Influence of exhaust gas recirculation on soot formation in diffusion flames

Abstract

A recently developed measuring technique for simultaneous imaging of two-dimensional fields of soot volume fractions, particle number densities, and mean particle radii was used to investigate the influence of exhaust gas recirculation on soot formation in laminar and turbulent co-flowing diffusion flames. These investigations are of special concern with respect to the use of exhaust gas recirculation in internal combustion engines. From direct ignition diesel engines, it is known that exhaust gas recirculation results in a decrease in the soot oxidation rate and, therefore, higher soot emission levels. The exhaust gas recirculation in several laminar and turbulent ethyne/nitrogen diffusion flames was simulated by the variation of the chemical composition of the oxidizer mixture in the co-flow. Soot formation was delayed and a lower maximum soot volume fraction was observed, if the ratio of the oxygen/nitrogen mole fractions in the oxidizer mixture of the co-flow decreased. Nevertheless, toward the flame tip, oxidation of soot particles also decelerated under the aformentioned situation, leading to a higher soot emission level. The addition of carbon dioxide to the oxidizer flow also reduced maximum soot volume fractions but did not affect soot oxidation. Moreover, slight temperature changes in the oxidizer co-flow led to significant changes in the observed profiles.