Thermal diffusion effects in hydrogen-air and methane-air flames

Publisher Website
Google Scholar
Abstract
The influence of thermal diffusion on the structure of hydrogen-air and methane-air flames is investigated numerically using complex chemistry and detailed transport models. All the transport coefficients in the mixture, including thermal diffusion coefficients, are evaluated using new algorithms which provide, at moderate computational costs, accurate approximations derived rigorously from the kinetic theory of gases. Our numerical results show that thermal diffusion is important for an accurate prediction of flame structure. Combustion models used in the study of pollutant formation, ignition processes and chemically controlled extinction limits combine complex chemical kinetics with detailed transport phenomena. One such transport phenomenon, often neglected in numerical flame models, is thermal diffusion which gives rise to the Soret and Dufour effects. In the governing equations of multicomponent reactive flows, both effects are expressed in terms of the same transport coefficients, namely the thermal diffusion coefficients. This originates from the self-adjointness of the linearized Boltzmann collision operator or also from the reciprocal relations in Onsager's theory. The Soret effect corresponds to an additional term in the species diffusion velocities which is proportional to the temperature gradient. On the other hand, the Dufour effect is accounted for by an additional term in the heat flux vector. From a qualitative viewpoint, it is well known that the Soret effect tends to drive light molecules towards hot regions and heavy molecules towards cold regions of the flow. It is thus particularly important in the presence of strong temperature gradients such as those found in flame fronts and chemical reactors where the wall temperature differs significantly from that of the surrounding gases. In particular, hydrogen, atomic or molecular, is a species which is likely to be strongly influenced by thermal diffusion. The influence of thermal diffusion on flame structure was first studied numerically for laminar one-dimensional hydrogen-nitrogen-oxygen flames (1-3). In (1), it was found that the laminar flame speed was slightly lower when thermal diffusion was taken into account. This result was obtained for both lean and rich flames, but thermal diffusion was modelled using semi-empirical expressions and only for the diffusion flux of atomic and molecular hydrogen. For rich flames, discrepancies in the thermal diffusion coefficients with respect to the values predicted by the kinetic theory were observed (1). On the other hand, it was