Flame hole formation

The creation of a steady flame hole was previously carried out by Fiou et al. [36]. In their experiments, a steady-annular premixed edge flame was formed by diluting the inner mixture below the flammability limit, for both methane/air and propane/air mixtures. They found that a stable flame hole was established when the outer mixture composition was near stoichiometry. Their focus, however, was on the premixed flame interaction, rather than on the edge-flame formation, extinction, or propagation. 

Using the coaxial configuration, for a given stretch rate and equivalence ratio in the outer tube, the critical lean and rich limits of the inner-core mixture leading to the onset of the hole formation can be systematically identified. The boundaries between the existence of the flame hole and the possible reignition can also be measured. Furthermore, it would be of interest to explore whether the retreat of the flame edge leads to extinction of the surrounding flame. 

In Test 2, the extent of vertical and horizontal spread of flame is observed with 10-s flame application. Flaming debris may be recorded, together with the duration of flaming and afterglow as well as the extent of any hole formation. If ignition does not occur but a hole is formed, the hole formation is further examined using fresh specimens, with a flame application time of 2 s. 

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