Computational and experimental study of steady axisymmetric non-premixed methane counterflow flames

We investigated computationally and experimentally the structure of steady axisymmetric, laminar methane/enriched-air diffusion flames. Experimentally, we imaged simultaneously single-photon OH LIF and two-photon CO LIF, which also yielded the forward reaction rate (RR) of the reaction CO + OH -> CO2 + H. In addition, particle image velocimetry (PIV) was used to measure the velocity in the proximity of the fuel and oxidizer nozzles, providing detailed boundary conditions for the simulations. Computationally, we solved implicitly the steady state equations in a modified vorticity-velocity formulation on a non-staggered, non-uniform grid. We compared the results along the axis of symmetry from the two-dimensional simulations with those from the one-dimensional model, and showed consistency between them. The comparison between the experimental and computational data yielded excellent agreement for all measured quantities. The field of these two-dimensional flames can be roughly partitioned into two regions: the region between the two reactant nozzles, in which viscous and diffusive effects are confined to the mixing layer and to the nozzle walls, where separation occurs; and a radial development region, which is initially confined by recirculation zones near the burner flanges. Buoyancy is virtually irrelevant in the first region at all but the smallest, and practically irrelevant, strain rates. Buoyancy, on the other hand, does play a role in the growth of the recirculation zones, and in determining the flame location in the outermost region.