Véronique Dias

Simulations of advanced combustion modes using detailed chemistry combined with tabulation and mechanism reduction techniques

Multi-dimensional models represent today consolidated tools to simulate the combustion process in HCCI and Diesel engines. Various approaches are available for this purpose, it is however widely accepted that detailed chemistry represents a fundamental prerequisite to obtain satisfactory results when the engine runs with complex injection strategies or advanced combustion modes. Yet, integrating such mechanisms generally results in prohibitive computational cost. This paper presents a comprehensive methodology for fast and efficient simulations of combustion in internal combustion engines using detailed chemistry. For this purpose, techniques to tabulate the species reaction rates and to reduce the chemical mechanisms on the fly have been coupled. In this way, the computational overheads related to the use of these mechanisms are significantly reduced since tabulated reaction rates are re-used for cells with similar compositions and, when it becomes necessary to perform direct integration, only the relevant set of species and reactions is taken into account. The proposed approach named tabulation of dynamic adaptive chemistry (TDAC) has been implemented in the Lib-ICE code, which is a set of libraries and applications for IC engine modeling developed using the OpenFOAM® technology. In particular, a modified version of the in-situ adaptive tabulation (ISAT) algorithm has been developed for systems with variable temperature and pressure, and the directed relation graph (DRG) method has been used to reduce the mechanism at run-time. The validation has been carried out with HCCI and Diesel cases both using a simplified case to compare the results obtained with and without TDAC, and a detailed case that is validated with experimental data. For each tested condition, a detailed comparison between computed and experimental data is provided along with the achieved speed-up factors compared to the use of direct-integration.

Lean and Rich Premixed Dimethoxymethane/oxygen/argon Flames: Experimental and Modeling

Experimental structures of two dimethoxymethane/oxygen/argon flames at equivalence ratios of φ = 0.24 and 1.72 have been studied by mass spectrometry. The detected species throughout the flame thickness were H2, CH3, CH4, H2O, C2H2, CO, CH2O, CH3O, O2, Ar, CO2, C2H4O2, and C3H8O2. The aim of this work was to extend an original model for ethylene combustion by building a sub-mechanism taking into account the formation and the consumption of oxygenated species involved in dimethoxymethane oxidation. By using kinetic data from the literature, the authors elaborated a new mechanism containing 480 reactions involving 90 chemical species in order to simulate these dimethoxymethane flames. The mechanism provides numerical results, which are in good agreement with experimental data for all species detected in both flames. Whatever the equivalence ratio of the flame, the two main degradation pathways of dimethoxymethane are the same: CH3OCH2OCH3 → CH3OCH2OCH2 → CH3OCH2 → CH2O and CH3OCH2OCH3 → CH3OCHOCH3 → CH3OCHO → CH3OCO → CH3O → CH2O, with the first being the fastest.

Modeling of Rich Premixed C2H4/O-2/Ar and C2H4/Dimethoxymethane/O-2/Ar Flames

Abstract Two rich premixed ethylene/oxygen/argon and ethylene/dimethoxymethane/oxygen/argon flat flames burning at 50 mbar were investigated experimentally by using molecular beam mass spectrometry to study the effect of methylal (dimethoxymethane) addition on species concentration profiles (C. Renard, P.J. Van Tiggelen and J. Vandooren, Proc. Combust. Inst., 29 (2002) 1277–1284). The replacement of 5.7% C2H4 by 4.3% C3H8O2, keeping the equivalence ratio equal to 2.50, is responsible for a decrease of the maximum mole fractions of most of the detected intermediate species. If this phenomenon is barely noticeable for C2 to C4 intermediates, it becomes more efficient for C5 to C10 species. Previously, a reaction mechanism has been validated against a premixed rich C2H4/O2/Ar flame (φ = 2.50) which describes in detail the formation of soot precursors and more precisely the main pathways involving benzene (V. Dias, C. Renard, P.J. Van Tiggelen and J. Vandooren, European Combustion Meeting, Orléans, France, p.221, 2003).The aim of this work is to extend this original model by building a sub-mechanism taking into account the formation and the consumption of oxygenated species involved in dimethoxymethane combustion. The new mechanism contains 474 elementary reactions and involves 90 chemical species in order to simulate both ethylene flames with and without methylal addition. The model leads to a good simulation for all species detected in these flames, and underlines the effect of methylal addition on species concentration profiles.According to this mechanism, the two main degradation pathways of methylal (CH3OCH2OCH3) in C2H4/methylal/oxygen/argon flame are: 1) CH3OCH2javascript:filterformular(´3´)OCH3 → CH3OCH2OCH2 → CH3OCH2 → CH2O 2) CH3OCH2OCH3 → CH3OCHOCH3 → CH3OCHO → CH3OCO → CH3O → CH3OH → CH2OH → CH2O with the first one being the fastest.

Experimental and Modeling Study of Propanal/H2/O2/Ar Flames at Low Pressure

ABSTRACT Propanal (C2H5CHO) is a crucial intermediate oxygenated species in hydrocarbons and oxygenated fuels combustion, thus motivating a better understanding of its kinetics. In this study, two premixed at flames of propanal/hydrogen/oxygen/argon, a stoichiometric and a rich, are stabilized at low pressure (27 mbar) on a burner. Species mole fraction profiles have been measured by molecular beam mass spectrometry (MBMS) for the reactants, products, and intermediate species: C2H5CHO (propanal), H2, O2, Ar, H, OH, HO2, CO, CO2, H2O, CH3, CH4, CH2O (formaldehyde), CH2CO (ketene), CH3CO (acetyl radical), C2H2 (acetylene), C2H4 (ethylene), and C2H6 (ethane). The mechanism from the Université catholique de Louvain (UCL) has been extended to the kinetics of propanal and its reliability has been validated against both of these propanal flames, at low pressure and in the temperature range up to 1530 K. According to the model, the main propanal consumption channel produces the ethyl radical (C2H5). This last radical is responsible for the hydrocarbons formation through C2H5 → C2H4 → C2H3 → C2H2; and also for the oxygenated compounds production through C2H5 → CH3CHO → CH3CO → CH2CO → CH2CHO. The presence of hydrogen, as a reactant, promotes the propanal consumption with the H radicals.

MASS SPECTROMETRY, GAS CHROMATOGRAPHY, AND COUPLING GC/MS AS COMPLEMENTARY TECHNIQUES FOR FLAME STRUCTURE ANALYSIS

Molecular beam mass spectrometry (MBMS) and gas chromatography (GC) are complementary methods that provide a detailed description of flame structures. MBMS can measure most stable and reactive species but mass overlapping (isomers, species at same m/e), isotopic, and ionic fragmentation interferences can be solved by using GC. To improve species identification, an experimental technique coupling both mass spectrometry and GC is developed. Rich flat premixed ethylene/oxygen/argon flames (φ = 2.25 and 2.50) have been investigated by both methods. After adequate calibrations, mole fraction profiles of several species measured by both techniques agree very well, but for methane, allene, propyne, and benzene, concentrations in burnt gases are somewhat larger when using GC than when using MBMS. C2H6, C2H4O, C3H6, and C3H8, which have similar masses as CH2O, CO2 or C3H8, CH2CO, and CO2, respectively, have been identified, separated, and calibrated by GC, which confirms that GC and MBMS are complementary techniques.

Experimental and Numerical Study of Ethyl Valerate Flat Flames at Low Pressure

ABSTRACT Three flat flames of ethyl valerate (C7H14O2) at different equivalence ratios, 0.81, 0.95, and 1.31, have been stabilized at 55 mbar and analyzed using gas chromatography. Based on the experimental results, a detailed kinetic model of ethyl valerate combustion, containing 211 species and 1368 reactions, has been elaborated. The model predictions agree well with the experimental results for the temperature range of 1100–2000 K. In the lean flame, the main decomposition pathways leads to C7H14O2 → CH3(CH2)3COOCHCH3 → CH3CHO → CH3CO → CH3 → CH2O → HCO. For the stoichiometric flame, the ethyl valerate consumption pathway produces C7H14O2 → C4H9COOH → CH3CHCH2CH2COOH → C4H7COOH → CH2COOH → CH2CO → CH3 → CH2O → HCO. In the rich flame, from ethyl valerate to pentenoic acid (C4H7COOH), the decomposition pathway is the same as in the stoichiometric flame. The pentenoic acid leads to the formation of C2H4 via C4H6 and C3H6. Finally, the new model has been tested on the experimental data obtained in a jet-stirred reactor at high pressure (10 atm) and low temperatures (560–1160 K). The mechanism predicts fairly well all the species, except H2 and C2H2. Future work should improve the mechanism to extend its validity range up to high pressure and low temperature range.