Laser-induced Incandescence Signal Research Articles

Inferring primary particle size distribution and thermal accommodation coefficient of soot aggregate across diverse ambient temperatures using laser-induced incandescence

Laser-induced incandescence (LII) technique has proven to be a powerful tool for non-invasive, in-situ particle sizing of soot aggregates in flames, and the key lies in interpreting LII signals with a heat transfer model. However, the reliability of model-derived primary particle size distribution (PPSD) is sensitive to the uncertainty of thermal accommodation coefficient (TAC), which is typically regarded as a pre-determined parameter in the LII heat transfer model. To overcome this problem, we have proposed a trivariate inversion strategy, i.e., inferring two PPSD parameters and TAC simultaneously from LII signals, which has been theoretically validated at room temperature, but its feasibility in the flame case remains to be investigated. In this study, the trivariate inversion strategy is extensively evaluated over a wide distribution of both ambient temperature and laser fluence. For different cases ranging from room temperature to flame temperature, numerical tests show that the inversion results of all three variables exhibit significantly high accuracy and low uncertainty with proper selection of laser fluence. Furthermore, our study shows that the trivariate inversion strategy can maintain robust performance in ambient temperature fluctuation caused by spatial inhomogeneity of the flame. All these results indicate that this trivariate inversion strategy has a promising application in combustion scenarios.

Comparison between Different Optical Configurations of Active-FRAME Setup in Multispectral Imaging of Flames

Snapshot multispectral imaging of chemical species in the flame is essential for improved understanding of the combustion process. In this article, we investigate the different configurations of a structured laser sheet-based multispectral imaging approach called the Frequency Recognition Algorithm for Multiple Exposures (FRAME). Using FRAME, a snapshot of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) excited by 283.5 nm laser and Laser-Induced Incandescence (LII) of soot particles excited by 532 nm laser are acquired simultaneously on a single FRAME image. A laminar diffusion flame of acetylene produced by a Gülder burner is used for the experiments. The standard FRAME approach is based on creating two spatially modulated laser sheets and arranging them in a cross-patterned configuration (X). However, the effect of using different configurations (angles) of the two laser sheets on the multispectral planar imaging of the flame has not yet been studied. Therefore, we have compared the FRAME approach in four different configurations while keeping the same flame conditions. First, we have compared the relation between laser fluence and LII signals with and without spatial modulation of the 532 nm laser sheet and found that both detections follow the same curve. When comparing the maps of flame species reconstructed from the standard FRAME configuration and other configurations, there are some dissimilarities. These differences are attributed to minor changes in the imaging plane, optical alignment, laser path length, different modulation frequencies of the laser sheet, laser extinction, laser fluence, etc.

Simultaneous Inversion of Particle Size Distribution, Thermal Accommodation Coefficient, and Temperature of In-Flame Soot Aggregates Using Laser-Induced Incandescence.

Measuring the size distribution and temperature of high-temperature dispersed particles, particularly in-flame soot, holds paramount importance across various industries. Laser-induced incandescence (LII) stands out as a potent non-contact diagnostic technology for in-flame soot, although its effectiveness is hindered by uncertainties associated with pre-determined thermal properties. To tackle this challenge, our study proposes a multi-parameter inversion strategy-simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial temperature of in-flame soot aggregates using time-resolved LII signals. Analyzing the responses of different heat transfer sub-models to temperature rise demonstrates the necessity of incorporating sublimation and thermionic emission for accurately reproducing LII signals of high-temperature dispersed particles. Consequently, we selected a particular LII model for the multi-parameter inversion strategy. Our research reveals that LII-based particle sizing is sensitive to biases in the initial temperature of particles (equivalent to the flame temperature), underscoring the need for the proposed multi-parameter inversion strategy. Numerical results obtained at two typical flame temperatures, 1100 K and 1700 K, illustrate that selecting an appropriate laser fluence enables the simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial particle temperatures of soot aggregates with high accuracy and confidence using the LII technique.

Investigation of the effect of hydrogen addition on soot and PAH formation in ethylene inverse diffusion flames by combined LII and PAH LIF

It is shown that signals from both laser-induced incandescence (LII) of soot and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAHs) decrease with hydrogen addition (in volume fractions of 3%, 6% and 9% with respect to the fuel mixture) to ethylene–air inverse diffusion flames (IDFs). The structure of the IDF suppresses soot oxidation and the effect of hydrogen addition under these conditions has been studied. Experiments were performed using a frequency-doubled, pulsed dye laser to perform planar LIF of PAH, and a pulsed fibre laser to perform LII. In relative terms, the LII signal decreases more sharply than the PAH LIF signal. This would be consistent with the dependence of soot inception on PAH concentration as well as the suppression of soot growth via the reduced concentration of PAH and perhaps other precursors such as acetylene. Similar trends in relative signal decrease are observed at a range of heights above the burner, despite the measurement locations encompassing a wide range of absolute signal levels. As a comparison, the influence of adding methane to the IDF in the same volume fractions was also studied and found to suppress PAH LIF and LII signals but to a far lesser extent than in the case of hydrogen.

Measurement of Soot Volume Fraction in Counterflow Diffusion Flames Up to 10 Atm Using Optical Diagnostics

ABSTRACTCombustion pressure has an influence on formation and destruction of soot. The balance of these processes determines the net soot emissions. In this work, the soot volume fraction (fv) in ethylene air counterflow flames has been investigated at elevated pressures using laser-induced incandescence (LII) and two-dimensional line of sight attenuation (2D LOSA) measurements. Pressure has been varied from 3 to 7 atm for fv measurements using 2D LOSA whereas fv has been reported from 3 to 10 atm using LII. Above 7 atm, counterflow flames were not perfectly axisymmetric, and therefore LII has been used instead of 2D LOSA above 7 atm. The fuel mole fraction varied between 0.2 and 0.3. Attenuation of LII signal has been corrected using LOSA measurements. Planar laser-induced fluorescence was used to study the location of polycyclic aromatic hydrocarbons (PAHs) at 3 and 7 atm. Locations of various sizes of PAHs have been reported by detecting the fluorescence signal at ranges of wavelengths centered at 350, 400, 450 and 510 nm. fv measurements by 2D LOSA and LII showed a good agreement. The LII signal showed significant attenuation due to the optical density of the counterflow flames, which highlights the importance of correcting the LII signal before inferring the values of the soot volume fraction. An increase in pressure resulted in an increase of the soot volume fraction and the overlap of the PAH zones and soot zone increased at elevated pressures.

Determination of the absorption function of laser-heated soot particles from spectrally resolved laser-induced incandescence signals using multiple excitation wavelengths

In this work, the optical properties of soot particles from a Gülder burner fueled with both ethylene or propane gas were investigated in situ using laser-induced incandescence. The particles in the flame were irradiated with four different laser wavelengths, namely 450 nm, 532 nm, 600 nm and 650 nm. The resulting laser-induced emissions were detected spectrally and temporally resolved by means of a spectrograph and an intensified camera at different delay times with respect to the laser pulse. To determine the optical properties of the particles from the data, the emitted spectra were simulated using a spectroscopic model with variable input parameters, and a regression was performed against the measured data. On the basis of an functional approach of the absorption function on wavelength, the dispersion exponent for soot was evaluated for a reference position on the centre axis at 40 mm height above the burner. It was found that the different fuel gases lead to similar values with regard to the absorption function, which can be expressed by a mean dispersion exponent with a value of 1.75 for ethylene and 1.68 for propane.

Initial interpretation of Laser-induced Incandescence (LII) signals from flame-generated TiO2 particles: Towards a quantitative characterization of the flame synthesis processes

Among various optical diagnostics for the characterization of particle formation in flames, laser-induced incandescence (LII), developed for soot particles, is attracting attention for the study of flame synthesis of metal-oxides. Among them, TiO2 nanoparticles are widely used for pigments and photocatalysts. Recent works have shown the feasibility of LII for flame-synthesized TiO2, but extensive research is still needed to quantitatively characterize TiO2 production in flames with LII measurements. In this work, the first attempt towards the characterization of TiO2 synthesis in flames is provided as a normalized volume fraction. To achieve this, TiO2 nanoparticles are generated in a laminar coflow diffusion flame of argon-diluted hydrogen and air with pre-vaporized titanium isopropoxide (TTIP). A 355 nm laser is used to irradiate the flame-generated particles. Spectral, temporal, and spatial measurements are performed at various flame heights. First, laser-induced emission (LIE) at prompt is investigated for different laser fluences to identify the operating conditions that ensure the LII-like nature of the measured signals. The LIE at high fluence presents sharp features that contain information on the atomic composition of the particles and of the vaporized species when compared to reference spectra of carbon black and high-purity TiO2 particles. Then, the LII signal at lower fluence is used to obtain an estimation of the spatial evolution of the normalized volume fraction and of the LII signal decay time. These results are finally used to discuss the major aerosol processes along the flame centerline.

Simultaneous application of soot and temperature measurements in a pressurized turbulent flame by laser-induced incandescence and coherent anti-Stokes Raman scattering for particle sizing

Simultaneous application of multi-channel laser-induced incandescence (LII) and shifted vibrational coherent anti-Stokes Raman scattering (SV-CARS) to study sooting flames is demonstrated for the first time. The potential of this diagnostics combination is evaluated on the basis of characterization of soot particles and correlation of soot presence with temperature. For that purpose, a sooting swirl flame operated at three bars has been employed with ethylene as fuel. The novel combination of CARS and time-resolved LII (TiRe LII) enables the estimation of particle size and correlation of this quantity with local gas temperature; simultaneously acquired 2D LII images provide information on the soot distribution in the ambience of the measurement volume which is used by CARS and TiRe LII. Even if the used LII model is approximative in some respect, the detected LII decay times indicate very small particle size throughout the flame relative to an atmospheric laminar diffusion flame which was used for comparison. In most instances, soot presence relates to local gas temperatures in a range between 1600 and 2400 K. Rare soot events at cooler temperatures occur near the nozzle exit and are attributed to transported soot. Comparison of the peak soot temperatures during the LII process shows a significant decrease in the turbulent pressurized flame relative to the laminar atmospheric reference flame. This is attributed to a less-efficient LII heat-up process at turbulent pressurized conditions due to beam steering. The background blackbody temperature, which can be derived by evaluating the signal captured in the different color channels of the LII system towards the end of the LII process, has been identified to be mostly controlled by hotter soot filaments between the laser plane and the detector. Thus, the LII signal tail is not a good measure of the local gas temperature in the measurement volume for this type of configuration.

Parameterization of a Refined Model Aimed at Simulating Laser-Induced Incandescence of Soot Using a Visible Excitation Wavelength of 532 nm

Laser-induced incandescence (LII) is one of the most powerful techniques for soot detection in combustion media. It is therefore commonly used to perform experiments in lab-scale flames and industrial combustors with a view to elucidating the formation mechanisms leading to combustion-generated fine carbonaceous particles while assessing their intrinsic properties. Quantitatively interpreting LII measurements, however, requires a firm knowledge of the optical properties of soot, including their wavelength-dependent absorption function (). Among the approaches used to evaluate such a crucial parameter, one can implement a LII model to derive the value which has to be set to reproduce a series of LII signals measured in a well-characterized environment. In this context, the present work aims at parameterizing a refined LII model built upon a comprehensive version of soot heat- and mass-balance equations for assessment when using a visible excitation wavelength of 532 nm. The proposed model integrates terms representing the saturation of linear, single- and multi-photon absorption processes, cooling by sublimation, conduction, radiation and thermionic emission, in addition to mechanisms depicting soot oxidation and annealing, non-thermal photodesorption of carbon clusters, as well as corrective factors accounting for the shielding effect and multiple scattering (MS) within aggregates. To parameterize this advanced simulation tool, an optimization procedure coupling design of experiments with a genetic algorithm-based solver was implemented. Doing so allowed to estimate the values of different factors involved in absorption and sublimation terms, including the multi-photon absorption cross-section for C2 photodesorption, the saturation coefficients for linear- and multi-photon absorption, as well as the value. Obtained parameters turned out to be well-suited to reproduce a set of LII signals acquired in a Diesel flame. While leading to predictions merging on a single curve with measured data, the so-parameterized model notably led to infer values of 0.3 and 0.38 when considering or neglecting MS within aggregates, respectively. Finally, the / ratio estimated based on data collected herein and in a former modeling work was found to be consistent with results issued from two-excitation wavelength LII measurements previously reported in the literature.

Soot-particle core-shell and fractal structures from small-angle X-ray scattering measurements in a flame

We have characterized soot particles measured in situ in a laminar co-flow ethylene-air diffusion flame using small-angle X-ray scattering (SAXS). The analysis includes temperature measurements made with coherent anti-Stokes Raman spectroscopy (CARS) and complements soot volume-fraction and maturity measurements made with laser-induced incandescence (LII). We compared the results of fits to the SAXS measurements using a unified model and a fractal core-shell model. Power-law parameters yielded by the unified model indicate that aggregates of primary particles are in the mass-fractal regime, whereas the primary particles are in the surface-fractal regime in the middle of the flame. Higher and lower in the flame, the primary-particle power-law parameter approaches 4, suggesting smooth primary particles. These trends are consistent with fits using the fractal core-shell model, which indicate that particles have an established core-shell structure in the middle of the flame and are internally homogeneous at higher and lower heights in the flame. Primary-particle size distributions derived using the fractal core-shell model demonstrate excellent agreement with distributions inferred from transmission electron microscopy (TEM) images in the middle of the flame. Higher in the flame, a second small mode appears in the size distributions, suggesting particle fragmentation during oxidation. Surface oxidation would explain (1) aggregate fragmentation and (2) loss of core-shell structure leading to smoother primary-particle surfaces by removal of carbon overlayers. SAXS measurements are much more sensitive to incipient and young soot particles than LII and demonstrate significant volume fraction from particles low in the flame where the LII signal is negligible.

Influence of rapid laser heating on differently matured soot with double-pulse laser-induced incandescence

For accurate laser-induced incandescence (LII) measurements of soot properties it is of great importance to understand the nature of the physical processes involved during rapid laser heating. In this work, we investigate how well-characterized differently matured fresh soot from a soot generator responds to rapid laser heating. For this purpose, a double-pulse LII setup is used with 10 μs time separation between the pulses using various combinations of two common LII wavelengths (532 and 1064 nm). Detection is performed at two wavelength bands for fluorescence analysis, and additionally elastic light scattering is used for mass loss analysis during heating. We investigate how the LII signal changes with pre-heating laser energy, specifically by fluence curve analysis to estimate the influence of thermal annealing, sublimation and laser-induced fluorescence interference. It is shown that extensive absorption enhancement occurs for all types of soot as the soot is thermally annealed, which is manifested through decreasing dispersion coefficient ξ and an increasing absorption coefficient E(m,λ). When comparing young and mature soot, a much larger impact of sublimation can be observed in the fluence curves of the mature soot. Also, we observe an enhanced contribution of laser-induced fluorescence for the young soot when performing LII measurements using 532 nm, which is suggested to originate from vaporized carbon fragments with an aromatic structure. This work further shows the potential of utilizing double-pulse arrangements for increasing the detectability of poorly absorbing soot, but also it highlights the impact of laser heating on soot, which may be important to avoid interferences when performing soot diagnostics.

A robust trivariate approach for characterizing the primary particle size distribution of soot aggregates using time-resolved laser-induced incandescence

This study proposes a new trivariate retrieval method based on a moderate-fluence laser-induced incandescence (LII) model. By changing the undesirable mathematical properties present in our previous work, this upgraded new method is able to obtain robust estimates of the two primary particle size distribution parameters and one assembled parameter, and still maintains the advantage of not requiring prior knowledge of thermal accommodation coefficient and aggregate structure. The robustness of the upgraded method is verified on a large number of numerical results, as evidenced by significantly improved retrieval accuracy and substantially reduced estimation uncertainty. The generality of the upgrade method is demonstrated over a large range of three variables. Furthermore, the influence of several key factors (the number of shots used for averaging noisy LII signal, the intensity ratio of the measurement termination, and the sublimation sub-model error) on the retrieval results of the upgraded method is investigated numerically.

A layer-peeling method for signal trapping correction in planar LII measurements of statistically steady flames

This work presents a layer-peeling (LP) algorithm to correct the signal trapping effect in planar laser-induced incandescence (LII) measurements of soot volume fraction. The method is based on measurements of LII signals captured by an intensified CCD camera at a series of parallel planes across a diffusion flame. A method based on presumed function (PF) of soot volume fraction is also proposed for comparison. The presented methods are numerically tested based on synthetic LII signals emitted from a simulated axisymmetric laminar diffusion flame using the CoFlame code. Numerical results showed that the LP method is able to correct the signal trapping effect, even for fairly large optical thicknesses and in a wide range of detection wavelengths. The correction decreases the relative errors induced by neglecting the trapping effect considerably. The signal trapping effect correction is less important for the determination of integrated soot quantities such as radially integrated soot volume fraction or total soot loading. Planar LII measurements were carried out and calibrated in order to test the method experimentally in a coflow flame. The LP, PF and a simplified analytical (SA) model were compared. The results indicate that the differences in soot volume fraction of 1 ppm or about 15% are obtained in zones of maximum soot loading of 6.5 ppm when the trapping effect is accounted for. Also, the LP and SA methods were found computationally efficient and accurate compared to the PF method. Although the study was performed in a canonical laminar axisymmetric flame, the proposed method can be applied to any statistically steady 3D flame.

3D mapping of polycyclic aromatic hydrocarbons, hydroxyl radicals, and soot volume fraction in sooting flames using FRAME technique

We report the three-dimensional (3D) mapping of polycyclic aromatic hydrocarbons (PAHs), soot, and hydroxyl radicals (OH) in ethylene/air diffusion flames. A structured illumination-based frequency recognition algorithm for multiple exposures (FRAME) approach is combined with sample translation to intersect the flame in several two-dimensional planes. The FRAME technique has been used for recording a snapshot of multiple species on a single camera. It relies on extracting the amplitude of spatial modulation of two or more probed species encoded on a single sub-image. Here, the FRAME technique is first applied for simultaneous imaging of PAH by laser-induced fluorescence (PAH-LIF) and soot by laser-induced incandescence (LII). Sequentially, it is employed for simultaneous mapping of OH-LIF and soot-LII. The LII signal is converted to absolute soot volume fraction (fv) maps using a line-of-sight light extinction measurement. Finally, we have demonstrated the approach for layer-wise 2D imaging of soot volume fraction and averaged 3D mapping of multiple species.

Application of planar auto-compensating laser-induced incandescence to low-sooting turbulent flames and investigation of the detection gate width effect

Laser-induced incandescence (LII) is an optical technique that is widely used for the in situ measurement of the soot volume fraction in flames. The low intensity of LII signals in low-sooting turbulent flames means that long camera acquisition times are needed to achieve sufficiently high signal-to-noise ratios. The direct application of auto-compensating LII (AC-LII) to infer the soot volume fraction from the measured LII signals can lead to large errors due to the significant decrease in the temperature of the soot particles over the duration of the camera gate. In order to reduce the measurement errors from AC-LII, we propose an improved approach in which the cooling of soot particles during the camera acquisition of LII signals is considered. The proposed methodology is applied to planar AC-LII measurements for the determination of soot volume fraction and primary particle diameter in a low-sooting turbulent flame, with the help of the open source LII software LIISim. This study represents the first application of planar AC-LII to a turbulent flame. A sensitivity analysis is also conducted in order to determine the main factors affecting the uncertainty of the proposed approach.Copyright © 2021 American Association for Aerosol Research

Generating planar distributions of soot particles from luminosity images in turbulent flames using deep learning

We report a computational method based on deep learning (DL) to generate planar distributions of soot particles in turbulent flames from line-of-sight luminosity images. A conditional generative adversarial network (C-GAN) was trained using flame luminosity and planar laser-induced incandescence (LII) images simultaneously recorded in a turbulent sooting flame with an exit Reynolds number of 15,000. Such a training built up the underlying relationship between the two types of images i.e., a predictive model which was then used to predict LII images from luminosity images and the accuracy was assessed using four different methods. Results show that the model is effective and capable of generating LII images with acceptable prediction accuracies of around 0.75. The model was also found to be applicable over a range of heights in the flames, as well as for the flames with a range of exit Reynolds numbers spanning from 8000 to 20,000. Besides, the probability density function (PDF) of LII signals in different flames can also be predicated using the model. This work, for the first time, demonstrates the feasibility of predicting planar signals from corresponding line-of-sight signals from turbulent flames, which potentially offers a much simpler optical arrangement for a modest trade-off in accuracy.

Two-dimensional laser-induced incandescence for soot volume fraction measurements: issues in quantification due to laser beam focusing

Two-dimensional laser-induced incandescence (LII) measurements usually involve the use of a cylindrical lens to illuminate the planar region of interest. This creates a varying laser fluence and sheet width in the imaged flame region which could lead to large uncertainties in the quantification of the 2D LII signals into soot volume fraction distributions. To investigate these effects, 2D LII measurements using a wide range of laser pulse energies were performed on a premixed flat ethylene–air flame while employing a cylindrical lens to focus the laser sheet. Using shorter focal length of the focusing lens resulted in larger variation of the LII signal profiles across the flame. A heat – and – mass – transfer - based LII model was also used to simulate the measurements and good agreement was found. The ratio between focal length (FL) and image length (IL) was introduced as a useful parameter for estimating the bias in estimated soot volume fractions across the flame. The general recommendation is to maximize this FL/IL ratio in an experiment, which in practice means the use of a long focal length lens. Furthermore, the best choices of laser fluence and detection gate width are discussed based on results from these simulations.

Correction of laser-induced incandescence signal trapping in soot measurement in a microgravity boundary layer laminar diffusion flame

This work presents a novel method to auto-correct the trapping effect of laser-induced incandescence (LII) signals inside the flame to obtain soot volume fraction distributions in laminar axisymmetric (2D) and three-dimensional (3D) flames using planar LII. The development of the proposed model is described with an estimation of the propagated uncertainties. The proposed model is firstly numerically validated in a canonical 2D coflow Santoro flame using synthetic LII signals and soot properties predicted by the CoFlame code. Secondly, experimental measurements of LII were carried out in the same 2D coflow Santoro flame and the LII signals were converted to soot volume fraction using the proposed model. Simultaneous LII and line-of-sight attenuation measurements were conducted to obtain the calibration factor. The proposed model automatically took into account the signal trapping effect in the conversion of the detected LII signal to soot volume fraction. Finally, the validated methodology is applied to a 3D laminar boundary layer diffusion flame established in microgravity. The significant differences in the measured soot volume fraction distributions with and without considering signal trapping, particularly in the 3D zone of the flame, demonstrate the importance of considering signal trapping to LII measurements of soot volume fraction in this flame. The model developed in this work can be readily applied in planar LII measurements of soot in any flame configuration as long as it is steady or statistically steady, to allow measurements to be performed at different positions inside a flame, such as for pool-fires or pulsating flames.

Simultaneous two-phase flame velocity measurement using laser-induced incandescence particle image velocimetry (LII-PIV)

In a previous study we demonstrated a novel two-phase PIV technique based on the laser-induced incandescence (LII) signal from black submicron tungsten carbide particles (WC), which achieved velocity measurements for both dispersed-form (large water droplet) and continuous-form (gas). Submicron WC particles are intentionally seeded into a two-phase flow, and heated by a light sheet generated by a double-pulsed PIV laser running at high energy. The 200 nm diameter, light absorbing WC particles are heated to several thousand degrees to emit strong incandescence signals, whilst the temperature rise in liquid droplets or large particles remains negligible. The small particles follow the gas phase flow, unlike the droplets which may have a different velocity. Droplets are detected via the Mie scatter signal at the same incident wavelength, whereas the LII signal from small WC particles is detected at a suitably different wavelength within the LII emission spectrum, thus allowing discrimination of velocities between phases. The LII-PIV technique had been implemented with a low-speed CCD PIV camera in non-reacting flows. In flames, the strong flame luminosity saturated the second frame due to the long exposure time as the characteristics of the device. To solve this problem, in the present study, we synchronized two high-speed CMOS cameras to a low speed laser. One records the LII signal and the other records the Mie scatter signal from 36.6 µm water droplets. The scattering from WC particles appears only as a weak background signal in the Mie image, which can be easily removed by applying a high-pass filter. Simultaneous velocity measurements for both gas and liquid phase are demonstrated in an air jet, a cold impinging flow, and finally in a Bunsen flame. The last two cases are repeated using the traditional two-phase PIV technique based on image segmentation so as to conduct a fair comparison of both techniques. We show that LII-PIV can achieve the same level of accuracy as the segmentation method in non-reacting flows, and can be applied to measure in flames with two-phase flows with less stringent requirements regarding seeding quality.

Soot formation characteristics in a pulverized coal flame formed in a swirling flow

To understand the soot formation characteristics in a pulverized coal flame with a swirling flow, simultaneous imaging of Mie scattering of coal particles and laser induced incandescence (LII) of soot were performed in this study. The pulverized coal flame was stabilized by a hydrogen diffusion pilot flame. The characteristic structures of soot formation in the pulverized coal flame with a swirling flow were analyzed based on a comparison of the experimental results and two-dimensional numerical simulation in the mixing region of coal particles and the oxidizer. The interactions between coal particle clouds and soot formation are discussed in detail. The results clearly show that the averaged radial dispersions of scattering signals from coal particles and of the LII signals from soot are overlapped. The overlapping region appeared nearby the nozzle exit due to the turbulent mixing and the high temperature region formed by swirl-induced recirculation flow. This overlapping region radially expands with increasing the height from the burner. Additionally, the characteristic areas of soot formation were observed in the results of simultaneous imaging of Mie scattering and LII. These areas are 1) streaky soot formation areas around the particle clouds, 2) soot formation areas inside of the particle clouds and 3) soot formation areas around the large coal particles.