Soot generation within radiating diffusion flames

Abstract

Soot volume fraction data on six fuels [C7H8, C6H10, C8H18, C6H12, C7H16 and (C5H8O2)n] in free, radiating, boundary layer, diffusion flames are presented. Approximate particle concentrations and size distributions, determined with previously reported optical techniques, are used. These sizes, r∼30 nm, indicate the particles are in the free molecular flow regime, Kn∼10, so that convective heat transfer dominates other thermal mechanisms and the particle temperature appears locked closely to the local gas temperature. Based on this coupling, soot radiation is incorporated in the boundary layer gas energy equation. The pertinent fields, i.e., temperature, velocity and species, are evaluated. Comparisons are made with corrected experimental temperatures obtained with thermocouples. Thermophoretic forces are included in particle trajectory determinations. The local soot mass flow rates within the boundary layer are found and net soot volume fraction generation rates, fv, are obtained for these well characterized laminar flames. An Arrhenius form appears to suffice to describe the temperature dependence of the soot growth rate over the temperature range, 1700 <2200 K. The energy parameter is 0 (50 kcal/mole) and the pre-exponential is 0 (1 GHz). Consistent with the goal of fv prediction for fire radiation calculations, analytical temperatures and velocities were used to obtain these empirical expressions. Future work will include mixed flow boundary layer systems where control over the free stream oxygen mass fraction in a combustion tunnel configuration provides greater flame stability and a wider temperature range for each fuel. © 1985 Combustion Institute.