A comparison of the structures of lean and rich axisymmetric laminar Bunsen flames: application of local rectangular refinement solution-adaptive gridding

Axisymmetric laminar methane-air Bunsen flames are computed for two equival ence ratios: lean (Phi = 0.776), in which the traditional Bunsen cone forms above the burner; and rich (Phi = 1.243), in which the premixed Bunsen con e is accompanied by a diffusion flame halo located further downstream. Beca use the extremely large gradients at premised flame fronts greatly exceed t hose in diffusion flames, their resolution requires a more sophisticated ad aptive numerical method than those ordinarily applied to diffusion flames. The local rectangular refinement (LRR) solution-adaptive gridding method pr oduces robust unstructured rectangular grids, utilizes multiple-scale finit e-difference discretizations, and incorporates Newton's method to solve ell iptic partial differential equation systems simultaneously. The LRR method is applied to the vorticity-velocity formulation of the fully elliptic gove rning equations, in conjunction with detailed chemistry, multicomponent tra nsport and an optically-thin radiation model. The computed lean flame is li fted above the burner, and this liftoff is verified experimentally For both lean and rich flames, grid spacing greatly influences the Bunsen cone's po sition, which only stabilizes with adequate refinement. In the rich configu ration, the oxygen-free region above the Bunsen cone inhibits the complete decay of CH4, thus indirectly initiating the diffusion flame halo where CO oxidizes to CO2. In general, the results computed by the LRR method agree q uite well with those obtained on equivalently refined conventional grids, y et the former require less than half the computational resources.