This review is subjected to impart light on the formation schemes of one ring aromatic compound to the polycyclic aromatic hydrocarbons (PAHs) and their further sequential growth into the first soot particle, i.e. soot nucleation. It mainly includes the role of stable and radical species in the formation of PAHs and soot. With some focus on limitations of previous results, new experimental investigations and related mechanisms are discussed in detail. A new and efficient kinetic mechanism for the formation of large PAHs via the active role of a phenyl radical, i.e. phenyl addition /cyclization (PAC), is proposed. An alternative mechanism, methyl addition/cyclization (MAC), is also proposed. This mechanism is especially applicable to the pyrolysis of aliphatics and alkylated aromatic hydrocarbons which produces methyl sufficient radicals. These new mechanisms are expected to be helpful especially for kinetic modeling in many theoretical works on hydrocarbons combustion. Recent progress in kinetic simulations of soot particle growth is also reviewed briefly.
The development of new fuels and combustion devices would be greatly accelerated if we could build and solve accurate predictive models of the combustion chemistry in these devices. This is the most important technical challenge facing the combustion community. Recent advances on several fronts suggest that this should soon be possible. Here we review some developments in automated mechanism construction, methods for estimating for the chemical parameters, numerical solution algorithms, and in procedures by which the entire combustion community can contribute to meeting this important technical challenge.
Reactions of alkyl radicals, R, with molecular oxygen (O2) are critical components in chemical models of autoignition phenomena. The fundamental kinetics of the R + O2 reactions is governed by a complex web of interrelated elementary physical chemistry processes. At low temperatures and moderate pressures the reactions form stabilized alkylperoxy radicals, RO2, but at higher temperatures thermal dissociation of the alkylperoxy radicals becomes more rapid, and formation of hydroperoxyl radicals (HO2) and the conjugate alkenes begins to dominate the reaction. Crucially for ignition chemistry, internal isomerization of the RO2 radicals produces hydroperoxyalkyl radicals, often denoted by QOOH, reactions of which are the key to low-temperature chain branching. Over the last decade the understanding of these important reactions has changed greatly, in part because of the combination of high-level theoretical studies with pulsed-photolytic kinetics experiments that probe HO2 or OH product formation. For example, it is now clear that HO2 elimination occurs directly from an alkylperoxy radical without first isomerizing to QOOH. In addition, the importance of including formally direct chemical activation pathways, especially for formation of products but also for formation of the QOOH species, in kinetic modeling of R + O2 chemistry has been demonstrated. Furthermore, it appears that the crucial rate coefficients for the isomerization of RO2 radicals to QOOH may be significantly larger than previously thought. This article reviews the status of general models of alkyl + O2 reactions in the context of some of the most recent pulsed-photolysis experiments.
Ignition enhancement effect by using a non-thermal (non-equilibrium) plasma generated by a nanosecond high-voltage discharge was numerically investigated. Electron-impact reactions and ion recombination reactions were added to an oxidation mechanism of Methane and Hydrogen fuels in order to consider an interaction between plasma and flame. The results showed that the non-thermal plasma drastically reduced ignition delay times of premixed gases of Methane and Hydrogen fuels and its effectiveness increased with discharge time and strength of the electric field. Moreover, it was confirmed that an efficiency of the non-thermal plasma for ignition enhancement was higher than that of a thermal plasma.
In order to observe and elucidate multiphase process of micro plastic resin particles under abrupt heating, some ingenious techniques are devised and introduced in this investigation; two devises for optical imaging and four devises for abrupt heating and extinguishment. One of the two optical devises is a mini optical composite which enables simultaneous photographing of direct silhouetted image and schlieren image, and another is a magnifying particle tracking system which realizes a wide and straight tracking range up to about 50 mm. In the latter four devises, on the other hand, reforming of a ragged resin particle into a spherical one around a fine tungsten wire of 5 μm diameter, a vertical type cylindrical mini burner for abrupt heating, a mini-puff generator for abrupt extinguishment of all flames around a PET particle at an arbitrary assigned time after the abrupt heating, and a slot burner having extremely fine two-dimensionality are introduced. According to various optical observations using these devised techniques, many characteristic behaviors, such as occurrence of internal multiple micro bubbling, micro explosions and jets, and transformation of a slender wake flame into a long and thin cluster consisting of multiple micro brilliant spots within a short passage about a few millimeters, are clarified with respect to multiphase process of plastic resin particles under abrupt heating.
In this study, we numerically investigated downward flame spread over a thin solid fuel in partially premixed atmospheres. For simplicity, we assumed that the pyrolysis gas consists of methane and nitrogen. To consider the partially premixed atmospheres, methane was added in the opposed air stream. The fuel concentration was kept below the lean flammability limit. For discussing the flame structure, the flame index was obtained to observe premixed and non-premixed flames. Results show that, in the partially premixed atmospheres, the flame spread rate is increased. The high temperature region is enlarged. The premixed flame region is expanded at the leading flame edge, with larger flame temperature. This is because the heat transfer to the unburned solid fuel is intensified. As a result, amount of ejected fuel velocity from the solid surface is increased, which supports the higher flame spread rate in the partially premixed atmospheres.
This paper describes the effects of outlet boundary conditions on the one-dimensional linear analysis of combustion oscillation in a DLE (Dry Low Emission) gas turbine. Ordinarily, the acoustic outlet condition of the combustor is treated as open end in conventional combustion oscillation analysis. However, in a gas turbine engine, this condition is over-simplified. In this study, an acoustic impedance of the combustor-connected plenum was introduced in the one-dimensional analysis to improve prediction accuracy. The acoustic impedance was measured by using the two-microphone method and approximated by a rational function of angular frequency. An analysis was carried out for the test gas turbine combustor with a scroll-shaped plenum. 285 Hz combustion oscillation occurred at 95 % load in the test gas turbine. The new boundary condition with acoustic impedance predicted an oscillation at 284 Hz, while the open-end condition did not indicate any oscillation occurrence. Parametric studies were then carried out to expand the margin for operation without combustion oscillation. Flame lifting and extension of LP (Lean premixed) burner were proved to be appropriate for the test gas turbine. Experiments using the new LP burners with these modifications were performed; operational margin increased more than 10 % of load. It was concluded that consideration of the acoustic impedance of combustor-connected plenum yields good results for the DLE combustor.
This paper theoretically discusses the stability of laminar jet diffusion microflame, defined as a flame established on a submillimeter-diameter burner. In addition to the ordinary liftoff/blowoff limits, insights into the lower extinction limit are of great practical importance for microflames; thus, this paper primarily focuses on the extinction limit of microflames. The applicability of the following two diffusion-flame models to microflames is first discussed: the Burke-Schumann (BS) theory and a self-similarity analysis. The BS theory is found to be suitable to study the stability of microflames. The extinction limit is then predicted using activation-energy asymptotics in the framework of the BS theory. The present theory qualitatively reproduces experimental observations, i.e., uL ∼ d-2, where uL is the jet velocity at the extinction limit (lower limit) and d the burner diameter.