Flame propagation in a vortex flow within small-diameter tubes

Abstract

A flame propagation in a small-diameter tube has been experimentally investigated with use of small-diameter tubes and swirl injectors for methane and propane/air mixtures. Results show that, in a 3.6 mm tube, a flame can propagate rapidly along the tube axis due to vortex bursting as observed in larger diameter tubes. The propagation range increases in equivalence ratio with increasing the air flow rate, whereas the range decreases with decreasing in the air flow rate, and further decreasing the air flow rate leads to a fail of propagation. As the swirl intensity is increased, the flame propagation range in the equivalence ratio and in the air flow rate is widened. The propagation range in equivalence ratio is shifted on the relatively fuel-lean side, between 0.75 and 1.1 for methane, whereas on the very fuel rich side between 1.1 and 2.2 for propane. These ranges are much narrower than those obtained in a 31-mm diameter tube. Thus, the Lewis number of a deficient species and the heat loss to the cold wall strongly affect the occurrence of vortex bursting in a small-diameter tube. Further experiments have shown that the flame speed increases with the maximum tangential velocity of the vortex flow, but the flame speeds are at highest one-third the maximum tangential velocity for rich propane/air mixtures because of small flame/core diameter ratio. The flame speed is also lowered down near the stoichiometric methane mixture, notwithstanding the large flame/core diameter ratio. This retardation of the flame speed seems to be resulted from an increase in the burned gas density due to cooling by the cold wall.