Georgii I. Tsvetkov

Different effects of active minor admixtures on hydrogen and methane ignitions

The methane combustion inhibitor CCl4 exerts no effect on the first ignition limit of hydrogen; therefore, the role of hydrogen atoms in hydrocarbon oxidation consists at least of participating in longer reaction chains than are observed in hydrogen oxidation. The upper limits of the rate constants of the reactions of hydrogen atoms with propylene and isobutylene molecules were estimated by the self-ignition limit method to be (1.0 ± 0.3) × 10−11 exp(−1450 ± 400/T) and (0.8 ± 0.3) × 10−11 exp(−550 ± 200/T) cm3 molecule−1 s−1, respectively, in the temperature range of 840–950 K. These data are evidence that the stronger inductive effect of the two methyl groups in isobutylene lowers the energy barrier to the H + iso-C4H8 reaction. It has been demonstrated experimentally that chemiluminescence in the hydrocarbon flame front at atmospheric pressure precedes heat evolution. Throughout the pressure and temperature ranges examined (5–750 Torr, 298–950 K), the chain mechanism determines the basic laws of combustion.

Effect of added reactive agents on the flame propagation velocity in rich hydrogen-air mixtures

An approximate analytical method for evaluating the efficiency of the action of an inhibitor on the velocity and propagation limits of a flame in rich hydrogen-air mixtures with small amounts of added propylene and isobutylene inhibitors is proposed. The method is based on the model of a narrow reaction zone and the distinct features of the branching chain mechanism of hydrogen oxidation reactions. Using this method, it is shown that the occurrence of flame propagation limits at higher concentrations of added reactive agents (inhibitors) is caused by the existence of a positive feedback between the flame front velocity and the number of active combustion sites, which break down in the inhibitor-added reaction. According to this feedback, the action of the inhibitor decreases the flame temperature and flame velocity.

Kinetic Features of Aerosol Formation in the Branched Chain Process of Dichlorosilane Oxidation under Static Conditions: I. Initiated Ignition

It is found experimentally that the initial pressure of aerosol formation progressively decreases with an increase in the initial concentration of dichlorosilane in the initiated ignition of dichlorosilane mixtures with oxygen at 293 K. The dependence of the maximal aerosol concentration on the total pressure is S-shaped. A generalized kinetic scheme is proposed that qualitatively describes the regularities observed in the experiments. The most important calculated parameters are the heat evolved in the chain process and the dependence of the pressure of the saturated vapor of a new phase on temperature. It is shown that the specific features of branched-chain processes under nonisothermic conditions determine the kinetic regularities of new phase formation. The optimal range of pressures is recommended for obtaining particles with as low dispersivity as possible.

Effect of chemically active additives on the detonation wave velocity and the detonation limits in lean mixtures

On the basis of the Zel’dovich-von Neumann-Doering detonation theory with allowance for heat losses, as well as the theory of chain processes through, the example of the oxidation of hydrogen-rich mixtures in the presence of an inhibitor, it is shown that taking into account inhibition reactions and the trimolecular chain termination has the consequence that there are not only heat losses but also “chemical” losses. It is demonstrated that the presence of only “chemical” losses alone can ensure that there is a concentration limit of detonation and that the combustion wave near the limit is supersonic. Theoretical estimates agree qualitatively with the experimental data on the inhibition of a developed detonation wave in H2-air mixtures with additives of a propane-butane mixture (0.5–4%) at a pressure of 1 atm.

Flame Emission Spectra in the Region 400–600 nm during Low-Pressure Silane and Dichlorosilane Oxidation

The flame emission in the region 400–600 nm during monosilane and dichlorosilane oxidation (initial pressures of 3–20 torr; T = 300 K) is caused by radical luminescence processes on the surface of aerosol microparticles of the SiO2 formed. The generation of energy by the interaction of gas-phase species with the SiO2 surface at the initial stages of the phase formation depends on the presence of the intrinsic structural defects Si+ and defects like Si+ implanted into SiO2. The addition of SF6 to the starting mixture results in the appearance of emission bands due to the Si+ defects in the radical luminescence spectrum.

Influence of inert and active additives on the initiation and propagation of laminar spherical flames in stoichiometric mixtures of methane, pentane, and hydrogen with air

The propagation of a laminar spherical flame in stoichiometric mixtures of methane, and pentane with air in the presence of argon and carbon dioxide and in hydrogen-air-propylene mixtures at atmospheric pressure in a constant-volume bomb is investigated using high-speed color cinematography. It is shown that, under the experimental conditions employed (at T0 = 298 K and a spark energy of E0 = 0.91 J), dilution of the combustible mixtures with these additives can cause a more than 10-fold increase in the time of formation of a steady flame front, with the inhibiting effect of carbon dioxide being stronger than that of argon. Small additives of propylene, a chemically active inhibitor, are demonstrated to substantially increase the time it takes to form a steady flame front and reduce the flame propagation velocity.

A long-lived intermediate product in the oxidation of monosilane and dichlorosilane at low pressures and 293 K

An electron-vibration structure of the UV spectrum of a long-lived intermediate is detected during oxidation of SiH4 and SiH2Cl2. This product is common to both reactions and exhibits the same promoting effect on them. It is shown that the formation of this promoting compound in the course of a branched chain reaction accounts for nonthermal flame propagation in reacting mixtures outside of the self-ignition region

Kinetic factors determining specific features of solid phase formation in dichlorosilane oxidation

Experimental data on the kinetic regularities of aerosol SiO2 formation in the course of dichlorosilane oxidation by oxygen at different initial pressures, compositions of the reaction mixture, and temperatures ranging from 380 to 578 K are presented. It is shown that the regularities of the process, including the specific feature of the transition from the regime of solid phase formation in the form of a film to the regime of aerosol formation can be explained on the basis of the Volmer–Weber–Frenkel–Zeldovich nucleation theory taking into account the branched chain nature of the reaction. The conditions for the transition of chain combustion into the regime of chain–thermal explosion almost coincide with the conditions of intensive formation of aerosol. The SF6 additives inhibit the process and thereby increase the dispersity of aerosol and the minimal pressure of its formation.

Features of the propagation of laminar spherical flames initiated by a spark discharge in mixtures of methane, pentane, and hydrogen with air at atmospheric pressure

Using high-speed digital color cinematography, we studied the propagation of a laminar spherical flame in stoichiometric mixtures of hydrogen, methane, and pentane with air in the presence of additives at atmospheric pressure in constant-volume reactors, and derived quantitative data on the time of formation of a stable flame front. Cellular flames caused by gas-dynamic instability attributable to convective flows arising during the afterburning of gas were observed in hydrocarbon-air stoichiometric mixtures diluted with inert additives. It was found that the effect of additives of carbon dioxide and argon (>10%) and minor additives of CCl4 on the combustion of hydrocarbons, and of propylene on the combustion of hydrogen-rich mixtures, lead to periods of delay in the development of a laminar spherical flame; in addition, additives of propylene promote the combustion of hydrogen poor mixtures.

Chain ignition of propane-air and pentane-air mixtures in a heated vessel

The spatial propagation of the chain ignition of propane-air and pentane-air mixtures with oxygen at a pressure of 1 atm and T = 600–800 K is studied. It is established that the features of the spatial propagation of chain ignition process are determined by the conditions of the reactor’s surface. It is shown that the site (or sites) of ignition are located on the surface of the reaction vessel; the flame front propagates from the site into the volume at a normal speed corresponding to the reactor temperature and the composition of the combustible mixture.