V. V. Azatyan

The influence of fluorinated hydrocarbons on the combustion of gaseous mixtures in a closed vessel

Abstract The influence of various fluorinated inhibitors (CF 3 H, C 2 F 5 H, C 3 F 7 H, C 3 F 6 H 2 , CF 2 ClH, C 2 F 5 Cl, C 2 F 5 I, C 4 F 8 , C 4 F 10 , C 2 F 4 Br 2 ) on the combustion characteristics of mixtures of hydrogen and air and also of methane and air in a closed vessel has been investigated experimentally. The flammability limits, maximum explosion pressure, and maximum rate of explosion pressure rise have been determined. In some cases, flames of lean mixtures of H 2 or CH 4 with air can be promoted by adding fluorinated hydrocarbons. Thus the maximum explosion pressure, Δ P max , and the maximum rate of pressure rise, ( dP / dt ) max , during the explosion of lean mixtures were both elevated by small additions (several vol. %) of fluorinated hydrocarbons; this is caused by heat release during the chemical conversion of the inhibitors. This heat release is high enough to increase Δ P max and ( dP / dt ) max by 50–100% above the values for mixtures without inhibitors.

Breakup of steady-state detonation in hydrogen-air mixtures under the action of propane admixtures

Minor amounts of propane effectively inhibit the detonation of hydrogen-air mixtures at atmospheric pressure. Controlled variation of the amount of the admixture provides means to break up the steady-state detonation wave at a preset distance from the place of its origination and to regulate its velocity in a certain range. This is possible due to the branched chain character of the combustion reaction in the detonation mode. Propane is not inferior to propylene in the effectiveness of action on detonation and, owing to its low cost and higher availability, is preferable as a means of preventing the explosion and detonation of hydrogen-air mixtures.

Promotion and inhibition of the combustion of methane in oxidative gases with various oxygen concentrations by fluorinated hydrocarbons

Experimental data on the combustion characteristics of methane-oxidative gas (O2 + N2)-inhibitor (CHF3, C2HF5, and C4F10) mixtures and an analysis thereof show that fluorinated hydrocarbons (HFC and FC) exhibit the properties of both flame inhibitors and promoters. The flammability concentration limits, maximum explosion pressure ΔPmax, maximum explosion pressure rise rate (dP/dt)max, and laminar flame speed Su are measured for near-limit methane-oxidative gas-fluorinated hydrocarbon mixtures. It is demonstrated that, when added to lean near-limit mixtures, HFC and FC behave as an additional fuel. Calculations of the thermodynamic characteristics of reactions involving fluorinated hydrocarbons capable of acting as both an inhibitor and oxidizer show that such reactions have significant heat effects, 150–700 kJ/mol, with the respective adiabatic temperatures being as high as 900–1800 K. The results of the present study suggest that the procedure of selecting fluorinated hydrocarbons for practical applications as fire and explosion suppressants should include careful tests of their promoting effect.

Scientific Foundations and Effective Chemical Methods of Combustion, Explosion, and Gas Detonation Control

The competition between the branching and termination of reaction chains is shown to determine all general laws of gas combustion and explosion not only at pressures much lower that atmospheric, but also at atmospheric and increased pressures upon self-heating. It is established that the role of any elementary reaction in combustion is determined primarily by its effect on the relation between the rates of chain branching and termination. Scientific foundations and effective chemical methods of combustion, explosion, and combustible gas detonation control are developed.

The role played by chain avalanches in the developed burning of hydrogen mixtures with oxygen and air at atmospheric pressure

The role of competition between reaction chain branching and termination at various burning stages accompanied by reaction mixture self-heating was studied by mathematically modeling combustion of hydrogen mixtures with air and oxygen at a 1 bar initial pressure. An algorithm was suggested that allowed chair branching to be switched off at various time moments during calculations with retaining all the other, including thermal, parameters. It was shown that the switching off of chain avalanches at any process stage resulted in virtually instantaneous burning termination no matter what level of reaction mixture self-heating was reached.

Kinetic Parameters for Direct Atomic Substitution Reactions

Experimental data (the rate constants and activation energies) for seven reactions of direct substitution of one atom for another D + CH3R → CH2DR + H, D + NH3 → DNH2 + H, D + H2O → HOD + H, F + CH3X → CH3F + X (X = F, Cl, Br, and I) involving atoms D and F and molecules C2H6, H2O, NH3, CH3F, CH3Cl, CH3Br, and CH3I are analyzed using the parabolic model of a bimolecular radical reaction. The activation energies for the thermally neutral analogs of these substitution reactions are calculated. Atomic substitution involving deuterium atoms has a lower activation energy of a thermally neutral reaction than radical abstraction or substitution.

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.

Simulation of the inhibition of hydrogen-air flame propagation

The results of simulation and experimental data presented here demonstrate that the competition between chain branching and chain termination is the key factor in hydrogen-air flame propagation, including the temperature regime of the process and the formation of concentration limits. Self-heating becomes significant in developed combustion. It enhances the chain avalanche and ensures the temperature necessary for layer-by-layer chain ignition. By varying the ratio between the chain branching and termination rates by means of an inhibitor makes it possible to control the flame propagation process.

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.

Effect of the molecular structure of olefin admixtures on the combustion and explosion of hydrogen-air mixtures

Small admixtures of propylene, hexene, and isobutylene efficiently inhibit the combustion and explosion of hydrogen-air mixtures at an initial pressure of 1 bar. The inhibition effect depends on the chemical properties of the admixture. The difference between the effects produced by inhibitors manifests itself in all combustion parameters. Since the chain avalanche plays the determining role in combustion, the inhibition efficiency depends on both the length and the structure of the hydrocarbon chain in the inhibitor molecule. Taking into account the competition between the branching and termination of reaction chains and the correlation between the molecular structure and reactivity of the admixture makes it possible to explain and describe all of the observed regularities of combustion.