Russian Journal of Physical Chemistry B最新文献

Small additions of hydrocarbons, such as propylene, which are widely studied as combustion and explosion inhibitors of hydrogen-air mixtures, sometimes exhibit specific properties. The known mechanism of the inhibitory effect of these additives is associated with the intensification of the termination of branching chains due to the addition of hydrogen atoms; however, conditions also exist in which these compounds, instead of inhibiting, have a neutral and even promoting effect. Such conditions and the reasons leading to the fact that inhibition is practically absent have not yet been studied. This article shows the results of numerical modeling, which make it possible to more fully outline the range of conditions where the addition of propylene practically does not inhibit hydrogen-air mixtures and outline possible reasons for this effect. The solution to three model problems is presented: self-ignition in a constant-volume reactor, laminar flame propagation, and gas ignition with a heated wire. The calculations are carried out with the detailed kinetic mechanism of chemical reactions NUIGMech 1.1 (2020). The objects of the study are three air mixtures containing hydrogen in amounts of 15, 29.6, and 50 vol % (lean, stoichiometric, and rich mixtures, respectively) without additives and with the addition of 1% propylene.

A numerical model for the POX steam-oxygen conversion of methane to synthesis gas in a reversed flow nonpremixed filtration combustion reactor with a reversed flow of a steam-methane mixture and a continuous supply of oxygen to the center of the reactor is carried out. The calculations were performed for the oxygen/methane molar ratio of 0.47 and steam/methane molar ratio of 0.5, in the parametric region close to the limit for the feasibility of the scheme. Various modes of initiation and control of flow reversal are considered, and dependences of the combustion temperature and the composition of products on the characteristics of the process are obtained. A comparison of the established cyclic mode of conversion with the predictions of the equilibrium model shows that the kinetic constraints lead to a higher combustion temperature and incomplete conversion of methane. At high temperatures, the conversion proceeds via the initial soot formation during the pyrolysis of methane and the subsequent reaction of soot with steam.

The ignition of the normal layer-by-layer burning (LB) and its transition to the convective burning (CB) regime in mixtures of ammonium nitrate (AN) with bulk density aluminum are studied. Experiments in a constant-volume bomb with pressure registration are carried out. The porosity of the samples is 0.55–0.59, the particle size of the AN varies from 20–40 to 250–630 µm, and the aluminum content varies from 8 to 47 wt %. Two brands of aluminum were used: ASD-4 and PAP-2. It is shown that the mixtures are capable of being ignited when the igniter pressure is close to or above the critical (minimum) value. The values of the critical pressure of the igniter, the pressure, and the time at which LB and CB occurs for mixtures with different particle sizes of AN and aluminum and different concentrations are measured. The replacement of aluminum ASD-4 with PAP‑2 leads to a significant (by an order of magnitude or even more) decrease in the values of critical pressure and pressures at which LB and CB begins.

A thermogravimetric analysis of the thermal decomposition of polymethylmethacrylate (PMMA) in a carbon dioxide flow is carried out. The kinetic constants of the process are determined. The heating rate of the sample varies over in wide range and amounts to 2, 5, 8, 20, 35, and 50 K/min. The values of the kinetic constants of PMMA decomposition are determined using the isoconversional method. For the degree of conversion of the substance ranging from 10 to 90%, the values of activation energy for the thermal decomposition of PMMA vary in the range from 213.5 to 194.3 kJ/mol, and the values of the preexponential coefficient change in the range from 1.62 × 10¹⁶ to 6.85 × 10¹² 1/s. The average activation energy for the thermal decomposition of PMMA in a carbon dioxide flow is 206 kJ/mol.

To verify the hypothesis of electrical hot spots (HSs)—channels of local electrical breakdowns—the analysis of the electrical properties of condensed explosives and the evaluation of the electric field strength in shock-loaded dielectrics are carried out. It is established that conditions for electrical breakdowns can be created in the compressed zone of explosives due to polarization phenomena (electric field) and shock-induced electrical conductivity (free electrons). The position of the electric model of detonation kinetics are formulated. The results of the experiments with liquid nitromethane (NM) and monocrystalline PENT are explained.

The neutralization of sulfur compounds during the filtration combustion of model mixture compositions containing iron sulfide or copper sulfate by adding marble (CaCO₃) is studied. It has been experimentally shown that during burning model charge compositions with additions of both iron sulfide and copper sulfate, replacing chemically inert sapphire with marble leads to a decrease in the combustion temperature by approximately 150–200°C. At the same time, the content of CO₂ in gaseous products increases, and the concentrations of CO and H₂ decrease. The greatest effect on the absorption of sulfur-containing substances when adding marble is shown in experiments where sulfur is present in the fuel in sulfide form: the addition of 50% marble makes it possible to capture 72% of the initial sulfur content, and for compositions with 90% marble in the charge, 85%. The absorption of the sulfur compounds formed during the combustion of model mixture compositions with copper sulfate is much worse. When the charge contains 50 and 85% marble, sulfur-containing compounds were absorbed by only 19 and 24%, respectively.

The uptake of O₃ on a salt film coating of MgCl₂·6H₂O at T = 254 and 295 K is studied in the range ([O₃] = 2.5 × 10¹³–1.6 × 10¹⁴ cm^–3) using a flow reactor with a movable insert and mass spectrometric recording. The time dependence of the uptake coefficient of the ozone at different O₃ concentrations is obtained in the relative humidity range from zero to 24%. Using the method of mathematical modeling, based on the shape of the dependence of the uptake coefficient and its time decay on the ozone concentration, the uptake mechanism is established and the elementary kinetic parameters are assessed, based on which it is possible to extrapolate the time behavior of the uptake coefficient to tropospheric conditions at arbitrary ozone concentrations. The ozone uptake at room temperature occurs through the reaction mechanism of an adsorbed molecule on the surface of the substrate. The mechanism includes the stage of reversible adsorption, formation of an adsorbed complex, and its unimolecular decomposition with the release of molecular chlorine into the gas phase. At low temperatures, the uptake proceeds through recombination via the Eley-Rideal reaction mechanism: it includes reversible adsorption, formation of a surface complex, its reaction with an ozone molecule from the gas phase, and the release of an oxygen molecule into the gas phase. In this case, no chlorine is formed. The dependence of the uptake coefficient on relative humidity in the range of values from 0 to 24% at T = 254 K is not detected.

The influence of atmospheric waves generated by a tropospheric convective source on the state of the upper atmosphere and ionosphere during the recovery phase of the geomagnetic storm on May 27–28, 2017 is studied. A new approach to accounting for atmospheric waves generated by tropospheric convective sources in large-scale atmospheric models without using wave parameterization is proposed and implemented. The developed approach makes it possible to comprehensively study the effects generated by atmospheric waves against the background of various geophysical events, including geomagnetic storms. The multimodel study shows that the proposed approach allows us to reproduce perturbations of the critical frequency of the ionosphere’s ionospheric F2 layer caused by the propagation of atmospheric waves generated by a tropospheric meteorological source. It is shown that the inclusion of a heat inflow source simulating the propagation of atmospheric waves from the lower atmosphere in the global model enhances the effects of a geomagnetic storm, which manifests itself as an additional decrease in the critical frequency of the F2 layer, which can reach 7% of the absolute values.

It is shown that the front of the flame of a thoroughly mixed diluted methane-oxygen mixture at 298 K and 100–300 Torr propagating to the ends of hollow cylindrical and conical obstacles does not form a von Kármán path (vortex shedding) behind them; however, this instability occurs under the same conditions in the flow of hot products after obstacles. The reason that vortex shedding is not observed behind an obstacle during flame propagation but appears in the course of propagation of a reflected stream of hot products is that thermal conductivity reduces the curvature of the flame and leads to its stabilization. Indeed, the convex areas of the chemical reaction zone in a combustible mixture give off more heat in relation to cold ones than in a flat flame: the heat from them is not only transmitted forward in the direction of flame propagation but also in the lateral directions. The resulting cooling of the reaction zone causes the flame regions that burst forward to lag behind. The opposite situation is observed in concave areas, where the temperature rises for the same reasons. The rate of reactions increases and they spread forward faster as the flame spreads. Thus, the surface of the curved front of the flame is evened out. In other words, thermal conductivity has a stabilizing effect on a curved flame. This effect is missing in non-reactive gas. This effect is absent in a nonreacting gas. Calculations based on the acoustic approximation of the Navier–Stokes equations for a compressible reacting medium make it possible to take into account the main observed feature of the flame front approaching an obstacle in the form of a cylinder: vortex shedding is not observed behind the obstacle during flame propagation. Thus, a qualitative model allows obtaining both the mode of the emergence of a von Kármán instability in a chemically inert gas and its absence during flame propagation.

It follows from the observational data that variations of the atmospheric electric field occur during geomagnetic storms. In this paper, we present the simulation results of ionospheric electric fields during the main phase of the geomagnetic storm on March 17, 2015, within the framework of a quasi-stationary model of a conductor consisting of the atmosphere and the ionosphere. For this purpose, the satellite data on the global distribution of currents between the magnetosphere and the ionosphere are used to describe the magnetospheric source of the electric field. A variation of the electric potential in the ionosphere leads to a variation of the electric field in the entire atmosphere, including its surface layer. It is important that during a geomagnetic storm, the observatory in which the atmospheric electric field is measured significantly changes its position relative to the direction of the Sun. This leads to significant changes in the ionospheric conductivity above the observatory, which affects both the ionospheric electric field and the atmospheric part of the global electrical circuit (GEC). Therefore, when assessing the effect of a geomagnetic storm on the atmospheric electric field in a particular observatory, it is necessary to take into account the local time when comparing the measurement data with the geomagnetic activity indices. For the storm on March 17–18, 2015, we found that taking into account the variations of the ionospheric electric field when calculating the atmospheric electric field allowed us to reproduce the disturbances of the fair weather electric field observed at the Borok Geophysical Observatory. Based on the simulation results, it is shown that during extremely strong magnetic storms, additional atmospheric electric field variations in some places on the Earth have the same scale as the fair-weather field itself.