Experimental Studies of the Micro-Structures of Opposed Flow Hydrocarbon Diffusion Flames: Methane, Ethane, and Propane

An Illustration of an Opposed Flow, Methane Diffusion Flame

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In this work, we report for the first time the micro-structures of atmospheric pressure, opposed flow, hydrocarbon diffusion flames acquired through heated micro-probe sampling followed by direct gas analysis of aromatic and PAH species. Mole fraction profiles of major products as well as trace aromatic, substituted aromatic, and polycyclic aromatic hydrocarbons (PAH up to C16H10, e.g. pyrene) were quantified by direct gas chromatography/mass spectrometry (GC/MS) analysis of samples withdrawn from within the flame without any pre-concentration.

Experimental System for Opposed Flow Diffusion Flames

Some of these new results are shown in the images below for a methane diffusion flame. Mole fraction profiles for a total of 27 chemical species ranging from major to trace aromatic and PAH were determined in the illustrated methane flame images. Mole fractions for these species range from 0.8 to 1.0x10^-7.

PAH Formation in an Opposed Flow Methane Diffusion Flame

As stated, flame structures wre obtained through direct, heated micro-probe sampling, without any pre-concentration. Similar to the sampling techniques developed for premixed flames, direct sampling of diffusion flames replaces pre-concentration sorbents, eliminating the concerns about PAH recovery and sample contamination.

Aromatic and Substituted Aromatic Species formed in
an Opposed Flow Methane Diffusion Flame

Motivation for Studies of PAH Formation within Opposed Flow Flames

Opposed flow flames of hydrocarbons, both in premixed and diffusion systems, have been extensively studied in the past with respect to soot, NOx, and flame extinction. In contrast, the detailed chemical structures of counterflow diffusion flames, especially with regards to the formation of trace aromatic and PAH have received little attention. Early research studies measured species profiles across a methane-air counterflow diffusion flame, but did not measure aromatics and PAH. Computational analysis of opposed flow flames also has been limited with most studies using reduced reaction mechanisms to describe the major features of the flames. However, the latter is due to the lack of detailed flame structure data. Recently, counterflow methane diffusion flames were studied using laser induced fluorescence (LIF). Although these investigators determined the total PAH levels, the concentration profiles of individual PAH were not provided. As stated previously in our laboratory objectives, distinction between individual PAH species is necessary in order to validate detailed chemical kinetic models. Health risk assessments also require the distinction between PAH species, because of the varying degree of toxicity and carcinogenicity of these pollutants.

Opposed Flow Diffusion Flame Research Program and Objectives

At the CRE, hydrocarbon fuels are of current interest in the area of diffusion flames. However, the diffusion flame experimental system at UCLA can be applied to other fuels such as;

The first phase of research at CRE involves the experimental analysis of diffusion flame structures to obtain the quantities of toxic by-products formed at various operating conditions. Currently, we are investigating the effect of fluid dynamic strain, fuel side additives, and oxidizer composition on PAH formation. Subsequently, the second phase involves the assembly of a detailed chemical kinetic model (DCKM) to simulate PAH formation in hydrocarbon diffusion flames. Upon completion, such a flame code will find application in the development of new cleaner combustion technologies and science based environmental regulations.

Chemical Kinetic Modeling Within Opposed Flow, Hydrocarbon Flames

Current theorectical work is focused on a developing a chemical kinetic model that includes chemistry for the production and consumption of aromatics and PAH species. Experimental results are compared to model predictions from a newly developed chemical kinetic mechanism. Results and key reaction pathways that lead to aromatics and PAH are are summarized in our Theoretical Modeling Section.

In conclusion, the GC/MS analysis of gases withdrawn from an opposed jet diffusion flame of methane indicate a rich chemistry with the formation of a large number of aromatics, and PAH that were also seen in fuel rich premixed flames. Detailed kinetic analysis of these flame structures will help to reveal the underlying chemical kinetics of fuel-rich hydrocarbon combustion.

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