Flames soot particle growth

Equation (42) cannot be used if NO concentrations approach their equilibrium values, since the net production rate then depends on the concentration of NO, thereby bringing bivariate probability-density functions into equation (40). Also, if reactions involving nitrogen in fuel molecules are important, then much more involved considerations of chemical kinetics are needed. Processes of soot production similarly introduce complicated chemical kinetics. However, it may be possible to characterize these complex processes in terms of a small number of rate processes, with rates dependent on concentrations of major species and temperature, in such a way that a function w (Z) can be identified for soot production. Rates of soot-particle production in turbulent diffusion flames would then readily be calculable, but in regions where soot-particle growth or burnup is important as well, it would appear that at least a bivariate probability-density function should be considered in attempting to calculate the net rate of change of soot concentration. 

The composition, properties, and size of soot particles collected from flame products vary considerably with flame conditions and growth time. Typically the C—H atomic ratio ranges from two to five and the particles consist of kregular chains or clusters of tiny spheres 10—40 nm in diameter with overall dimensions of perhaps 200 nm, although some may agglomerate further to much larger sizes. 

For premixed fuel-air systems, results are reported in various terms that can be related to a critical equivalence ratio at which the onset of some yellow flame luminosity is observed. Premixed combustion studies have been performed primarily with Bunsen-type flames [52, 53], flat flames [54], and stirred reactors [55, 56], The earliest work [57, 58] on diffusion flames dealt mainly with axisymmetric coflow (coannular) systems in which the smoke height or the volumetric or mass flow rate of the fuel at this height was used as the correlating parameter. The smoke height is considered to be a measure of the fuel s particulate formation and growth rates but is controlled by the soot particle bumup. The specific references to this early work and that mentioned in subsequent paragraphs can be found in Ref. [50],... 

Flame turbulence should not affect soot formation processes under premixed combustion conditions, and the near correspondence of the results from Bunsen flames [52] and stirred reactors [55] tends to support this contention. However, the effect of turbulence on sooting diffusion flames can be very complex and unclear in most experiments unless the effect of the intensity (and scale) of turbulence on the flame structure, the temperature-time history of the pyrolyzing fuel, the rate of incipient particle formation and particle growth, and, in the case of some fuels, the transport of oxygen to the fuel stream are known. 

Soot forms in a flame as the result of a chain of events starting with the oxidation and/or pyrolysis of the fuel into small molecules. Acetylene, C2H2, and polycyclic aromatic hydrocarbons (PAHs) are considered the main molecular intermediates for soot formation and growth (McKinnon and Howard 1990). The growth of soot particles involves first the formation of soot nuclei and then their rapid growth due to surface reactions (Harris and Weiner 1983a,b). 

Harris, S. J., and Weiner, A. M. (1983a) Surface growth of soot particles in premixed ethylene/air flames, Combust. Sci. Technol. 31, 155-167. 

Quantitative Chemical Mechanism for Heterogeneous Growth of Soot Particles in Premixed Flames... 

Depletion of growth species does not occur in our flames. Therefore, the final size reached by the soot particles, when sur-... 

Di Stasio, S., Konstandopoulos, A.G and Kostoglou, M. (2002). Cluster-cluster aggregation kinetics and primary particle growth of soot nanoparticles in flame by fight scattering and numerical simulations. J. Colloid Interface Sci., 247, 33 6. 

Smoke is a mixture of black carbon (soot) and aerosol [26,27]. It has been suggested that soot nucleation and growth occur near the highly ionized regions of the flames in combustion processes, and that some of the charges are transferred to smoke particles. Multimodal distributions show that the soot particle radii belong to three modes [26] ... 

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