HAL Id: jpa-00226639
https://hal.archives-ouvertes.fr/jpa-00226639
Submitted on 1 Jan 1987
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
STUDY OF THE HEAD ON DETONATION WAVE STRUCTURE IN GASEOUS EXPLOSIVES
H. Presles, P. Bauer, C. Guerraud, D. Desbordes
To cite this version:
H. Presles, P. Bauer, C. Guerraud, D. Desbordes. STUDY OF THE HEAD ON DETONATION
WAVE STRUCTURE IN GASEOUS EXPLOSIVES. Journal de Physique Colloques, 1987, 48 (C4),
pp.C4-119-C4-124. �10.1051/jphyscol:1987407�. �jpa-00226639�
STUDY OF THE HEAD ON DETONATION WAVE STRUCTURE IN GASEOUS EXPLOSIVES H.N. PRESLES, P. BAUER, C. GUERRAUD
andD. DESBORDES
Laboratoire dfEnergetique et de Detonique, U.A. 193, E.N.S.M.A., 86034 Poitiers Cedex, France
R6sumt
Une mGthode optique a 6tt d6veloppte pour Gtudier la structure d'une onde de dttonation sur toute sa surface. Elle est comparte 2 la methode classique des traces sur dGp6t de noir de carbone.
Abstract
An optical method was designed to analyze the detonation wave structure over its whole area. It is compared with the classical soot tracks method.
I - INTRODUCTION -
It is presently acknowledged that the front of a steady detonation wave in gaseous explosives exhibits a three dimensional structure (an extensive review is given in
/ I / ) .Some work in the field of condensed explosives, and mainly those involving homogeneous liquids 12 -
6 1 ,lead to a similar conclusion. However most of the studies carried out on the detonation wave structure have been done with gaseous explosives at a low initial pressure. They state that the cell width
A ,as exhibited on the soot coating (see Fig. 1) is merely related to the mean induction distance length
A / 7 /Therefore measurements of
A,which is a characteristic parameter for each given explosive mixture, yields straight way an insight of the chemical reaction rate.
Moreover it was shown that the main dynamic parameters of the detonation (review in
1 8 1 )are closely connected to
1.For instance the critical tube diameter d for the transition of a detonation into an unconfined g,eometry, the minimum tube diameter d for the onset of a stable detonation and the critical energy Ec for direct initiation are respectively provided by the following relationships
:Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987407
JOURNAL DE PHYSIQUE
Therefore the knowledge of
Ais required for the assessment of detonation hazards.
Due to the difficulty of examining the fine structure of the detonation wave of condensed explosives, none has ever been able to provide similar relations. This is the reason why we undertook an investigation on the detonation of gaseous explosive mixtures at a high initial pressure
19, 101.This was aimed at reaching initial densities of the same order as that of condensed explosives. Therefore we expect to fill the gap between two ranges of densities
:on the one hand, gaseous explosive mixtures at a low initial pressure and condensed explosives on the other hand.
Evolution of the kinetics as a function of pressure, namely that of the detonation wave structure is one out of several investigation purposes. However the soot tracks method that has, to date, been widely used for gaseous explosives mixtures becomes useless as soon as the initial pressure exceeds a few bars. We thus developed previously an optical method that was based on the record of brightness temperature peaks connected to the movement of the triple points within the detonation front
/9/.However this method was limited to an initial pressure lesser than
20bars for most of hydrocarbon-air mixtures since the upper limit of resolution of our device was allowing the measurement of cell width greater than
2 mm.In order
topursue the study of the detonation wave structure at higher initial pressures we designed an optical device. This method is based on the record of the deformation of an aluminized mylar mirror hit by a detonation front.
The detonation wave structure of a mixture H
-0-Ar at low initial pressures
2 2was studied with this method together with that of soot tracks. The comparison of these two set of data provides plenty of confidence in this project aimed at the study of dense gases detonations.
I1 - OPTICAL METHOD -
This method, already reported in
/11/is derived from that of MALLORY
/ 1 2 /better known as the named "impedance mirror technique". It was aimed at the study of the structure of the detonation front propagating in liquid explosives. The deformation of a mirror parallel to the detonation front and undergoing its effect, is recorded by means of a fast camera. MALLORY used an aluminized PMMA slab as a mirror.
In the case of gaseous explosive mixtures at a low initial pressure the use of such a mirror is irrelevant. Likewise, a 25
pmaluminized mylar sheet was used (Fig.
1).
It was enlightened by means of a parallel beam of a
50um duration. The
reflected beam was recorded with a BECKMAN and WHITLEY image converter with an
exposure time of
100ns.
,
,
, incident-
L-'Fig.
ITwo dimensional description of the optical method
:a) prior to any interaction
b) during the interaction
As soon as the mirror gets deformed, the beam reflected from this region of the mirror is withdrawn from the field of the camera. The record displays a lack of light at the corresponding location. The meansize x of these perturbations are
P
compared to the average cell width
1derived from the soot tracks method.
111
-
EXPERIMENTAL CONDITIONS-
The experimental device (Fig. 2) was a 6 m long and 52 mm i.d. steel tube connected to a 250 mm i.d. and 300
mmlong cylindrical vessel. The opposite side of this chamber was holding two 70-mm-diameter portholes. The mylar sheet was stretched on a proper piece at the end of the tube perpendicular to its axis.
The detonation tube may be equipped with a thin metallic plate along its axis.
That was used for soot tracks records of the structure. The average cell width x of
the detonation front in a
2H2+
O2+ 7 Ar mixture is reported on Fig. 3 as a function of the initial pressure in a range of 0.15 to 0.5 atm. Using Ar as a diluent enables a rather good regularity of the detonation structure as it has often been shown on the basis of soot tracks records.
Fig. 2 Experimental setup.
For each experiment the steadiness of the detonation wave was checked using 8
JOURNAL
DE
PHYSIQUEI o soot trucks method I \
Fig.
3Mean cell size
Aand mean perturbation size
AP versus the initial pressure.
ionization gages along the two last meters of the detonation tube. This control is important since it was shown that the size of the cellular structure is strongly dependant on the detonation velocity
1 1 3 , 1 4 1 .IV - DESCRIPTION OF THE STRUCTURE OBTAINED WITH THE OPTICAL METHOD -
The records displayed on Fig. 4 are related to the
2H2+ O2 +
7Ar mixture and correspond to four different values of the initial pressure.
One may notice that the shape of the perturbations is irregular. Nevertheless it exhibits a prominent characteristic width. Thus the mean width
Aof the
P perturbations may be evaluated. The corresponding values are reported on the Fig.
3 .According to the normal dispersion on the cell size
A ,a satisfying agreement is obsewed between these two types of data.
Therefore, it will be further assumed that the mirror perturbations may be
representative of the cells and that the optical method may be regarded as reliable
for providing data on the mean cell width in the detonation front.
V
-
CONCLUSION-
The optical method presented in this paper is based on the record of the deformation of an aluminized mylar mirror hit by a detonation front.
At that stage it allows the study of the stru:ture of the detonation front on its whole area and is expected to lead to a better understanding of its three-dimensional evolution even in the most complex features.
This method should allow the study of the structure in any situation where the initial pressure is greater than a few bars. While increasing this parameter, such a study, apart from its own objectives, might lead to an improved understanding of the structure of the detonation in high explosives.
JOURNAL
DE
PHYSIQUEREFERENCES -
/I/ FICKETT W., DAVIS W.C., Detonation, California Press, 1979.
/ 2 /
