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Diffusion flame
About: Diffusion flame is a research topic. Over the lifetime, 9266 publications have been published within this topic receiving 233522 citations.
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TL;DR: In this article, a review of the current understanding of soot morphology and optical properties is reviewed, emphasizing non-premixed flame environments, and the results show that soot consists of nearly spherical primary particles, having diameters generally less than 60 nm, which collect into open structured aggregates that are mass fractal objects.
Abstract: Motivated by the importance of soot to the emission of particulates and other pollutants from combustion processes, current understanding of soot morphology and optical properties is reviewed, emphasizing nonpremixed flame environments. The understanding of soot morphology in flames has grown rapidly in recent years due to the development of methods of thermophoretic sampling and analysis by transmission electron microscopy (TEM). The results show that soot consists of nearly spherical primary particles, having diameters generally less than 60 nm, which collect into open structured aggregates that are mass fractal objects. Aggregates grow by cluster/cluster aggregation to yield broad aggregate size distributions with the largest aggregates containing thousands of primary particles and reaching dimensions of several urn. The optical properties of soot aggregates generally are not suited for the Rayleigh and Mie scattering approximations which has led to the development of approximate Rayleigh-Deby...
108 citations
TL;DR: In flames produced by freely burning fuel, buoyancy may play an important role in determining the speed of the gases in the flame zone and hence the flame height as mentioned in this paper, which is consistent with a dimensionless analysis, leading, for one particular fuel system, to the functional equation L/D = f(Q2/gD5).
107 citations
01 Jan 2002
TL;DR: In this paper, a digital particle image velocimetry technique that is appropriate for the experimental derivation offundamental flame properties was implemented, and measurements were conducted for the CH 4 and C 2 H 2 flames by independently varying the equivalence ratio and flame temperature to distinguish between temperature and concentration effects.
Abstract: A digital particle image velocimetry technique that is appropriate for the experimental derivation offundamental flame properties was implemented. The technique allows for the determination of the instantaneous flowfield and is essential for fluid mechanics measurements in reduced gravity environments. Measurements of laminar flame speeds were conducted in the stagnation flow configuration just before a flame undergoes a transition from planar to Bunsen flame. Results obtained for lean CH 4 /air and C 2 H 2 /air flames were found to be in close agreement with prefious laser Doppler velocimetry data. Subsequently, measurements were conducted for the CH 4 and C 2 H 2 flames by independently varying the equivalence ratio and flame temperature to distinguish between temperature and concentration effects. The laminar flame speeds were also calculated using the GRI-Mech 3.0 mechanism. It was convincingly shown that under high-O 2 and low-temperature conditions, the experimental laminar speeds are over predicted by the simulations especially for C 2 H 6 flames. Additional experiments were conducted by adding H 2 to lean C 2 H 6 /air flames and by diluting those mixtures by either He or N 2 to vary the flame temperature. While for the He dilution case, the predictions noticeably overpredict the experiments, for N 2 dilution, close aggrement was observed. Analyses of the flame structures revealed that for those fuel-lean flames, the burning rate largely depends on the competition of the two-body branching and three-body termination reaction between H and O 2 . It was not possible to point to possible kinetic deficiencies other than referring to uncertainties associated with the rates and collision efficiencies of three-body reactions. The high-O 2 low-temperature region is of interrest not ouly to lean-premixed combustion, but also to flame ignition, and requires further exploration.
106 citations
TL;DR: In this paper, a tri-variate model for the formation of soot particles is presented and further validated in high temperature premixed ethylene flames, premixed benzene flames, an acetylene counterflow diffusion flame and toluene pyrolysis in shock-tubes.
106 citations
01 Jan 2000
TL;DR: In this paper, the GRI 2.11 mechanism was used with a modified version of the Lawrence Livermore National Laboratory HCT code to describe the reaction kinetics, and the laminar flame speed was strongly affected by the equivalence ratio gradient and by the burned gas composition and temperature.
Abstract: Freely propagating laminar methane/air flames were modeled for spatially stratified equivalence ratio conditions at atmospheric pressure. The GRI 2.11 mechanism was used with a modified version of the Lawrence Livermore National Laboratory HCT code to describe the reaction kinetics. Results showed that the laminar flame speed was strongly affected by the equivalence ratio gradient and by the burned gas composition and temperature. Production of molecular hydrogen from the original fuel and its transport to the reaction zone as well as heat transfer from the burned to the fresh gases are key factors in understanding the influence of stratification on laminar flame speed. Combinations of different mixture stratification conditions can either enhance or reduce the flame velocity when compared with homogeneous mixtures. Mixture stratification is also responsible for higher flame resistance to extinction both on the lean and rich sides of the equivalence ratio. The importance of heat and mass transfer on the observed results implies that their extrapolation to high pressure and to turbulent systems must be made with care.
106 citations