Effects of gas-band radiation on soot kinetics in laminar methane / air diffusion flames

Abstract

A coupled radiation and soot kinetics calculation of laminar methane/air diffusion flame properties is described. Transport equations for mass, momentum, mixture fraction, enthalpy (sensible + chemical) including gas-band radiation, soot mass fraction, and soot number density are solved. A simplified soot kinetics model incorporating nucleation, growth, oxidation, and agglomeration processes is used. The reaction rates in the simplified kinetics model depend on the temperature and the local concentrations of C 2H 2, O 2, and OH. The major gas species and the C 2H 2 and OH concentrations are obtained using state relationships. The local temperature is obtained by solving the energy equation, taking radiation loss and gain from gas species and soot particles into consideration. The radiative source/sink term in the energy equation is obtained using a multiray method in conjunction with the narrow-band algorithm RADCAL. The results of the soot kinetics model are compared with existing laser-induced incandescence (LII) measurements of soot volume fractions. Reasonable comparison can be obtained only with an arbitrary downstream shift of 20 mm in the origin of the predictions from the burner exit. This highlights the need for improved chemical kinetics, but does not affect the following conclusions: 1) the contribution of participating gas (CO 2 and H 2O) radiation dominates that of soot radiation by an order of magnitude in the present methane/air flames, and 2) even for the present weakly radiating flames, the local radiative heat loss/gain strongly influences the soot nucleation, formation, and oxidation rates.