Flame propagation in planar droplet arrays and interaction phenomena between neighboring droplet streams

Abstract

Well-defined burning droplet systems consisting of three or five parallel monosized droplet streams located in a plane and forming planar droplet arrays were examined. The droplet streams produced with a vibrating orifice droplet generator were directed vertically upward. Sizes were measured in order to characterize the burning droplet arrays and to study the interaction between the droplets. To get qualitative information on the development of droplet heating along the droplet streams, the position of the first rainbow was determined. This rainbow position gives the droplet temperature if the temperature is uniform. During droplet heating with nonuniform temperature qualitative results may be obtained. The droplet streams were ignited by a heating coil. Measurements were performed both with ignition of all streams at the same location above the droplet generator and with ignition of one stream at the edge of the configuration. The latter type of ignition allows investigation of the propagation of the flame across the droplet array due to the flame spread from one droplet stream to its neighboring stream. Because the flame is quasistationary, the velocity of flame propagation is determined from the ignition points in the different droplet streams. The velocity of flame propagation is defined as the velocity of flame spread from droplet stream to droplet stream. The influence of initial droplet temperature, spacing between the droplet streams, droplet size, and volume flux were studied at constant droplet velocity. An increasing velocity of the flame propagation was found for increasing initial droplet temperature. At constant volume flux, no influence of droplet size on the velocity of flame propagation could be detected. However, for increasing volume flux at constant droplet velocity, an increasing velocity of flame propagation was found in most cases of the present experiment.