Detonation Dynamics Research Articles

Structure of Gaseous Detonation Waves and Chemical Kinetics

The tridimensional unsteady character of the flow field for detonations has been established about thirty years ago. The shock pattern is rather complex and the complete description of the phenomenon remains beyond the power of fast computer even today. However the classical thermodynamic approach allows to grasp what are the dominating factors of the whole process: leading shock inducing an exothermic reaction governed by the chemical kinetic mechanism of the heat release. The dynamics of gaseous detonation is coupled therefore closely to the rate of the heat release which in turn influences the time-dependent shock structure of the front. Measurements of the detonation velocity (D) are close to the values of the Chapman-Jouguet model of a detonation which is based on full thermodynamic equilibrium. But, the structure as evidenced from the soot records is more sensitive to the initial conditions :pressure, equivalence ratio, diluent nature and percentage, as well as the type of fuels, promoters and inhibitors of combustion processes. From systematic measurements of the length (L) of the detonation cell imprinted on soot for several mixtures, it has been possible to demonstrate the promoting role of hydrogen, and the inhibiting role of halocarbons on the detonation of CO/O2/Ar mixtures. Some detailed studies of OH emission, shock velocity of the unsteady leading shock, pressure inside a cell have shown the self-similar character of the flow field inside one cell. It demonstrates that the detonation phenomenon can be viewed as a periodic reinitiation of reactive shocks at a frequency identical to the reciprocal of the characteristic time as defined by the ratio: L/D. Irregular cell structure can occur in more complex mixtures such as CH4/O2. In that case the higher values of the activation energy of the conversion process induce a much higher sensitivity to the elaborate temperature fluctuations of the flow field. More recently, numerical simulations have allowed to model the unsteady bidimensional model of a detonation. The following papers could be referred to for more details about this approach of the gaseous detonations: -J.C.LIBOUTON, M.DORMAL, and P.J.VAN TIGGELEN, Progress in Astronautics and Aeronautics, Vol. 75 pp 358-369 (1981) -P.J.VAN TIGGELEN et J.C.LIBOUTON, Annales de Physique, Vol. 14 pp 649-660 (1989) -M.H.LEFEBVRE, E.S.ORAN, K.KAILASANATH, and P.J.VAN TIGGELEN, Progress in Astronautics and Aeronautics, Vol. 153 pp 64-76 (1992) -M.H.LEFEBVRE, E.S.ORAN, K.KAILASANATH,and P.J.VAN TIGGELEN, Combustion and Flame Vol. 95 pp 206-218 (1993).

Molecular dynamics of detonation. I. Equation of state and Hugoniot curve for a simple reactive fluid.

A simple microscopic model of chemical reactions is explored for diatomic exchange reactions 2AB?${\mathit{A}}_{2}$+${\mathit{B}}_{2}$. A pair of atoms bind chemically when they lie within a ``square'' attractive well. The presence of a bond, however, leaves the bound atoms in a state in which they have only hard-sphere repulsion for other atoms. A calculation combining the canonical-ensemble Monte Carlo technique with molecular dynamics is used to determine the equilibrium composition and thermodynamic properties of equimolar binary mixtures of A and B atoms, subject to this square-well, hard-sphere interaction potential for temperatures and densities accessible to detonation waves in a fluid consisting of metastable AB molecules at an initial temperature ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{A}\mathit{B}}$/20${\mathit{k}}_{\mathit{B}}$ (in which ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{a}\mathit{b}}$ is the well depth of the ab interaction and ${\mathit{k}}_{\mathit{B}}$ is the Boltzmann constant), with ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{A}\mathit{A}=3}$${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{A}\mathit{B}}$ and ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{B}\mathit{B}}$=${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{A}\mathit{B}}$. The calculations for systems of both 216 and 1728 atoms include the pressure, the internal energy, and the mean free times for various types of collisions. The Hugoniot curve for states behind detonation waves is determined for six values of the density for the smaller systems and one value for the larger; finite-system effects are found to be small. The Hugoniot curve is shown to have a concave shape in the pressure-volume plane, typical of exothermic materials, and the Chapman-Jouguet state is determined.

Determining curvature effect on detonation velocity from rate stick experiment

Analysis of the reactive Euler equations has shown that there is a correction to the CJ detonation velocity due to the curvature of a diverging detonation wave. We show that this determines the diameter effect and failure diameter of an explosive. The curvature correction to the detonation velocity is sensitive to the reaction rate and equation of state of the explosive. A rate stick experiment, in which both the steady-state detonation velocity and the shape of the detonation front are measured, is well suited to the empirical determination of the curvature effect. This is important in applications of detonation dynamics for propagating a detonation front and is an improvement over the Huygen construction used in programmed burn.

Thermal gasdynamic parameters of detonation spray deposition

1. An examination is made, using equations of the theory of detonation and gas dynamics, of the dependence of detonation wave parameters on the composition of the gas mixture. Recommendations are given on the permissible mixture composition fluctuations in the detonation spray deposition of coatings, together with calculated values of some parameters of detonation products near the front of a detonation wave. 2. The field of the gasdynamic characteristics of the stream of detonation products in the barrel of a detonation spray-deposition apparatus is calculated. Data obtained are employed for estimating the length of the path necessary for accelerating a particle being deposited.

The dynamics of the combustion products behind plane and spherical detonation fronts in explosives

The flow behind the detonation front of an explosive contained in a tube strong enough to confine the motion to one dimension is shown to be a progressive wave of finite amplitude of the type studied by Riemann. The wave is similar at all stages of its progress if the initiation of the explosion is instantaneous, the linear scale of the whole field of flow increasing at a uniform rate. If the products of combustion obey the lawPρ¯γ= constant the distribution of gas velocity along the tube is linear. If the initiation end of the tube is closed a fixed pro­portion of the whole detonating column is at rest. This last case has an analogy in three dimensions. The dynamics of spherical detonation from a point in an explosive is analyzed. As in the one-dimensional case, a fixed proportion of the whole volume of burnt gas is at rest. The radial rate of change of the variables, velocity, pressure and density become infinite at the detonation front, but it is unlikely that this result would be true in a real explosive where the time of reaction is not zero. The results are applied in both linear and spherical cases to the detonation of T. N. T., using data given by Jones & Miller (1948).