Over the last two decades, our understanding of soot formation has evolved from an empirical, phenomenological description to an age of quantitative modeling for at least small fuel compounds. In this paper, we review the current state of knowledge of the fundamental sooting processes, including the chemistry of soot precursors, particle nucleation and mass/size growth. The discussion shows that though much progress has been made, critical gaps remain in many areas of our knowledge. We propose the roles of certain aromatic radicals resulting from localized π electron structures in particle nucleation and subsequent mass growth. The existence of these free radicals provides a rational explanation for the strong binding forces needed for forming initial clusters of polycyclic aromatic hydrocarbons. They may also explain a range of currently unexplained sooting phenomena, including the large amount of aliphatics observed in nascent soot formed in laminar premixed flames and the mass growth of soot in the absence of gas-phase H atoms. While the above suggestions are inspired, to an extent, by recent theoretical findings from the materials research community, this paper also demonstrates that the knowledge garnered through our longstanding interest in soot formation may well be carried over to flame synthesis of functional nanomaterials for clean and renewable energy applications. In particular, work on flame-synthesized thin films of nanocrystalline titania illustrates how our combustion knowledge might be useful for developing advanced yet inexpensive thin-film solar cells and chemical sensors for detecting gaseous air pollutants.

Formation of nascent soot and other condensed-phase materials in flames

Method of moments with interpolative closure

Soot formation in laminar counterflow flames

Soot particle emission from combustion processes is a significant environmental and health problem. Optical detection is clearly the only viable choice for in situ PAH and soot measurement in turbulent flames. This paper reviews the considerable progress made in optical diagnostics of soot and its precursors formed this last decade. Advances include the spectroscopy of polycyclic aromatic hydrocarbons (PAHs) and optical diagnostics for PAHs in flames, new approaches to identify the structure and composition of incipient soot, and the latest refinements for soot particle detection and quantification. Improvements of selective and quantitative PAH measurements have been made possible with supersonic expansion of flame extracted samples, coupled with laser induced fluorescence (jet-cooled LIF) and mass-spectroscopy combined with resonance enhanced or synchrotron selective photoionisation. More accurate description of all facets of soot particles properties (optical properties, morphology, and volume fraction) has guided recent research. In particular, optical exploration has led to new insight into soot inception, including the identification of particular soot precursors from large PAHs to nanoparticles passing through stacked PAHs, polymeric-like structures and aliphatic-linked PAHs. Despite the various improvements, the ultimate accuracy of the soot diagnostic methods such as laser induced incandescence, light extinction and elastic light scattering relies on our knowledge of the soot optical properties which may vary as a function of soot age in flames. Nonetheless, with well designed experiments, comparative measurement of sooting trends within or between flames can be acquired with accuracies needed for the validation of numerical simulations of soot formation. These optical methods have been successfully adopted to classify the sooting tendencies of a large number of hydrocarbons and to examine the effect of pressure on soot formation. The ultimate objective is certainly to use optical diagnostics to provide useful data for soot formation modelling in turbulent flames and under practical combustion.

Study of the formation of soot and its precursors in flames using optical diagnostics

The evolution of soot morphology in a laminar coflow diffusion flame of a surrogate for Jet A-1

In this paper we make use of a detailed particle model and stochastic numerical methods to simulate the particle size distributions of soot particles formed in laminar premixed flames. The model is able to capture the evolution of mass and surface area along with the full structural detail of the particles. The model is validated against previous models for consistency and then used to simulate flames with bimodal and unimodal soot particle distributions. The change in morphology between the particles from these two types of flames provides further evidence of the interplay among nucleation, coagulation, and surface rates. The results confirm the previously proposed role of the strength of the particle nucleation source in defining the instant of transition from coalescent to fractal growth of soot particles.

Numerical simulations of soot aggregation in premixed laminar flames

Particle aggregation with simultaneous surface growth was modeled using a dynamic Monte Carlo method. The Monte Carlo algorithm begins in the particle inception zone and constructs aggregates via ensemble-averaged collisions between spheres and deposition of gaseous species on the sphere surfaces. Simulations were conducted using four scenarios. The first, referred to as scenario 0, is used as a benchmark and simulates aggregation in the absence of surface growth. Scenario 1 forces all balls to grow at a uniform rate while scenario 2 only permits them to grow once they have collided and stuck to each other. The last one is a test scenario constructed to confirm conclusions drawn from scenarios 0-2. The transition between the coalescent and the fully developed fractal aggregation regimes is investigated using shape descriptors to quantify particle geometry. They are used to define the transition between the coalescent and fractal growth regimes. The simulations demonstrate that the morphology of aggregating particles is intimately related to both the surface deposition and particle nucleation rates.

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Particle aggregation with simultaneous surface growth.

Soot aggregation with simultaneous surface growth was modeled using a dynamic Monte Carlo method. Simulations were performed in the environment of a 10-bar laminar premixed flame of ethylene. The Monte Carlo algorithm begins in the particle inception zone and constructs soot particles via ensemble-averaged collisions between small spheres and deposition of gaseous species on the sphere surfaces. Simulations were conducted using three scenarios. Scenario 1 forces all spheres to grow at a uniform rate, whereas scenario 2 only permits them to grow once they have collided and stuck to each other. Results from 1 and 2 were compared to a reference, labeled scenario 0, which simulates aggregation in the absence of surface growth. The simulations demonstrate that aggregation with sufficiently small spherical particles in the presence of surface growth leads to a spheroidal shape. The results show that the spheroidal shape of particles is attributed to rapid surface growth and intense particle nucleation. In addition, particle shape may also be affected by rearrangement of the internal structure of colliding aromatic clusters.

Pablo Mitchell

Papers

Numerical simulations of soot aggregation in premixed laminar flames

Particle aggregation with simultaneous surface growth.

Monte carlo simulation of soot aggregation with simultaneous surface growth-why primary particles appear spherical