Key Factors of Combustion

The inhibition and promotion of non-thermal NCl₃ flame by NOCl and H₂, respectively, are explained. The crossover from the non-thermal mode of flame propagation to the thermal one is analyzed for NCl₃–He mixtures. Calculations based on a kinetic mechanism taking into account energy chain branching are performed, and qualitative agreement between the calculated and observed data is demonstrated. Nonlinear chain branching shortens the time needed for thermal ignition and increases the flammability of the combustible mixture. It is shown that the mechanism of reaction of NCl₃ decomposition proposed above is in good qualitative agreement with experimental data. It is established that the conditions sufficient to obtain oscillating solutions are the following: (a) accounting for adsorption—desorption of NCl₃ on reactor walls, (b) accounting for nonlinear chain termination Cl + Cl₂ ³П_ou → Cl + Cl₂ ¹Σ_g ⁻, (c) accounting for energy chain branching. Therefore, accounting for processes of desorption of NCl₃ from the reactor surface during oscillations and the change in a surface state leads to the occurrence of the oscillation modes in a numerical experiment, which are in qualitative agreement with experimental data.

The promotion of the branched-chain decomposition of nitrogen trichloride by molecular hydrogen additives manifests itself in a decrease in the induction period and the acceleration of initial reactant consumption. The emission spectrum of the H₂ + NCl₃ flame contains the intense bands of NCl (b¹Σ⁺ − X³Σ⁻, v = 1–0, v = 0–1, and v = 0–0), and the bands of a hydrogen-free compound. The latter bands can be assigned to electronically excited NCl₂ radicals formed in the H + NCl₃ reaction. The promotion effect in the system studied should be due to the side reaction of linear branching. The occurrence of the H + NCl₃ reaction via two pathways (NHCl + 2Cl and NCl₂ + HCl) ensures the qualitative agreement between the experimental data and calculation.

It has been found that the time delay τ of thermal ignition of dichlorosilane—chlorine mixtures occur in the presence of more than 4 % of propylene; the consumption of inhibitor leads to ignition, in which absorption spectrum of dichlorosilylene radicals is detected along with the emission of SiHCl (A¹B₁–X¹A₁). The inhibiting effect is due to the fast reactions of propylene with silylenes as chain carriers. In the presence of both inhibitor and of more than 45 % inert additive (sulfur hexafluoride) the dependence of τ on the concentration of deactivator undergoes drastic change. Thus, deactivation processes have marked influence on the flammability. Experimental data are in agreement with calculations based on the generalized kinetic model of the branched-chain process including chain termination via both inhibitor and deactivator. It is shown that non-thermal flame propagation is inherent both to monosilane and dichlorosilane oxidation. Deactivating properties of an inert additive influence on the velocities of non-thermal flame propagation in the case of dichlorosilane oxidation. That may be the evidence of participation of excited particles in nonlinear branching reaction. 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 provides non-thermal flame propagation in reacting mixtures outside of the thermal ignition region.

Chemiionization was revealed in the chlorination of dichlorosilane, the lower limit of the concentrations of charged particles was estimated as ~10⁷ charged units cm⁻³. The detected relationship between chemiionization and phase formation in low-temperature heterophaseous BC gives grounds to consider the chemical nature of a reaction zone of silanes oxidation over the region of flame propagation as weakly ionized plasma. The use of the properties of the plasma in an external electric field has allowed developing an essentially new technique of low-temperature deposition. Evolution of thermal ignition and induced ignition of dichlorosilane-oxygen mixtures over the pressure range from 4 to 500 Torr at initial temperatures from 300 to 400 K was studied by means of framing Schlieren cinematography. It was shown that the ignition is of non-thermal nature; the reaction originates on the reactor surface with the generation of adsorbed chain carriers, which subsequently escape into the volume; a visible flame velocity makes ~50 m/s. SF₆ additives to the combustible mixture suppress thermal ignition, in this case the concentration of SiO₂ aerosol decreases dramatically. SF₆ molecules presumably take part in a competing reaction of chain termination involving also charged species.

The influence of a constant electric field on kinetic regularities of dichlorosilane oxidation near the lower limit of thermal ignition was established. The features of this influence on both the lower limit and the delay period of thermal ignition, as well as on the period and quantity of chemical oscillations are determined by the material and a surface state of the reactor, as well as by the reactions of the long-lived intermediate. The new critical phenomenon is revealed: a sharp decrease of integrated intensity of chemiluminescence at thermal ignition of DCS + O₂ mixes over CuSO₄ coating over a small interval of electric field strength.

It is shown that the flame emission in the region 400–600 nm in monosilane and dichlorosilane oxidation (initial pressures of 3–20 Torr; T₀ = 300 K) is caused by radical luminescence processes on the surface of aerosol ultra-disperse particles of SiO₂. The generation of energy by the interaction of gas-phase species with the SiO₂ surface at initial stages of the phase formation depends on the presence of both the intrinsic structural defects =Si: and defects of Si⁺ implanted into SiO₂. The addition of SF₆ to the initial mixture results in the appearance of the emission bands due to the Si⁺ defects in the radical luminescence spectrum. Electronically excited HO₂ radicals (A ¹ A′–X ² A′′), OH radicals (ν = 2-0), and HCl molecules (ν = 3-0) are identified using the emission spectra at 0.8–1.6 μm in the rarefied flame in dichlorosilane combustion at 293 K and low pressures. The spectrum also contains the composite bands of the H₂O (0.823 μm) and H₂O₂ (0.854 μm) molecule vibrations. The maximum intensity of emission of these species is reached behind the front of the chemical transformation, and the equilibrium between the vibrational and translational degrees of freedom is established in the region of the regular thermal regime of cooling. SF₆ additives act as a reservoir that accumulates the vibrational energy in the developed ignition.

Molecules of H₂O₂ and H₂O are detected in gas phase in deuterium—oxygen flame by their near IR emission spectra in the reactor, which surface was previously treated with atomic hydrogen. It was shown that both the formation of these compounds and the observed decrease in the lower limit of spontaneous ignition of D₂ + O₂ mix under influence of adsorbed hydrogen atoms are caused by heterogeneous elementary reactions providing chain propagation. The initial stage is the reaction of adsorbed atomic hydrogen with O₂ from the gas phase. The emission spectra of hydrogen–oxygen and hydrogen–air flames at 0.1–1 atm exhibit a system of bands between 852 and 880 nm, which are assigned to the H₂O₂ molecule vibrationally excited into the overtone region. This molecule results from the reaction HO₂ + O₂. The overtone region also contains bands at 670 and 846 nm, which are assigned to the vibrationally excited HO₂ radical. The radical is the product of the reaction between H and O₂. The HO₂ radicals resulting from H₂ or D₂ oxidation inhibited by small amounts of propylene are initially in vibrationally excited states.

The experimental conditions considered when the certain estimations of the character of the flow in the installation must be performed to exclude the factors, which should hinder obtaining the results required. The evidence are obtained for the occurrence of the ignition of diluted stoichiometric methane-oxygen mix (total pressure up to 200 Torr) behind a single opening at the transition of the laminar flow to the turbulent one rather than after a delay period of ignition. The features of FF penetration through rectangular openings in comparison with circular ones with the use of both color speed cinematography and visualization of gas flows by the illumination of fine powder with a laser sheet are experimentally investigated. It is shown that the length of the “flame jump” after the opening in an obstacle is mostly determined by the time of occurrence of the transition from the laminar flow to the turbulent one rather than the time of an ignition delay period. The results are important both for 3D modeling and for the solution of explosion safety problems for volumes with complex geometry. It is experimentally shown that at the penetration of a flame through obstacles gas dynamic factors, for example, flame turbulization can determine the kinetics peculiarities of combustion, for instance transition of low-temperature hydrocarbon combustion to the high-temperature mode.

The formalism of the one-dimensional detonation theory taking into account both thermal losses and the theory of branched chain processes was applied to hydrogen oxidation in the presence of hydrocarbon additive. It is shown that accounting for both reactions of termination of the active centers of combustion via molecules of hydrocarbon additive, and chain oxidation of hydrocarbon additive allows qualitative interpreting of the main features of the process. They are both passing of detonation velocity through a maximum at an increase in the content of the additive in a lean mix and the existence of two detonation limits on the concentration of the additive.

A cellular combustion regime of 40 % H₂—air mixture in the presence of Pt wire over the interval 270–350 °C was observed for the first time. It is shown that the regime is caused by the catalytic action of Pt containing particles formed by decomposition of volatile platinum oxide in the gas phase. It is experimentally revealed that the emergence and participation of chemically active surface during gas combustion (by the example of H₂ combustion over Pt surface) significantly complicates the understanding of the process due to the occurrence of a number of new governing parameters. These include the dependence of chemical activity of the catalyst on its chemical composition, temperature and conditions of mass transfer. It is shown that under certain conditions Pt catalyst can suppress combustion and thereby show the opposite effect due to the high efficiency of Pt surface coated with a Pt oxide layer in the reaction of chain termination. Therefore, kinetic factors could be the determining ones even under conditions of high turbulence.