Olivier Herbinet

Experimental Confirmation of the Low-Temperature Oxidation Scheme of Alkanes†

The control of auto-ignition can allow to increase the efficiency of internal combustion engines with clear potential positive effects on the problem of global warming.[1] The design of internal combustion engines,[2] as well as the improvement of safety in oxidation processes,[3] rely on a good understanding of the kinetic mechanism of the auto-ignition of organic compounds. Here we experimentally demonstrate a key assumption of this mechanism, which has been accepted for more than 20 years but never proven.[4-6] A detailed speciation of the hydroperoxides responsible for the gas-phase auto-ignition of organic compounds has been achieved for the first time, thanks to the development of a new system coupling a jet stirred reactor to a molecular-beam mass spectrometer combined with tunable synchrotron vacuum ultraviolet (SVUV) photoionization. The formation of alkylhydroperoxides (ROOH) and of carbonyl compounds including a hydroperoxide function (ketohydroperoxide) has been observed under conditions close to those actually observed before the auto-ignition. This result gives the experimental confirmation of an assumption made in all the detailed kinetic mechanisms developed to model auto-ignition phenomena.

Towards cleaner combustion engines through groundbreaking detailed chemical kinetic models

In the context of limiting the environmental impact of transportation, this critical review discusses new directions which are being followed in the development of more predictive and more accurate detailed chemical kinetic models for the combustion of fuels. In the first part, the performance of current models, especially in terms of the prediction of pollutant formation, is evaluated. In the next parts, recent methods and ways to improve these models are described. An emphasis is given on the development of detailed models based on elementary reactions, on the production of the related thermochemical and kinetic parameters, and on the experimental techniques available to produce the data necessary to evaluate model predictions under well defined conditions (212 references).

Quantification of OH and HO2 radicals during the low-temperature oxidation of hydrocarbons by Fluorescence Assay by Gas Expansion technique

Significance The design of internal combustion engines relies on a good understanding of the kinetic mechanism of the autoignition of hydrocarbons. •OH and •HO2 radicals are known to be the key species governing all stages of the development of ignition. A direct measurement of these radicals under low-temperature oxidation conditions has been achieved by coupling the fluorescence assay by gas expansion technique, an experimental technique designed for the quantification of these radicals in the free atmosphere, to a jet-stirred reactor, an experimental device designed for the study of low-temperature combustion chemistry. •OH and •HO2 radicals are known to be the key species in the development of ignition. A direct measurement of these radicals under low-temperature oxidation conditions (T = 550–1,000 K) has been achieved by coupling a technique named fluorescence assay by gas expansion, an experimental technique designed for the quantification of these radicals in the free atmosphere, to a jet-stirred reactor, an experimental device designed for the study of low-temperature combustion chemistry. Calibration allows conversion of relative fluorescence signals to absolute mole fractions. Such radical mole fraction profiles will serve as a benchmark for testing chemical models developed to improve the understanding of combustion processes.

Jet-Stirred Reactors

The jet-stirred reactor is a type of ideal continuously stirred-tank reactor which is well suited for gas-phase kinetic studies. It is mainly used to study the oxidation and the pyrolysis of hydrocarbon and oxygenated fuels. These studies consist in recording the evolution of the conversion of the reactants and of the mole fractions of reaction products as a function of different parameters such as reaction temperature, residence time, pressure, and composition of the inlet gas. Gas chromatography is classically used for the analysis of the species in the gas phase, but recent studies aimed at coupling new types of analytical devices to a jet-stirred reactor to observe new types of species and to gain accuracy in the identification and the quantification of species.

Numerical Simulations and Experimental Results of Endothermic Fuel Reforming for Scramjet Cooling Application

The last years saw a renewal of interest for hypersonic research in general and regenerative cooling specifically, with a large increase of the number of dedicated facilities and technical studies. In order to quantify the heat transfer in the cooled structures and the composition of the cracked fuel entering the combustor, an accurate model of the thermal decomposition of the fuel is required. This model should be able to predict the fuel chemical composition and physical properties for a broad range of pressure, temperature and cooling geometry. For this purpose, an experimental and modeling study of the thermal decomposition of generic molecules (long-chain or polycyclic alkanes) that could be good surrogates of real fuels, has been started at the DCPR laboratory located in Nancy (France). This successful effort leads to several version of a complete kinetic model. The last version for n-dodecane will be described in this paper and now include a detailed mechanism for Poly-Aromatics Hydrocarbons synthesis. Validation have been done for a wide range of temperature and pressure (6501500 K and 1-100 bar) and good agreement was generally found between the numerical model and available experimental results. In addition to this model, other molecules have been studied to investigate the differences between linear alkanes and cyclanes hydrocarbons. The unimolecular decomposition of cyclic hydrocarbons involves either the breaking of a C-H bond, which has a large bond energy, or the opening of the ring and the formation of a diradical. The behavior of the diradicals is very different from the one of linear radicals and should be investigated in order to gain a good knowledge of the thermal stability of such cyclanes.

Laminar Flame Velocity of Components of Natural Gas

Laminar burning velocities are important parameters in many areas of combustion science such as the design of burners or engines and for the prediction of explosions. They play an essential role in the combustion in gas turbines for the optimization of the nozzles and of the combustion chamber. Adiabatic laminar flame velocities are usually investigated in three types of apparatus which are currently available for that type of measurements: constant volume bombs in which the propagation of a flame is initiated by two electrodes and followed by shadowgraphy, counterflow-flame burners with axial velocity profiles determined by Particle Imaging Velocimetry, and flat flame adiabatic burners which consist of a heated burner head mounted on a plenum chamber with the radial temperature distribution measurement made by a series of thermocouples (used in this work). This last method is based on a balance between the heat loss from the flame to the burner required for the flame stabilization and the convective heat flux from the burner surface to the flame front. It was demonstrated that this heat flux method is suitable for the determination of the adiabatic flame temperature and flame burning velocity. The main hydrocarbon in natural gas is methane, with smaller amounts of heavier compounds, mainly species from C2 to C4 . New experimental measurements have been performed by the heat flux method using a newly built flat flame adiabatic burner at atmospheric pressure. These measurements of laminar flame speeds are presented for components of natural gas, methane, ethane, propane and n-butane, as well as for binary and tertiary mixtures of these compounds representative of different natural gases available in the world. Results for pure alkanes were compared successfully to the literature. The composition of the investigated air/hydrocarbon mixtures covers a wide range of equivalence ratios, from 0.6 to 2.1 when it is possible to sufficiently stabilize the flame. Empirical correlations have been derived in order to predict accurately the flame velocity of a natural gas containing C1 up to C4 alkanes as a function of its composition and the equivalence ratio.Copyright