On the Radiative, Diffusion and Chemical Effects of Soot Formation in a Nonsmoking Laminar Ethylene Diffusion Flame

Abstract

ABSTRACT As one of the important by-products of incomplete combustion due to a high local equivalence ratio, soot particles can affect the flame structure and species distributions through their radiative, diffusion, and chemical effects based on the conservation equation of energy. In this work, a fully step-by-step decoupling method was employed to numerically isolate the various effects of soot formation on flame properties and soot evolution in a laminar nonsmoking ethylene diffusion flame. The combustion process was solved by a detailed kinetic model coupled with a sectional soot model. Results show that, the peak temperature of the flame increases when the radiative effect of the particles is neglected; however, it decreases when the chemical effect is excluded due to the absence of particle oxidation. The flame height, which depends on OH concentration, increases under both the radiative and chemical effects of soot particles. When soot radiation is not considered, the peak concentration of CO increases because of a lower conversion of CO to CO2 under higher local temperature, and a further increase is observed when soot reactions are removed, since the formation of CO is the only pathway of carbon conversion under this condition. When the radiation of soot particles is not considered, soot volume fraction (SVF) is reduced owing to a lower hydrogen-abstraction-carbon-addition (HACA) rate caused by a shorter residence time. In addition, soot particles move inside the flame where both SVF and temperature are lower due to their thermophoresis diffusion. The inception and coagulation rates increase without the radiative effect of soot, therefore, the average diameters of primary particles are smaller and the aggregate characteristics are higher.