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Concentration Measurements of CH and OH Radicals in Laminar Biofuel Flames

An investigation to determine the dominant route of NOx formation in biofuel flames and to confirm the relationship of the NOx increase with iodine numbers in fuels has been presented. This is done through the measurement of the concentration of OH and CH radicals, indicators of the formation of NOx through the Zeldovich and Fenimore mechanisms. A laminar partially pre-mixed flame at an initial fuel equivalence ratio (φ) = 7 was used to minimize the effects of fluid mechanics and isolate the effects of fuel chemistry. Three biofuels, with different iodine numbers, were studied: soy methyl ester (SME), canola methyl ester (CME) and methyl stearate (MS). Planar Laser-Induced Fluorescence (PLIF) images of hydroxyl radicals (OH) and CH radicals were captured with a diagnostic system consisting of a pulsed Nd:YAG laser and an Optical Parametric Oscillator (OPO) with frequency doubler option (FDO) using proper wavelengths. It was found that the population of OH radicals was low in the flames of all fuels, but significant CH radical concentrations were detected in all the flames, with the maximum population occurring in the SME fuel flame. The presence of high concentrations of CH measured in the regions of peak NOx indicate that NOx formation is primarily through the Fenimore mechanism, rather than the thermal mechanism, at this fuel-rich condition. Moreover, the fuel with highest iodine number, SME, produced significantly more NOx because of its tendency to facilitate the production of more soot, C, and CH radicals.