On the structure of an aluminum dust flame

In the present work, two experimental approaches were carried out involving the combustion of suspensions of fine aluminum particles in an oxidizing gas. First, a microgravity investigation into the quenching distance of aluminum dust flames was performed for two different particle sizes. As part of this investigation, two sizes of iron powder were also tested for comparative purposes. Experiments were carried out in a vertical Pyrex tube in which two sets of parallel plates of a predetermined spacing were inserted. Laminar flames were ignited in the dusty suspension by a burning wire and failure or successful transmission of the flames through the parallel quenching plates was determined by visual observation. The quenching limits were determined as a function of mean particle diameter in the attempt to uncover the dominant rate-determining mechanism: kinetic or diffusive. The second investigation entailed an analysis of the spectrum of light emitted from a stable Bunsen-style flame in a continuous flow of dusty mixture. By analyzing the emitted light at a discrete series of points from the outside of the flame right through to the center, it was possible to extract information regarding the spatial variation of temperature of gaseous emitters as well as solid emitters through the flame zone.The quenching experiments suggested that the combustion of iron particles in the range of 3 to 9 mum might be primarily controlled by diffusion of species to the reaction zone, whereas in aluminum particles in the range of 5 to 15 mum might be kinetically controlled. The spectral analysis revealed an unusual structure within the flame zone in which the temperature of the solid emitters peaks at the beginning of the flame zone and decreases quickly thereafter, whereas the temperature of the gaseous AIO initially increases, and then falls off about half way through the flame. This is consistent with an initially diffusive flame that due to oxygen consumption cools down and transitions to kinetically controlled consumption of the aluminum