Search

In this work, a multi-wavelength pyrometry method is proposed in order to evaluate soot temperature for laminar axisymmetric non-premixed flames. A single radiative model that combines three detection wavelengths is developed to increase the spectral information used in the soot temperature determination. A simple set-up is used in order to capture the soot emission at two or three wavelengths. The robustness of the methodology is assessed by a radiative model, which includes a sensitivity and relative error analysis for the soot temperature, and accounts for self-absorption effects. The model and procedure are verified by employing numerical temperature and soot volume fraction fields to recreate the convoluted soot emissions. Soot emission measurements involve different combinations of wavelengths that are employed to evaluate soot temperature without requiring additional measurements of the soot absorption coefficient when soot self-absorption is neglected. The error on the determined soot temperature was estimated to be lower than 60 K under this assumption for soot volume fractions under 10 ppm. In addition, an analysis of the signal noise effect on the temperature shows that the modulated absorption/emission (MAE) technique is more sensitive than the emission (EMI) technique. A sensitivity analysis showed that the presented three-wavelength EMI model is less sensitive to small deviations on the measured soot emission ratio than the classical two-wavelength approach. This feature makes the model suitable to improve the accuracy on the determination of soot temperature when the noise level of the signal is significant. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the effects of oxygen index on the flame geometry, soot production and radiative loss in laminar over-ventilated co-flow buoyant axisymmetric diffusion flames at atmospheric pressure. Three gaseous hydrocarbon fuels, namely ethylene, propane, and butane, were considered. The oxygen index was varied from 21% to 37% and several mass flow rates were used. These conditions were chosen to keep the flames under the smoke point. Soot volume fraction and temperature were deduced from line-of-sight attenuation and two-color emission measurements, respectively. A scaling analysis based on the smoke point height was developed in order to unify the sooting behavior of the flames investigated. This analysis produced correlations for the flame height, the maximum soot volume fraction, the maximum integrated soot volume fraction and the radiant fraction at the smoke point. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the effects of the Oxygen Index (OI) in an ethylene laminar inverse diffusion flame (IDF). The OI was varied from 21% to 37% and its influence was measured in terms of the flame height, soot volume fraction, soot temperature and radiant fraction. The stoichiometric flame height was measured by the spontaneous emission of CH ∗ radicals and was found to decrease when the OI increases. In contrast, the luminous flame height increases with OI because soot can still form and grow beyond the reaction zone. Radial profiles of soot volume fraction and soot temperature were obtained by means of a Modulated Absorption/Emission (MAE) technique. The line-of-sight intensities, integrated along the optical path and captured by ECCD camera at two wavelengths were inverted using deconvolution and regularization techniques in order to obtain radial profiles of soot volume fraction and temperature. The increase in OI enhances soot volume fraction due to higher temperatures and soot formation rates. Both the local and integrated soot quantities increase with OI. The radiant fraction of IDF increases with the OI in a similar way to the integrated soot volume fraction. [ABSTRACT FROM AUTHOR]

• Numerical framework for simulating experimental measurements for axisymmetric flames. • The framework enables generation of rich datasets for reference and measured signals. • A Convolutional Neural Network (CNN) is used to retrieve soot volume fraction fields. • CNN outperforms traditional inversion methods based on numerical deconvolution. • CNN trained with synthetic data can retrieve the fields from real experimental data. Typical procedures for estimating soot volume fraction distribution in laboratory flames require solving ill-posed inverse problems to recover the fields from convoluted signals that integrate light extinction from soot particles along the line-of-sight of a photo-detector. Classical deconvolution methods are highly sensitive to noise and the choice of tunable regularization parameters, which prevents obtaining consistent estimations even for the same reference flame settings. This paper presents a novel approach based on Convolutional Neural Networks (CNNs) for estimating the soot volume fraction fields from 2D images of line-of-sight attenuation (LOSA) measurements in coflow laminar axisymmetric diffusion flames. Using a set of reference synthetic soot volume fraction fields of canonical flames and their corresponding projected LOSA images, we trained a CNN for reconstructing soot fields from images representing the data captured by a camera. Experimental results show that the proposed CNN approach outperforms classical deconvolution methods when reconstructing the flame spatial soot distribution from noisy images of LOSA. [ABSTRACT FROM AUTHOR]

Anisole fuel has gained increased attention, since it is considered a biofuel, and its addition to gasoline increases the octane number, improving the engine performance. This study focuses on the comprehensive characterization of soot maturity and sooting propensity of a well-controlled laminar coflow diffusion flame fueled with anisole and using methane as the carrier gas. Four laminar non-premixed flames were established below the smoke point. The oxidizer composition ranged from a molar fraction of oxygen (oxygen index, OI) of 21% to 35% (doped with pure oxygen), while keeping constant the total volumetric flow rate of oxidizer stream. Multi-wavelength line-of-sight attenuation experiments were performed from visible to infrared wavelengths, and the measurements of the extinction coefficient and flame temperature were used to retrieve two dimensional fields of soot temperature, volume fraction, radiative intensity and degree of soot maturity. In general, the sooting propensity and radiative behavior of the anisole flames follows a similar trend than typical hydrocarbon fuels when increasing the OI. However, the spatial distribution of soot volume fraction is enhanced, particularly near the flame centerline. In contrast, the soot production is promoted by the OI, near the flame wings. The same effect can be observed for the soot temperature. Indeed, temperature increases, promoting formation and oxidation processes, specially at the locations near the maximum soot volume fraction. In general, the maturity of soot particles is affected as the OI is changed. Also, it is observed that the degree of maturity is enhanced by flame temperature. In order to support these local observations, a statistical analysis was carried out. Finally, the results obtained yield a curated database for validating detailed soot production models of oxygenated fuels, and to gain further insights on the evolution of soot maturity and propensity of these particular fuels under different oxidizer conditions. • Spatially-resolved characterization of anisole sooting and radiative properties. • Oxygen boosts local soot temperature, volume fraction, and radiation. • Increased oxygen content enhances soot particle's maturation. • Increased temperature correlates with increased soot maturity. • These effects are more pronounced near the flame wings. [ABSTRACT FROM AUTHOR]

A canonical laminar coflow non-premixed flame was produced using a Jet A-1 fuel burning in coflowing ambient air environment using a Gülder type burner configuration. The aviation fuel was vaporized and carried by methane to the reaction zone. Non-intrusive laser-based multi-wavelength line-of-sight attenuation and emission measurements were employed to quantify soot properties in a spatially-resolved fashion. The experimental methodology allowed to evaluate soot properties such as temperature, absorption function, soot maturity, percentage of organic and graphitic compound contents, soot volume fraction and the divergence of the radiative heat flux. The distribution of soot temperature was found to be similar to that of alkenes and alkanes fuels, increasing radially toward the flame edge and axially toward the tip, peaking at 1950 K. The level of maturity was found to be higher (increased proportion of graphitic contents) in high soot temperature regions, revealing a relationship between temperature and the maturation process. Also, graphitic soot particles were predominantly found at the top of the flame, near the centerline zone and in the vicinity of the flame wings. As expected, the maturity influenced the absorption function, which was found to vary from 0.2 to 0.37 as the percentage of graphitic compounds changes from approximately 60% to 80% (mean values). This spatial variation of the absorption function led to significant differences on the retrieved soot volume fraction, around 1 ppm for the maximum values, as compared to the use of a constant value assuming fully graphitic soot particles. Despite these observations, the radiative heat flux released by the flame was not significantly affected, even considering the importance of taking into account the maturity for soot experimental diagnostics. • Detailed characterization of soot properties of commercial Jet A-1 fuel. • Spatial distribution of soot absorption function was obtained. • Higher organics compounds found near the flame centerline. • Mature soot particles found near the high temperature zone. • Overall, maturity is enhanced with increasing temperature. [ABSTRACT FROM AUTHOR]

In combustion research, optical diagnostics often necessitate Abel inversion to deconvolve the measurements of axisymmetric flames, especially to determine local soot temperature fields from integrated flame radiative emission measurements. However, traditional Abel inversion methods can considerably amplify experimental noise, especially towards the flame centerline. Although various strategies exist to mitigate this issue, they typically do not address the correction of signal trapping within the flame. The present paper introduces an adapted Abel inversion tool that combines resistance to experimental noise with a mechanism to correct signal trapping effects. The tool employs noise-free Abel inversion using piecewise spline functions. Its effectiveness is validated through numerical comparisons with conventional methods. The practical application is demonstrated in the measurement of soot temperature in a canonical laminar diffusion ethylene flame. This is achieved by using multi-wavelength line-of-sight attenuation (LOSA) and emission measurements. The tool corrects for the signal trapping effect in emission data by integrating extinction profiles, thus enabling a more precise soot temperature analysis. The current study also compares two techniques of temperature measurement based on thermal emission: traditional two-color pyrometry and a calibrated emission and absorption ratio method. The study further explores the effects of signal trapping, scattering contribution, and the absorption function ratio on soot temperature determination based on experimental profiles. The findings suggest that signal trapping and scattering contributions have a negligible impact on measurements at the wavelengths examined. However, the choice of absorption function ratio significantly influences the outcomes of two-color pyrometry, highlighting the advantage of the one color emission/absorption technique for the determination of sooting flames temperature measurements. • A new Abel transform based on cubic splines considering signal trapping is proposed. • This method, slightly slower, is more robust than classical Onion-Peeling. • The method is applied to determine soot temperature in a canonical ethylene flame. • One-color SAT method is compared to classical 2C pyrometry. • The one-color approach is advantageous because it is independent of optical index. [ABSTRACT FROM AUTHOR]

The macroscopic simulation of soot production in flames by CFD approaches often assumes of spherical particles or considers ad-hoc formulas in order to take the particle fractal aggregates' morphology into account. On the other hand, numerically simulated aggregates can be generated by Discrete Element Modeling (DEM) simulations at the nanoscopic scale. However, the change in thermodynamic conditions, surface growth, oxidation, and nucleation mechanisms are commonly neglected by this approach. This work combines these complementary approaches to investigate the detailed morphology of soot along 4 different particle trajectories in a diffusion flame. The proposed multi-scale approach shows remarkably larger and more compact aggregates near the wings of the flame as compared to the centerline, in agreement with previous experimental studies. This approach allows to analyse the particle polydispersity and morphological parameters beyond the fractal dimension, such as anisotropy coefficient and monomers overlapping coefficient. It becomes possible to reveal clear morphological signatures of soot formed along different streamlines in the flame. Finally, we have also observed a positive correlation between primary particle diameter and aggregate gyration diameter due to the predominance of surface reactions over aggregation. The proposed post-processing of CFD results based on DEM methods, bringing new information on soot morphology, is a proof of concept of a more accurate procedure for validating soot models used in CFD codes as compared to existing methods in the literature. [ABSTRACT FROM AUTHOR]