Formation of ultra-lean comet-like flame in swirling hydrogen-air flow

In this study, a hydrogen-air premixed flame in a partially tapered swirl burner in which a stable counterflow of unburned and burned gases is expected to be formed, was investigated. The experimental results indicate the formation of almost steady flames at equivalence ratios of as lean as 0.084, and the resulting ultra-lean flames in the swirling flow had a comet shape. Furthermore, the flame was numerically reproduced, and the mechanisms behind the phenomenon were identified by checking the balance among the chemical enthalpy through diffusion, heat flux by conduction, and transport of these parameters by convection. It was determined that the region around the tip of the flame head was almost dominated only by diffusion and heat conduction similar to a flame ball, but its formation mechanism was found to be essentially different from that of a flame ball because the comet-like flame can be numerically reproduced even without a radiative heat loss, in contrast to a flame ball.