Coflow Diffusion Flames Research Articles

Effect of ammonia addition on suppressing soot formation in methane co-flow diffusion flames

Due to issues surrounding carbon dioxide emissions from carbon-containing fuels, there is growing interest in ammonia (NH3) as an alternative combustion fuel. One attractive method of burning NH3 is to co-fire it with hydrocarbons, such as natural gas, and in this case soot formation is possible. To begin understanding the influence of NH3 on soot formation when co-fired with hydrocarbons, soot volume fractions and mole fractions of gas-phase species were computationally and experimentally interrogated for CH4 flames with up to 40% NH3 by volumetric fuel fraction. Mole fractions of gas-phase species, including C2H2 and C6H6, were measured with on-line electron impact mass spectrometry, and soot volume fractions were obtained via color-ratio pyrometry. The simulations employed a detailed chemical mechanism developed for capturing nitrogen interactions with hydrocarbons during combustion. The results are compared to findings in N2CH4 flames, in order to separate thermal and dilution effects from the chemical influence of NH3 on soot formation. Experimentally, C2H2 concentrations were found to decrease slightly for the NH3CH4 flames relative to N2CH4 flames, and a stronger suppression of C6H6 was found for NH3 relative to N2 additions. The measured results show a strong suppression of soot with the addition of NH3, with soot concentrations reduced by over a factor of 10 with addition of up to 20% or more NH3 by mole fraction. The model satisfactorily captured relative differences in maximum centerline C2H2, C6H6, and soot concentrations with addition of N2, but was unable to match measured differences in NH3CH4 flames. These results highlight the need for an improved understanding of fuel-nitrogen interactions with higher hydrocarbons to enable accurate models for predicting particulate emissions from NH3/hydrocarbon combustion.

Experimental and numerical study on soot formation in laminar diffusion flames of biodiesels and methyl esters

Biodiesel and blends with petroleum diesel are promising renewable alternative fuels for engines. In the present study, the soot concentration generated from four biodiesels, two pure methyl esters, and their blends with petroleum diesel are measured in a series of fully pre-vapourised co-flow diffusion flames. The experimental measurements are conducted using planar laser induced-incandescence (LII) and laser extinction optical methods. The results show that the maximum local soot volume fractions of neat biodiesels are 24.4% - 41.2% of pure diesel, whereas the mean soot volume fraction of neat biodiesel cases was measured as 11.3% - 21.3% of pure diesel. The addition of biodiesel to diesel not only reduces the number of inception particles, but also inhibits their surface growth. The discretised population balance modelling of a complete set of soot processes is employed to compute the 2D soot volume fraction and size distribution across the tested flames. The results show that the model also demonstrates a reduction of both soot volume fraction and primary particle size by adding biodiesel fuels. However, it is not possible to clearly determine which factors are responsible for the reduction from the comparison alone. Moreover, analysis of the discrepancies between numerical and experimental results for diesel and low-blending cases offers an insight for the refinement of soot formation modelling of combustion with large-molecule fuels.

Effects of ethanol and 2, 5-dimethylfuran addition on the morphology and nanostructure evolution of soot in gasoline primary reference fuel-air coflow diffusion flames

The sediment morphology and oxidation activity of soot at different heights above the burner are studied for three fuels. The three fuels are n-heptane/iso-octane mixtures (0% 2, 5-dimethylfuran and 0% ethanol, DE0), n-heptane/isooctane mixture with 15% ethanol (E15) and n-heptane/iso-octane mixture with 15% 2, 5-dimethylfuran (DMF15). Oxygenated fuel increases flame height and decreases flame brightness. DMF15 has the highest flame and has a brighter waist edge. From the field emission scanning electron microscopy (FESEM) images, ethanol and 2–5, dimethylfuran (DMF) make the deposits particle size of soot smaller and uniform. The soot deposits at the height of the flame became more orderly, E15 soot with the smallest fibrous deposits and pore structure. “Donut” and “Steamed bun” structures are found in soot sediments of DE0 and DMF15, which had not been previously reported. The results of thermogravimetric analysis (TGA) show that the oxidation activity of soot in different flame positions is very different. Generally, the activation energy of soot increases with the increase of sampling position, but the activation energy of DMF15 soot decreases first and then increases. Finally, the soot activation energy of E15 and DMF15 is much less than that of DE0, and E15 has the highest oxidation activity. It is inferred that the structure of the special soot deposits is caused by the inhomogeneous oxidation activity on the surface of the deposits, therefore, a free radical erosion sediments model is proposed.

On the Effects of Carbon Dioxide as a Diluent on Precursor Nanoparticles and Soot in Axi-symmetric Laminar Coflow Diffusion Flames

ABSTRACT The impact on the use of carbon dioxide (CO2) as a diluent in the fuel and oxidizer streams on the formation and evolution of precursor nanoparticles and soot in axisymmetric ethylene laminar coflow diffusion flames is experimentally explored using optical diagnostics in this paper. The optical diagnostics employed are based on temporally and spectrally resolved point measurements of laser-induced fluorescence, elastic scattering, and laser-induced incandescence. The measurements provide insight to indicative structure and concentration of precursor nanoparticles, and enable the determination of volume fraction and mean diameter of soot. Previous studies have indicated that the use of CO2 as a diluent compared to nitrogen can reduce soot formation through both chemical and thermal effects. However, the effect of CO2 addition on precursor nanoparticles’ concentrations and structure is not well understood, hence exploring the impact of CO2 on precursor nanoparticles is one of the primary aims of this study. Flames with CO2 as a diluent in the jet only, coflow only, and both streams are identified and investigated. For the flames studied with respect to the replacement of N2 with CO2: i), the soot volume fraction and mean soot diameter is reduced, and ii) precursor nanoparticles concentration is reduced, and the structural transformation is delayed.

Pressure dependence of sooting characteristics of m-xylene and n-octane doped laminar methane diffusion flames from 2 to 10 bar

The work reported here contributes to a wider effort that focuses on the issue of whether single ring aromatics and their same carbon number alkane counterparts have the same pressure dependence of their sooting characteristics. Influence of doping methane base fuel with either m-xylene or n-octane in laminar high-pressure co-flow diffusion flames was studied from 2 to 10 bar pressure in current work. Two liquid hydrocarbons added to the base fuel methane contributed 3% of the total carbon mass flow in the fuel stream, which was kept fixed at 0.524 mg/s at all pressures considered, to have tractable measurements. The methane flames doped with either m-xylene or n-octane were stabilized on a co-flow diffusion burner installed inside of a high pressure combustion chamber with optical access. Temperatures and soot volume fractions were inferred from spectrally-resolved measurements of soot radiation. Line of sight soot emission intensities were inverted at each measurement plane along the flame axis to obtain radially-resolved temperature and soot volume fraction. Soot yields, calculated from soot volume fractions, of m-xylene doped flames were found to be higher than those of n-octane doped flames in the pressure range of 2–10 bar; however, the ratio of soot yields of m-xylene to n-octane decreased with pressure, from 2.9 at 2 bar to 1.2 at 10 bar. This finding implies that m-xylene and n-octane, both with eight carbons in their molecular structure, do not have the same sooting propensity with increasing pressure; m-xylene’s pressure dependence is weaker than that of n-octane within the pressure range of 2–10 bar.

The impact of cyclic fuels on the formation and structure of soot

This paper investigates the impact of cyclic fuels on the nanostructure, nucleation and overall production of soot in an n-heptane (C7H16) laminar coflow diffusion flame. The fuels selected to dope the n-heptane flame are cyclopentene (C5H8), cyclohexene (C6H10) and methylcyclohexane (C7H14). These fuels were chosen for their differences in their structure and sooting tendency. The flame structure was studied with Differential Mobility Spectrometry (DMS) for particle size distribution determination, two-colour ratio pyrometry to calculate the soot volume fraction and soot temperature. The soot nanostructure was investigated using Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). The addition of cyclic fuels was found to promote the formation of soot nanoparticles earlier in flames. In addition, the soot volume fraction was increased significantly by the addition of the cyclic fuels, especially by the addition of cyclopentene. The addition of 20% of cyclopentene increased the soot volume fraction by a factor of 2. HRTEM results suggest a significant influence of cyclopentene on the soot nanostructure; cyclopentene addition promotes the incorporation of five-membered rings (pentagonal rings) leading to highly curved fringes. This suggests cyclopentene could be used as a fuel to promote curvature in different carbonaceous materials to modify their properties.

Numerical investigation into the decoupling effects of hydrogen blending on flame structure and soot formation in a laminar ethylene diffusion flame

Numerical calculations were conducted to explore the various effects of hydrogen blending on flame properties and soot behaviors in an ethylene coflow diffusion flame, based on a fully step-by-step decoupling method, by introducing several virtual species into the gas-phase mechanism. Results show that the concentration of OH increases under the chemical effect of hydrogen due to an enhanced rate of H2 + O ↔ OH + H. The soot yield, primary number density, and average primary number per aggregate decrease under dilution effect while these increase under chemical effect. The enhancements of hydrogen-abstraction-carbon-addition (HACA) rates and polycyclic aromatic hydrocarbon (PAH) condensation rates are responsible for soot mass addition under chemical effect. Both the oxidation rates by O2 and OH are delayed under the chemical effect because of lower concentrations of O2 and OH in the sooting zone. The overall effect of higher surface growth rates and delayed oxidation rates results in an increased soot volume fraction (SVF).

Effects of Ethanol Blending on the Formation of Soot in n-Heptane/Air Coflow Diffusion Flame

Laminar diffusion flame was used to study the effect of ethanol on n-heptane flame in terms of the morphology and microstructure of soot under atomization combustion. For the same carbon mass flux at the outlet of the burner, the ratio of ethanol doping in n-heptane was changed, and the soot was collected from the axial positions of the flame at different heights using the thermophoresis probe method. The results showed that the flame height increased significantly with the increasing ratio of ethanol doping. When the ratio of ethanol and n-heptane (CE/CN) was 1.5, the flame height increased by 10 mm compared with that of pure n-heptane flame. Besides, the temperature in the center of the flame decreased with the increasing ratio of ethanol doping, but the temperature in the low position was higher than that in the pure n-heptane flame, and the temperature in the high position was lower than that in the pure n-heptane flame. However, the flame temperature was the highest when the proportion of ethanol in the mixture was greater than that of n-heptane. The temperature at the flame center decreased with the increasing ratio of ethanol doping, while the temperature at the flame edge increased with the ratio. The primary particle size of soot (soot size hereafter) in all working conditions increased with the increase of flame height, which was in line with the general growth law of soot. Moreover, the soot size at the same height decreased with the increasing ratio of ethanol doping, and this trend was most obvious at the flame height of 20 mm and 30 mm. Compared with pure n-heptane, when CE/CN was 1.5, the soot size at 20 mm and 30 mm decreased by an average of 34.83%, indicating that ethanol could inhibit the surface growth of soot particle. Furthermore, the density of soot particles collected by a single copper net decreased significantly, indicating that ethanol could reduce the production amount of soot.

Soot primary particle sizing in a n-heptane doped methane/air laminar coflow diffusion flame by planar two-color TiRe-LII and TEM image analysis

Soot primary particle size distribution along the centerline of a laminar coflow methane/air diffusion flame doped with vaporized n-heptane at atmospheric pressure was studied using the planar two-color time-resolved laser-induced incandescence (TiRe-LII) technique and analysis of transmission electron microscope images. An improved thermophoretic probe sampling procedure was used to collect samples of soot particles. The LII signals captured at two wavelength bands in the visible are used to determine the soot effective temperature by two-color pyrometry. The methodology was first validated against the literature data obtained in a laminar coflow ethylene/air diffusion flame. The same methodology is then applied to the n-heptane doped methane flame along the flame centerline. The Sauter and geometric mean diameters of soot primary particles were obtained. Good agreement is found between the soot primary particle size distributions obtained by the two techniques.

A Maximum Entropy-Inspired Interpolative Closure for the Prediction of Radiative Heat Transfer in Laminar Co-Flow Diffusion Flames

ABSTRACT An interpolative-based first-order maximum entropy (M1) moment closure for providing approximate solutions to the equation of radiative transfer in non-gray participating media is proposed and described. This newly developed non-gray moment closure technique results in significant computational savings compared to an approach that makes use of the direct numerical solution of the optimization problem for entropy maximization. Its predictive capabilities are also assessed by comparing its solutions to those of the more commonly adopted first-order spherical harmonics (P1) moment closure technique, as well as the popular discrete ordinates method (DOM), which is used as a benchmark for the model comparisons. The evaluations are performed for sooting co-flow laminar diffusion flames for blends of ethanol and methane fuels at atmospheric as well as at elevated pressures and include comparisons to available experimental data for soot volume fraction and flame temperature. The strong spectral dependence exhibited by the absorption coefficient of real gases is treated herein using the statistical narrow-band correlated-k method, and the chemical kinetics of the underlying species are modeled using a reduced mechanism for methane and ethanol fuels. Theoretical details of the proposed interpolative M1, along with a description of the proposed Godunov-type finite-volume scheme developed for the numerical solution of the resulting system of hyperbolic moment equations are briefly discussed. The finite-volume method makes use of limited second-order solution reconstruction, multi-block body-fitted quadrilateral meshes with anisotropic adaptive mesh refinement (AMR), and an efficient Newton–Krylov–Schwarz (NKS) iterative method for solution of the resulting nonlinear algebraic equations arising from the finite-volume discretization procedure. The numerical results for laminar co-flow flames show that the flame solutions and predictions of soot formation of the non-gray M1 maximum-entropy moment closure are very promising and in very good agreement with those of both the DOM and P1 spherical harmonics model, while offering a substantial reduction in the number of dependent solution variables, compared to the DOM.

Effects of Water Vapor Addition on the Flame Structure and Soot Formation in a Laminar Ethanol/air Coflow Flame

ABSTRACT Ethanol is a promising alternative fuel to gasoline engines; however, the problem of particulate matter (PM) emissions still exists when ethanol is applied to gasoline direct injection (GDI) engines, especially under high load. Water injection is proved to be an effective way to reduce the PM emissions and engine knocking, simultaneously. In this work, a numerical calculation was carried out in an ethanol/air coflow diffusion flame to investigate the effects of water vapor addition on the flame structure and soot formation. A complete step-by-step decoupling method was proposed to numerically isolate the dilution, density, transport, thermal, radiative and chemical effects of water vapor by introducing several virtual species. Results show that the temperature distribution of the flame is mainly affected by the dilution effect and thermal effect of water vapor, followed by radiative effect and chemical effect. The mole fraction of OH radical increases because of the enhancement of the reaction H2O + H <=> OH + H2 and O + H2O <=> OH + OH, while the conversion of CO to CO2 is promoted through the reaction CO + OH <=> CO2 + H, due to the chemical effect. Soot volume fraction is also reduced, where the dilution and chemical effects play the most important roles. Hydrogen-abstraction-carbon-addition (HACA) surface growth and OH oxidation are the processes controlling the soot mass addition and consumption, respectively. HACA rate is mainly affected by the dilution and chemical effects. However, chemical effect has little influence on the OH oxidation rate, since the collision probability of OH with soot particles remains almost unchanged as a result of the increase of OH concentration and the decrease of soot volume fraction under the chemical effect.

Influence of water-vapor in oxidizer stream on the sooting behavior for laminar coflow ethylene diffusion flames

The effects of adding water-vapor to the oxidizer stream on soot production in laminar coflow diffusion flames under different oxygen indices (OI) were investigated both experimentally and numerically. A modified coflow Gülder-type burner was employed to produce the laminar ethylene flames for fifteen different conditions of the oxidizer stream from oxygen-deficient (OI 17%) to oxygen-enriched (OI 25%) conditions, and also without and with adding water-vapor into the oxidizer stream up to 10% on mole basis. The measured soot volume fractions were compared with numerical predictions obtained using the CoFlame code and a chemical kinetic mechanism that consists of reaction pathways up to 5-ring PAHs. The sectional soot model used to simulate the soot particles dynamics considers soot nucleation, surface growth, PAH condensation, oxidation, particle coagulation and fragmentation. Fairly good agreement between experimental and numerical results was found, as close as 1% and 5% of difference in the peak soot volume fraction for OI21% with 0% and 10% of water-vapor addition, respectively. Clear trends of increasing the soot volume fraction (peak and also overall) was observed with increasing OI, while a significant reduction was obtained with the addition of the water-vapor. For the cases analyzed, a reduction of the total soot content was up to 60%. The chemical effects were numerically isolated using non-reacting water-vapor and analyzed, contributing especially to the soot oxidation rates. Finally, a study of the main reaction pathways was performed to better understand the chemical effects of water vapor. The results show that water vapor addition affects the concentrations of H and OH radicals and alters the formation and oxidation of soot precursors.

Evolution of the soot particle size distribution along the centreline of an n-heptane/toluene co-flow diffusion flame

A newly developed experimental set-up for studying liquid hydrocarbon combustion in the well-established Yale burner was used to investigate the correlation between fuel composition and its sooting propensity. Soot particle size distributions (PSDs) and flame temperatures along the centreline of an n-heptane/toluene co-flow diffusion flame are reported. The results are compared to soot temperature and volume fraction profiles obtained using colour ratio pyrometry. The addition of toluene (0, 5, 10, and 15 mol%) to heptane moved soot inception to lower heights above the burner (HAB). The earlier inception extended the soot growth zone in the toluene-laden flames, leading to larger soot primary and agglomerate particles. Toluene addition had little influence on the maximum soot number density, indicating that the observed increase in soot volume fraction can mainly be attributed to the increase in particle size. The reported PSDs inside a vapour-fed diffusion flame are the first of their kind and provide a comprehensive dataset for future studies of combustion chemistry and soot particle models.

CARS thermometry in laminar sooting ethylene-air co-flow diffusion flames with nitrogen dilution

Temperature measurements were performed in nonpremixed, sooting jet flames using coherent anti-Stokes Raman scattering (CARS). The jet fluid was ethylene diluted with different concentrations of nitrogen, and the co-flow fluid was dry air. The flames were stabilized on a Yale burner, also known as Smooke/Long burner, and the operating conditions were chosen to match previous experiments on the burner by the Yale group and other research groups. The selected flames have also been identified as target flames by the International Sooting Flame Workshop. CARS measurements of temperature provide insight into the structure of these sooting flames and serve as benchmark data for comparison with computations. A dual-pump vibrational CARS system was used here in order to avoid soot interferences. We were able to obtain high quality CARS spectra in all regions of the flame including the highly sooting regions and near the exit of the fuel nozzle, where the nonresonant modulation of the nitrogen CARS spectrum is strong due to the high ethylene concentrations. The measurements are compared with CFD results in literature and discussed in context of measured soot volume fraction, also, from literature.

Dilution effects of N2 and CO2 on flame structure and reaction characteristics in CH4/O2 flames

The excited radical chemiluminescence spectra of flames contain a variety of combustion characteristic information such as flame structure and temperature field, and are therefore widely used in the flame spectral diagnosis. In this work, the two-dimensional OH* distribution in the methane/oxygen co-flow diffusion flames was obtained by an UV imaging system, and an Abel inversion algorithm was performed as well. Based on the variation of OH* distribution characteristics, the effects of the addition of N2 and CO2 as diluent on the flame reaction zone and structural characteristics were investigated, and the differences in the flame combustion characteristics between the N2-diluted flame and the CO2-diluted flame were further compared. As the diluent concentration increases in the oxidant, the flame becomes elongated. At the same diluent concentration, the CO2-diluted flame has a narrower flame front and the chemical reaction intensity is significantly lower than the N2-diluted flame. The continuous addition of the diluent leads to a transition in the position of flame core reaction zone, which gradually close to the burner exit. The transition is affected by the flow field, and the reaction mechanism is also one of the influencing factors. The dominant reaction to generate OH* gradually changes from R2 (H + O + M → OH* + M) to R1 (CH + O2 → CO + OH*) with the dilution level increases. At a low dilution level, the CO2-diluted flame shows a thinner and longer flame shape. However, with the dilution level increasing, the height and reaction zone thickness in the N2-diluted flame become greater, compared with the flame of CO2 dilution.

Palladium-based catalysts for methane oxidation by co-flow diffusion flame synthesis

Pd supported on mixed ceria-titania oxide catalysts are synthesized with a co-flow diffusion flame, and the catalytic activities for CH4 oxidation are evaluated in a lab-scale fixed bed. The as-prepared catalysts exhibit high catalytic activities and good Pd cluster distribution. Among them, Pd supported on pure CeO2 is found to show the best performance. The results show that it has fairly low T10, and complete oxidation occurs as low as 400 °C. It is deduced that three factors facilitate the reaction between the adsorbed CH4 and O2, namely, solid-solution-like PdxCe1-xO2 structures, highly active sites provided by atomic-level assembly during the flame synthesis, and the presence of lattice oxygen. Additionally, the deactivation of the catalyst may be caused by the decreasing of lattice oxygen, the reduction of PdO and the instability of PdxCe1-xO2. These results may aid in the development of better catalytic nanomaterials for clean methane oxidation.

Effect of oxygen-rich combustion on soot formation in laminar co-flow propane diffusion flames

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. In this work, the effects of oxygen-rich combustion on soot formation in the propane/(O2+N2) laminar flow coaxial jets diffusion flame were numerically investigated by using the detailed gas-phase chemical reaction model with the mechanism of tetracyclic aromatic hydrocarbons and the complex thermodynamic properties and transport characteristics parameters. Soot surface growth follows the hydrogen-abstraction-carbon-addition (HACA) model. A hybrid gas-phase mechanism was adopted, which contains a DLR-based polycyclic aromatic hydrocarbons (PAHs) formation, growth model and a gas-phase model. Results show that the oxygen-rich combustion has a great influence on the flame temperature, especially the high temperature region. With the increase of oxygen concentration, the soot formation region of flame broadens and the maximum of soot volume fraction increase from 3.95 ppm to 10.87 ppm. The extra oxygen makes PAHs increased around the nozzle, leading to larger rate in early soot nucleation and surface growth, eventually more soot yield.

Soot inception in laminar coflow diffusion flames

This paper focuses on the soot inception region in laminar coflow diffusion flames of methane and ethylene stabilised on the Yale diffusion burner. Earlier studies of these flames have focused on the downstream regions where soot has already developed. Laser-induced fluorescence (LIF) and elastic scattering measurements from 266 nm excitation are combined with laser-induced incandescence (LII) excited at 1064 nm. The structure and evolution of the soot precursor particles are characterised using the LIF intensity, decay time and the relative spectral emission in the ultraviolet and visible. The LIF decay times indicate that the majority of 266 nm excited LIF originates from nanostructures rather than gas phase polycyclic aromatic hydrocarbons (PAH). A similarity in the particle evolution for the low and high sooting flames in the upstream regions is found, indicating a general transition towards larger structures with more aromatic features as the nanostructures advect downstream in the fuel rich pyrolytic conditions. Higher nanostructure concentrations are found to precede the higher soot volume fractions (SVF) found in fuel rich sootier flames, although not proportionally, suggesting that surface growth strongly contributes to SVF in the high sooting flames. In the heavier sooting flames the majority of particle formation shifts from the centreline to the wings at the outer edges of the flame closer to stoichiometry. Particle formation in the wings of the flames occurs in the presence of oxygen and higher temperatures, resulting in particles with spectroscopic properties resembling those formed toward the oxidiser side of counter-flow diffusion flames. In the heavier sooting flames, particles produced in the wings appear to mix with particles formed along the centreline of the flame at the flame tip, resulting in a broad range of nanostructures and soot occurring in this region.

Morphological evolution of soot emissions from a laminar co-flow methane diffusion flame with varying oxygen concentrations

The effects of oxygen-enriched atmosphere on the morphological evolution of soot emitted from a methane co-flow laminar diffusion flame were studied using a 12-μm SiC fiber deposition sampling method combined with field emission scanning electron microscopy (FESEM) analysis. The temperature distribution and the soot morphological evolution along the flame radial and axial directions at different oxygen concentrations were systematically investigated. Results showed that the soot morphology was strongly dependent on its position and oxygen concentration in flames. At the same flame height, the soot morphology changed significantly when the oxygen concentration increased from 21% to 31%. The soot generation rate increased rapidly under the elevated oxidative condition, and the position of soot inception was closer to the burner nozzle. As the flame height and oxygen concentration increased, soot particles in the flame centerline were enlarged and evolved to fiber-like depositions. Furthermore, the large clusters of soot particles gradually evolved to even denser spongy and fibrous particles around the flame edges. In addition, unique sponge-like soot deposits were observed for the first time in methane flames.

Improved methodology for performing the inverse Abel transform of flame images for color ratio pyrometry.

A new method is presented for performing the Abel inversion by fitting the line-of-sight projection of a predefined intensity distribution (FLiPPID) to the recorded 2D projections. The aim is to develop a methodology that is less prone to experimental noise when analyzing the projection of axisymmetric objects-in this case, co-flow diffusion flame images for color ratio pyrometry. A regression model is chosen for the light emission intensity distribution of the flame cross section as a function of radial distance from the flame center line. The forward Abel transform of this model function is fitted to the projected light intensity recorded by a color camera. For each of the three color channels, the model function requires three fitting parameters to match the radial intensity profile at each height above the burner. This results in a very smooth Abel inversion with no artifacts such as oscillations or negative values of the light source intensity, as is commonly observed for alternative Abel inversion techniques, such as the basis-set expansion or onion peeling. The advantages of the new FLiPPID method are illustrated by calculating the soot temperature and volume fraction profiles inside a co-flow diffusion flame, both being significantly smoother than those produced by the alternative inversion methods. The developed FLiPPID methodology can be applied to numerous other optical techniques for which smooth inverse Abel transforms are required.