Experimental investigation of flame propagation in long, narrow, and open tubes

An experimental investigation of premixed propane-air flames propagating in long, narrow tubes open at both ends was performed in order to establish flame phenomenology and make comparisons to theoretical predictions. Fuel-air equivalence ratios of 0.8, 1.0, 1.2, and 1.4 were studied, and the effect of tube length was investigated using lengths of 59.1 and 104 cm. Quantitative comparisons to theoretical studies provided support to the hypothesis that a pressure gradient is established in the tube because of the Poiseuille flow induced in the gas by the flame, leading to flow away from the flame in both directions. Several regimes of propagation were established, including initial smooth propagation at constant speed, regions of high-amplitude oscillation accompanied by flame acceleration, and regions of small-amplitude oscillation during which the mean propagation speed was nearly uniform. The most intense flame oscillations and consequent acceleration occurred in mixtures with equivalence ratios of 1.0 and 1.2. This oscillatory behavior is hypothesized to be the result of an acoustic pressure field set up in the gas by the outflow of gases at both open ends of the tube. These standing pressure waves excited flame oscillations and ultimately drove the flames to instability. The instabilities were particularly intense in near-stoichiometric rich flames, for which Le<1 and the flame was diffusionally unstable. Lean propane flames (Le>1) are diffusionally stable, and this tends to mitigate flame oscillations but does not always suppress them. The general conclusion is that smooth, laminar flame propagation in long, narrow, open tubes will almost never occur in practice, due to the inherent destabilizing effect of the acoustic pressure field set up by the flame.