Behavior of flames propagating through lycopodium dust clouds in a vertical duct

The structure of flame propagating through lycopodium dust clouds has been investigated experimentally. Upward propagating laminar flames in a vertica l duct of 1800 mm height and 150x150 mm square cross-section are observed, and the leading flame front is also visualized using by a high-speed video camera. Although the dust concentration decreases slightly along the height of duct, the leading flame edge propagates upwards at a constant velocity. The maximum upward propagating velocity is 0.50 mis at a dust concentratio n of 170 g/m(3). Behind the upward propagating flame, some downward propaga ting flames are also observed. Despite the employment of nearly equal sized particles and its good dispersability and flowability, the reaction zone i n lycopodium particles cloud shows the double flame structure in which isol ated individual burning particles (0.5-1.0 mm in diameter) and the ball-sha ped flames (2-4 mm in diameter; the combustion time of 4-6 ms) surrounding several particles are included. The ball-shaped flame appears as a faint fl ame in which several luminous spots are distributed, and then it turns into a luminous flame before disappearance. In order to distinguish these ball- shaped flames from others with some exceptions for merged flames, they are defined as independent flames in this study. The flame thickness in a lycop odium dust flame is observed to be 20 mm, about several orders of magnitude higher than that of a premixed gaseous flame. From the microscopic visuali zation, it was found that the flame front propagating through lycopodium pa rticles is discontinuous and not smooth. (C) 2000 Elsevier Science Ltd. All rights reserved.