New insights into strobe reactions: An intriguing oscillatory combustion phenomenon

Abstract

Strobes are self-sustained oscillatory combustions that have various applications in the fireworks industry and also in the military area (signaling, missile decoys and crowd control). However, most of the strobe compositions were discovered using trial and error methods. The fundamentals mechanisms involved in strobe remain unclear. A few oscillatory chemical systems were reviewed in in this thesis and compared to strobe reactions. First, the oscillatory behavior observed in some of the systems (BZ type reactions and cool flames) was explained by the presence of intermediate species that are created and consumed by two chemical pathways. They compete with each other and it results in an oscillation of the concentration of the intermediate species. Then, to explain the oscillatory behavior observed in some SHS (Self-propagating High-temperature Synthesis) combustion, the assumption made was that if the reaction time is much larger than the characteristic time of heat transfer. Those two features were considered in the analysis of strobe reaction. The classical ternary strobe composition made of ammonium perchlorate / magnalium / barium sulfate was studied in this thesis. The following physical mechanisms were observed. After ignition of the composition, there is the formation of a layer on top of the pellet. This layer is heated up by a reaction and begins to melt. When the flash is imminent, parts of the surface layer are ejected indicating the formation and rapid expansion of gas below the surface. A bright flash follows after which the process is repeated. The pellet is consumed almost linearly, layer by layer. Some of the chemical mechanisms of this strobe were identified. The decomposition of ammonium perchlorate present in the layer ignited produces the heat and the oxygen molecules necessary for the strobe reaction to proceed. Then the magnalium begin to melt and decomposes into aluminum and magnesium. Finally, the oxidation of liquid and gaseous aluminum and magnesium were observed. The same reactions occur in the next layer. Parameters that influence the strobe effect were emphasized. First, the effect of potassium dichromate was studied. A small amount of this compound in the compositions has an important impact on the regularity and sharpness of flashes. It was hypothesized that the decomposition of this compound during the dark phase absorbs part the energy released, restraining the increasing reaction rate. Then, the influence of the melting point of the metal sulfates or nitrates was studied. It appears that the compositions containing high melting point metal sulfates (barium and strontium sulfates) produce sharper and more regular flashes. Those compositions also have a slower frequency. Finally, the influence of the heat transfer of the composition on the strobe behavior by varying the fuel weight fraction and its particle size. It appears that compositions with a lower heat transfer in the composition results in a lower flash frequency. Besides, it was noticed that none of those parameters seemed to have a strong influence on the temperature of the compositions while burning.