Self‐Propagating Reactive Fronts in Compacts of Multilayered Particles

Abstract

Reactive multilayered foils in the form of thin films have gained interest in various applications such as joining, welding, and ignition. Typically, thin film multilayers support self-propagating reaction fronts with speeds ranging from 1 to 20 m/s. In some applications, however, reaction fronts with much smaller velocities are required. This recently motivated Fritz et al. (2011) to fabricate compacts of regular sized/shaped multilayered particles and demonstrate self-sustained reaction fronts having much smaller velocities than thin films with similar layering. In this work, we develop a simplified numerical model to simulate the self-propagation of reactive fronts in an idealized compact, comprising identical Ni/Al multilayered particles in thermal contact. The evolution of the reaction in the compact is simulated using a two-dimensional transient model, based on a reduced description of mixing, heat release, and thermal transport. Computed results reveal that an advancing reaction front can be substantially delayed as it crosses from one particle to a neighboring particle, which results in a reduced mean propagation velocity. A quantitative analysis is thus conducted on the dependence of these phenomena on the contact area between the particles, the thermal contact resistance, and the arrangement of the multilayered particles.

Influence of Nanoscaled Surface Modification on the Reaction of Al/Ni Multilayers

“…Some indications can be found in the literature: experiments reveal that stacking faults and defects cause a decrease of the chain reaction efficiency if the bilayer thickness fall below 20 nm [16]. A reduced cross section of the RML can hinder the propagation of the reaction front and can be used to slow down the velocity [17,18]. Molecular dynamic investigations suggest that amorphous aluminum present at grain boundaries promotes the reaction [19].…”

Regime of validity of a spatially homogenised reaction model for multilayered particles

“…For this purpose, we define two measures that reflect the extent of temperature gradients within a particle, given by where T f and T 0 refer to the adiabatic flame temperature and room (initial) temperature, respectively. Expression (16) corresponds to the normalised peak value of S(t), the instantaneous maximum temperature difference in a given particle, whereas expression (17) corresponds to the normalised time average of S(t). S max and S(t) are calculated for each particle in the chain, but in our analysis we rely only on values from particles that are appreciably away from the domain boundaries.…”

“…Following Sraj et al [17], coupling between the particles is achieved through thermal conduction between touching particles across a certain surface contact area. In both the meshed and the homogenised particle models, accounting for heat transfer across particle interfaces occurs via the heat flux q, but with a reduced κ value given by the sum of the internal particle resistance (which varies with time) near the interface and a specified (constant) thermal contact resistance value R c .…”

“…In particular, Fritz et al observed that the front velocity in the fabricated particle compact can be about two orders of magnitude smaller than in the corresponding film. Motivated by these experiments, Sraj et al [17] simulated self-propagating fronts in idealised multilayered particles consisting of quasi-2D rectangular foils that are in thermal contact. In accordance with the analysis of Fritz, the computations revealed that an ignition delay occurs as the reaction front crosses from one particle to the next, which consequently results in an overall reduction in the average flame velocity.…”

Design analysis for optimal calibration of diffusivity in reactive multilayers

“…In order to overcome limitations due to the stiffness of the governing equations, Salloum and Knio recently proposed a reduced reaction formalism, and demonstrated orders of magnitude enhancements in computational efficiency for transient multidimensional simulations of Ni-Al multilayers [31][32][33]. The reduced model was further generalized to account for variable thermal transport properties, and used to examine the behavior of self-propagating reaction fronts [34,35]. Recently, Fritz [36] studied homogenous reactions in Ni-Al nanolaminate foils that are triggered using a uniform current pulse.…”