Chain Thermal Explosion Research Articles

Characteristics of the Development of a Chain Thermal Explosion when Burning Gas Mixtures under Atmospheric Pressure

An experimental study of the features of the critical transition between two kinetic combustion regimes of gas-air mixtures reacting by a chain mechanism is carried out. The presence of two radically different combustion modes is experimentally confirmed by the example of the combustion of propane–air mixtures, both containing an inhibitor (trifluoromethane) and without it, and in complete agreement with the predictions of the theory of A chain thermal explosion (CTE). It is shown that the pattern observed in this work is characteristic not only for slow-burning but also for fast-burning gas mixtures of various compositions. The two distinct and radically different regions observed on the pressure–time variation curve during the combustion of gas mixtures are a manifestation of the general laws which combustible gas mixtures reacting by the chain mechanism follow in a wide range of pressures. It is found that fluorine-containing hydrocarbon is capable of induced oxidation in rich combustible gas mixtures. Trifluoroethane CF3H has an inhibitory effect on the chain combustion process of the propane–air mixture before it enters the CTE and promotes combustion in the CTE mode. If the transition to the CTE is not possible during the propane–air mixture combustion, the fluorinated agent has only an inhibitory effect on combustion.

Features of the Physicochemical Mechanisms and Kinetic Laws of Combustion, Explosion, and Detonation of Gases

This review generalizes the results of integrated studies, which showed that, contrary to previously accepted concepts, the gas-phase processes of combustion, explosion, and detonation of gases at atmospheric and elevated pressures occur by chain mechanisms and under laws of nonisothermal chain reactions. Owing to the determination of the chain nature of these processes, their previously unexplainable main characteristics were explained. The law of the temperature dependence of the rates of chain combustion reaction was derived, which determines features of the kinetics and macrokinetics of these processes, including chain thermal explosion, a necessary step of deflagration-to-detonation transition. Scientific foundations and chemical methods of control of various combustion modes were developed. Examples of using these methods to prevent explosions in mines and suppress deflagration-to-detonation transition in power units were presented.

Propane burning in argon, carbon dioxide, and water vapor at increased pressure

The paper presents investigation results on propane combustion in argon, carbon dioxide, and water vapor under the conditions of O2 deficiency and high reagent density when they are uniformly heated up to 620 K. Based on time dependences of reaction mixture temperature, the self-ignition temperature of propane was determined. It has been established that the oxidation of propane in an Ar and H2O medium proceeds according to the mechanism of chain-thermal explosion. The results of mass spectrometric analysis showed that oxidation in a CO2 medium is characterized by the lowest degree of propane conversion. It was also found that at a low density of water vapor, the oxidation of propane is accompanied by a high yield of H2. The mechanisms of CO2 and H2O molecule participation in the oxidation of propane are discussed in the paper.

Effect of H2O and CO2 on propane, propene, and isopropanol oxidation at elevated pressures

The article discusses research results on oxidation features of high-density fuel-enriched C3Н8/О2, C3Н6/О2, and C3Н7OH/О2 mixtures (ρfuel = 0.22–0.25 mol/dm3, ρO2 = 0.76–0.88 mol/dm3) diluted with argon, carbon dioxide, or water vapor (from 59 to 72% mol.) at the uniform heating (1 K/min) of tubular reactor to 640 K. Proceeding from the time dependences of the reaction mixtures temperature, it was revealed that the self-ignition temperature of fuels in the Ar medium increases in the following sequence: C3H6 < C3H7OH < C3H8. The self-ignition temperature of propane and propene is almost independent of the diluent nature, while the self-ignition temperature of isopropanol in the diluents medium increases in the following sequence: Ar < CO2 < H2O. The chain-thermal explosion was observed during the propane oxidation in Ar and H2O media, as well as during the isopropanol oxidation in Ar and CO2 media. Using mass spectrometric analysis, it was shown that the oxidation of propane in CO2 medium is characterized by a low degree of fuel conversion at almost complete consumption of O2, while the oxidation of propene is characterized by a low degree of fuel conversion and O2 consumption. Oxidation of fuels in the H2O medium occurs in several stages and is characterized by almost complete consumption of O2. In this paper we consider the mechanisms of chemical involvement of CO2 and H2O molecules in fuel oxidation.

Features of low-temperature oxidation of hydrogen in the medium of nitrogen, carbon dioxide, and water vapor at elevated pressures

The article discusses novel research results on combustion features of high-density Н2/О2 mixtures (ρH2 = 0.70–1.89 mol/dm3, ρO2 = 0.32–0.81 mol/dm3) diluted with nitrogen, carbon dioxide, or water vapor (from 46 to 76% mol.) at the uniform heating (1 K/min) of tubular reactor. Based on time dependencies of temperature increment in the reaction mixtures caused by the heat release during oxidation of H2, it is found that the self-ignition temperature of Н2/О2/N2 and Н2/О2/H2O mixtures is by ≈ 30 K lower than that of the Н2/О2/СО2 mixture. Unlike combustion of H2 in the N2 medium, in the CO2 and H2O media a chain-thermal explosion is observed at a certain concentration of reagents. The influencing mechanisms of diluents on the H2 oxidation dynamics, as well as the contribution of homogeneous and heterogeneous reactions in the heat release are revealed. It is established that high heat capacity of H2/O2/CO2 mixture, chemical interaction between its components, and presence of CO2 molecules adsorbed on the reactor inner surface, are the factors determining the H2 oxidation dynamics in CO2 medium. At oxidation of H2 in the H2O medium, the process takes place against the background of water evaporation and, as a consequence, is characterized by increased heat capacity and thermal conductivity of the H2/O2/H2O reaction mixture.

Chain-Thermal Explosions and the Transition from Deflagration Combustion to Detonation

The transition from combustion to a chain-thermal explosion, a necessary step in the transition from deflagration combustion into detonation, is studied using the example of hydrogen oxidation. Differences between the kinetic modes of ignition and a chain-thermal explosion are discussed.

Characteristics of a chain thermal explosion as a function of the kinetic properties of reaction chains

Study of the combustion and explosion of hydrogen‒carbon oxide‒air mixtures shows that the sharpness of a chain thermal explosion depends on the frequency of branching in a given branch of a reaction chain. It is established that varying the СО: Н2 concentration allows us to observe and eliminate the degeneration of an explosion while maintaining the regimes of ignition and deflagration.

ChemInform Abstract: Non-Isothermal Regimes and Chemical Control of Branching-Chain Processes

Non-isothermal modes of branching-chain reactions are analysed. Characteristic features of these reactions caused by the simultaneous action of the reaction chain avalanche and self-heating are considered in terms of concepts of the modern theory. New data are presented indicating that chain branching is the predominant factor in gas combustion processes not only at very low pressures, as it was assumed earlier, but also at atmospheric and higher pressures. The specific character of the temperature dependence of the rate of chain reactions and data dealing with the discovery of a special chain combustion mode, chain thermal explosion, are considered. The efficiency of chemical control of chain combustion in any mode including explosion, flame propagation and its transition to detonation by virtue of highly efficient non-corrosive inhibitors is demonstrated. The bibliography includes 135 references.

Role of the HO 2 • radical in hydrogen oxidation at the third self-ignition limit

It is demonstrated by simultaneous analysis of experimental data and the results of numerical simulation that, at 100 kPa, the H2/O2 mixture self-ignites only owing to the occurrence of a chain avalanche. Self-heating becomes significant in the course of chain combustion and strengthens the chain avalanche. The accelerating decomposition of H2O2, which results from the reaction between HO 2 • and H2, contributes to chain branching and determines the character of the chain thermal explosion. The role of the HO 2 • radical depends on the chain process conditions, consisting of chain termination as a result of the partial loss of HO 2 • at the initial self-acceleration stage, H atom regeneration, and participation in the second branched cycle under conditions of accelerated H2O2 decomposition.

Disappearance of criticality in a branched-chain thermal explosion with heat loss

In the framework of the currently developed branched - chain thermal explosion theory, the equation governing leakage through a hole of a reaction vessel is given. The critical ignition, extinction and transition temperature excess, activation energy parameter and modied Semenov’s number are estimated employing this equation. We calculated numerically and obtained analytically these non-dimensional parameters with and without initiation respectively. The similar solution for Semenov model appear as a limiting case of our solution. We also obtained the ignition times.

Chain-thermal explosion and degree of ionization of hydrogen-air flames

Using a passive probe to record the accumulation of electrical noise signals of hydrogen-air flames, it is established that within the concentration limits of intense combustion of these mixtures, which are approximately 18–60% (vol.) hydrogen and are due to the interaction mechanism in the burning mixtures, there is a considerable increase in the degree of flame ionization.

Transition of Flame Propagation from Isothermal to Thermal Regimes in Chain Processes with Nonlinear Chain Branching

The transition of an isothermal flame to a thermal combustion regime is considered experimentally and theoretically for the thermal decomposition of nitrogen trichloride. Experimental data on the transition of a thermal regime of flame propagation to a chain-thermal explosion are presented for dichlorosilane oxidation. One of the ways for the development of the chain-thermal explosion is shown to be the appearance of a local center of intense combustion in the thermal flame front.

Kinetic Regimes of Developed Chain Combustion

It is shown that chain thermal explosion is an inherent property of chain branching combustion that is due to simultaneous action of a chain avalanche and self-heating, which under certain conditions also becomes a progressively accelerating process. An attempt to deny this phenomenon is in fact equivalent to denying thermal explosion. This phenomenon is also observed in the absence of convection. The chain branching reaction mechanism is involved in all regimes of gas-phase combustion of hydrogen-containing compounds with oxygen. This results in inhibition or promotion of flame propagation, indicating the necessity of taking into account competition between reaction chain branching and termination.

Existence of Critical Conditions of Chain Thermal Explosion in Flames

The paper discusses the possibility of the existence of two kinetic regimes and critical conditions of thermal chain explosion during wave propagation of flame over hydrogen–air and methane–air mixtures. It is shown that experimental data do not confirm the existence of t he two kinetic regimes. For fuel mixtures in sealed vessels under gravity, two combustion regimes exist that are not related to kinetic characteristics.

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.

Critical Conditions of Chain Thermal Explosion

This paper reports results of experimental investigation of the critical conditions of chain thermal explosion of hydrogen–air and methane–air mixtures are introduced. The observed regularities, including the difference in critical conditions between the ignition and explosion of these combustibles, are explained by considering the chemical mechanisms of oxidation of hydrogen and methane in the context of the nonlinear theory of nonisothermal chain processes. Examples of regularities of combustion processes at atmospheric pressure are given that cannot be described without considering the leading role of the competition between the branching and termination of reaction chains. It is shown that in solving the equations used to describe combustion processes, neglect of the variation in the heat effects of reactions with temperature can lead to large errors.

Theory of branching reactions with chain interaction

A number of branching schemes involving quadratic interactions is examined using the methods of non-linear stability theory. A critical value for the branching factor is derived exactly for a chain thermal explosion involving radical-radical termination and self-heating. A self-inhibited branching scheme is shown to exhibit oscillatory behaviour and a resemblance to the CO + O2 kinetic scheme, where many successive glows have been observed, is pointed out.