Expansion Behavior and Temperature Mapping of Thermites in Burn Tubes as a Function of Fill Length

Densmore, John M.; Sullivan, Kyle T.; Gash, Alexander E.; Kuntz, Joshua D.

doi:10.1002/prep.201400024

Cited by 22 publications

(16 citation statements)

References 21 publications

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“…From experimental measurements of pressure rise times and flame speeds, the reaction front thickness in a burn tube has been calculated to be 10 mm thick, which lines up reasonably well with values of 10-40 mm measured from temperature rises observed in pyrometry experiments [5,14,18]. From this, we estimate a thermal gradient of 2.7 Â 10 5 K/m based on a temperature drop from 3000 K to 300 K over this distance.…”

Section: Conductionmentioning

confidence: 78%

“…As diatomic gas molecules have higher heat capacities, this point is worth considering. As these species are non-equilibrium, we use results of Al/CuO burn tube experiments that have observed pressures of $1900 PSI (130 atm) and temperatures of $3000 K [9,14]. Therefore based on our assumed tube diameter, a 1 mm section at those conditions will contain 4.2 Â 10 À6 mol of gas based on the ideal gas law.…”

Section: Convection Of Gasesmentioning

confidence: 99%

“…To make this process as simple as possible, all the calculation parameters are derived from an Al/CuO burn tube experiment, which involves loosely packed material in a large length/diameter ratio tube [5,9,10,14]. When ignited, the pressure and/or luminous front can be observed as it propagates forward through unreacted material.…”

Section: Calculation Parametersmentioning

confidence: 99%

See 2 more Smart Citations

Commentary on the heat transfer mechanisms controlling propagation in nanothermites

Egan

Zachariah

2015

Combustion and Flame

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a b s t r a c tThe combustion of nanothermites is a complex multiphase process that is still not well understood. One important aspect that is in need of further examination is the heat transfer mechanisms that drive combustion. Here we present some simple calculations to critically analyze the viability of conduction, convection, and radiation mechanisms of heat transfer as it relates to reaction propagation in nanothermites. While convection has generally been accepted as the critical mechanism for heat transfer, we show that the movement of hot gases cannot account for the required energy flow. Instead, it is illustrated that the movement of condensed phase material plays the critical role in heat transfer and should be accounted for in future models.Published by Elsevier Inc. on behalf of The Combustion Institute.

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Section: Conductionmentioning

confidence: 78%

Section: Convection Of Gasesmentioning

confidence: 99%

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Commentary on the heat transfer mechanisms controlling propagation in nanothermites

Egan

Zachariah

2015

Combustion and Flame

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“…10 %) has occurred when the luminous front reaches the end of the tube. In ar ecent work, we showed that even as mall amount of mass packed into one end of the tube, with the remainder of the tube left unfilled, could yield luminous propagation velocities faster than that observed in af ully packed tube [26].T he open question, then, is what does the luminous front signify when it is propagating through am aterial?D oes it indicate that material behind the luminous front has fully reacted, or is it possible that reacting material can flow forward through the void space at high velocity even though only af raction of the total energy may have been released?…”

mentioning

confidence: 78%

Quantifying Dynamic Processes in Reactive Materials: An Extended Burn Tube Test

Sullivan

Cervantes

Densmore

et al. 2015

Propellants Explo Pyrotec

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[a] 1I ntroductionReactive materials (RM) are as ubset of energetic materials, and include intermetallic or metal/metal oxide (thermite) composites. Although these systems are the most common, the term could be further extended to encompass other composite materials;w hich gives rise to ar apid burst of energy in the form of ap ressure, or thermal pulse. This energy release, combined with the high mass and volumetric energy densities of such materials, makes them attractive candidates for several applications demanding ar apid, and tunable, energy release profile.It has been known for some time that when one reduces the particle size in RM, the apparent reactivity can increase significantly [1].T his has largely been attributed to reduced transport distances between the constituents. However, there are other known features of small particles;w hich could also be af actor;s uch as reduced ignition temperature, activation energy,m elting point, etc. brought forth by the increasing surface to volume ratio. Another property of small, metal particles is that the thin, native oxide shell represents ar elatively larger fraction of the total mass. Several authors have suggested that the reaction mechanism for fine Al particles occurs via diffusional transport of species across this oxide shell [2][3][4][5],w hile others have suggested the large increase in reactivity warrants the need for an ew theory,a nd have speculated that the core-shell interaction under rapid heating can activate am elt dispersion mechanism [6,7].R ecent work has demonstrated that rapid condensed-phase reactions can occur early in the reaction, but it remains unclear to what extent the reaction has completed during this step [8,9].Q uantitative measurements, such as the material burn time, can greatly help improve our fundamental understanding of reaction pathways and mechanisms.Abstract:Acommon method for measuring the reactivity of rapidly deflagrating materials has been to loosely pack ad esired mixture into an approx. 10 cm long 3 mm diameter acrylic tube, ignite the material on one end, and report the observed self-propagating flame velocity through the material. While this method can yield qualitative information, linking this to quantitative intrinsic properties, such as particle burn time, has remained challenging. In this work, we significantly redesign the traditional burn tube experiment. Between 25 and 250 mg of nanocomposite aluminum/copper oxide (Al/CuO) thermite is loosely packed into the capped end of a1 .8 ml ong tube, and the remainder of the tube is left unfilled. The material is ignited using ah ot wire, resulting in as teadily-propagating luminous front, which extends part, or all, of the way down the length of the tube. We suggest the behavior is ar esult of "reactive entrainment", which occurs when the reaction time scale becomes longer than the characteristic momentum relaxation time scale. When this criterion is met, and when there are significant pressure gradients present or produced during the reaction, material will be ...

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“…The high speeds associated with nanothermite formulations are attributed to convective burning. A recent study has highlighted the issues with reactive wave speed measurements following the luminous zone in nanothermite formulations (Densmore et al, 2014), which found that the luminous front propagation could be affected strongly by dispersion of the reactive material and does not necessarily correspond with a propagating reactive wave. However, the structural integrity of PS can be expected to prevent this sort of erroneous measurement, as evident from Figure 8.…”

Section: Downloaded By [University Of Western Ontario] At 00:05 09 Fementioning

confidence: 99%

Reactive Wave Propagation Mechanisms in Energetic Porous Silicon Composites

Parimi

Tadigadapa

Yetter

2014

Combustion Science and Technology

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Propagating reactive waves through porous silicon (PS)-sodium perchlorate composites and the generation of shock waves in the gaseous medium above the PS surface were studied using high-speed shadowgraphy. Propagation speeds were varied by changing the PS specific surface area (SSA) and the dopant type and level, and by the addition of organized microstructures along the wave propagation direction. Shadowgraph analysis showed that upstream permeation of hot gaseous combustion products was responsible for a two order of magnitude enhancement in the reactive wave propagation speeds obtained by the presence of organized microscale patterns on PS samples with low SSA (∼ 300 m 2 /g), which nominally exhibit baseline speeds of ∼ 1 m/s. Shadowgraph analysis and sound speed measurements on PS samples with high SSA (∼ 700 m 2 /g), which exhibit fast reactive wave propagations of ∼ 1000 m/s, indicated that neither the strong shock over the PS surface nor detonation of the porous layer were the mechanisms by which the wave propagated. Thermal analysis of PS showed that the heat release from exothermic reactions between PS and the oxidizer within the pores shifted to lower temperatures as the SSA of PS increased, which was accompanied by a reduction in the activation energy associated with the lowest temperature exothermic reaction between PS and the oxidizer. The combined experiments indicated that a combination of conductive and convective burning, possibly assisted by fast crack propagation within the silicon/porous silicon substrate, was responsible for the observed difference in propagation speeds and was the mechanism by which the reactive wave propagated with speeds on the order of a km/s within the porous layers.

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Expansion Behavior and Temperature Mapping of Thermites in Burn Tubes as a Function of Fill Length

Cited by 22 publications

References 21 publications

Commentary on the heat transfer mechanisms controlling propagation in nanothermites

Commentary on the heat transfer mechanisms controlling propagation in nanothermites

Quantifying Dynamic Processes in Reactive Materials: An Extended Burn Tube Test

Reactive Wave Propagation Mechanisms in Energetic Porous Silicon Composites

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