Soret effects in laminar counterflow spray diffusion flames

The impact of vapour-phase Ludwig-Soret effects on the structure of steady laminar counterflow spray diffusion flames is investigated numerically using complex chemistry and multicomponent molecular transport laws of Chapman-Enskog kinetic theory. These Ludwig-Soret effects, which tend to drive light molecular species towards hot regions and heavy molecular species towards cold regions of the flow, not only influence streamwise diffusion within the planar diffusion flame structure, but also the structure of the fuel vapour diffusion boundary layer near each suspended fuel droplet. Our numerical results provide a quantitative assessment of the impact of Ludwig-Soret effects on the flame structures and on droplet boundary layer structures for typical experimental flames. Computationally, Ludwig-Soret effects are found to vary from being negligible for the experiments considered with nitrogen as the diluent to decreasing the peak temperature by nearly 125 K for the experiment considered with helium as the diluent. In addition, Ludwig-Soret effects do not significantly influence vaporization rates in the zones where the vaporization process is fully developed. Only the droplet boundary layer structure is modified with higher surface fuel vapour concentrations approximately compensating reduced fuel diffusion around the droplet.