Propagation and extinction behavior of methane/air premixed flames through straight and converging-diverging microchannels

Propagation and extinction behavior of a CH₄/air premixed flame passing through straight and converging-diverging (C-D) microchannels (diameters ranging from 1 to 10 mm) were investigated both experimentally and numerically. The dynamic behavior of flame propagation inside the channels was experimentally studied by using CH^∗ chemiluminescence and direct imaging. Three patterns of flame propagation were observed, i.e., the flame can survive, partially extinguish and then re-ignite downstream, or completely extinguish. Regime diagrams showing these different patterns as functions of channel geometry and equivalence ratio were generated for both the straight and C-D channels. Numerical simulations were carried out to explain the experimentally observed flame dynamics inside the channels using detailed CH₄/air chemistry. In general, flames were easier to extinguish in C-D channels than in straight channels for a fixed channel diameter and equivalence ratio. Additionally, flames were harder to survive in C-D channels with larger exit-to-throat area ratios. Both heat loss and flame stretch were key factors that can generally cause flame extinction in narrow C-D channels. Heat loss was found to be the primary reason for flame extinction inside the microchannel in comparison with the stretch effect.