Burning velocities in fuel-rich aluminum dust clouds

Abstract

A new experimental apparatus that can produce Bunsen-type premixed dust flames stabilized on a water-cooled ring has been contructed. The burning velocities in rich aluminum dust clouds of particle size d 32 =5.4 μ m have been measured by determining the surface area of the inner flame cone and the total volumetric flow rate. It was found that in rich Al−O 2 −N 2 mixtures, the burning velocity is a weak function of dust concentration. At the same time, the dependence of the burning velocity on the oxygen concentration is considerably stronger in rich mixtures than that reported in literature for the fuel-lean aluminum dust clouds. The burning velocity changes from 12–13 cm/s to about 32 cm/s when the oxygen content in the mixture increases from 11% to 30%. The burning velocity was also found to be a strong function of the molecular transport properties of the carrier gas. Thus, replacing the argon in an Al−O 2 −Ar mixture with helium increases the burning velocity from 20 to 65 cm/s. The oxygen limit for the flame propagation (9.5±0.5%) was also found by measuring the minimal oxygen concentration in the dust carrier gas when a premixed flame cone is formed inside the diffusive dust flame. A simple model of the dust flame with molecular heat transfer that has been proposed is based on the assumption that the particle burning rate in the flame front is controlled by the oxygen diffusion. It was shown that the weak dependence of the flame speed on dust concentration is a direct result of the weak dependence of the particle burning rate in the diffusive regime on the flame temperature. The model also predicts dependence of the burning velocity in rich dust mixtures on the Lewis number in accordance with the experimental observations.