A two-phase mixture theory for the deflagration-to-detonation transition (ddt) in reactive granular materials

A Multiphase Godunov Method for Compressible Multifluid and Multiphase Flows

Advances in science and technology of modern energetic materials: an overview.

Metal-based reactive nanomaterials

Metal combustion has received renewed interest largely as a result of the ability to produce and characterize metallic nanoparticles. Much of the highly desirable traits of nanosized metal powders in combustion systems have been attributed to their high specific surface area (high reactivity) and potential ability to store energy in surfaces. In addition, nanosized powders are known to display increased catalytic activity, superparamagnetic behavior, superplasticity, lower melting temperatures, lower sintering temperatures, and higher theoretical densities compared to micron and larger sized materials. The lower melting temperatures can result in lower ignition temperatures of metals. The combustion rates of materials with nanopowders have been observed to increase significantly over similar materials with micron sized particles. A lower limit in size of nanoenergetic metallic powders in some cases may result from the presence of their passivating oxide coating. Consequently, coatings, self-assembled monolayers (SAMs), and the development of composite materials that limit the volume of non-energetic material in the powders have been under development in recent years. After a brief review of the classifications of metal combustion based on thermodynamic considerations and the different types of combustion regimes of metal particles (diffusion vs. kinetic control), an overview of the combustion of aluminum nanoparticles, their applications, and their synthesis and assembly is presented.

Metal particle combustion and nanotechnology

The paper describes a quasi-one-dimensional model of combustion of sandwich heterogeneous solid rocket propellant designed to predict the burnup rate of individual components (oxidizer and binder) and the average mass rate of combustion of the entire propellant. This model includes the heat- and mass-transfer and kinetic-gasdynamic processes in the gas phase above the surface of burning solid propellant, as well as the heat-transfer processes in solid material. Included in the calculations are the thermochemical properties of ammonium perchlorate (AP, oxidizer) and of polybutadiene rubber (PBR, fuel). The thermochemical processes are included in the kinetic scheme of two overall chemical reactions involving five effective chemical components. Systematic calculations are performed of the process of solid fuel combustion with different mass ratios of oxidizer to fuel. It is demonstrated that the employed kinetic model of thermochemical processes provides an adequate description of the available experimental data on the rate of combustion of AP- and PBR-based solid propellants.

Quasi-one-dimensional model of combustion of sandwich heterogeneous solid propellant

The results of the ongoing investigation of laser-sustained argon plasmas in the forced convective flow regime are reported. Plasma absorption and thermal efficiency are the key parameters measured. Flow velocities of up to 30 m/s at one atm to 13.6 m/s at 2.5 atm are imposed upon plasmas sustained at 2.5, 5.0, and 7.0 kW input laser powers, using either f/4 or f/7 beam focusing geometries. Absorption ranged from 55 to 91% and thermal efficiency ranged from 11 to 46%, depending upon the operating conditions. Absorptions and efficiencies this high have never before been reported and demonstrate that laser propulsion could be a practical orbital transfer concept. Spectroscopic analysis of the plasma core is used concurrently with conventional techniques to analyze plasma behavior.

Laser-sustained argon plasmas, for thermal rocket propulsion

Interpretations of emission measurements from aluminized explosive fireballs

Nano-aluminum combustion remains poorly understood, as evidenced by recent paradoxical measurement results. While nanoscale Al is often assumed to burn in the kinetic limit, optical temperature measurements of nano-aluminum particles combusting in a heterogeneous shock tube at pressures near 20 atm showed peak temperatures above 3000 K in ambient air at 1500 K. This temperature overshoot and the simultaneous burning times measurements greater than 100 μs cannot be described from an energy balance considering continuum heat transfer losses from the particle. A conservative estimate assuming all reaction energy is used for sensible heating of the aluminum particle requires complete combustion in less than 1 μs to reach peak temperatures of 3000 K suggesting deviation from continuum mechanics. Similar heat transfer effects are seen in laser induced incandescence (LII) experiments of various nano-particles. In the shock tube and LII experiments the particle Knudsen number is near unity and non-continuum heat transfer effects and slip conditions can be expected. Previous work by Igor Altman suggests a formula which predicts an upper limit on the energy accommodation coefficient (EAC) which characterizes the energy transfer between a gas particle and surface. A simple model considering surface combustion and an energy balance using various heat transfer loss mechanisms shows that an upper limit of 0.005 for the EAC predicted by Altman’s theory agrees well with experimental data. An EAC of 0.0035 produces a best fit of the thermal profile for varying nano-aluminum particle sizes and predicts the temperature overshoot at high pressures. The decrease in the EAC for nano-aluminum has significant implications in the application and modeling of nano-aluminum used in explosives and solid rocket motors where ideally the particle transfers energy to the surrounding atmosphere. In such cases of low EAC radiation plays a more significant role in particle heat transfer.

Herman Krier

Papers

Quasi-one-dimensional model of combustion of sandwich heterogeneous solid propellant

Laser-sustained argon plasmas, for thermal rocket propulsion

Interpretations of emission measurements from aluminized explosive fireballs

Heat transfer effects in nano-aluminum combustion

Shock tube ignition of AL/MG alloys in water vapor and argon