Daniel Desbordes

Formation and evolution of two-dimensional cellular detonations

This paper reports the results of numerical simulations of cellular detonations generated by using numerical noise as a source of initial fluctuations imposed on a strong planar shock propagating through the reactive medium. The calculations show that a plane detonation wave moving at Chapman-Jouguet (CJ) velocity is unstable to transverse perturbations with wavelength greater than one or two half-reaction-zone lengths. The numerical noise affects the initial cell formation process, but it has no influence on the cell size and regularity of the structures developed. Increasing the activation energy results in more irregular structures characterized by stronger triple points, larger variations of the local shock velocity inside the detonation cell, and higher frequency of appearance and disappearance of triple points. These features of the systems with irregular cellular structures can account for the experimental observation that such systems are less affected by boundary conditions. For the two-dimensional detonation, the average reaction zone is larger and maximum reaction rate is lower than in the one-dimensional case. This means that the formation of detonation cells reduces the maximum entropy production in the reaction zone, and slows down the approach of the system to the equilibrium state. This effect is shown to increase with activation energy due to larger unreacted gas pockets, and deeper penetration of the pockets into the region of mostly burned material.

Gaseous nitromethane and nitromethane-oxygen mixtures: A new detonation structure

Detonation in gaseous nitromethane (NM) and mixed with O2 has been studied. Experiments were performed in a preheated steel tube at an initial temperatureT0∼=390 K for different initial pressuresP0 (1.7≥P0≥5 10−2 bar). Different selfsustained detonation regimes were obtained, from multiheaded mode to spinning and galloping mode in marginal conditions. These chemical systems were characterized by a specific detonation cellular structure very different from that currently observed with classical gaseous CnHm/O2/N2 mixtures. All modes of detonation propagation in rich NM/O2 mixtures exhibit a double scale cellular structure. The pattern of this double scale structure is particularly clear in the case of spinning mode.

Direct initiation of detonation in cryogenic gaseous H2-O2 mixtures

Critical conditions for the direct initiation of self-sustained detonation in cryogenic hydrogen-oxygen mixtures are examined experimentally. These initial conditions are expected to depend mainly on four parameters: the equivalence ratio of the mixture, the amount of the initial energy deposition, the initial temperature and pressure of the mixture. These critical conditions are determined by fixing alternatively three of these parameters and varying the fourth one from subcritical to supercritical detonation conditions. Results are presented for initial pressuresPo and equivalence ratios Φ ranging from 0.3 to 1 bar and from 1 to 2 respectively, for the two initial temperaturesTo, 123 K and 293 K. These results indicate that for the lowest values of the initial pressure, a decrease of initial temperature may favour the onset of detonation. Whatever the initial conditions, the measured detonation pressures and velocities are in reasonably good agreement with the corresponding Chapman-Jouguet values computed using the ideal-gas equation of state.

Supersonic H2-air combustions behind oblique shock waves

In order to study the mechanisms of initiation and stabilization of H2-Air combustions (stoechiometric mixture initially atT0=293 K andp0=0.5 bar) in supersonic flow conditions behind an oblique shock wave (OSW), an original technique is used where OSW is generated in this mixture by the lateral expansion of the burnt gas behind a normal CJ gaseous detonation propagating into a bounding reactive mixture. Four Mach numberM of propagation of OSW are considered in the study, namelyM=7.7-6.1-4.4 and 3. Depending on the Mach numberM and inclinaison angleθ of OSW different regimes of combustion may occur in the driven mixture. For high values ofM (6.1 and 7.7) delayed steady overdriven oblique detonation waves (SODW) were obtained with a near CJ detonation wave as the critical regime. It was found that SODW obtained correspond quite well to prediction of the polar method. When thermal conditions behind the OSW are lower, either for high Mach number 6.1 and 7.7 for smaller angleθ than the previous case, or for lower Mach number, 4.4 and 3, the flame initiated at the apex is stabilized as a turbulent oblique flame behind the OSW. With much lower conditions, no combustion appears in the H2-Air mixture.

An optical method for the study of the detonation front structure in gaseous explosive mixtures

Abstract An optical method aimed at the study of the shape and spreading of the cells within the detonation front of gaseous explosive mixtures is described. It is based on photography of the deformation of a thin plane mylar mirror hit by the detonation front. The agreement between the mean size of the perturbation zones of the mirror and the mean size of the cells provided by the soot tracks method leads to the conclusion that the optical method can be very useful in the study of the detonation front structure.

Detonation structure and detonability of C\(_2\)H\(_2\)-O\(_2\) mixtures at elevated initial temperature

Abstract. We have studied the influence of initial pressure (

The terminal phase of an unconfined hemispherical deflagration

P_0=0.05

Operating Limit of a Pulsed Detonation Engine. The Marginal Case of Detonation Propagation.

–1 bar) and temperature (

Detonation cellular structure in NO{sub 2}/N{sub 2}O{sub 4}-fuel gaseous mixtures

T_0= 300

Optimization of the deflagration to detonation transition: reduction of length and time of transition

–500 K) on the detonation structure of three acetylene-oxygen based mixtures. Two were stoichiometric, C