EXPERIMENTAL INVESTIGATION ON QUENCHING DISTANCE FOR ALUMINUM DUST FLAMES

摘要

An experimental investigation on quenching distance for Al dust flames have been done in improved flow system which can yield stable, controlled, and uniform dust mixtures. Experiments were performed with 18 micron aluminum particles diameter. Dust dispersion technique uses an annular high-speed jet which disperses dust continuously supplied via a piston-type dust feeding system. Laminarized dust flow ascending in a vertical Pyrex tube (d=4.6cm, L= 150cm) was ignited at the open tube end. Constant pressure flames propagating downwards were observed. A set of thin, evenly spaced steel plates was installed in the upper third part of the tube in order to determine the flame quenching distance. Three different stages of flame propagation were observed: laminar, oscillating (transition region), and turbulent accelerating flames. Quenching distance as a function of dust concentration were determined during the laminar stage of flame propagation in dust-21% Oxygen-79% Nitrogen, dust-30% Oxygen-70% Nitrogen, and in dust-21% Oxygen-79% Argon mixtures. Furthermore, this research studies the effects of bed-gas on quenching distance and lean limit of combustion.It was found that the minimum quenching distance decreases when concentration of oxygen increases in the mixture. The minimum quenching distance is found to be about 4mm in air and decreases to 2mm in mixture of 30% O_2. Also, it was found that the amount of lean limit of combustion decreases with increasing of oxygen percentage in mixture. The substitution of Argon for Nitrogen in air decreases the minimum quenching distance from about 4 to 3mm, and the amount of lean limit of combustion increases. The results is discussed with a mechanism of diffusive oxygen transport to the surface of burning Al particles in which a higher rate of oxygen transport in the N_2/O_2 , as compared to the Ar/O_2 gas mixtures.