hand, it increases the surface temperature at which the decomposition.

Searching for energetic materials with balanced detonation performance and sensitivity is a long-standing goal in the development of high energy density materials (HEDMs). In this work, density functional theory calculations are carried out to characterize the structure--property relationships of four linked 1,2,4-oxadiazole/1,2,4-oxadiazole and 1,2,4-oxadiazole/1,3,4-oxadiazole derivatives. Our results show that all these designed compounds possess good oxygen balance, positive heats of formation, high crystal densities, remarkable detonation performance, and acceptable impact sensitivity. Particularly, the first of these compounds has the best balanced detonation performance and sensitivity, with excellent detonation performance superior to that of 1,3,5-trinitro-1,3,5-triazinane (RDX) and lower impact sensitivity than that of 1,3,5,7-tetranitro- 1,3,5,7-tetrazocane (HMX). Given these exceptional properties, it is expected that all these designed compounds are potential HEDM candidates with low sensitivity.

The method of probe sampling of gas samples is actively used to study high-temperature oxidation processes. This method directly provides information on the chemical composition of the reaction volume; however, the probe inevitably introduces thermal and gas-dynamic perturbations into the system. Surface temperature profiles of a quartz probe in a premixed CH₄/O₂/Ar flame were measured in the pressure range 1–5 atm. Measurements were made by a non-contact method using a thermal imager, which was calibrated by thermocouple measurements taking into account the shielding by the flame. The obtained data can be used as boundary conditions for numerical simulation of the sampling process, which will significantly increase the accuracy of error estimation when using probe methods.

This paper describes the influence of mechanical activation of coals on ignition and thermal-oxidative degradation using two independent methods: thermogravimetric analysis and ignition of a dust suspension in a vertical pipe furnace. It is determined that the ignition temperature of mechanically activated coals decreases and that mechanical activation affects the further process of thermal-oxidative degradation.

This paper describes the experimental study of amplitude (with a frequency of up to 50 kHz) and integral characteristics of the self-magnetic field of high-temperature combustion products of hydrocarbon fuel, which flow out of the nozzle of a model liquid-propellant engine (LPE) with simulation of the heat of a combustion chamber with injection of aluminum-magnesium alloy particles (k-phase) into the combustion chamber. It is determined that the amplitude of the magnetic field intensity generated by high-temperature (up to 3500 K) combustion products depends on the LPE operation regimes and the presence of k-phase particles in the particle flow. The magnetic field amplitude increases by 20% during the LPE burnout ~0.2 s earlier than the pressure drop in the combustion chamber. The total volumetric electric charge generated by the combustion product flow with the k-phase is estimated.

A physicomathematical formulation of the problem of ignition of the forest canopy based on the laws of mechanics of reacting media is presented, and the results of numerical calculations of the time and radius of ignition of the forest exposed to a thermal radiation source - a fireball resulting from a hydrocarbon explosion. Forest canopy (set of tree crowns) is considered as a homogeneous two-temperature porous reactive medium. Temperature and concentration changes at the upper boundary of the forest canopy from the beginning of its heating to ignition are described.

Cellular detonation in monodisperse suspensions of submicron and nano-sized aluminum particles is numerically simulated. Approaches of mechanics of heterogeneous media are applied. The transition from the continuum to free-molecular regime of the flow around the particles is taken into account in the processes of interphase interaction. Particle combustion is described within the framework of the semi-empirical model developed previously. Results calculated for two-dimensional flows in a plane channel for suspensions of aluminum particles with the particle size ranging from 1 μm to 100 nm. The regular structure of cellular detonation is found to transform to an irregular structure as the particle size decreases. An increase in the peak pressure and enlargement of the detonation cell are also noted, which is attributed to enhancement of the activation energy of reduced kinetics caused by the transition to the kinetic regime of combustion of aluminum particles.

Regimes of continuous detonation of heterogeneous mixtures of aviation kerosene and air with addition of hydrogen or syngas are studied in a flow-type annular cylindrical combustor 503 mm in diameter. With variations of the flow rates of air, liquid kerosene, hydrogen, and their relationships, regimes of continuous spin detonation are obtained in the following ranges: number of detonation waves 1-5, detonation wave velocity 1.15-1.67 km/s, and wave rotation frequency 0.73-4.86 kHz. In the case with addition of syngas with the composition CO + 3H₂, regimes with two opposing transverse waves are obtained; the mean velocity of wave rotation is 0.66-1.47 km/s, and the frequency is 0.85-1.87 kHz. Bubbling of the gaseous fuel (hydrogen or syngas) through liquid kerosene in the fuel injection system makes it possible to reduce the mass fraction of the gas in the two-phase fuel down to 8.4% for hydrogen and 47% for syngas with the composition CO + 3H₂. It is demonstrated that the minimum fraction of syngas in kerosene that still ensures the detonation regime is determined by the amount of hydrogen. Based on the stagnation pressure measured at the combustor exit, the specific impulse in the case of continuous detonation is determined as a function of the two-phase fuel composition. The maximum value of the specific impulse (about 4000 s) is obtained for the mass fraction of hydrogen in the two-phase fuel equal to 42%. The minimum diameter of the annular detonation combustor is estimated as a function of the specific flow rate of the heterogeneous mixture.

Infrared (IR) guided missile is known to cause 90\% of aircraft damage. Magnesium/Teflon/Viton (MTV) decoy flares are customized materials, which are capable of yielding thermal signature to interfere with IR guided missile seekers. In this study, the IR-signature of an MTV flare is measured in a jet engine nozzle. The jet engine IR-signature is characterized with two characteristic peaks over the α-band (2-3 λm) and β-band (3-5 λm); this is correlated to black body emission at 690 °C. The MTV decoy flare with 65% Mg offers an increase in the average intensity of the α- and β-bands by 21 and 4~times, respectively, to that of the jet engine. Quantification of emitting species in the combustion flame is conducted by using the ICT thermodynamic code. The developed MTV formulation offers the relative intensity ratio of the α- and β-bands equal to 0.96, which is comparable to that of an aircraft (0.7).

This paper presents experimental data on the parameters of an air shock wave resulting from initiation of BC-2 light-sensitive composition by the radiation of a laser diode. The propagation of the air shock wave front was recorded by the background-oriented schlieren method. The air shock wave front was visualized using cross-correlation processing. Empirical dependences characterizing the propagation of the air shock wave front resulting from initiation of a light-sensitive composition of arbitrary mass. Experiments confirmed the possibility of initiating BC-2 light-sensitive composition using an EV-15 electric igniter.

Flash radiography was used to study the propagation of detonation in semi-ring charges of plasticized TATB with a steel shell inside when initiating normal detonation along a line on the outer surface of the charge. The shape of the detonation wave front at different times was determined. The shape of the front is found to be influenced by a layer of RDX based plastic explosive located on the surface of the main charge and having a detonation velocity ~10% higher than that of TATB. It is shown that the position and shape of the detonation wave front in a cylindrical TATB charge is not described by the laws of geometric optics (Huygens principle) due to the features of detonation initiation in the initial section and the presence of the steel shell.

A simple phenomenological model of electrical resistance of metals at high pressures and temperatures is considered on the basis of the Bloch-Gruneisen equation of electrical resistance and Mie-Gruneisen equation of state. Two free parameters of the model for copper are found through comparisons of model predictions with experimental data on isothermal compression and isobaric heating. The dependence of the specific electrical resistance of copper on the shock pressure in the range up to 20 GPa is determined on the basis of experiments including measurements of electrical conductivity of foil specimens. Comparisons of the experimental shock wave results with the formulated model reveal the difference in the specific electrical resistance values. It is proposed to attribute the observed difference between the model and experimental results to the nonequilibrium nature of the physical state under shock compression, leading to generation of defects of the crystal structure of the metal. The electrical resistance component caused by the crystal structure defects is identified, and its dependence on the shock pressure is determined. The concentration of point defects in shock-compressed copper is estimated. The contribution of defects to electrical resistance of the shock-compressed metal is found to increase with increasing pressure. This effect should be taken into account in determining the equilibrium specific electrical conductivity and the derivatives of the physical variables (e.g., thermal conductivity).

The kinetic parameters of thermal decomposition of furazano-1,2,3,4-tetrazine-1,3-dioxide (FTDO) and the FTDO/dinitrazapentane (DNP) (75/25%) system were experimentally investigated by differential scanning calorimetry. Kissinger kinetic analysis was performed using differential scanning calorimetry data obtained at heating rates of 0.1–5.0 K/min. It was found that the thermal decomposition of FTDO and the FTDO/DNP system occurs by a first-order reaction with an activation energy of 185 (FTDO) and 171 kJ/mol (FTDO/DNP).