M. Larini

Ignition and growth of a detonation by a high energy plasma

Many experimental and numerical studies have been achieved to describe the transition process of deflagration to detonation when a projectile impacts an explosive. Also a large work has been done for the determination of various parameters — such as the impact pressure, the efficiency factors, etc. of the laser — material interaction. When a laser beam impacts an explosive, the P2τ criterion, characteristic of shock initiated detonations, is no longer valid due to the generated hot plasma whose effect is to decrease the DDT (Deflagration to Detonation Transition) duration. The present paper deals with a modelling of the plasma-explosive medium allowing the determination of distances and times of the DDT process. The two phase modelling of the granular explosive takes into account the creation of hot spots. The pressure of the plasma is computed using a semi empirical model, while the temperature is obtained from Maxwell Boltzmann statistics. The authors focused their attention on the equation of state for the detonation products and the numerical process.

An exact Riemann solver for detonation products

Numerical methods based upon the Riemann Problem are considered for solving the general initial-value problem for the Euler equations applied to real gases. Most of such methods use an approximate solution of the Riemann problem when real gases are involved. These approximate Riemann solvers do not yield always a good resolution of the flow field, especially for contact surfaces and expansion waves. Moreover, approximate Riemann solvers cannot produce exact solutions for the boundary points. In order to overcome these shortcomings, an exact solution of the Riemann problem is developed, valid for real gases. The method is applied to detonation products obeying a 5th order virial equation of state, in the shock-tube test case. Comparisons between our solver, as implemented in Random Choice Method, and finite difference methods, which do not employ a Riemann solver, are given.

Solid-propellant fire in an enclosure fitted with a ceiling safety-vent

Abstract The response of an enclosure having a ceiling safety-vent to a fire of solid propellant located on the floor is investigated numerically. The full Navier—Stokes equations are solved along with the species continuity equations. A recent method is used to compute chemical equilibria and the coupling between chemistry and thermodynamics is treated according to a new strategy. The particular boundaries, which are the combustion zone of the propellant and the outflow section, require an original treatment by solving a set of ‘full’ or ‘half’ Riemann problems taking into account the transport of chemical species. The SOLAICE algorithm is successfully developed for the reactive-diffusive case dealing with particular boundaries. A fire of a standard hot homogeneous propellant in a rectangular cavity initially filled with air is simulated for two opening conditions of the safety-vent. They predict the increase in the rates of energy release and CO2/H2O production in the reaction zone caused by afterburning processes involving the air of the enclosure. The course of the compartment fire is described in terms of time evolution of the average gas temperature and pressure, and oxygen depletion for both opening configurations.

Gun propellant break-up effects on transient combustion processes

A two-dimensional interior ballistics computer code has been developed to solve the viscous two-phase flow in a combustion chamber of a gun tube during the ignition phase beginning with primer discharge and ending with projectile motion. The motivations are to achieve a better understanding of the physical processes occuring during this phase and to predict overpressures which can occur under certain ignition conditions and configurations of loading. Calculations are presented for axisymmetric two-phase flow in a model combustion chamber assuming grain fracture or not. Predicted results indicate that grain fracture due to high compaction may generate substantial local pressure peaks along the combustion chamber. In extreme cases of severe grain fracture, the result is a catastrophic overpressure which can explain structural gun-damage and spontaneous deflagration-to-detonation transition.

Transport phenomena in a nonequilibrium, partially ionized gas in a magnetic field

A small-parameter method in which the gas and electron temperatures can be different is used to solve the Boltzmann equation. The zeroth-approximation solutions are Maxwellian with different temperatures Te and Ts. Transition to the BGK formalism on the basis of an extremely crude estimate of the frequency of electron collisions leads to numerical results which agree well with the available data. Then an extension of the Eucken method leads to analytic expressions for the nonequilibrium quasi-Lorentz transport coefficients.