G. L. Agafonov

Kinetic modeling of solid carbon particle formation and thermal decomposition during carbon suboxide pyrolysis behind shock waves

The kinetic modeling of carbon suboxide (C 3 O 2 ) thermal decomposition and formation, transformation, and thermal decomposition of solid fullerene-like and soot-like carbon particles is carried out by one-step heating for the mixture of 0.33% C 3 O 2 in Ar within the temperature range of 1200 to 2250 K at the pressure of 5 MPa and for a two-step heating for the mixtures of 1%, 2%, and 4% C 3 O 2 in Ar within the temperature range of 2400 to 3750 K at 2 MPa. The formation of small carbon clusters up to C 30 in the gas phase and the formation, growth, transformation, coagulation, and thermal decomposition of the precursors, fullerene-like and solid-like solid carbon particles, are modeled with the help of a detailed gas-phase kinetic scheme combined with the kinetic scheme for heterogeneous reactions in the context of the discrete Galerkin method. The experimentally measured and calculated values of the induction period of solid carbon particle formation, the total yield of solid carbon particles Y , and the observable growth rate constant of solid carbon particles k f are compared and demonstrate a good agreement. The possible pathways of transformation of the precursors, fullerene-like and soot-like solid carbon particles, are analyzed.

Soot Formation During Pyrolysis of Methane and Rich Methane/Oxygen Mixtures Behind Reflected Shock Waves

The pyrolysis of CH4/Ar and rich CH4/O2/Ar mixtures behind reflected shock waves were studied both experimentally and theoretically. The experiments were carried out behind reflected shock waves. Simulations were performed within the framework of a detailed kinetic model involving more than 260 species and 2500 elementary steps. The measured and calculated time profiles of soot yield and soot particle temperatures for CH4/Ar and CH4/O2/Ar mixtures proved to be in good agreement. The proposed kinetic model can also describe the temperature and pressure dependences of the ignition delay time for stoichiometric CH4/O2 mixture at low pressures and for rich CH4/O2 mixtures at elevated pressures over a wide pressure range, as well as the promoting effect of C2H6 and C3H8 additives.

A Shock-Tube and Modeling Study of Soot Formation During Pyrolysis of Propane, Propane/Toluene and Rich Propane/Oxygen Mixtures

The authors studied soot formation during the pyrolysis of propane/Ar, propane/toluene/Ar, and rich propane/oxygen/Ar mixtures behind reflected shock waves. The time profiles of soot yield and temperature were obtained. The soot-suppressing effect of oxygen and soot-promoting effect of toluene were revealed. The results were simulated within the framework of a detailed kinetic model composed of 2760 direct and reverse elementary steps and involving 284 species. As compared to the authors’ previous studies, the model was refined and tested against experimental data on pyrolysis and oxidation of a number of hydrocarbons. The measured and calculated time profiles of soot yield and soot particle temperatures, as well as soot yield temperature dependences, for the tested mixtures were found to be in close agreement.

Effect of Iron Pentacarbonyl on Soot Formation Behind Shock Waves

The effect of iron pentacarbonyl on soot formation during the pyrolysis of propane/Ar and acetylene/Ar mixtures behind reflected shock waves was studied. Time profiles of the soot yield and temperature were obtained. The soot-promoting effect of iron pentacarbonyl was revealed. The results for propane/Fe(CO)5/Ar mixtures were simulated within the framework of a detailed kinetic model composed of 3390 direct and reverse elementary steps involving 295 species. A novel kinetic mechanism of the thermal decomposition of Fe(CO)5 and the formation of free iron atoms and iron nanoparticles was tested. This mechanism correctly describes the available experimental data. A qualitative explanation of the experimentally observed effects of Fe(CO)5 additives on soot formation was proposed. It was suggested that the nascent iron nanoparticles serve as soot precursors for further surface growth with the formation of soot particles. The influence of small Fen(CO)m fragments and small Fen clusters on soot formation is less important because of a rather short lifetime of these species.

Initiation of laminar flames in near-limit H2/O2/H2O mixtures

A one-dimensional spherically symmetrical flame model including a detailed chemical mechanism for hydrogen oxidation, multispecies transport, and thermal radiation is used to examine the ignition and flame propagation properties for homogeneous hydrogen/oxygen/steam mixtures at elevated temperatures up to about 500 K and pressures ranging from 0.15 to 3 MPa. The model takes into account 26 elementary reactions and absorption of thermal radiation by water molecules. Time-dependent conservation equations of mass, species, and energy are integrated numerically to investigate the details of flame structure and calculate the flame speed. Kinetic sensitivity analysis of the burning velocity is used to elucidate the importance of individual elementary reactions at various pressures. Laminar flame propagation in stoichiometric mixtures heavily diluted with steam is investigated near the flammability limit through numerical simulation of the evolution of the spherical flame kernel initiated by a spatially localized ignition source. Critical conditions for flame propagation are analyzed in terms of ignition energy and flame kernel radius. The effect of pressure on these critical conditions is revealed and investigated. The predicted ignition nergy and limiting hydrogen concentration nonmonotonically depend on pressure. The computed results are in reasonable agreement with available experimental data on flammability limits.

Shock Tube and Modeling Study of Chemical Ionization in the Oxidation of Acetylene and Methane Mixtures

ABSTRACT Chemical ionization during the oxidation of methane and acetylene is experimentally and theoretically studied behind reflected shock waves over a wide temperature range and atmospheric pressure. The results of experimental measurements of the concentration of free electrons by microwave interferometry and electric probe method during the oxidation of acetylene and methane behind reflected shock waves are presented. A detailed kinetic model of the process of chemical ionization was developed. The results of experimental measurements and kinetic simulations are in good qualitative and quantitative agreement. The kinetic model of chemical ionization makes it possible to improve the kinetic description of the experimentally measured time histories of free electrons for the hydrocarbons studied.

Chemical Ionization of n-Hexane, Acetylene, and Methane behind Reflected Shock Waves

ABSTRACT Chemical ionization of n-hexane, methane, and acetylene is experimentally studied behind reflected shock waves over a wide temperature range at nearly atmospheric pressure and simulated within the framework of the unified kinetic mechanism developed in the present work. The dependences of the concentration of free electrons measured with a microwave interferometer and an electric probe on the temperature and mixture composition are reported. The experimental data and the results of kinetic simulations are demonstrated to be in close qualitative and quantitative agreement.

Shock-tube study of the formation of iron, carbon, and iron–carbon binary nanoparticles: experiment and detailed kinetic simulations

ABSTRACT An experimental and computational study of the formation of pure iron nanoparticles, carbon nanoparticles (soot), and binary carbon-coated iron nanoparticles during the pyrolysis of iron pentacarbonyl–argon, ethylene–argon, and iron pentacarbonyl–ethylene–argon mixtures, respectively, behind reflected shock waves is carried out. The shape and size distribution of these nanoparticles are examined on a Zeiss Ultra plus ultrahigh-resolution field-emission scanning electron microscope. The binary iron–carbon particles were also investigated by high-resolution transmission electron microscopy and high-angle annular dark-field imaging (HAADF STEM) on a FEI Osiris transmission electron microscope equipped with a Bruker SuperX detector. Detailed kinetic simulations of the formation of these three types of particles are performed, which predict the concentration, average size, and size distribution of particles.

Soot Formation During Pyrolysis and Oxidation of Aliphatic and Aromatic Hydrocarbons in Shock Waves: Experiments and Detailed Kinetic Modeling

Experimental and kinetic modeling studies of soot formation in the pyrolysis and oxidation of various mixtures of aliphatic and aromatic hydrocarbons in argon were carried out for shock tube conditions. The soot yield and the soot particle temperature were determined using the double-beam absorption-emission technique. Our previous kinetic model of soot formation is augmented by introducing an additional subset of reactions of soot nucleation, involving both polyaromatic and unsaturated aliphatic hydrocarbons. The proposed kinetic model was successfully tested by describing the published data on the products yield in the pyrolysis and oxidation of acetylene and diacetylene in shock-tube experiments. It closely reproduces our experimental data on the time histories of the soot yield and soot particle temperature, as well as the temperature and concentration dependences of the soot yield at fixed reaction times, for the pyrolysis and oxidation of C2H2/Ar, C2H6/Ar, C2H4/Ar, C2H4/O2/Ar, CH4/Ar, CH4/O2/Ar, C3H8/Ar, C3H6/Ar, toluene/Ar, and benzene/Ar mixtures under fuel rich conditions in reflected shock waves (T50 = 1400–2850 K, P50 = 2.5–5.5 bar).