Vol 42, No 8 (2023)

Recent publications on the combustion of a granular charge intended for the synthesis of compositions based on titanium carbide have revealed significant changes in the characteristics and combustion rate with an inert gas coflow. The authors of the studies associated these changes with the convective burning regime. This paper presents a theoretical model that makes it possible to analyze the contribution of convective heat transfer during the combustion of a granular charge in a cocurrent gas flow in self-propagating high-temperature synthesis (SHS) processes. It is shown that, depending on the rate of the hot gas flow blown through a granular sample, three combustion regimes are possible. In the absence of gas or a low gas flow rate (up to a level of 1 kg/(m2 s)), the combustion front is predominantly flat and convection does not play a significant role. At moderate flow rates (at the level of 10 kg/(m2 s)), the effect of convection becomes noticeable, the combustion rate doubles, and the combustion front is no longer flat, since the surface layers of the granules heat up faster than the layers in the center. Finally, at high flow rates (at the level of 50 kg/(m2 s)), the contribution of convection becomes predominant, the combustion rate exceeds the base rate (in the absence of gas blowing) by more than an order of magnitude, and the structure of the combustion wave is significantly rearranged.

The fast combustion process of nanosized porous Al + CuO mixtures placed in glass tubes is studied using a high-speed video recording. Mathematical processing of the high-velocity frame sequence obtained using neutral filters of different thicknesses made it possible to determine the nanothermite (NT) burning rate in different parts of the tube and experimentally estimate the sizes of the ignition and combustion zones of NT. To explain the mechanism of combustion propagation, a simple model based on Darcy’s law is proposed for the filtration of hot products through the macropores. Based on the results of the model experiments on the combustion of NT in glass-tubes filled by portions of the mixture separated by inert barriers (glass microspheres, air gaps), it was possible to develop a simple procedure to estimate the permeability of a nanosized mixture and pressure in the combustion zone.

The reasons for the observed propagation velocities of stationary laser-supported combustion (LSC) waves in laser plasmatron scheme in argon and air to exceed the calculated ones in assumption of heat-conductive propagation mechanism are considered. Earlier obtained analytical solution of the hydrodynamic problem of flowing around model low density heated gas volume with step-like spherical boundary is used for interpretation experimental results. It is shown that when laser power is 2–3 times above LSC threshold power heat-conductive mechanism with correction factor predicted by the model gives satisfying description of the LSC wave velocities observed. At higher laser power radiative heat transfer factor should be taken into account. It is shown that flowing around spherical hot gas boundary model can also be applied to describe gas flow in thermal gravitational convection around continuous optical discharge (COD). An estimate is given for the pulsation frequency of the convective plume from COD, leading to the similarity relation common for optical discharges and flickering flames.

This paper carries out a numerical study of the parameters of the shock waves (SWs) generated during the expansion of cylindrical volumes of compressed methane in the case when the compressed gas is in water. This problem simulates the rupture of underwater gas pipelines. Compressed methane is considered at a pressure of 10.1 MPa, which is typical for trunk pipeline systems. The dependences of pressure on the distance from the axis of the pipeline in waves in the surrounding water mass are obtained. At the same time, a hump-shaped profile characteristic of cylindrical geometry is observed in the SWs in water. The interaction of the waves at the exit to the free surface and when traversing the steel layer is considered.

It is found that the determining factor for the catalytic ignition of mixtures of hydrogen with ethane and ethylene is not only the material of the catalyst but also the chemical nature of the C2 hydrocarbon in the mixture with H2. It is shown that the limits of the catalytic ignition of the synthesis gas over metallic rhodium (Rh) are qualitatively different from the dependences for a hydrogen–hydrocarbon mixed fuel. The dependence of the lower limit of catalytic ignition on temperature has a distinct maximum, which indicates a more complex mechanism of the catalytic process than in the case of hydrogen–methane mixtures; the Arrhenius dependence of ln [H2]lim on 1/T does not hold. Therefore, the interpretation of the upper and lower limits of catalytic ignition (ULCI, LLCI) used in the literature, taking into account catalyst poisoning by CO molecules, needs to be clarified. The relatively long delay periods of the catalytic ignition of hydrogen–n-pentane mixtures (tens of seconds) and the absence of dependence of the delays on the initial temperature allow us to conclude that the catalytic ignition of hydrogen–propane/n-pentane mixtures is determined by the rate of transfer of hydrocarbon molecules to the surface of the catalytic wire. Thus, in the oxidation of hydrogen–hydrocarbon mixtures for “short” hydrocarbons, the main factor determining the catalytic ignition is the oxidation reaction of hydrogen on the catalytic surface. With an increase in the number of carbon atoms in the hydrocarbon, the factors associated with the chemical structure, i.e., the reactivity of the hydrocarbon in catalytic oxidation, begin to play a significant role; and then the rate of oxidation is determined by the rate of transfer of the hydrocarbon molecules to (or within) the catalyst surface.

Compact samples with diameters of 3 and 5 mm are prepared from pyrophoric iron nanopowder in a glove box in an argon atmosphere, which are placed in bottles with a ground-in lid. The iron nanopowder is obtained by the chemical-metallurgical method. The average size of the nanoparticles is 85 nm. It is established that during the exposure of the bottles with the samples to air, the samples were passivated with the preservation of their high chemical activity, since a combustion wave was launched over the sample at a rate of about 0.25 mm / s when the oxidation reaction was initiated by a high-temperature source. Exposure of passivated samples with a diameter of 5 mm for 60 min at a temperature of 110°С did not lead to a change in the phase composition of the sample. Exposure at a temperature of 180°С for 30 min led to a change in the color of the sample and its oxidation. The experiments with passivated samples with a diameter of 3 mm show that, under conditions of programmed heating, ignition of the samples occurs at a temperature of about 100°С. The conducted studies allow us to consider the obtained compact samples from an iron nanopowder thermally stable at temperatures below 100°C, when special temperature conditions are not required for their safe storage and transportation.