The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at ...

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

Metal-based nanoenergetic materials: Synthesis, properties, and applications

Nanofluids are described as a relatively new kind of colloidal solutions with a particle size smaller than one billionth of a meter (1–100 nm) suspended in the base fluid so as to enhance the thermophysical properties, which makes them an obvious choice for use in number of commercial applications including engineering, medical sciences, biotechnology, agriculture technology, transportation etc. With the advancement in nanotechnology during last few years, scientific community focuses on improvising combustion behavior, stability aspects, various engine performance parameters and emission characteristics of conventional diesel engine using nanoparticle laden diesel biodiesel fuel blends. Most recently few experimental works on above issues using nanosized metallic, non-metallic, organic and mixed particles in the base liquid fuel for diesel engine have appeared in the open literature. The obtained results are very encouraging due to multifold enhancement in thermo physical and chemical properties of modified fuel such as high surface to volume ratio, high reactive medium for combustion, enhanced heat and mass transport properties due to high thermal conductivity, improvement in flash point, fire point, pour point etc depending upon the type of nanoparticles used, their particle size and concentration with base fuel. Despite having all superiorities, somewhat unclear and contradictory results are found in the literature, further the experimental results of different researchers are not generalized so far as to reach at common consensus about this new approach of fuel modification. Keeping all these facts in mind, a serious attempt has been made to summarize the important published work on combustion and stability aspects of nanoparticle laden diesel, biodiesel fuels and their blends, and its effects on fuel and engine overall characteristics with the objective to provide a pathway to conduct further research in this area for utilizing maximum potential of nanoparticle fuel emulsion technology and to provide a promising future fuel for diesel engine.

A comprehensive review on combustion and stability aspects of metal nanoparticles and its additive effect on diesel and biodiesel fuelled C.I. engine.

Nano aluminum particles have received considerable attention in the combustion community; their physicochemical properties are quite favorable as compared with those of their micron-sized counterparts. The present work provides a comprehensive review of recent advances in the field of combustion of nano aluminum particles. The effect of the Knudsen number on heat and mass transfer properties of particles is first examined. Deficiencies of the currently available continuum models for combustion of nano aluminum particles are highlighted. Key physicochemical processes of particle combustion are identified and their respective time scales are compared to determine the combustion mechanisms for different particle sizes and pressures. Experimental data from several sources are gathered to elucidate the effect of the particle size on the flame temperature of aluminum particles. The flame structure and the combustion modes of aluminum particles are examined for wide ranges of pressures, particle sizes, and oxidizers. Key mechanisms that dictate the combustion behaviors are discussed. Measured burning times of nano aluminum particles are surveyed. The effects of the pressure, temperature, particle size, and type and concentration of the oxidizer on the burning time are discussed. A new correlation for the burning time of nano aluminum particles is established. Major outstanding issues to be addressed in the future work are identified.

https://www.yang.gatech.edu/publications/Journal/FGV%20(2015,%20Sundaram).pdf

Combustion of Nano Aluminum Particles (Review)

We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35−14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.

/pdf/functionalized-graphene-sheet-colloids-for-enhanced-fuel-28gnzsi50s.pdf

Functionalized Graphene Sheet Colloids for Enhanced Fuel/Propellant Combustion

The oxidation of Al-particles down to nano-scale was investigated by TG, SEM and in-situ X-ray diffraction. Al particles are usually coated by a 2-4 nm layer of Al{sub 2}O{sub 3} which can be derived from the degree of weight increase on complete oxidation by TG-curves. The low temperature oxidation of Al particles occurs at least in two steps. The first step builds a layer of 6 to 10 nm thickness composed of crystallites of the same size independent on the initial particle size. This reaction is dominated by chemical kinetics and converts a substantial fraction of the particle if the particle sizes decrease below 1 {mu}m, an effect carefully to be taken into account for nano-particles because of safety reasons. The second step combines diffusion and chemical reaction and proceeds therefore slowly, the slower the bigger the particles are. The kinetic parameters of these two steps can be obtained by a model taking into account both reaction steps, chemical kinetics and diffusion for spherical particles when fitting it to TG-curves. X-ray diffraction shows that particles smaller than 1 {mu}m build {gamma}- and {theta}-Al{sub 2}O{sub 3} in the first step with nano-crystalline structures which are then transformed to {alpha}-Al{sub 2}O{sub 3}. (Abstract Copyright [2004], Wiley Periodicals, Inc.)

On the Mechanism of Low Temperature Oxidation for Aluminum Particles down to the Nano-Scale

Summary In combustion research, nitromethane is an important nitro-compounded liquid propellant offering high performance. Additionally it can be used as a model substance for understanding mechanisms of combustion of solid propellants. The objectives of investigations were the burning behaviour and the flame spectrum in the UV=VIS spectral range because of the appearance of strong radical emissions also present in flames of other energetic materials. The evaluation is based on a detailed chemical kinetics and a linear combustion model including a reduced mechanism.

/pdf/modelling-nitromethane-combustion-55s832poyn.pdf

Modelling Nitromethane Combustion

Pyrotechnic mixtures react mostly fast and under a high release of thermal energy and heat radiation. A spectral analysis of this emission allows deep insight into the reaction process by the non-intrusive determination of the reaction temperatures and observation of the main reaction species and their concentrations. In the presentation various pyrometric and spectroscopic measurement techniques and data analysis and interpretation procedures are introduced using the spectral range form the ultraviolet, visible and near to medium infrared in combination with image processing of video and IR-camera signals.

Fast Emission Spectroscopy for a Better Understanding of Pyrotechnic Combustion Behavior

Aluminum hydride is a promising candidate for application in energetic materials and hydrogen storages. E.g. an AP/HTPB rocket propellant filled with alane was calculated for a 100 N s kg−1 higher specific impulse compared to the same concentration of aluminum. Different investigations on α-AlH3 polyhedra using thermoanalytical methods and X-ray diffraction were performed to receive a better understanding of dehydration at about 450 K, passivation of the remaining porous aluminum particles and further oxidation. A modeling approach to describe these conversions including diffusion processes, Avrami-Erofeev mechanism and Arrhenius type reaction steps of n-th order were introduced. Results were discussed in comparison to experimental investigations under pressure with model propellants on the base of gelled pure nitromethane and also filled with alane or pure aluminum in concentrations of 5%, 10% and 15%. Both alane and aluminum increase the burning rate on a factor of two correlated with a temperature increase up to 500 K and more. A mesa burning effect at 6 to 10 MPa was indicated by the mixtures with alane.

Volker Weiser

Papers

On the Mechanism of Low Temperature Oxidation for Aluminum Particles down to the Nano-Scale

Modelling Nitromethane Combustion

Fast Emission Spectroscopy for a Better Understanding of Pyrotechnic Combustion Behavior

On the Oxidation and Combustion of AlH3 a Potential Fuel for Rocket Propellants and Gas Generators

Burn Rate Models for Gun Propellants