Nikolai M. Rubtsov

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.

The thermal explosion of the detonating mixture is impossible without a chain avalanche

The fundamental regularities of hydrogen/oxygen combustion are considered, which unambiguously indicate the branched chain character of the process at atmospheric pressure. It is noted that, in the general case, the ignition conditions are determined by the competition between chain termination and both chain branching and chain propagation reactions. Some publications ignoring this important point are considered.

Gas-phase spontaneous ignition of hydrocarbons

The ignition of hydrocarbons at low temperatures is experimentally studied in a rapid-mixture-injection static reactor. The ignition process was monitored using a high-speed color video camera. It was found that, at low temperatures, ignition starts in kernels, a feature also characteristic of methods for measuring the ignition delay time at high and medium temperatures (shock tube, rapid compression machine). Kernel-mode ignition is associated with gas-dynamic phenomena inherent in different techniques of heating the gas to the desired temperature. Ignition in the kernel is of chain-thermal nature. The emergence of a visible kernel can be considered the beginning of hot flame propagation. It is shown that, in the self-ignition mode, the propagation of the flame front from the initial kernel occurs by the induction mechanism, proposed by Ya.B. Zel’dovich, rather than by the diffusion-heat-conduction mechanism. Introduction of a platinum wire into the reactor produces a catalytic effect in the negative temperature coefficient region, while virtually unaffecting the ignition delay at lower temperatures.

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.

Autowave distribution of nitric oxide and its endogenous derivatives in biosystems strongly enhances their biological effects: A working hypothesis

It is hypothesized that in cells producing nitric oxide (NO), NO and its endogenous derivatives (low-molecular S-nitrosothiols and dinitrosyl iron complexes (DNIC) with thiol-containing ligands) can move in the intracellular space not only by diffusion but also in an autowave mode. This hypothesis is based on the previously obtained data on autowave distribution of DNIC with glutathione following application of a drop of a solution of Fe(2+)+glutathione onto the surface of a thin layer of a S-nitrosoglutathione solution. The appearance of autowaves is conditioned by a self-regulating self-sustained system arising in the process. This system consists of self-convertible DNIC and S-nitrosothiols as well as free ferrous iron ions, thiols and NO and can function in the autowave regime for several seconds with subsequent passage to a steady state maintained by chemical equilibrium between DNIC and their constituent components (free Fe(2+) ions, thiols, S-nitrosothiols and NO). Possible advantages of autowave distribution of NO and its endogenous derivatives in the intracellular space over free diffusion, which might entail higher efficiency of their biological action, are discussed.

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