Combustion of boron powders through bunsen flame

Abstract

I N the present work, an attempt has been made to investigate the combustion of the boron powders passing through a premixed flame which is intended to simulate, as closely as possible, the local conditions which may arise in a real combustor of an air augmented rocket. ContentsBoron powders, having average diameters of 0.9 and 15 /xm, are fed through a conventional Bunsen burner at atmospheric pressure, and the temperature fields and the variation of the burning velocity are measured. The premixed gases, blended with methane, hydrogen, oxygen, and nitrogen, are introduced into the burner. The mixture compositions were arbitrarily selected so as to sustain the premixed flame stabilization. The experiments were carried out for the equivalence ratio $ g in the range from 0.5 to 1.2. Figure 1 shows the temperature distributions across the premixed flame at 6 mm above the burner rim which is at approximately half height of the flame cone. The data show that the addition of the boron powders substantially reduces the flame temperature. In the Macek data quoted in Ref. 1, the ignition delay for the 17.5 and 22 jum particles lies in the range of about 4-6 ms, while in the present experiment the residence time of the particles in the premixed flame is estimated to be about 2 ms. Therefore, the particles may not be able to ignite in the flame. Figure 2 shows the variation of the burning velocity of the premixed flame with ps, density of boron powders in unit volume. Notice that the burning velocity considerably decreases as ps increases, a trend which is appreciable for small particle size. This feature is qualitatively identical with the results of Mitani's flame inhibition theory. 2 It is suggested that the boron powders do not contribute to the gas phase reaction in a premixed flame, acting instead merely as a heat sink.

Effect of Initial Oxide Layer on Ignition and Combustion of Boron Powder

“…Based upon previous work, the influencing factors of boron combustion can be summarized in two aspects, the properties of boron particles and external conditions. The physical and chemical properties of boron particles such as: crystallinity 1, surface morphology 2, and particle size 3 are very important for their ignition and combustion. External conditions such as temperature 4, pressure 5 and reaction atmosphere 6 are also important.…”

Combustion of Boron Particles in Premixed Methane/Air Flames

“…The first one containst he heating of the boron particle and the removalo ft he oxide layer.T he second stage comprises the combustion of the clean boron particle. Due to the high boilingt emperature of boron (between 3931 Ka nd 4139 K), combustionm ostly occurs through surface reactions.Gaponenko et al first proposedacombustion model for dry and water-vapor containing environments [ 4].T he model contains diffusion processes, three reaction processes, and the heat transfer.I shikawa and Matsumoto investigated the combustion behavioro fb oron particlesw ith as ize range between 1-15 mm [ 5].T hey could show that combustion takes place at the particle surface as well as in the gas phase.B ased on their experimental results, they developed af irst combustion modelf or the second combustion stage, includingh eterogeneous reactions, and compared their calculations by varying several parameters such as reaction rates or temperature witht he experiment.One of the most comprehensive efforts towards the understanding of boron particle combustion was made by Maček et al [6,7].E xperiments were conducted using particles with as ize range of 35-125 mm. These particles were ignited at different flame operating parameters (gas temperature, oxygena nd water-vapor concentration, pressure).…”