The role of particles in environmental and engineering applications is two-fold. On the one hand, particles may pose potential hazard for human activity (deposition of aerosols in human lungs). On the other hand, they can be successfully used in engineering solutions (to induce working processes in energy systems and to suppress detonation). Processes that control transport and combustion of particles remain unresolved, and introduce significant uncertainties into modelling and simulation. One of the most important parameters for engineering applications is the minimum pulse energy (MPE) required to induce detonation of the mixture.
The metal particles (typically aluminium) suspended in an oxidizing or combustible gas form a reactive gas-particle mixture. Reacting two-phase flows attract particular interest because of their applicability in various technological processes and energy conversion systems. As a result, numerous investigations have been devoted to these flows.
The reactive metal particles are used to enhance blast performance. Although the total energy released by the metal combustion is significant and comparable to the total energy released by the explosive itself, the timescale of this energy release (timescale of particle reaction) for typical particle sizes (from 1 to 100 microns), is too long to contribute directly to the detonation front itself. The metal particles react with gas or detonation products behind the blast wave. It has been shown that the metal particle reaction significantly increases the strength of the blast and the total impulse delivered to nearby objects or structures.
When a power laser pulse interacts with a gas, the gas breaks down and becomes highly ionized. This process is always accompanied by a light flash and generation of sound. The development of electron cascade requires the existence of initial free electrons in a gas. Particles, trapped by a laser beam, considerably influence results of the process. There is threshold intensity of laser pulse at which the particle material converts into the meta-stable condition and its intense evaporation leads to heat destruction of the particle, either by means of local jetting of the essential part of the particle mass or due to explosion of the particle (optical breakdown). The injection of metal particles with low evaporation temperature and low ionization potential (for example, aluminium) leads to drop of detonation MPE. Vapour aureole around metal particle is a source of free electrons, and optical breakdown in the gas-particle mixture comes for lower intensity of laser pulse than in pure gas.
Many experimental, theoretical and numerical studies have been performed for the past years. However, some fundamental and practical problems are yet to be resolved. They include qualitative and quantitative description of processes around individual particle of non-spherical shape, knowledge in particle microphysics and optical properties of particles, sub-models of heating and evaporation, transport of aggregates of complex morphology, threshold values of optical breakdown, dependence of MPE on the contributing factors (time and shape of laser pulse, composition of gas mixture, volume fraction of particles).
Computational fluid dynamics (CFD) tools have the potential to model the physics and dynamics of explosion and detonation, but without relevant physical and mathematical models, choice of appropriate numerical methods, adjustment of computational parameters and proper user guidelines based on extensive validation work, poor prediction capability is expected. Initiation or suppression of detonation by the use of different ways and techniques (for example, by using a cloud of inert particles) is on of challenging tasks in CFD applications to hydrogen safety engineering.
In this study, physical and mathematical models of optical breakdown on individual aluminium particle and numerical methodology for computer modelling of laser-induced detonation in the particulate mixture are developed. Laser-induced detonation in gas-particle mixture is simulated, and contribution of parameters of laser pulse and composition of the mixture to MPE is studied. Comparison of some numerical results with experimental data is made.
There is also growing interest in using finer metal particles and nano-particles to increase the reactive surface area and achieve better combustion of heterogeneous mixtures. As a consequence of high specific surface area of nano-particles, they exhibit many features distinct from micron-sized particles, including lower ignition temperatures and faster burning rates. The continuum hypothesis for the gas phase around the nano-particle becomes invalid. A more advanced model is required to appropriately describe the combustion mechanism of nano-particles.
