P. A. Vlasov

DETAILED KINETIC MODELING OF SOOT FORMATION DURING SHOCK-TUBE PYROLYSIS OF C6H6: DIRECT COMPARISON WITH THE RESULTS OF TIME-RESOLVED LASER-INDUCED INCANDESCENCE (LII) AND CW-LASER EXTINCTION MEASUREMENTS

The results of calculations of the main parameters of the soot formation process (τ, k f, SY, and r m) carried out with the use of the detailed kinetic model of soot formation are compared with the experimental measurements of these parameters by the continuous-wave (CW)-laser extinction technique and by the time-resolved laser-induced incandescence (LII) method during C6H6 pyrolysis behind reflected shock waves. The detailed kinetic model of soot formation that is developed incorporates the gas-phase mechanisms of acetylene pyrolysis and the mechanisms of formation of polyaromatic hydrocarbons, polyyne molecules, and pure carbon clusters. It combines the H abstraction/C2H2 addition and polyyne pathways of the soot formation process. The formation, growth, and coagulation of soot precursors and soot particles are described within the framework of the discrete Galerkin technique based on an error-controlled expansion of the size distribution function of heterogeneous species into the orthogonal polynomials of a discrete variable (in particular, the number of monomers in the heterogeneous particle) that makes it possible to preserve a discrete character of any elementary transformations of heterogeneous particles and to describe them as elementary chemical reactions for the heterogeneous particles of all sizes. The comparison of the calculations with the experimental measurements of the induction time τ, observable rate of soot particle growth, k f, and soot yield SY by the CW-laser extinction method in the pyrolysis of benzene/argon mixtures in shock-tube experiments clearly demonstrates that the coincidence is quantitatively good for all the main parameters of soot formation. A particular difference between the values of the mean soot particle radius r m experimentally measured by the time-resolved LII technique and calculated with the help of the detailed kinetic model is observed at the low and high temperatures. The results presented demonstrate the current level of the predictive capabilities of the detailed kinetic model of soot formation and the reliability of the time-resolved LII technique for the quantitative determination of the soot particle sizes.

Kinetic Modeling of Carbon Suboxide Thermal Decomposition and Formation of Soot-Like Particles Behind Shock Waves

Abstract The kinetic modeling of thermal decomposition of C3O2 and formation of soot-like solid particles behind shock waves is carried out. The transformation of C atoms up to the cluster C30 in the gas phase and the formation, growth, transfer, and coagulation of carbon solid particles are modeled with the help of a detailed gas phase kinetic scheme combined with a discrete Galerkin method. Together with the calculation of concentration profiles of small carbon clusters, precursors of solid panicles, soot-like solid particles, and fullerene-like solid particles, the temperature dependencies on the induction period, the growth rate constant, the yield, and the mean size of carbon solid particles are obtained for a mixture of 0.33 % C3O2 in argon for the following experimental conditions: A pressure of 5.0 MPa and a temperature range of 1200 to 2250 K. The comparison of these calculated values with the experimentally measured ones is performed and the possible pathways of transformation of C atoms from C3O2 to soot-like solid particles are analyzed.

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.

Luminescent Characteristics of the Shock-Wave Ignition of an Ethylene−Oxygen Mixture

ABSTRACT The ignition of a stoichiometric ethylene−oxygen mixture diluted with argon was experimentally and computationally studied to gain new insights into the nature of the chemiluminescence that accompanies this process and to obtain some of its quantitative characteristics. The experiments were performed behind reflected shock waves at temperatures of 1270−1820 K and a pressure of ~1 bar. The time evolution of the luminescence intensity of the electronically excited C2*, CH*, and OH* radicals and CO2* molecule was monitored photometrically. The measured temperature dependence of the ignition delay time was found to be in satisfactory agreement with the published data and the results of simulations within the framework of the ChemphysMech_v.1 (Tereza et al., 2010) and AramcoMech_1.3_C4 (Metcalfe et al., 2013) reaction mechanisms. The possible reaction pathways of formation of C2*, CH*, OH*, and CO2* were analyzed. It was shown for the first time that, along with the recombination reaction CO + O → CO2*, CO2* is formed by the reaction CH + O2 → CO2* + H, which dominates during fuel burnout, whereas the former becomes the main channel of CO2* formation at later stages. The contribution from the reaction C2H + O2 was demonstrated to play a minor role in the formation of CH* as compared to the C2H + O reaction, at least under the conditions tested.

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.