Combustion Characteristics of Condensed Phase Reactions in Sub-Centimeter Geometries

Abstract

In this work, the combustion characteristics of the exothermic systems Ti/C, Ni/Al, 3Ni/Al and combinations thereof, are examined in small diameter aluminum tubes (3-6 mm ID). Tailoring the overall exothermicity of the reactant system is accomplished by varying the reaction combination, stoichiometry, and the addition of alumina as a diluent. For the combined reactive system, Ti/C-3Ni/Al, it is shown that at 40 wt.% Ti/C content, the failure diameter lies between 3 and 4 mm. While at a Ti/C content of 30 wt.%, complete combustion front propagation is only observed for tube diameters of 6 mm. For the more exothermic system, Ti/C-Ni/Al, studied at a diameter of 4.8 mm, the addition of low levels of alumina as a diluent is shown to drastically alter the combustion front velocity, resulting in extinction with an increase from 2 to 2.5 wt.% addition. The addition of a thermal barrier (Grafoil TM) for Ti/C-Ni/Al (15/85 wt.%) diluted with 2 wt.% Al 2 O 3 , results in an increase in propagation rate and the range of packing densities that complete propagation is observed; the effects of Grafoil TM on the reactive composition Ti/C-3Ni/Al (35/65 wt.%) are not as pronounced.

Ti/C‐3Ni/Al as a Replacement Time Delay Composition

“…Additionally, direct comparison of microchannels with an effective inner diameter of 4.8 mm (with and without Grafoil) also resulted in little difference being observed in the combustion velocity ( Figure 3B). This is in contrast to our previous results for the Ti/C-Ni/Al-Al 2 O 3 system where the overall propagation rate increased along with the range of consolidated densities where full propagation was observed [19]. Therefore, for these compositions the added benefits of reducing the radial heat loss with Grafoil was not significant enough to observe a change in combustion velocity.…”

“…At low densities, the reacting compact may not transfer enough energy to react subsequent layers, while at high densities the increased thermal conductivity of the compact may conduct heat away from the reaction zone too quickly resulting in self-quenching of the reaction. Initial screening of the Ti/C-3Ni/Al system in 4.8 mm Al microchannels found that the reaction was self-sustaining only at compositions containing in excess of 35 wt.-% Ti/C [19]. This result is specific to the powders used in this study since the overall reactivity is dependent on the particle size, morphology, and intimacy of the reactants.…”

“…As shown in Table 2, compositions varying from 30-40 wt.-% Ti/C fell within this range. Initial screening of these compositions resulted in the slowest combustion velocities without failure at the diameters of interest [19]. In reality, the adiabatic combustion temperatures will never be reached due to the very small scale of the experiments.…”

“…5 W m À1 K À1 radially [24]) 0.25 mm thermal barrier (Grafoil) is illustrated in Figure 1A. Previously, such a barrier was shown to have several beneficial effects on the overall stability and combustion front velocity for the Ti/C-Ni/Al-Al 2 O 3 system [19]. It should be noted that when introducing the barrier, the inner diameter of the microchannel is effectively reduced (i.e., 6.0 mm becomes 5.5 mm) and the quantity of the reactive composition is also reduced.…”

“…The objective of this work was to extend our preliminary efforts on two condensed phase reactions, Ti/C and 3Ni/Al, as potential time delay compositions [19]. Specifically, failure diameter and combustion wave velocity were investigated by varying the overall mixture exothermicity with combinations of the highly exothermic Ti/C reaction and the less exothermic 3Ni/Al reaction.…”