Nabiha Chaumeix

Chemical Kinetic Model for Monomethylhydrazine/Nitrogen Tetroxide Gas Phase Combustion and Hypergolic Ignition

A detailed kinetic model devoted to the gas phase hypergolic ignition of MMH/NTO mixtures below 298 K and their combustion above 1000 K is presented in this study. It consists of 403 equilibrated reactions among 82 species. This mechanism has been confronted with theoretical data available in the literature. The agreement between theory and predictions is found to be good. Important reactions for ignition at low initial temperature and pressure have been identified through sensitivity analyses. Two competing pathways can explain hypergolic ignition at low temperature. The formation of molecular preignition products is shown to promote the ignition. This conclusion was unexpected, as the formation of these products was generally considered to inhibit the ignition, by reactant consumption. Important reactions for combustion above 1000 K have been preliminarily identified. Some reactions are important both for low-temperature ignition and combustion, whereas some others are important either for ignition only or combustion only. The study is focused on the need for three reduced kinetic models.

Experimental and Chemical Kinetic Modeling Study of 3-Pentanone Oxidation

Shock tube ignition delay times have been measured for 3-pentanone at a reflected shock pressure of 1 atm (±2%), in the temperature range 1250-1850 K, at equivalence ratios of 0.5-2.0 for O(2) mixtures in argon with fuel concentrations varying from 0.875 to 1.3125%. Laminar flame speeds have also been measured at an initial pressure of 1 atm over an equivalence ratio range. Complementary to previous studies [Pichon S., Black, G., Chaumeix, N., Yahyaoui, M., Simmie, J. M., Curran, H. J., Donohue, R. Combust. Flame, 2009, 156, 494-504; Serinyel, Z.; Black, G.; Curran, H. J.; Simmie, J. M. Combustion Sci. Tech., 2010, 182, 574-587], laminar flame speeds of 2-butanone have also been measured, and relative reactivities of these ketones have been compared and discussed. A chemical kinetic submechanism describing the oxidation of 3-pentanone has been developed and detailed in this paper; rate constants for unimolecular fuel decomposition reactions have been treated for falloff in pressure with nine-parameter fits using the Troe Formulism. Both compounds treated in this work may be used as fuel tracers, thus further ignition delay time measurements have been carried out by adding 3-pentanone to n-heptane in order to test the effect of the blend on ignition delay timing. It was found that the autoignition characteristics of n-heptane remained unaffected in the presence of 15% 3-pentanone in the fuel, consistent with results obtained using acetone and 2-butanone [Pichon S., Black, G., Chaumeix, N., Yahyaoui, M., Simmie, J. M., Curran, H. J., Donohue, R. Combust. Flame, 2009, 156, 494-504; Serinyel, Z.; Black, G.; Curran, H. J.; Simmie, J. M. Combustion Sci. Tech., 2010, 182, 574-587].

Visualizations of Gas-Phase NTO/MMH Reactivity

The objectives of this investigation are to study the details of the nonignition, preignition, ignition, and combustion sequences of the NTO (nitrogen tetroxide)/MMH (monomethyl hydrazine) bipropellant combination. Several techniques have been used to visualize NTO/MMH reactivity at room temperature and recreate some failure scenarios. NTO/MMH flammability diagrams have been established experimentally.

Search for Green Hypergolic Propellants: Gas-Phase Ethanol/Nitrogen Tetroxide Reactivity

Monomethylhydrazine (MMH)/nitrogen tetroxide (NTO) is a well-known bipropellant combination system, the main advantage of which is low temperature ignition without ignition device, also called hypergolicity. The use of other chemical systems, less toxic, less hazardous, less corrosive, and more environmentally friendly than MMH/NTO are nowadays desirable. The replacement of the fuel MMH by ethanol (EtOH) is examined here. It is shown that ethanol/NTO mixtures are not hypergolic, but that reactions take place at room temperature and that these reactions form ethyl nitrite. Chemical reasons able to explain this reactivity, which does not lead to ignition, are discussed. In particular, it is shown that the EtOH/NTO initiation reactions going through H abstraction by NO2, whatever this initiation reaction is, and especially the one leading to ethyl nitrite, are considerably more endothermic than the corresponding MMH/NTO initiation reactions.

Induction Delay Times and Detonation Cell Size Prediction of Hydrogen-Nitrous Oxide-Diluent Mixtures

Silane-nitrous oxide mixtures are widely used in some industries such as semiconductor manufacturing. Since the decomposition of silane is faster than that of N2O and involves the formation of H2, the H2-N2O system might be an important sub-system of the silane oxidation mechanism. The induction delay times of this system have been widely studied in the low pressure range. Aim of the present study is to investigate the high-pressure behaviour of H2-N2O-Ar. Induction delays behind reflected shock waves have been measured between 1300–1860 K, at the pressure of 910±50 kPa for mixtures with equivalence ratios of 0.5, 1, and 2. It has been shown that equivalence ratio variations have no effect on induction delays. The modeling of delays has been improved by including an excited OH* kinetic sub-mechanism. Finally, various techniques of detonation cell size prediction have been evaluated in comparison with available experimental data.

Ignition Delays of Heptane/O2/Ar Mixtures in the 1300-1600 K Temperature Range

Ignition time measurements of lean, stoichiometric, and rich n-heptane/oxygen/argon mixtures have been studied behind reflected shock waves in the temperature range 1300‐1600 K and pressure range 2‐4 atm. The experimental data were compared to previously published results and have been found to be consistent with them. The experimental data were compared to ignition delay predicted using some of the kinetic models available in the literature. Discrepancies have been interpreted kinetically, thus leading to a slightly modified detailed kinetic model able to predict accurately high-temperature ignition delay such as encountered in the heptane/oxygen detonation wave.

Ignition by Electric Spark and by Laser-Induced Spark of Ultra-Lean CH4/Air and CH4/CO2/Air Mixtures at High Pressure

This study investigates the lean flammable limit (LFL) of CH4/air and CH4/CO2/air mixtures, using two different ignition devices: electrical discharge and laser-induced spark (LIS). The experiments were performed for CH4/air and (0.6 CH4 + 0.4 CO2)/air mixtures in ultra-lean conditions (0.48 ≤ φ ≤ 0.67) at different initial pressures (P = 100–700 kPa) and at an initial temperature of 298 K. In these conditions, all observed flames were ascending. The minimum laser pulse energy for ignition (MPE) and the maximum overpressure (MO) were determined. The LFL increases with the initial pressure and with the addition of CO2.

Polycyclic Aromatic Hydrocarbon Growth by Diradical Cycloaddition/Fragmentation

The recent theoretical and experimental investigations on the growth of polycyclic aromatic hydrocarbons in pyrolytic environments highlight the possible role of the 1,4-cycloaddition/fragmentation (1,4-CAF) steps in the formation of PAH intermediates and consequently soot. The present theoretical study explores the possibility to generalize such mechanism to reactions involving various diradical compounds and stable multiring structures. The calculations were performed using the uB3LYP/6-311G(d,p) method and different composite methods, when possible, for more accurate energy estimates. First, the complex potential energy surface for the reactions between o-benzyne and naphthalene was investigated, including the 1,4-CAF mechanism to form anthracene and acetylene through the dibenzobicyclo[2.2.2]octatriene intermediate. Moreover, the products of the addition reactions to the α- and β-carbons and to the ring-junction atoms were determined. The energies for the optimized CAF structures, which constitute the most-favorable pathway from an energetic point of view, were calculated using CBS-QB3, G3(MP2)B3, and G3B3 methods and compared to the corresponding values for the o-benzyne + benzene reactions. Additional calculations were focused on the possible CAF reactions between o-benzyne and larger multiring structures, such as anthracene, phenanthrene, pyrene, and the four-ring PAHs. The results indicate how the energetics of such reactions is influenced by both the size of the PAH compound and the position of the carbon atoms involved. In the second part of the study, the energy barriers necessary to form multiring diradicals from the corresponding radical molecules were analyzed at a G3(MP2)B3 level of theory. Such calculations are preliminary for the subsequent study on the CAF reactions between the different diradical intermediates and benzene. While the size of the diradical does not affect significantly the energy barriers, the position of the diradical site is critical. The concerted Diels-Alder reactions between the naphthynes and naphthalene were also studied in order to further clarify the analogies between the reactions involving different diradicals. Based on these results, kinetic considerations were provided based on the comparison with the simpler o-benzyne + benzene system, although further higher-level calculations and master equation kinetic analyses will be required to derive the general kinetic rules.

Autoignition of n-Decane/n-Butylbenzene/n-Propylcyclohexane Mixtures and the Effects of the Exhaust Gas Recirculation

In the present work, the autoignition of single-component and binary mixtures composed of n-decane, n-butylbenzene, and/or n-propylcyclohexane has been investigated using shock-tube techniques. In addition, the effects of the presence of the exhaust gas recirculation (EGR) components on the autoignition behavior of a 4:3:3 molar n-decane: n-butylbenzene:n-propylcyclohexane surrogate mixture have been investigated experimentally. The experiments have been performed at highly diluted conditions in argon bath gas, over a wide range of temperatures (1250–1750 K), equivalence ratios (Φ = 0.2–1.5), and nominal pressures of 10–20 bar. A chemical kinetic model was developed to simulate the newly obtained experimental data by blending sub-models from the literature. In particular, the experimental and numerical analyses suggest that the observed increase in the ignition delay times in the presence of EGR is mainly due to the dilution levels and not to the chemistry of the fuel. Additional kinetic analyses were performed to compare the high-temperature autoignition properties of the different components considered herein.

Numerical assessment of accurate measurements of laminar flame speed

In combustion, the laminar flame speed constitutes an important parameter that reflects the chemistry of oxidation for a given fuel, along with its transport and thermal properties. Laminar flame speeds are used (i) in turbulent models used in CFD codes, and (ii) to validate detailed or reduced mechanisms, often derived from studies using ideal reactors and in diluted conditions as in jet stirred reactors and in shock tubes. End-users of such mechanisms need to have an assessment of their capability to predict the correct heat released by combustion in realistic conditions. In this view, the laminar flame speed constitutes a very convenient parameter, and it is then very important to have a good knowledge of the experimental errors involved with its determination. Stationary configurations (Bunsen burners, counter-flow flames, heat flux burners) or moving flames (tubes, spherical vessel, soap bubble) can be used. The spherical expanding flame configuration has recently become popular, since it can be used...