Soot Precursor Species Research Articles

Development of a Practical Soot Modeling Approach and Its Application to Low-Temperature Diesel Combustion

The author's developed a practical soot model and implemented in the multidimensional computational fluid dynamics code, KIVA-3vr2 for use in low temperature diesel combustion simulations. The model framework is based on four fundamental steps: soot inception through a four-ring polycyclic aromatic hydrocarbon species, surface growth through acetylene, soot coagulation, and oxygen- and OH-induced soot oxidation. Diesel combustion was simulated by using a reduced n-heptane chemistry mechanism. A reduced polycyclic aromatic hydrocarbon chemistry mechanism was formulated from the literature and coupled with the n-heptane mechanism. Improvements were made in the chemistry mechanism for better predictions of ignition delay, liftoff length and soot precursor concentrations. The CHEMKIN-II code was used to solve the combustion chemistry. However, in order to reduce the computational time of the coupled soot and chemistry calculations, a semi-implicit solver was also implemented and used for the soot precursor species. Soot model performance and computational efficiency was evaluated by comparing the model predictions with available optical spray chamber experimental data. The model performance was also evaluated by comparing the model predictions with experimental results from a heavy-duty optical engine as well as light- and heavy-duty metal engines.

Towards a detailed soot model for internal combustion engines

In this work, we present a detailed model for the formation of soot in internal combustion engines describing not only bulk quantities such as soot mass, number density, volume fraction, and surface area but also the morphology and chemical composition of soot aggregates. The new model is based on the Stochastic Reactor Model (SRM) engine code, which uses detailed chemistry and takes into account convective heat transfer and turbulent mixing, and the soot formation is accounted for by SWEEP, a population balance solver based on a Monte Carlo method. In order to couple the gas-phase to the particulate phase, a detailed chemical kinetic mechanism describing the combustion of Primary Reference Fuels (PRFs) is extended to include small Polycyclic Aromatic Hydrocarbons (PAHs) such as pyrene, which function as soot precursor species for particle inception in the soot model. Apart from providing averaged quantities as functions of crank angle like soot mass, volume fraction, aggregate diameter, and the number of primary particles per aggregate for example, the integrated model also gives detailed information such as aggregate and primary particle size distribution functions. In addition, specifics about aggregate structure and composition, including C/H ratio and PAH ring count distributions, and images similar to those produced with Transmission Electron Microscopes (TEMs), can be obtained. The new model is applied to simulate an n-heptane fuelled Homogeneous Charge Compression Ignition (HCCI) engine which is operated at an equivalence ratio of 1.93. In-cylinder pressure and heat release predictions show satisfactory agreement with measurements. Furthermore, simulated aggregate size distributions as well as their time evolution are found to qualitatively agree with those obtained experimentally through snatch sampling. It is also observed both in the experiment as well as in the simulation that aggregates in the trapped residual gases play a vital role in the soot formation process.

Large eddy simulation of soot formation in a turbulent non-premixed jet flame

A recently developed subgrid model for soot dynamics [H. El-Asrag, T. Lu, C.K. Law, S. Menon, Combust. Flame 150 (2007) 108–126] is used to study the soot formation in a non-premixed turbulent flame. The model allows coupling between reaction, diffusion and soot (including soot diffusion and thermophoretic forces) processes in the subgrid domain without requiring ad hoc filtering or model parameter adjustments. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. A relatively detailed but reduced kinetics for ethylene–air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. Acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. The diffusion effect is found to be most important in the nucleation regime, while the thermophoretic forces become more influential downstream of the potential core in the oxidation zone. The particle size distribution shows a log–normal distribution in the nucleation region, and a more Gaussian like distribution further downstream. Limitations of the current approach and possible solution strategies are also discussed.

Small-Ion and Nano-Aerosol Production During Candle Burning: Size Distribution and Concentration Profile with Time

The characteristics of small-ions and aerosols in the diameter range 0.4 nm to 1.1 μm, produced during burning of paraffin wax tea-light candles, were investigated using a custom-built aspiration condenser ion mobility spectrometer (ACIMS) and a sequential mobility particle sizer and counter (SMPS+C) system. Peaks in the number concentration were observed at diameters 10–30 nm and 100–300 nm, consistent with “normal” and “sooting” burn modes. In addition, a smaller mode in the size range 2.5–9 nm was observed, interpreted as a soot-precursor species. When a fan was placed behind the burning candle a “modified small-ion” signal was seen at sizes 1.1–2.0 nm. This was not observed without the fan present or when a lamp chimney was used. During burning, aerosol concentration was elevated and small-ion counts were low. However after extinction of the flame, this trend was reversed and the number of small-ions increased to levels higher than those observed prior to burning, remaining so for several hours.

Modeling Study on Soot Formation at High Pressures

Detailed modeling of soot formation and destruction was carried out using a model which consists of a relatively large reaction gas phase mechanism and of a mechanistic soot model. The objective of the present study is to validate a comprehensive gas phase reaction mechanism for soot precursor species from experiments of sooting flames for pressures up to 15 bar and to identify dominant pathways in the soot model. From the simulation of experimental soot absorption profiles measured in rich premixed laminar ethylene/air flames at elevated pressures, dominant reaction steps are identified by sensitivity analysis. The response of the proper soot model with respect to variations of rate coefficient values of relevant reactions in the gasphase and in the particle inception and oxidation regime is investigated

Nitric-oxide emissions scaling of buoyancy-dominated oxygen-enriched and preheated methane turbulent-jet diffusion flames

Theoretical predictions and measurements of nitric-oxide emissions from buoyancy-controlled weak (≤11%) oxygen-enriched as well as fuel-preheated ( 400 K) preheat and are confirmed by measurements of total radiant fluxes. Measurements of soot-precursor species and soot loading suggest enhanced radiation through enhanced soot loading as potential NOx-abatement strategy.

Effects of metal additives on soot precursors and particulates in a C 2H 4/O 2/N 2/Ar premixed flame

Parameters which characterize soot precursor and particulate species in a C 2H 4/O 2/N 2/Ar premixed flame have been measured both with and without alkali and alkaline earth salts present as fuel additives. The principal experimental techniques which were used include Mie scattering and optical extinction for particulate size, number density and volume fraction, and gas Chromatographic analysis of quartz probe collected samples for the determination of precursor concentrations. Other critical parameters such as flame temperature and metal species concentrations were addressed as well. Temperature was determined from the Wien equation using measured values of the flame's absorptivity and brightness. Species concentrations were estimated from analytical expressions known to be reliable for premixed flames. Data are presented which give the effects of the above metals on the particulates for three well-defined but different ethylene flames. The efficiency of a given metal is shown to be dependent almost exclusively on temperature and the metal atom's ionization potential; the higher the temperature and the lower the ionization potential, the greater is the soot removal. As such, only the alkali metals were effective. For them, the perturbation of soot takes the form of decreased size and volume fraction and increased number density, the extent of the perturbation increasing with residence time. The relative effectiveness of individual alkalis was shown to be correlated with their respective cation concentrations as estimated from the Saha equation with temperature and atom density as input parameters. With one or two intriguing exceptions, the precursors were, for the most part, unaffected by the metal. Accordingly, alkalis are principally effective at a late combustion stage where they induce increased numbers of smaller particulates which then burn out rapidly. Evidence here for intervention at an earlier stage, if any, is marginal.

A simple nucleation/depletion model for the spherule size of particulate carbon

One of the interesting aspects of particulate carbon generated during combustion is the limited size range (radii ∼10–100 nm) of the fundamental spheroidal units (spherules) which agglomerate to form characteristic soot clusters. In this communication is described a simple physical model based on classical nucleation theory, invoking depletion of soot precursor species as the mechanism which limits spherule size. Calculations of spherule radius for particulate carbon and other matter are in reasonable agreement with experiment.

Optogalvanic Spectroscopy-Application To Combustion Systems

This paper reviews the application of optogalvanic spectroscopy to flames. Optogalvanic spectroscopy is a method of obtaining absorption spectra of atomic and molecular species in flames and electrical discharges by measuring voltage and current changes upon laser irradiation. The technique alleviates problems associated with optically monitoring either small absorptions or weak fluorescence in the presence of strong laser light. Optogalvanic spectroscopy in discharges has been useful in characterizing laser linewidths as well as provid-ing a convenient frequency calibration for tunable dye lasers. In addition, opto-galvanic signals have been used to frequency stabilize continuous-wave dye lasers. Optogalvanic spectroscopy has also been possible on some molecular species which exist only in flame or discharge environments. Analytical flame spectrometry utilizing the optogalvanic effect for trace metal detection shows significant promise for many metallic elements. The optogalvanic effect can also serve as a probe of ionization effects in flames. Ionization cross sections and ion mobilities may be determined from an analysis of optogalvanic signals. This technique has been extended to include determination of mobilities for soot precursor species. Flow velocities may also be determined in laminar flames by following the residual depletion of neutrals due to laser-enhanced ionization.