Sooting structure of methane counterflow diffusion flames with preheated reactants and dilution by products of combustion

Abstract

This paper presents the results of an experimental investigation into the sooting structure of methane counterflow diffusion flames where the fuel and the oxidizer streams were preheated and/or diluted by products of combustion such as CO2 and H2O. Low strain rates were employed to spatially resolve the inner structure of these flames and detailed measurements of temperature, species, PAH, soot particle number density and volume fraction were made. Strain rate, methane concentration and fuel flow rate were held constant throughout this work. In these fuel-rich flames a three-color (blue-yellow-orange) structure was observed. Primary combustion reactions were found to occur in the blue zone and measurable size soot particles (>5 nm) were observed only in the orange zone. In the yellow zone, which lies between the blue and the orange zones, blue-green fluorescence was measured but soot particle scattering was found to be negligible. Thus, it seems that large PAHs responsible for fluorescence may also be responsible for yellow emission. Soot inception occurs at the diffuse interface between the yellow and the orange zones whose location depends on the local thermochemical conditions. It was found that an increase in the reactants preheating temperature (while simultaneously reducing the O2 concentration to hold the temperature throughout the reaction zone at or below the original flame temperature) led to an early inception and increased soot volume fraction. However, addition of CO2 and H2O (while holding all other conditions constant) resulted in delayed soot nucleation and a significant reduction in the soot volume fraction. These two observations are consistently explained by the mechanism of OH interference with soot inception. An increase in the CO2 and/or H2O concentrations (brought about either by an increase in the O2 concentration or by direct addition) result in an increase in the OH concentration. This reduces the PAH and C2H2 concentrations and delays soot inception. Thus, thermochemical conditions that favor soot inception substantially increase the ultimate soot loading because they control the residence time available for surface growth.