Home – Home – Jornals – Combustion, Explosion and Shock Waves 2016 number 4

This paper presents a numerical and experimental study of the effect of flame-retardant additives on the autoignition of methane behind shock waves. It is shown that at a temperature of 11300-1900 K, the compounds CCl4, CF3H, and (СН3О)3РО not only do not suppress ignition but significantly reduce the induction time of methane-oxygen mixtures. A kinetic mechanism is proposed which relates the promoting effect to the reactivity of the pyrolysis products of the additives.

The influence of alkanes С₃–С₅ on the ignition of their binary mixtures with methane in air at a temperature 523–1000 K and a pressure of 1 atm is studied. It is shown that the presence of only 1% alkanes С₃–С₅ considerably reduces the ignition delay of methane. At a concentration of 10–20%, the ignition delay practically corresponds to the autoignition delay of the added alkane. The effect of additives of heavy alkanes becomes less noticeable with increasing initial temperature. These results can be used to estimate the permissible content of the C₅₊ heavy species in the gas turbine engine fuel at which their influence on the fuel knock resistance is sufficiently low. It is only 0.5%.

This paper describes the method for calculating the concentration limits of combustion with the use of equations for the boundaries of the diffusion-thermal stability. The key parameter responsible for the existence of the combustion limits is the heat effect of the gas mixture combustion. The thermal effect and the equation of the boundary of the diffusion-thermal stability are used to determine the minimum flame temperature below which combustion is not possible. A significant impact on the concentration limits is produced by the heat capacity of the components of the mixture if it is strongly dependent on temperature. For the upper and lower concentration limits, the minimum flame temperature generally varies and depends on the relative concentration and properties of the diluent. The theoretical methods for calculating the concentration limits are tested according to the experimental data on the combustion of the methane/air/diluent mixture.

The mechanism of formation of a nano-sized aerosol during mechanical destruction of coal from Kuzbass mines is studied. The concentration and size spectrum of aerosol particles in the mine tunnel with an operating shearer was measured using an aerosol spectrometer. It is found that 90% of the particles are less than 200 nm. In the nanometer range, there are two peaks corresponding to average diameters of 20 150 nm: the first of them is due to single particles, and the second to aggregates consisting of single particles. The formation of aerosol during mechanical crushing of coal in a continuous flow mill was studied. The spectrum and morphology of the particles produced in the laboratory mill are in qualitative agreement with those for the nanoaerosol formed in the mine. The effect of the coal aerosol on the combustion of gas mixtures was studied. Laboratory experiments have shown that the presence of nanoaerosol in a lean methane-air mixture significantly increases its explosiveness. This is manifested in an increase in the maximum pressure and a significant increase in the pressure rise rate during explosion. The study leads to the conclusion that the source of nanoaerosol is the organic components contained in coal and released into the gas phase during local heating of coal on the cutter teeth.

The process of chromium powder combustion in a concurrent flow of nitrogen-containing gas in a range of specific flow rate of up to 20 cm³/(c x cm²) is studied. The use of forced filtering intensifies the combustion wave propagation in the Cr-N₂ system. In this case, the combustion rate increases simultaneously with the decreasing degree of nitriding. The modes of superadiabatic heating with a flow of pure nitrogen and a hydrogen-argon mixture. It is shown that the use of a gaseous mixture promotes the formation of the inverted combustion wave. The tempering mode used in forced filtering allows fixating the high-temperature single-phase nonstoichiometric nitride Cr₂N.

Experimental data demonstrating the correlation of parameters in the power-law dependence of the burning rate of composite solid propellants on pressure are reported. The reasons for changes in the burning rate due to changes in conditions of mixing of the propellant are discussed. The scatter of the pressure in the combustor of a solid-propellant rocket motor is analyzed with due allowance for the correlation of parameters in the burning rate law. It is shown that the relative scatter of the burning rate depends on pressure at which propellant combustion occurs. Moreover, for each propellant, there exists a pressure level at which the burning rate scatter is theoretically equal to zero, regardless of the differences in propellant compositions and properties.

Results of a comprehensive numerical and experimental study of oscillatory processes in combustors of small-scale combustion heaters of air are reported. Methods for prediction and experimental determination of regular features of changes in spectral characteristics of pressure oscillations in the combustor in the nominal operation mode are presented. The data obtained in the study can be used for the development of various combustion heaters, including those for testing combustors of air-breathing engines for promising flying vehicles.

Regimes of continuous spin detonation in a plane-radial combustor with an external diameter of 80 mm with peripheral injection of a hydrogen-oxygen mixture in the range of specific flow rates of the mixture 3.6-37.9 kg/(s x m²) are obtained for the first time. Depending on the diameter of the exit orifice in the combustor (40, 30, or 20 mm), specific flow rate of the mixture, its composition, and counterpressure, one to seven transverse detonation waves with a frequency from 6 to 60 kHz are observed. It is found that the number of detonation waves increases, while their intensity decreases owing to reduction of the exit orifice diameter or to an increase in the counterpressure. The flow structure in the region of detonation waves is analyzed. The domain of detonation regimes in the coordinates of the fuel-to-air equivalence ratio and specific flow rate of the mixture is constructed. A physicomathematical model of continuous spin detonation in a plane-radial combustor is formulated. For parameters of hydrogen and oxygen injection into the combustor identical to experimental conditions, the present simulations predict similar parameters of detonation waves, in particular, the number of waves over the combustor circumference and the wave velocity.

A new physicomathematical model is proposed for describing the process of melting of nano-sized aluminum samples, which takes into account the dependences of thermophysical variables on the temperature and particle size obtained by the molecular dynamics method. The study is performed for samples with spherical, cylindrical, and plane symmetry. The times of melting of aluminum nanoparticles are found as functions of the nanoparticle radius and ambient temperature. Two-front melting modes are observed for the first time; these modes are the result of the scale factor in the dependence of the melting temperature on the particle size.

The results of experiments on the breakup dynamics of droplets and jets moving in a gas medium after their shock-wave ejection from the surface of a liquid and molten metal. Velocities of jets, the cloud of droplets, and the free surface, and the deceleration parameters of the droplets and jets in the gas were measured using a heterodyne laser interferometer (photon Doppler velocimetry, PDV). The induction period and the droplet sizes after breakup were determined.

This paper presents the results of experimental studies of the cavitation mechanism of fluid breakdown in a wide range of shock-wave loading. The velocity of the free surface of liquids and the velocity spectrum of the cloud of particles and jets was measured using a laser heterodyne interferometer (photon Doppler velocimetry method), and their size was determined. The spall strength of distilled water was determined.

This paper presents the comparative analysis of the results of more than fifty experiments on measuring the electrical conductivity of detonation products of RDX, HMX, PETN, TNT, and TATB-based explosives. It is revealed that there is a correlation between the electrical conductivity and the mass fraction of carbon both in the chemical spike and at the Chapman-Jouget point.

The dynamic compression and localized adiabatic shear in samples of an HMX based explosive using the Hopkinson split bar technique was studied. Experiments to study dynamic compression were performed at strain rates (0.3-2.0) × 103 с^-1. Fracture of the explosive samples was found to occur at stresses of 60-80 MPa. The behavior of HMX based samples was also studied in experiments on localized shear at different strain rates (200-2500 s^-1). The issues of the initiation of explosive transformations under the dynamic loads are discussed.