Quenching distance of laminar flame in aluminum dust clouds

Abstract

Quenching distances for aluminum dust flames have been measured in an improved flow system which can yield stable, controlled, uniform dust mixtures. Experiments were performed with fine atomized aluminum dust ( d 32 = 5.4 μm). The 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 = 0.05 m, L = 1.2 m) 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, and turbulent accelerating flames. Flame speed and quenching distance as a function of dust concentration were determined during the laminar stage of flame propagation in dust-oxygen-nitrogen and in dust-oxygen-helium mixtures. It was found that the quenching distance and flame speed are very weak functions of dust concentration in rich mixtures. The minimum quenching distance is found to be about 5 mm in air and increases to 15 mm in mixtures of 11% O 2. The substitution of nitrogen for helium in air increases the minimum quenching distance from 5 to 7 mm. A simple analytical model of a quasi one-dimensional dust flame with heat losses was developed with an assumption that the particle burning rate in the flame front is controlled by the process of oxygen diffusion. Algebraic equations defining flame speed were obtained in two limiting cases: lean and rich mixtures. The model predicts wide plateaus in the flame speed and quenching distance versus dust concentration plots in rich mixtures. These plateaus were observed experimentally. Calculated values of minimum quenching distances are in good agreement with experimental data.