Effect of Lewis number on ball-like lean limit flames

The lean limit flames for three different fuel compositions premixed with air, representing three different mixture Lewis numbers, stabilized inside a tube in a downward flow are examined by experiments and numerical simulations. The CH* chemiluminescence distribution in CH4-air and CH4-H2-air flames and the OH* chemiluminescence distribution in H2-air flames are recorded in the experiments. Cell-like flames are observed for the CH4-air mixture for all tested equivalence ratios. However, for CH4-H2-air and H2-air flames, ball-like lean limit flames are observed. Flame temperature fields are measured using Rayleigh scattering. The experimentally observed lean limit flames are predicted qualitatively by numerical simulation with the mixture-averaged transport model and skeletal mechanism of CH4. The results of the simulations show that the entire lean limit flames of CH4-H2-air and H2-air mixtures are located inside a recirculation zone. However, for the lean limit CH4-air flame, only the leading edge is located inside the recirculation zone. A flame structure with negative flame displacement speed is observed for the leading edges of the predicted lean limit flames with all three different fuel compositions. As compared with 1D planar flames, the fuel transport caused by convection is less significant in the present 2D lean limit flames for the three different fuel compositions. For the trailing edges of the three predicted lean limit flames, a diffusion dominated flame structure is observed.