Studies of aromatic hydrocarbon formation mechanisms in flames: Progress towards closing the fuel gap

This paper provides an overview of a workshop focused on fundamental experimental and theoretical aspects of soot measurements by laser-induced incandescence (LII). This workshop was held in Duisburg, Germany in September 2005. The goal of the workshop was to review the current understanding of the technique and identify gaps in this understanding associated with experimental implementation, model descriptions, and signal interpretation. The results of this workshop suggest that uncertainties in the understanding of this technique are sufficient to lead to large variability among model predictions from different LII models, among measurements using different experimental approaches, and between modeled and measured signals, even under well-defined conditions. This article summarizes the content and conclusions of the workshop, discusses controversial topics and areas of disagreement identified during the workshop, and highlights recent important references related to these topics. It clearly demonstrates that despite the widespread application of LII for soot-concentration and particle-size measurements there is still a significant lack in fundamental understanding for many of the underlying physical processes.

Laser-induced incandescence : recent trends and current questions

Soot particle emission from combustion processes is a significant environmental and health problem. Optical detection is clearly the only viable choice for in situ PAH and soot measurement in turbulent flames. This paper reviews the considerable progress made in optical diagnostics of soot and its precursors formed this last decade. Advances include the spectroscopy of polycyclic aromatic hydrocarbons (PAHs) and optical diagnostics for PAHs in flames, new approaches to identify the structure and composition of incipient soot, and the latest refinements for soot particle detection and quantification. Improvements of selective and quantitative PAH measurements have been made possible with supersonic expansion of flame extracted samples, coupled with laser induced fluorescence (jet-cooled LIF) and mass-spectroscopy combined with resonance enhanced or synchrotron selective photoionisation. More accurate description of all facets of soot particles properties (optical properties, morphology, and volume fraction) has guided recent research. In particular, optical exploration has led to new insight into soot inception, including the identification of particular soot precursors from large PAHs to nanoparticles passing through stacked PAHs, polymeric-like structures and aliphatic-linked PAHs. Despite the various improvements, the ultimate accuracy of the soot diagnostic methods such as laser induced incandescence, light extinction and elastic light scattering relies on our knowledge of the soot optical properties which may vary as a function of soot age in flames. Nonetheless, with well designed experiments, comparative measurement of sooting trends within or between flames can be acquired with accuracies needed for the validation of numerical simulations of soot formation. These optical methods have been successfully adopted to classify the sooting tendencies of a large number of hydrocarbons and to examine the effect of pressure on soot formation. The ultimate objective is certainly to use optical diagnostics to provide useful data for soot formation modelling in turbulent flames and under practical combustion.

Study of the formation of soot and its precursors in flames using optical diagnostics

In-cylinder CO and UHC imaging in a light-duty diesel engine during PPCI low-temperature combustion.

Calculations of intermolecular potentials are presented for homo-molecular and hetero-molecular clusters of 24 peri-condensed PAH spanning monomer masses ranging from 78 to 1830 Da. Binding energie...

Intermolecular Potential Calculations for Polynuclear Aromatic Hydrocarbon Clusters

Aspects of soot dynamics as revealed by measurements of broadband fluorescence and flame luminosity in flickering diffusion flames

One-dimensional fluorescence imaging measurements of CO concentrations have been made in steady and flickering axisymmetric, methane/air diffusion flames burning at atmospheric pressure. These experiments extend fluorescence detection of CO to flames that contain significant soot volume fractions, approximately 1–2×10 −6 . Our aim is to quantify changes in the CO levels that occur for flickering conditions, where increased soot production and subsequent oxidation may have important effects. The Q branch (0,0) band of the B 1 Σ + ←←X 1 Σ + transition was excited near 230 nm in a two-photon process, and the B 1 Σ + →A 1 II (0,1) band fluorescence was detected at 483.5mm. Quenching-independent data were obtained, and interferences from broadband molecular fluorescence and soot incandescence were accounted for by subtracting profiles measured for excitation at a nearby, nonresonant wavelength. Maximum CO concentrations are found to be approximately equal in the steady and flickering flames burning with the same fuel flow rate. For the flickering flames, the greater radial extent of the burning flamelet following clip-off yields approximately 50–65% larger volume-integrated CO levels. Overall, this increase in CO production is modest compared to the factor of 4 enhancement observed in the time-averaged, volume-integrated, soot volume fraction, indicating that soot oxidation does not appear to appreciably impact CO levels in these methane flames.

Quantitative two-photon laser-induced fluorescence imaging of CO in flickering CH4/air diffusion flames

The recent halt in the production of CF 3 Br because of its deleterious effect on stratospheric ozone levels has intensified the search for new suppressants with comparable properties. In contrast to most proposed alternatives, earlier investigations of CF 3 I have reported excellent extinction efficiencies, sometimes superior to CF 3 Br on a molar basis. These findings have spurred an interest in elucidating more clearly the chemical effects produced by iodine-containing suppressants. In the present study, OH· and soot concentrations have been measured using fluorescence imaging and laser-induced incandescence methods, respectively, in a co-flowing, axisymmetric, atmospheric-pressure propane/air diffusion flame inhibited by CF 3 Br and CF 3 I. In addition, broadband molecular fluorescence (attributed to polycyclic aromatic hydrocarbons) has been monitored, and peak temperatures have been measured using two-line OH· laser-induced fluorescence. Overall, the two suppressants behave similarly when added to both the air and fuel streams, with the most notable exception being the greater enhancement of soot production for CF 3 Br addition at subextinction concentrations. CF 3 I is found to be slightly superior to CF 3 Br under our experimental conditions in terms of (I) requiring smaller mole fraction concentrations at extinction and (2) producing less within-flame soot. The reductions in the OH· concentrations with agent additions are essentially the same for CF 3 I and CF 3 Br.

David A. Everest

Papers

Aspects of soot dynamics as revealed by measurements of broadband fluorescence and flame luminosity in flickering diffusion flames

Quantitative two-photon laser-induced fluorescence imaging of CO in flickering CH4/air diffusion flames

The effect of CF3I compared to CF3Br on OH· and soot concentrations in CO-flowing propane/air diffusion flames