Alexis Coppalle

Influence of Sampling and Storage Protocol on Fractal Morphology of Soot Studied by Transmission Electron Microscopy

The aim of this work was to compare the fractal characteristics, Df and kf, the primary particle diameter, Dpp, the gyration diameter of aggregates, Dg, and the overlap coefficient, Cov, of carbon nanoparticle aggregates produced by an ethylene diffusion flame and sampled by means of four commonly used techniques. The first method involves a thermophoretic piston probe (TPP) which inserts a TEM grid into the flame. Three other methods were applied at the outlet of a dilution device, also inserted in the flame. The first of these used a nuclepore filtration sampler (NFS), and is based on filtration of particles onto a polycarbonate membrane. The second, post dilution method, the insertion particle sampler (IPS), inserts a TEM grid, perpendicular to the aerosol flow. Similar to TPP, the last method is a thermophoretic particle sampler (TPS) sampling directly onto a TEM grid. After collection, the samples are stored in the dark either, (1) in a nitrogen filled cell at low humidity or, (2) in ambient air for studying atmospheric ageing. Good agreement was observed between TPP, TPS, and IPS indicating that the dilution induced for TPS and IPS does not significantly change the morphology of soot. On the other hand, the NFS protocol tended to overestimate the overlap coefficient and the size of primary particles and aggregates. Finally, with regard to the aging effect, we found that kf and Dpp evolve slowly during storage in the atmosphere while Df, was insensitive to the storage conditions. However, the overlap coefficient increased and the gyration diameter decreased as a function of storage duration, while storage under nitrogen tended to reduce these changes.

Automated Determination of Aggregate Primary Particle Size Distribution by TEM Image Analysis: Application to Soot

In many applications, nanoparticles appear to be in an aggregated form. Thus, a complete description of their morphology involves an analysis of their size at different scales, from the aggregate size to the primary particle size. In this study, we present an automated method for the determination of the primary particle size distribution. It is based on an image-processing algorithm operating Euclidian distance mapping leading to a function that contains information about aggregates morphology. This algorithm is first applied to virtual aggregates with point contact between spheres, generated by diffusion limited cluster aggregation code. It shows that obtained functions can be used to retrieve the parameters of the primary particle diameter size distribution. However, it also demonstrates that overlapping or necking effects between primary spheres can have an impact on the shape of the function. The analysis is also performed on real soot images generated by ethylene diffusion flame and civil aviation engine that are different from a physicochemical point of view. This leads to the definition of a calibrated morphological function that can be used for the determination of the parameters of the primary diameter distributions of real soot particles. Finally, the same method is applied to TEM images of soot generated by a commercial soot generator miniCAST. Three operating conditions were investigated showing that generators can be used to produce soot particles with morphologies similar to those produced by jet engines. Copyright 2014 American Association for Aerosol Research

Comparison of Numerical Studies Characterizing Optical Properties of Soot Aggregates for Improved EXSCA Measurements

In order to compare EXSCA measurements with light-scattering calculations, numerical studies characterizing the optical properties of soot aggregates were compared by using different approaches: [1] the Rayleigh-Debye-Gans theory for the fractal aggregate model (RDG-FA), studied by Faeth and Koylu, [2] the rigorous solution model (RS) proposed by Xu, and [3] the discrete dipole approximation model (DDA), developed by Draine and Flatau. The extinction, absorption and scattering cross-sections, C, C and C, and matrix scattering coefficients, |S1|2, |S2|2, |S3|2 and |S4|2, were studied, emphasizing the extinction coefficient C and the scattering coefficient |S1(90°)|2. First, these coefficients for a panel of six aggregates with 64 or 128 primary spheres were compared using the three models. For the absorption and extinction cross-sections, the results are close and RDG-FA may be adequate to determine these parameters. For the total scattering cross-section, the DDA model is close to RS whereas the RDG-FA model shows limitations with strong relative differences. For the scattering coefficients, we focused on |S1|2 and |S2|2, |S3|2 and |S4|2 being negligible. For the 64-sphere aggregates, the relative differences between DDA and RDG-FA are generally great and higher for RDA-FA than for DDA. These deviations are especially significant for backscattering. If, on the contrary, we focus on |S1(90°)|2, all of the models give a good prediction. To complete this study, computation times for DDA and RS are indicated and cross-section distributions for a panel of 28 aggregates obtained using RDG-FA and DDA are presented.

Inversion method and experiment to determine the soot refractive index: application to turbulent diffusion flames

Experimental and numerical studies have been performed to determine the soot refractive index in methane turbulent diffusion flames with two oxidizers: air and oxygen. In the flame zone, soot particles were sampled with a cooled probe. Measurements of optical soot properties have been carried out to obtain extinction and vertical-vertical (90°) scattering coefficients. The size distributions were obtained by electrical mobility analysis. Using these distributions, the optical properties have been computed with the Rayleigh-Debye-Gans theory for fractal aggregates by considering the morphology of soot aggregates and using morphological parameter values based on literature reports for other similar systems. Then, the refractive index has been obtained from a numerical inversion method by matching the measured and computed optical coefficients. This refractive index determination method is new to our knowledge. In turbulent diffusion methane oxygen flames the soot refractive index averaged value found is m = 1.95(±0.13)-0.51i(±0.12), and in the air flame m = 2.10(±0.12)-0.48i(±0.06). In view of the uncertainties, the refractive index is independent of the oxidizer type, the aerodynamic conditions and the flame zone location for the sampling. A sensitivity analysis has been carried out to study the influence of some morphological and experimental parameters on the refractive index value.

First in-flight synchrotron X-ray absorption and photoemission study of carbon soot nanoparticles

Many studies have been conducted on the environmental impacts of combustion generated aerosols. Due to their complex composition and morphology, their chemical reactivity is not well understood and new developments of analysis methods are needed. We report the first demonstration of in-flight X-ray based characterizations of freshly emitted soot particles, which is of paramount importance for understanding the role of one of the main anthropogenic particulate contributors to global climate change. Soot particles, produced by a burner for several air-to-fuel ratios, were injected through an aerodynamic lens, focusing them to a region where they interacted with synchrotron radiation. X-ray photoelectron spectroscopy and carbon K-edge near-edge X-ray absorption spectroscopy were performed and compared to those obtained for supported samples. A good agreement is found between these samples, although slight oxidation is observed for supported samples. Our experiments demonstrate that NEXAFS characterization of supported samples provides relevant information on soot composition, with limited effects of contamination or ageing under ambient storage conditions. The highly surface sensitive XPS experiments of airborne soot indicate that the oxidation is different at the surface as compared to the bulk probed by NEXAFS. We also report changes in soot’s work function obtained at different combustion conditions.

Pool Fire Behavior in a Small and Mechanically Ventilated Compartment

Several fire tests were conducted with heptane and in a chamber of 1 m3, in which the smoke exhaust was done by a fan. The aim was to analyze the rate of mass loss of fuel and the behavior of the flame in the case of under-ventilated fires. For all tests, the oxygen concentration in the flow which feeds the base of the flame decreases continuously. However the mass loss rate increases or decreases depending on the ventilation in the compartment. This can be explained by the respective contribution of the flame and of the smoke layer in the heat transfer toward the liquid fuel. If the ventilation of the room is sufficient, a periodic phenomenon of propagation/expansion of the flame is also observed, with a characteristic frequency that has been determined by an analysis of the flame images and which is close to 1 Hz.

Measurement of the Mass Specific Extinction Coefficient of Acetylene, Toluene and Polymethyl Methacrylate Soot Particles In Visible and Near-Infrared Wavelengths

An experimental set-up has been developed to determine the mass specific σs and the dimensionless extinction Kt coefficients of soot particles at visible (632 nm) and near-infrared (1064 nm) wavelengths. Near-infrared measurements have been carried out with a multiple-path extinction cell to increase the measurement accuracy. The extinction coefficient Kext has been analysed as a function of the mass concentration, measured by a Tapered Element Oscillating Microbalance (TEOM 1105 R&P) in order to retrieve the soot mass specific extinction coefficient, σe, in the smoke plume of acetylene, toluene and PolyMethyl MethAcrylate (PMMA) burning under turbulent, well-ventilated and small-scale conditions. Results for mass specific extinction coefficient σ e are consistent with previous measurements (Mulholland & Croarkin [1] and Newman & Steciak [2]) obtained for a variety of fuels for small-scale fires. The mobility diameter distribution of the soot particles is measured using a Scanning Mobility Particle Sizer (SMPS 3936 TSI). The soot aggregate morphology is determined from Transmission Electronic Microscopy picture analysis. From these measurements, the soot volume fraction is determined and so the dimensionless extinction coefficient. Values at 632 and 1064 nm are given and discussed in comparison to other published data. It is found the scattering contribution to the extinction process is important for the present experimental conditions. For this reason, the use of the dimensionless coefficient is expected to provide a more accurate soot volume fraction value than the calculation which assumes the scattering is negligible.