Andrea D'Anna

A wide-range modeling study of iso-octane oxidation

Abstract This paper presents a semidetailed kinetic scheme for the oxidation of iso-octane (2,2,4-trimethyl-pentane). Both the low- and high-temperature primary mechanisms are reduced to a lumped kinetic model involving only a limited number of intermediate steps. This primary reaction scheme, similar to the one already presented for n -heptane [1], is flexible enough to maintain accurate prediction of intermediate components, heat release, and ignition delay times for a wide range of operating parameters. General criteria for the reduction of intermediate species allow an efficient coupling, with a detailed kinetic model of C 1 C 4 oxidation. Thermochemical oscillations and the negative temperature coefficient (NTC) region of the reaction rate of the low-temperature oxidation of iso-octane in a jet-stirred reactor are reproduced quite well by the model. Several comparisons with experimental data, obtained under very different operating conditions, including a shock tube, a rapid compression machine, flow and jet-stirred reactors, support the applicability of this model of iso-octane oxidation over a wide range of pressures, temperatures, and mixture compositions.

Combustion generated fine carbonaceous particles

Soot is of importance for its contribution to atmospheric particles with their adverse health impacts and for its contributions to heat transfer in furnaces and combustors, to luminosity from candles, and to smoke that hinders escape from buildings during fires and that impacts global warming or cooling. The different chapters of the book adress comprehensively the different aspects from fundamental approaches to applications in technical combustion devices.

A kinetic model for the formation of aromatic hydrocarbons in premixed laminar flames

A detailed chemical kinetic mechanism, including 340 elementary steps and 90 species, has been developed to simulate the formation of aromatic compounds in rich premixed flames of aliphatic hydrocarbons. The mechanism can reproduce the concentration profiles and net rates of benzene and larger aromatic hydrocarbons (two- and three-ring polycyclic aromatic hydrocarbons [PAHs]) in a wide range of temperatures for a slightly sooting, premixed ethylene-oxygen flames. Key sequences of reactions in the formation of aromatics are the combination of resonantly stabilized radicals, whereas the alternative mechanism that involves acetylene addition does not seem to be fast enough to explain the observed formation rates of aromatics in the flames examined. The main routes involved in the formation of the first aromatic ring are the propargyl self-combination and its addition to 1-methylallenyl radicals. Cyclopentadienyl radical combination, propargyl addition to benzyl radicals, and the sequential addition of propargyl radicals to aromatic rings are the controlling steps for the formation of larger aromatic species. The predicted concentration and formation rate of larger aromatics are higher than those of PAHs identified by gas chromatography but similar to those of total aromatic species collected in flames, namely, soot and tar-like material. This result suggests that tar-like material can be considered as the result of a fast reactive coagulation of two- and three-ring PAHs, which form structures with aromatic compounds, eventually connected by aliphatic bonding. Soot inception may be the result of an internal rearrangement of tar without significant contribution of light hydrocarbons, at least in the conditions examined. The model has been used to explore the effect of C/O ratio and temperature on the formation rate and concentration of aromatic hydrocarbons. The net formation rate of aromatics increases monotonically with C/O ratio, and at a fixed C/O ratio, it passes through a maximum as temperature is increased. A comparison with the “bell-shaped” concentration profile of soot collected in flames at different temperatures supports the hypothesis that the formation of two- and three-ring structures may be the rate-determining step in soot formation for slightly sooting regimes.

Coagulation and carbonization processes in slightly sooting premixed flames

UV absorption spectra in the 200–400 nm range and transient thermocouple measurements are used tocharacterize the process of nanoparticle formation and their carbonization to soot particles in slightly sooting premixed flames of ethylene. The UV absorption technique allows a quantitative determination of the concentration of both soot and nanoparticles. In the examined flame, the total particulate is initially present as nanoparticles, but even in the sooting region, where the yellow luminosity is dominant, soot particles represent less than 50% of the total mass of particulate. This technique can be easily used for the determination of soot and nanoparticles at the exhaust of real combustion devices and in urban polluted areas. Time-resolved thermocouple measurements are used to follow the variation along the flame axis of the emissivity of the particulate deposited by thermophoresis on the thermocouple junction. Thesevalues are used for the evaluation of an apparent rate of carbonization, which has been compared with the coagulation rate of the particles evaluated by scattering and extinction measurements. Both processes, carbonization and coagulation, show a second-order kinetic with a constant of the order of 5×10 12 cm 3 /s. This technique has shown to be a very simple method for detecting nanoparticles, which can be applied also in diffusion flames and other practical systems. In addition, it furnishes emissivity values, which might be useful for a more realistic assessment of the radiative heat transfer of slightly luminous flames.

The Diesel Exhaust Aftertreatment (DEXA) Cluster: A Systematic Approach to Diesel Particulate Emission Control in Europe

The DEXA Cluster consisted of three closely interlinked projects. In 2003 the DEXA Cluster concluded by demonstrating the successful development of critical technologies for Diesel exhaust particulate after-treatment, without adverse effects on NO x emissions and maintaining the fuel economy advantages of the Diesel engine well beyond the EURO IV (2000) emission standards horizon. In the present paper the most important results of the DEXA Cluster projects in the demonstration of advanced particulate control technologies, the development of a simulation toolkit for the design of diesel exhaust after-treatment systems and the development of novel particulate characterization methodologies, are presented. The motivation for the DEXA Cluster research was to increase the market competitiveness of diesel engine powertrains for passenger cars worldwide, and to accelerate the adoption of particulate control technology.

Controlling steps in the low-temperature oxidation of n-heptane and iso-octane

The low-temperature oxidation of n-heptane and iso-octane in mixtures with air in a jet-stirred-flow reactor have been compared under suitable high-pressure conditions, such that the two mixtures of hydrocarbon and air showed comparable fuel conversions and phenomenologies. The large presence of aldehydes in the products of the low-temperature oxidation of n-heptane was attributed to a degenerate chain-branching path involving the addition of molecular oxygen to heptylhydroperoxy radicals and isomerization by internal H-atom abstraction. The latter step is particularly favored in linear alkanes where easy-to-abstract H-atoms are available. On the other hand, cyclic ethers and fuel-conjugate olefins were the dominant products of the low-temperature oxidation of iso-octane. This is due to a lack of H-atoms for internal abstraction; this limits the degenerate chain-branching route and favors the propagation path toward species having the same skeleton of the fuel, such as cyclic ethers and fuel-conjugate olehns. The prevalence of a degenerate chain-branching path for n-heptane compared with the propagation reactions in iso-octane oxidation is responsible for the different autoignition tendencies of n-heptane and iso-octane.

Evidence and characterization of nanoparticles produced in nonsooting premixed flames

The size distribution of nanoparticles generated in nonsooting ethylene/air premixed flames and collected in water samples has been evaluated using two particle-sizing techniques that show high sensitivity to particles in the nanometer size range: dynamic light scattering and electrospray-differential mobilty analysis. A d 63 of 2.85 nm has been estimated. By combining the results of ex situ size determination and in situ scattering and extinction measurements in UV, the complex refractive index of nanoparticles has been determined. The result is in good agreement with the value we previously used for the elaboration of in situ measurements and is also consistent with the conceptual model that nanoparticles formed in flames contain 2-, 3-ring polycyclic aromatic hydrocarbon functionalities as their building blocks. This work shows that nanoparticles produced in combustion systems are partially soluble in water, which is particularly relevant for determining their possible effects on human health and climate.

Optical and Spectroscopic Characterization of Rich Premixed Flames across the Soot Formation Threshold

ABSTRACT Phenomenological aspects of formation, destruction, and coagulation of high molecular mass structures formed in the main oxidation zone of rich premixed flames and in rich flames well below the soot threshold limit have been examined. High molecular mass structures transparent to the visible radiation, previously detected in the preinception region of soot forming flames, are also present in flames below the soot formation limit. The onset of ultra-violet fluorescence within the main oxidation zone implies that the formation of these species is a very fast process and can be considered as a “polymerization” of small aromatic groups activated by the presence of oxidizing agents. The final concentration of this material falls down as the C/O ratio is decreased and below C/O = 0.35 it is not anymore present. It appears that rich premixed flames present two critical C/O ratios: a first one for soot formation and a lower second one for high molecular mass structure formation. Ultra-violet scattering/e...

SOOT AND NANOPARTICLE FORMATION IN LAMINAR AND TURBULENT FLAMES

A new optical diagnostic method has been developed based on the interaction of a pulsed UV laser source with combustion-generated aerosols. This method allows characterization of nanoparticles of organic carbon (NOC) and soot by point measurements. Fluorescence and incandescence measurements induced by the fifth harmonic of a Nd-YAG laser at 213 nm are used for the determination of the volume fractions of particulates in a laminar premixed flame and in a turbulent non-premixed flame of ethylene/air. The selected light source enhances the fluorescence of NOC, which exhibit a large absorption band between 200 and 250 nm and also heats up soot particles to give incandescent emission. Ultraviolet emission signals are correlated with NOC extinction coefficients, while LII signals are correlated with extinction coefficients in the visible region. Laser light scattering measurements are used to estimate the mean sizes of both classes of particles.

Modeling aerosol formation in opposed-flow diffusion flames

The microstructures of atmospheric pressure, counter-flow, sooting, flat, laminar ethylene diffusion flames have been studied numerically by using a new kinetic model developed for hydrocarbon oxidation and pyrolysis. Modeling results are in reasonable agreement with experimental data in terms of concentration profiles of stable species and gas-phase aromatic compounds. Modeling results are used to analyze the controlling steps of aromatic formation and soot growth in counter-flow configurations. The formation of high molecular mass aromatics in diffusion controlled conditions is restricted to a narrow area close to the flame front where these species reach a molecular weight of about 1000 u. Depending on the flame configuration, soot formation is controlled by the coagulation of nanoparticles or by the addition of PAH to soot nuclei.