Jürgen Warnatz

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

The Mechanism of High Temperature Combustion of Propane and Butane

Abstract By combination of a mechanism describing lean and moderately rich combustion of alkanes and alkenes with a mechanism describing rich combustion and formation of soot pre-cursors in acetylene flames, a general reaction scheme is developed for the simulation of lean and rich high-temperature combustion of hydrocarbons up to C4 - species. Results of these simulations are compared to experimental data, and some consequences of this reaction scheme are discussed with respect to future experimental work on rich flames of propane and butane.

Chemistry of high temperature combustion of alkanes up to octane

Alkanes are initially attacked by H, O, and OH radicals generated in the oxyhydrogen reaction. The alkyl radical formed in this way decomposes to smaller alkyl radicals by fast thermal elimination of alkenes. Only the relatively slow thermal decomposition of the smallest alkyl radicals, CH3 and C2H5, competes with recombination and with oxidation reactions by O atoms and O2. This part of the mechanism is rate-controlling in the combustion of alkanes (and alkenes) and must be described by a detailed mechanism consiting of elementary reactions. Alkyl radical decomposition and the reactions leading to C1-and C2-fragments are too fast to be rate-limiting and can therefore be described by simplified reaction schemes disregarding alkyl isomeric structures. Simulations of flames of higher alkanes (up to octane) using these simplifying assumptions show agreement with the experimental material available. The mechanism derived by these considerations can then be used to explain of phenomena (like non-Zeldovich NO formation or formation of soot precursors), which can be interpreted from a detailed knowledge of the C1/C2-chemistry.

Development of a parallel direct simulation code to investigate reactive flows

Abstract Solving the Navier-Stokes equations with detailed modeling of the transport and reaction terms remains at the present time a very difficult challenge. Direct simulations of two-dimensional reactive flows using accurate models for the chemical reactions generally require days of computing time on todays most powerful serial vector supercomputers. Up to now, realistic three-dimensional simulations remain practically impossible. Working with parallel computers seems to be at the present time the only possible solution to investigate more complicated problems at acceptable costs, however, lack of standards on parallel architectures constitutes a real obstacle. In this paper, we describe the structure of a parallel two-dimensional direct simulation code using detailed transport, thermodynamic and reaction models. Separating the modules controlling the parallel work from the flow solver, it is possible to get a high compatibility degree between parallel computers using distributed memory and message-passing communication. A dynamic load-balancing procedure is implemented in order to optimize the distribution of the load among the different nodes. Efficiencies obtained with this code on many different architectures are given. First examples of application conceding the interaction between vortices and a diffusion flame are shown in order to illustrate the possibilities of the solver.

A re-evaluation of the means used to calculate transport properties of reacting flows

Simulations of laminar combustion and other reactive flow processes (like chemical vapor deposition, plasma etching, etc.) are presently carried out in most cases using the transport code TRANFIT attached to the CHEMKIN package. The approach used is based on experimental data from 1975 and is now outdated, especially in view of recent work presented in the literature. The new approach described here seeks to remove the deficiencies of former transport models by using the following features: (1) representation of transport data of light species at high temperature by switching to an exponential repulsive potential, (2) use of effective potential parameters to handle the intermolecular forces in an easy and elegant way, if polar molecules are considered, and (3) use of a simplified formula for binary thermal diffusion factors, based on an expansion for large values of the mass ratio of the species included. This paper presents the new transport model in terms of a complete set of equations. The molecular parameters provided allow a complete treatment of the oxidation of H2 and H2/CO mixtures (data for species taking place in the oxidation of hydrocarbons and in other reaction systems are not yet available). To demonstrate the consequences of the new transport model for combustion processes, results have been generated by implementing the model in a code for the simulation of premixed laminar flames.

Numerical simulation of spark ignition including ionization

A detailed understanding of the processes associated with spark ignition, as a first step during combustion, is of great importance for clean operation of spark ignition engines. In the past 10 years, a growing concern for environmental protection, including low emission of pollutants, has increased the interest in the numerical simulation of igniton phenomena to guarantee sucessful flame kernel development event for lean mixtures. However, the porcess of spark ignition in a combustible mixture is not yet fully undrstood. The use of detailed reaction mechanisms, combined with electrodynamical modeling of the spark, is necessary to optimize spark ignition for lean mixtures. This work presents the simulation of the coupling of flow, chemical reactions, and transport with discharge processes including ionization in order to investigate the development of a stable flame kernel initiated by an electrical park in methane/air mixtures. A transport model taking into account the interactions of charged particles has been incorporated in the flow model. This model is based on the Chapman-Enskog theory with an extension for polyatomic gases and considers resonant charge transfer and ambipolar diffusion for the computation of the transport coefficients. A two-dimensional code to simulate the early stages of flame development, shortly after the breakdown discharge, has been developed. The modeling includes an equation for the electrical field. The spark plasma channel left behind by the breakdown is incorporated into the initial conditions. Due to the fast expansion of the plasma channel, a complicated flowfield develops after the emission of a shock wave by the expanding channed. The second phase, that is, the development of a propagating flame and the flame kernel expansion, can last up to several milliseconds and is dominated by diffusive processes and chemical reactions.

Experimental and computational investigation of the structure of a sooting C2H2-O2-Ar flame

Mole fraction profiles have been measured by molecular beam-mass spectrometer technique in a sooting C2H2-O2-Ar flame (27.5%-27.5%-45%) stabilized under reduced pressure (2.6 kPa) on a flat flame burner. Emphasis was put on the detection and concentration measurement of the intermediate species which play a role in the formation of the first aromatic rings. In addition to the major products and the radicals usually involved in acetylene oxidation mechanisms, C2H, C2H3, C3, C4 species and benzene have been measured. The mole fraction profiles have been compared with predictions from a simulation model. Care was taken to use as much as possible a detailed mechanism known to model acetylene oxidation in a wide range of experimental conditions. The mechanism proposed by Warnatz23, for the oxidation of alkanes and recently checked by Westmoreland38 for the modelling of a rich C2H2/O2 flame was adopted as a starting point. This tested mechanism was complemented by formation and consumption reactions for C4H3, C4H4, C4H5 and benzene. The satisfactory agreement between calculated and measured profiles was turned to account to specify the main steps in the route to benzene.

Experimental investigation and computational modeling of hot filament diamond chemical vapor deposition

A joint investigation has been undertaken of the gas-phase chemistry taking place in a hot-filament chemical vapor-deposition (HFCVD) process for diamond synthesis on silica surfaces by a detailed comparison of numerical modeling and experimental results. Molecular beam sampling using quadrupole mass spectroscopy and resonance-enhanced multiphoton ionization time of flight mass spectroscopy (REMPI-TOF-MS) has been used to determine absolute concentrations of stable hydrocarbons and radicals. Resulting species of a CH4/H2, a CH4/D2 (both 0.5%/99.5%) and a C2H2/H2 (0.25%/99.75%) feedgas mixture were investigated for varying filament and substrate temperatures. Spatially resolved temperature profiles at various substrate temperatures, obtained from coherent anti-Stokes Raman spectroscopy (CARS) of hydrogen, are used as input parameters for the numerical code to reproduce hydrogen atom, methyl radical, methane, acetylene, and ethylene concentration profiles in the boundary layer of the substrate. In addition,...

Reaction mechanism reduction for higher hydrocarbons by the ILDM method

Because of its generality and simple concept, the intrinsic low-dimensional manifolds (ILDM) method is used to reduce the number of concentration variables from about 50–100 to only 1–3. Typical results for flame propagation and premixed flame-front structure are presented for n -heptane and n -dodecane combustion. The new features are that (1) ILDMs for higher hydrocarbons can be determined the first time, with sufficient accuracy to determine NO (which is sensitively dependent on the O-atom concentration) by use of a newly developed stiff-stable algorithm: (2) an effective in situ tabulation has been developed, which can save orders of magnitude of computer time and storage: and (3) use of a set of a key species leads to a very simple interface to (even commercial) codes where source terms determined from the ILDM simply replace source terms calculated from a reaction mechanism, so that the problematic projection of the conservations to the ILDM is no longer necessary. A secondary result is that flames at increasing pressure (1 bar, 20 bar, and 80 bar are considered in this paper) become more and more determined by a thermal rather than a chain-branching mechanism. One important consequence is that at elevated pressures, similar results for Lewis numbers Le =1 and Le ≠1 are produced, leading to a considerable simplification of the transport model to be applied. ILDMs generated in this way have been successfully applied to turbulent combustion simulation and pollutant formation in diesel and two-stroke engines.

Impact of the Inlet Flow Distribution on the Light-Off Behavior of a 3-Way Catalytic Converter

Numerical simulations are increasingly assisting research and development in the field of emission control of automotive vehicles. Our work focuses on the prediction of the tail-pipe emissions, based on a numerical simulation of the automotive catalytic converter. Besides the prediction of the tail-pipe emissions, an understanding of the processes occurring inside a monolithic catalytic converter implies new opportunities for the design of the optimum exhaust gas system. In this paper, we present a three-dimensional transient numerical study of the influence of the velocity distribution in front of the inlet face on the thermal behavior of the monolith during the light-off of a 3-way catalytic converter. The differences in the thermal and chemical behavior due to the shape of the velocity distribution are discussed. The recently developed code DETCHEM M O N O L I T H /1/ is used for the numerical simulation. This code, for the first time, combines two-dimensional simulations of the reactive flow inside a large number of single monolith channels including a heterogeneous multi-step reaction mechanism with a transient simulation of the three-dimensional temperature field of the entire converter.