Experimental and numerical study of soot formation in counterflow diffusion flames of gasoline surrogate components

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.

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

Retrieving soot volume fraction fields for laminar axisymmetric diffusion flames using convolutional neural networks

Structural effects of C3 oxygenated fuels on soot formation in ethylene coflow diffusion flames

Abstract Soot formation is experimentally and numerically investigated in laminar counterflow diffusion flames burning ethylene and three typical gasoline surrogate components; n-heptane, iso-octane, and toluene. Laser-induced incandescence and a light extinction technique are employed to determine the soot volume fraction within the well-controlled region of the burner. The experiments are performed across a wide range of strain rates and stoichiometric mixture fractions. From the experimental data, sensitivities of soot formation on strain rate and stoichiometric mixture fraction are derived for each fuel. The fuels show significantly different sensitivities. For iso-octane and n-heptane, a higher sensitivity of soot production on the strain rate is observed as compared to ethylene and toluene. Moreover, the sensitivities of soot formation on the strain rate increase with increasing stoichiometric mixture fraction. One-dimensional simulations of the flames investigated experimentally were performed using two different detailed chemical kinetic mechanisms, detailed chemical soot models, and the hybrid method of moments as well as a discrete sectional method to describe soot dynamics. The models are capable of predicting the soot volume fraction of the ethylene flames with remarkable accuracy, whereas for the gasoline surrogate components, the overall soot volume fractions are overpredicted for all tested models. In iso-octane flames, soot nucleation and PAH condensation rates are particularly enhanced. A reaction pathway analysis shows that in ethylene flames, the formation of benzene mostly originates from acetylene, while for iso-octane, large amounts of iso-butenyl form propyne, propargyl, and then benzene.

Control of soot emission raises fundamental issues and has important practical implications requiring a full understanding of soot production and oxidation processes. The research reported in the present paper intends to contribute to the studies carried out within the frame of the International Sooting Flame workshop (ISF) on laminar sooting flames. The objective is to identify and quantify sources of experimental errors and to extend the existing database for the Yale laminar diffusion burner flame. This will especially enable more comprehensive comparisons among different experimental techniques and numerical simulations. To this end, a combined use of Modulated Absorption/Emission (MAE) and Laser Induced Incandescence (LII) techniques is presented in this work. Results are compared with already existing experimental data in terms of soot volume fraction, soot temperature and primary particle size distribution, highlighting the high variability of the experimental data depending on the measurement techniques as well as the underlying assumptions and post-processing methods. These complementary original data may serve to guide the validation of numerical modeling in this configuration.

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M. Roussillo

Papers

Multi-diagnostic soot measurements in a laminar diffusion flame to assess the ISF database consistency