Michael Frenklach

A detailed kinetic modeling study of aromatics formation in laminar premixed acetylene and ethylene flames

A computational study was performed for the formation and growth of polycyclic aromatic hydrocarbons (PAHs) in laminar premixed acetylene and ethylene flames. A new detailed reaction mechanism describing fuel pyrolysis and oxidation, benzene formation, and PAH mass growth and oxidation is presented and critically tested. It is shown that the reaction model predicts reasonably well the concentration profiles of major and intermediate species and aromatic molecules in a number of acetylene and ethylene flames reported in the literature. It is demonstrated that reactions of n-C4Hx + C2H2 leading to the formation of one-ring aromatics are as important as the propargyl recombination, and hence must be included in kinetic modeling of PAH formation in hydrocarbon flames. It is further demonstrated that the mass growth of PAHs can be accounted for by the previously proposed H-abstraction-C2H2-addiction mechanism.

KINETIC MODELING OF SOOT FORMATION WITH DETAILED CHEMISTRY AND PHYSICS: LAMINAR PREMIXED FLAMES OF C2 HYDROCARBONS

Abstract The present study reports an updated detailed chemical kinetic model for soot formation. The model combines recent developments in gas-phase reactions, aromatic chemistry, soot particle coagulation, soot particle aggregation, and develops a new submodel for soot surface growth. The model was tested against experimental profiles of major and minor chemical species, aromatics, soot volume fractions, and soot particle diameters reported in the literature for nine laminar premixed flames of ethane, ethylene, and acetylene. The numerical agreement between the model predictions and experimental data is generally within a factor of 3. This level of agreement is very encouraging, considering the current uncertainties in the thermodynamics and kinetics of aromatics and soot chemistry. The principal accomplishment of the present study is that the demonstrated level of agreement all around—main flame environment, aromatics, and soot—can be attained with a single reaction model.

Reaction mechanism of soot formation in flames

Chemical reactions and physical processes responsible for the formation of polycyclic aromatic hydrocarbons and soot in hydrocarbon flames are reviewed. The discussion is focused on major elements in the present understanding of the phenomena, clarification of concepts central to the present state of the art, and a summary of new results.

DETAILED MODELING OF SOOT PARTICLE NUCLEATION AND GROWTH

Detailed modeling of soot particle nucleation and growth in laminar premixed hydrocarbon flames is presented. The model begins with fuel pyrolysis, followed by the formation of polycyclic aromatic hydrocarbons, their planar growth and coagulation into spherical particles, and finally, surface growth and oxidation of the particles. The computational results are in quantitative agreement with experimental results from several laminar premixed hydrocarbon flames. A detailed analysis of soot particle inception and surface growth processes is presented. Surface growth was described in terms of elementary chemical reactions of surface active sites. The density of these sites was found to depend on the chemical environment. The model predicts the classical picture of soot particle inception and the classical description of soot particle structure.

Substrate-Free Gas-Phase Synthesis of Graphene Sheets

We present a novel method for synthesizing graphene sheets in the gas phase using a substrate-free, atmospheric-pressure microwave plasma reactor. Graphene sheets were synthesized by passing liquid ethanol droplets into an argon plasma. The graphene sheets were characterized by transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, and electron diffraction. We prove that graphene can be created without three-dimensional materials or substrates and demonstrate a possible avenue to the large-scale synthesis of graphene.

Detailed kinetic modeling of soot formation in shock-tube pyrolysis of acetylene

The chemical reaction pathways to soot were investigated by experimenting with detailed kinetic models of soot formation under the conditions used in shock-tube pyrolysis experiments. The analyses of the computational results revealed a single dominant route for the main soot mass growth. Fused polycyclic aromatics play a particularly important role: their formation reactions are essentially irreversible and have the effect of “pulling” chains of reversible reactions. Hydrogen atoms reactivate aromatic molecules to radicals by abstraction reactions. The main bottleneck appears at the formation of the first aromatic ring. The model explains the time scale of soot formation and soot yields obtained in shock-tube pyrolysis of acetylene and also is in accord with product distributions observed in flames.

Detailed Modeling of PAH Profiles in a Sooting Low-Pressure Acetylene Flame

Abstract A detailed modeling study of the formation of polycyclic aromatic hydrocarbons in a burner-stabilized low-pressure sooting 23.6 % C2 H2-21.4% 02-Ar flame of Bockhorn and co-workers is reported. The model predicts the correct orders of magnitude and relative appearances of the concentration peaks, but overstates the decline of the species concentrations in the post-flame zone. Imprecise knowledge of the thermochemical data and unknown details of the oxidation of hydrocarbon radicals are the reasons identifed for the latter. The main reaction pathways for cyclization and growth of polycyclic aromatics and the results of the sensitivity tests are in close agreement with those of the previous modeling study of acetylene oxidation under shock-tube conditions. An additional factor that is important in the. flame environment is the diffusion of hydrogen atoms from the main reaction zone into a cooler preflame region

Growth mechanism of vapor-deposited diamond

An elementary-reaction mechanism of diamond growth by a vapor deposition process is proposed. The central postulate is that the main monomer growth species is acetylene. The mechanism basically consists of two alternating steps: surface activation by H abstraction of a hydrogen atom from a surface carbon and the addition of one or two acetylene molecules. During the addition reaction cycle a number of solid C–C bonds is formed and hydrogen atoms migrate from a lower to an upper surface layer. The mechanism is in general agreement with the macroscopic views of the Russian researchers and is consistent with the numerous experimental observations reported in the literature.

Aerosol dynamics modeling using the method of moments

Abstract A numerical approach to the solution of an aerosol dynamics system is presented. It is accurate, extremely economical, applicable to any combination of aerosol processes, and it does not require prior knowledge of the particle-size distribution function. The approach is based on the solution of differential equations for the moments of the particle-size distribution function. Two numerical techniques are discussed. The first consists in approximating the collision-frequency coefficient with a single “effective” value and the second uses interpolation techniques to evaluate time derivatives of the moments of the distribution function. The methods are tested with a variety of examples in the free-molecular regime against the results computed with a discrete model. As a severe test, an example of simultaneous particle nucleation, coagulation, and surface reaction is considered.

Optimization and analysis of large chemical kinetic mechanisms using the solution mapping method—combustion of methane

Abstract A method of systematic optimization, solution mapping, as applied to a large-scale dynamic model is presented. The basis of the technique is parameterization of model responses in terms of model parameters by simple algebraic expressions. These expressions are obtained by computer experiments arranged in a factorial design. The developed parametrized responses are then used in a joint multiparameter multi-data-set optimization. A brief review of the mathematical background of the technique is given. The concept of active parameters is discussed. The technique is applied to determine an optimum set of parameters for a methane combustion mechanism. Five independent responses—comprising ignition delay times, pre-ignition methyl radical concentration profiles, and laminar premixed flame velocities— were optimized with respect to thirteen reaction rate parameters. The numerical predictions of the optimized model are compared to those computed with several recent literature mechanisms. The utility of the solution mapping technique in situations where the optimum is not unique is also demonstrated.