Different Trends of Premixed Flame Propagation in Obstructed Channels with both Ends Open

Premixed flames accelerate extremely fast and may even trigger a detonation when propagating in semi-open obstructed channels (here one end of a channel is closed; the flame is ignited at the closed end and propagates towards the open one). However, industrial pipes often have both ends open, with a flame ignited at one the ends. The present work is devoted to this configuration. Specifically, premixed flame propagation through a comb-shaped array of obstacles, in-built in a two-dimensional (2D) channel of half-width R with both ends open, is studied by solving fully-compressible hydrodynamic and combustion equations with Arrhenius chemical kinetics. The channels of half-widths R/Lf = 12, 24, 36 (Lf is the thermal flame thickness) are considered, with various blockage ratios, α = 1/3, 1/2, 2/3, for each R. It is found that in the relatively narrow channels, R/Lf =12 and 24, the instantaneous total burning rate oscillates with time for all α considered, with the oscillation period and amplitude varying with R and α. In fact, such an oscillating regime resembles that observed previously in unobstructed channels with both ends open. The oscillations can potentially be treated as fluctuations around a quasi-steady solution. In contrast, in a wider channel, R/Lf = 36, the oscillations are terminated soon, followed by flame acceleration, which is weaker (though similar) to that in semi-open channels. The results support the recent experiment, modelling and theory yielding steady or quasi-steady flame propagation prior to the onset of flame acceleration in obstructed pipes with both ends open.