Brief communicationSoot particle measurements in diffusion flames
Abstract
The formation and growth of soot particles in a coannular diffusion flame have been studied using a laser extinction/scattering technique for particle size measurement. Measurements have been obtained with ethene as the fuel for various fuel flow rates. The results reveal that the flame can be broadly divided into two regions. The first is characterized as a region of growth where soot formation processes dominate, while in the second region oxidation processes are dominant. Measurements show that soot is first observed to form low in the flame in an annular region inside the main reaction zone. At higher locatikons this annular region widens until the entire flame is observed to contain paricles. The spatial distributions of particle volume fraction, mean particle size, and particle number concentration are mapped throughout the flame using the Rayleigh theory for the scattering of light by absorbing particles. Measurements of depolarized scattered light and fluorescence have also been obtained and indicate a correlation between the species responsible for these processes and soot growth. Results indicate that the particle formation region obeys closely the Burke-Schumann scaling for flow rate dependence, whereas substantial differences occur in the oxidation region. Measurements have also been obtained using ethane as the fuel for an initial comparison of fuel structure effects.
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Cited by (596)
On the intricacies of soot volume fraction measurements in counterflow diffusion flames with light extinction: Effects of curtain flow
2024, Journal of Aerosol ScienceCounterflow diffusion flame is an attractive platform for fundamental research on kinetics of soot formation. Accurate determination of soot volume fraction in the flame is a prerequisite for in-depth analysis of the sooting characteristics and assessment of predictive soot models. Light extinction has been proven to be an efficient technique for measuring soot volume fraction thanks to its non-intrusiveness and its simple optical setup. Nevertheless, tomographic inversion needs to be performed if spatially resolved soot volume fraction is to be obtained from the measured light extinction data which is essentially a projection along the line-of-sight. In this regard, radial distribution of soot volume fraction would affect the accuracy of the measurement through its influences on the inversion processes. In this work, we show that the curtain flow, which is necessary to avoid the formation of the undesired secondary diffusion flame and to keep the core counterflow from ambient disturbances, has notable effects on spatially resolved soot volume fraction measurements with line-of-sight measurements. In particular, different flow rate settings of the curtain flow can result in different soot distributions at the edges of soot fields: upwards curved, outwards extended, and downwards curved, which may influence the measurement of centerline soot volume fraction distribution. The necessity of tomographic inversion, the minimal region of the projection image necessary for tomographic inversion (when necessary), the quasi-one-dimensional feature of soot distribution, and the sensitivity of measurement to slight flame asymmetry were investigated where possible to determine the most suitable curtain flow configuration for soot volume fraction measurements by light extinction. Recommendations on curtain flow setting are finally made.
Influence of sub-atmospheric pressure on flame shape and sooting propensity in ethylene laminar coflow non-premixed flame
2024, Combustion and FlameSooting flames have been a longstanding research topic and an extensive literature has been developed at both atmospheric and high pressure. In contrast, studies of sooting flames at subatmospheric pressures are relatively scarce. As pressure decreases buoyancy, and consequently buoyancy-driven convective flow, decreases as well. So one could expect characteristic residence times to be longer. To assess the intuitive finding, steady coflow non-premixed ethylene/air flames were established at different pressure conditions, ranging from 0.2 to 1 bar. The configuration was documented by both numerical and experimental works. By the Modulated Absorption Emission (MAE) technique, fields of temperature, soot volume fraction, and dispersion exponent as a measure of soot maturity were extracted. Extending the MAE setup from 2 to 4 spectral ranges allows a more accurate evaluation of the dispersion exponent together with the temperature calibration factor. Numerical simulations were conducted using the CoFlame code, giving access to the flow topology and the governing characteristic times. According to numerical simulations, with increasing pressure, while the buoyancy-driven convective flow does increase, the flow velocities do decrease. It seems consistent with experimental results, finding higher maturity, expected with higher residence time, when increasing pressure. In addition to the important database produced for flames under sub-atmospheric conditions, this paper also couples originally experimental and numerical results, leading to (i) the reconstruction of the synthetic signals that a camera would capture, and (ii) the tracking of the quantities of interest experienced along the streamlines. Most of the global trends are well-captured by CoFlame, i.e. decreasing pressure leads to the decrease of soot volume fraction, maturity, and flame height, together with the increase in temperature. Meanwhile, significant discrepancies can be noticed, i.e. the numerical simulations overestimate the soot volume fraction, especially for the lower pressure levels, together with an underestimated flame temperature leading to an overestimation of the flame height.
On the mechanisms affecting soot production in oxygen-depleted buoyant flames
2023, Fire Safety JournalThe objective of this article is to apply a detailed polycyclic aromatic hydrocarbons (PAH)-based soot production model to investigate the effects of oxygen depletion on the overall soot production processes in laminar coflow ethylene diffusion flames. This task is accomplished by simulating the flames, studied experimentally by Sun et al. (Combust. Flame 211:96–111, 2020), burning under oxygen concentration in the oxidizer down to 16.8% and characterized by a constant volumetric stoichiometric air to fuel ratio to keep the residence time unchanged. This configuration allows isolating the effects of reducing oxygen on both soot formation and oxidation processes and avoiding the complex interactions between soot production and turbulence. Model predictions are in reasonable agreement with the experiments in terms of temperature, soot volume fraction, and primary particle diameter and capture quantitatively the effects of oxygen depletion on soot production. In all the flames, both the H-abstraction and acetylene addition (HACA) and PAH condensation contribute significantly to the soot mass growth, with the HACA slightly dominating the overall process. Reducing the oxygen concentration affects similarly all the formation and oxidation processes with a reduction in the range 30–40% from 21.0% to 18.9% and in the range from 60 to 70% as it is further reduced to 16.8%. Among the formation processes, the PAH condensation and surface growth by HACA have a similar sensitivity to oxygen depletion, followed by soot inception.
Overview of methods to characterize the mass, size, and morphology of soot
2023, Journal of Aerosol ScienceCombustion and other high-temperature processes can produce solid aerosol nanoparticles with complex morphologies, including fractal-like aggregates of primary particles. Characterizing these morphologies, as well as particle mass, is key to understanding their behavior in natural and engineered systems, and it can provide clues to the origin of the particles. We focus here on the characterization of soot, although most of the techniques apply to other aerosol aggregates. A complete description of these aerosols would include the mass and morphology of every particle. In practice, it is possible to obtain detailed information on individual particles from microscopy of extracted samples. A particular focus of this review, tandem classifier/detector systems can determine 2-dimensional mass and mobility distributions that may be interpreted through the lens of fractal models. Very fast in situ light scattering measurements can be used to determine the structure factor, related to fractal dimension, and the aggregate and primary particle size distributions. These approaches are complementary when there are appropriate models to connect morphological details to optical and transport characteristics of the particles. Over the last few decades these models have become more sophisticated, requiring more information on the particle structure and properties, but also facilitating more sophisticated inferences from in-situ and online measurement techniques.
Experimental study on soot formation and primary particle size in oxy-combustion ethylene diffusion flames under CO<inf>2</inf> substitution for N<inf>2</inf>
2023, Case Studies in Thermal EngineeringThis paper reports an experimental study on the effects of O2 concentration and CO2 concentration on soot volume fraction and primary particle size of flame centre in ethylene laminar co-diffusion flame. The incandescent light was induced by a two-color band method and laser-excited by wavelength 1064 nm. Previous studies have shown that the oxygen concentration affects the soot volume fraction in the flame. However, the effects of O2 concentration on the primary particle size under CO2 atmosphere are unclear and need to be further discussed. Seven flames were optically detected, in which the oxygen concentration of the accompanying gas varied from 21% to 50% by volume. One primary flame was burned in co-flowing ethylene/air. Six ethylene flames were burned in an oxygen-rich O2/N2 or O2/CO2 atmosphere with an oxygen mole fraction from 30% to 50%. The C2H4 flow rate remained constant in all operating conditions. The results show that the SVF observed by the LII method and spectroscopy in the oxygen-enriched flame are in good agreement. Furthermore, the soot's primary particle diameter measured by the time-resolved LII method is in good agreement with the transmission electron microscope image analysis.
With the increase of oxygen concentration, the flame height becomes shorter, the luminosity becomes brighter, and the soot yield increases. Compared with an N2 atmosphere with the same oxygen concentration, the soot yield in a CO2 atmosphere was significantly inhibited. In all the flames studied, the SVF along the flame axis showed a similar distribution: it first increased to the maximum value and then decreased rapidly until the flame tip. The maximum SVF on the axis appears between 0.64 and 0.78 based on the normalized height of the visible flame. In all ethylene/(O2/N2) flames, the maximum primary particle diameter and average primary particle diameter increase with oxygen content. For ethylene/(O2/CO2) flame, the maximum diameter of primary particles decreases with the increased oxygen content. The uncertainty of primary diameter is mainly caused by the thermal adaptation coefficient, the lower LII signal of smaller particles and the uncertainty of local flame temperature.
Can laser-induced incandescence calibrated by laser extinction method be used for quantitative determination of soot volume fraction in laminar flames?
2023, Applications in Energy and Combustion ScienceThis review is conducted to assess the effectiveness of the laser extinction method (LEM) calibrated laser-induced incandescence (LII) to quantitatively determine the soot volume fraction in flames. This article mainly focuses on the discussion and analysis of the existing experimental results in typical co-flow laminar diffusion flames. Initially, a brief introduction of the background and application of sooting tendency in co-flow laminar diffusion flames by using the combination of LII and LEM is presented. Then, the general theoretical backgrounds of LII and LEM techniques used for soot diagnostics is introduced. This is followed by a detailed summary and comparison of the maximum soot volume fraction in laminar diffusion flames obtained by a combination of LII and LEM techniques as well as the various optical methods. The maximum soot volume fraction in flames measured by the combined technique of LII and LEM exhibits high consistency with those of other optical methods, but the uncertainty derived from this approach is relatively large in soot concentration measurement. Minimizing the measurement uncertainty of soot volume fraction obtained by the combination of LII and LEM is still an important and non-negligible issue. The determination of the soot refractive index function E(m) for mature soot is one of the major sources of measurement uncertainty when using the combined technique of LII and LEM. Whereas it is of great challenge to confirm an accurate E(m) for mature soot of flames in the visible and near-infrared spectrum, and it still needs a large number of experimental investigations to reveal the accurate value of E(m). Finally, the conclusion and perspective for investigating sooting tendency in flames by the LEM-calibrated LII technique are presented.
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