THE PROBLEMS IN DETONATION PHYSICS

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J. Lee

To cite this version:

J. Lee. THE PROBLEMS IN DETONATION PHYSICS. Journal de Physique Colloques, 1987, 48

(C4), pp.C4-417-C4-432. �10.1051/jphyscol:1987432�. �jpa-00226671�

JOURNAL DE PHYSIQUE

Colloque C4, suppl6ment au n-9, Tome 48, septembre 1987

THE PROBLEMS IN DETONATION PHYSICS

J. LEE

Department of Mech. Engineering, Mc GiZZ University, Montrba 2 , Canada

1. Introduction

This Workshop has brought together specialists from various disciplines. The objective is to explore how the techniques in these different fields can contribute towards the solution of the detonation problem. This is not an easy task for each of us has to learn the special languages of the different disciplines in order to communicate. I hope that the presentations of the past four days have achieved this to a certain degree. What remains now is to get into the detonation problem itself. As an introduction to the subequent discussions that follow, I thought it best to outline in a fundamental way the major outstanding problems in detonation physics.

Detonation is essentially a macroscopic phenomenon. It is a strong compression shock wave driven (or sustained) by the energy release in the chemical reactions which are initiated by the shock wave itself. Chemical reactions occur at the molecular level where unstable molecules (reactants) break

forln more stable species (products) and release potential energy in the process. The quantum chemical study of the molecular bond structure is obviously important at this microscopic level of the detonation process. On the other hand, the potential energy released by the reacting molecules must thermally equilibrate bringing the products to a very high local thermodynamic rate (i. e. pressure and temperature). It is the subsequent relaxation of this high local thermodynamic state that generates and sustains the shock wave that propagates into the reactants.

The shock wave in turn initiates further chemical reactions: either thermally via adiabatic compressional heating or mechanically via the generation of shear (and turbulence). Shock wave compression, shock wave interactions, and the generation of shear and turbulence are all macroscopic processes, belonging to the realm of hydrodynamics and thermodynamics. It is a formidable task to bridge the gap between

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987432

microscopic and macroscopic processes differing in length scales by at least five orders of magnitude. Molecular dynamical calculations which describe the detailed dynamics of typically about lo4 particles do not provide a true macroscopic description. In fact the thermodynamic quantities (such as pressure, temperature, particle velocity, etc.) and the macroscopic phenomena (i.e. sound and shock waves, heat transport, etc.) as deduced from molecular dynamical computations must be interpreted with great caution. The mechanical equations of motion (Hamiltonian, Lagrangian or Newtonian) used to describe the dynamics of the molecules are invariant to time reversal. Strictly speaking, the concept of thermal equilibrium, irreversibility and dissipative processes have no significance in molecular dynamics.

Numerical modelling of the hydrodynamic equations is on the macroscopic level, the results of which are directly comparable to experimental measurements. However, empirical input must be provided in the form of the equation of state, transport properties as well as chemical kinetic constants for the chemical reactions. If one is not careful there are sufficient degrees of freedom provided by these empirical constants for the numerical results to fit any experimental data even when the mechanisms are incorrect. Thus good agreement with experiments does not necessarily provide the correct test for the validity of the numerical model.

In the past four days, we have heard presentations on quantum chemistry spectroscopy, chemical kinetics, molecular dynamics, numerical modelling at the hydrodynamic level, as well as experiments on actual detonation processes. Let us now focus on how to tie these various fields together to solve the outstanding problems. In what follows, I hope to outline briefly the fundamental detonation problems and open the floor to discussion.

2. The Basic Ouestions

Almost all research in detonation physics is centered around the following questions: Given an explosive at given initial and boundary conditions, i) what are the steady detonation states (i.e. detonation velocity, pressure, temperature, products concentration, etc.)? ii) what are the dynamic detonation parameters (i.e. critical diameter, initiation energy, shock sensitivity, etc.)? and iii) is transition from deflagration to detonation

possible and if so, what is the transition distance?

Initial condition refer to the type of explosive, its composition density,

grain size, physical dimensions, geometry and other physical,

thermodynamical and chemical properties. The type and the energy time characteristics of the ignition or initiation source can be included in the specification of the initial conditions. Boundary conditions mainly refer to the degree of confinement and properties of the confining surfaces.

Current experimental work is aimed at direct measurements of the detonation states, dynamic detonation parameters, and transition distances. The ultimate objective of detonation research is to achieve a-priori predictions of all these quantities, preferably from first principles.

3. Current Status

i) Steadv detonation states

According to classical theory, the Chapman-Jouguet criterion (i.e.

detonation velocity equals the sound speed in the product gases, D

U1 +

C1) provides the missing link to close the set of conservation equations across the detonation front. For given explosive and initial conditions, the steady detonation states can be determined via an equilibrium thermodynamic calculation provided that the equation of state is specified.

The Chapman-Jouguet theory assumes equilibrium at the C-J surface where D

-

U1 + C1 and the steady detonation states computed on the basis of this

criterion are referred to as Chapman-Jouguet (C-J) detonations. Three

problem areas arise in the determination of the steady detonation states in

general. The first is associated with the equation of state for condensed

media. Current equations of state are empirically obtained from fitting

with experimental Hugoniot data. The accuracies are uncertain when they

are extended beyond the range of the data base itself. The second problem

arises from the equilibrium of the product gases. For so called non-ideal

explosives, different components of the explosive may have quite different

equilibrium times. Thus overall chemical equilibration may not be obtained

at the C-J surface and the steady detonation states may be based only on

partial chemical equilibration. A typical example would be the

condensation of carbon in the product gases. The non-equilibrium

condensation kinetic processes must be accounted for in the C-J

calculations. The third difficulty in computing the steady detonation

states is due to the influence of boundary conditions. Strictly speaking,

C-J states are independent of boundary conditions. However, boundary

conditions control the non-equilibrium hydrodynamic processes in the

products which may influence the energetics of the reactions and hence the

detonation state. Low velocity detonation is an example of the influence

of boundary conditions on the steady detonation speed.

ii) Dynamic Parameters

The Chapman-Jouguet theory does not take into account the structure of the detonation wave. It is independent of the propagation mechanism.

The dynamic detonation parameters, on the other hand, are functions of the detonation wave structure or the non-equilibrium rate processes. The fundamental mechanism of detonation is not known at present. The classical Zeldovich-D8ring-Von Neumann (ZDN) model which assumes a one-dimensional shock followed by a reaction zone provides a plausible mechanism of propagation, i.e. adiabatic shock heating leading to thermal decomposition and subsequent chemical reactions. For the case of gaseous explosives where the equation of state is well known and the details of the chemical kinetic processes can also be adequately described, the ZDN detonation structure can be determined theoretically. However, one-dimensional ZDN

detonation structures have not been observed experimentally because they

have been proven to be unstable. In practice the detonation structure is cellular, formed by a series of interacting incidents, reflected (or transverse) shock and Mach waves. For gaseous explosives the cellular structure has been well documented using different techniques and the averaged dimensions of the detonation cells (or transverse wave spacing) have been measured for many common gaseous explosives. The use of the one- dimensional ZDN structure has failed to provide quantitative theories for the prediction of the dynamic parameters. However, using the experimentally measured cell size (transverse wave spacing) as a length scale to characterize the reaction zone thickness (instead of the idealized ZDN reaction zone length), good correlations between the dynamic parameters and the cell size can be obtained. In other words, empirical relationships and analytical theories exist to give the dynamic parameters as a function of the cell size. Thus for gaseous explosives, the knowledge of the cell size enables the dynamic parameters to be computed to acceptable accuracies. The prediction of the cell size from first principles (using more fundamental parameters such as kinetic rate constants) is still an outstanding problem.

For condensed phase detonations, there is strong experimental evidence

of a similar cellular structure as gaseous detonations. However, no

systematic attempts have been made to measure the cell size and attempt

correlations with the dynamic parameters as yet. The cell size for

condensed phase detonations may be an order of magnitude (or more) smaller

than that of gaseous explosives at normal pressures. This may present

experimental difficulties in the measurement of the cell size. However the

critical diameter (which in general is an order of magnitude greater than

the cell size) could be used as the fundamental length scale in this case.

It would be of interest to pursue such correlations between cell size (or critical diameter) with the other dynamic parameters. Prediction of cell sizes from first principles may not be forthcoming for some time. Since the initiation mechanism in condensed explosives may not be unique and may depend on the particular explosive and its structure, it is doubtful that a universal theory (or model) for condensed phase detonation can be formulated. Measurement of a length scale that can characterize the reaction zone may replace the difficult task of searching for the correct initiation mechanism, at least from a practical point of view.

iii) Transition

Why a deflagration transits to a detonation is not known. Perhaps it is a consequence of some fundamental law that an explosive always tends to maximize its burning rate. Thus depending on the boundary conditions, a deflagration may accelerate to become i) a fast deflagration, ii) a low velocity detonation or iii) a normal Chapman-Jouguet detonation. A maximum burning rate principle is essentially

statement

instability in the same spirit as transition from laminar to turbulent flow or a transition from conduction to cellular convection in the Bernard cell problem.

Transition from deflagration to detonation is a macroscopic phenomenon involving the non-linear coupling between hydrodynamics, thermodynamics and chemical reactions which provide the energy to drive the non-linear processes. The transition problem can be divided into two phases: the acceleration phase and the onset of detonation. The mechanisms for flame acceleration in gaseous explosives are well known although their quantitative descriptions are not likely to be forthcoming for a while.

The mechanisms for the onset of detonation in gaseous explosives have also

been identified and again their quantitative description is extremely

difficult . For condensed explosives neither the mechanisms of flame

acceleration nor the mechanisms responsible for the onset of detonation

have been clearly identified. Difficulty in direct optical observations of

condensed phase combustion and the rapid time and spatial resolutions of

the diagnostics required for other indirect measurements severely hinder

progress. The DDT problem is not likely to be resolved even for gaseous

explosives in the near future. Yet the DDT problem is perhaps the most

important one in detonation reserch for it is directly related to the

question as to why and how explosives detonate. The occurence of DDT

ensures that the explosive (and its boundary conditions) can sustain a

detonation. Quite often, very strong shocks are used to initiate a

detonation even though the explosive and its boundary conditions are

outside the limits. The ability to undergo a transition implies the ability to self zelzera- the conditions necessary for the rapid detonation processes in the explosive. The detonability of the explosive is thus unambiguously demonstrated.

4. Concludinp: Remarks

I have attempted to synthesize the wide spectrum of research efforts in

detonation physics into a few fundamental issues. I have also given a

brief account as to what I see is the current state of knowledge in the

resolution of these fundamental detonation problems. This I hope will

serve as an adequate starting point for the subsequent discussions that

follow. It may be redundant to point out that the central problem is

detonation and the various disciplines are to be applied to the resolution

of the detonation problem. It is really a question of finding the right

shoes to fit the feet and not vice versa. The success of any cooperative

effort between specialists of different disciplines depends very much on

the commitment of all concerned towards the solution of the central

problem. We have learned what each discipline can do, now we have to

critically examine their applicability to the detonation problem.

Remarques de

l o La c e l e r i t e D e s t determinee p a r l l @ n e r g i e li b e r e e l o r s de l a r e a c t i o n chimique, diminuee eventuellement de "pertes". C e l l e s - c i sont & imputer &

l a presence des p a r o i s ( p e r t e de chaleur, f r o t t e m e n t ...) ou a l e u r absence ( d e t e n t e l a t e r a l e ) .

E l l e s ne peuvent e t r e totalement absentes que dans l e cas de detonation spherique ( l a q u e l l e e s t en quelque s o r t e auto-confinee). Par consequent pour

" c o n t r o l e r " t e l l e ou a u t r e t h e o r i e ( t e l ou a u t r e modele) du mecanisme de l i b e r a t i o n de l ' e n e r g i e , en se r e f e r a n t a l a c e l e r i t e de l a detonation, c ' e s t l a c @ l e r i t e de l ' o n d e de d e t o n a t i o n sph6rique d i v e r g e n t e q u ' i l f a u t connai- t r e .

2' Au s u j e t de l a comparaison des d e f l a g r a t i o n s e t de detonation.

a ) L ' e n e r g i e l i b e r e e e s t l a mPme, mais l e "couplage" e n t r e l e choc e t l a combustion e s t totalement d i f f e r e n t e t il n ' e x i s t e que t r e s exception- nellement des regimes de propagation des d e f l a g r a t i o n s

c & l @ r i t e SF cons t a n t e .

b ) La t r a n s i t i o n des d e f l a g r a t i o n s en detonation i m p l i q u e l a f o r m a t i o n d'une onde de choc suffisamment f o r t e e t donc l ' a c c e l e r a t i o n du f r o n t de combustion. C ' e s t une c o n d i t i o n necessaire, mais nullement, en p r i n c i p e , s u f f i s a n t e . La t r a n s i t i o n se p r o d u i t l o r s q u e e n t r e l e f r o n t de choc e t c e l u i de combustion sont r e a l i s e e s l e s c o n d i t i o n s de p, de T e t de turbulence encore insuffisamment connues. Dans l e cas des ex-.

p l o s i f s condenses, e t a n t donne l e s c 6 l e r i t e s de propagation du f r o n t de combustion (mm/sec a quelques m/sec & haute pression), l a f o r m a t i o n d'une onde de choc f o r t e p a r a i t problematique,

moins q u ' i l

a i t un mecani sme.

R e m a r q u e d e C.

La t h e o r i e thermo-hydrodynamique de l a d e t o n a t i o n ne p a r a i t pas d e v o i r & t r e remise en cause s i l a charge explosive a des dimensions s u f f i s a n t e s pour qu'un regime monodimensionnel s t a t i o n n a i r e puisse & t r e e t a b l i e t que l e s diverses r e a c t i o n s chimiques a i e n t des c i n e t i q u e s v o i s i n e s .

Par contre, 1 'analyse microscopique (au sens p r e c i s e dans 1 ' a t e l i e r ) e s t ne- c e s s a i r e pour comprendre l e s processus de d i s s o c i a t i o n de l a molecule explo- s i v e d e r r i e r e l e f r o n t :

- La mise en evidence du champ e l e c t r i q u e dans l e f r o n t ( v o i r expose de M. PRESLES) e s t i n t e r e s s a n t e e t c o n s t i t u e 1 a premiere demonstration d'une r e g i o n hors e q u i l i b r e .

- Des analyses de ce tyDe a i n s i que des observations e t mesures de spec- t r o s c o p i e a c t i v e d e v r a i e n t c o n s t i t u e r l e s o u t i l s de l a recherche f u t u r e

& c o n d i t i o n d ' 6 t r e appl iquees & des e x p l o s i f s homogenes ( 1 iq u i d e s ) ou

& o r i e n t a t i o n ster6ochimique connue (monocristaux) .

R e m a r k s f r o m R. CHERET

When d e a l i n g w i t h e x ~ l o s i v e s , a PRACTICAL ISSUE i s a q u a n t i t a t i v e e s t i m a t i o n o f HAZARDS and EFFECTS o f a f i n i t e e x p l o s i v e s t r u c t u r e under a given e x c i t a - t i o n . The u l t i m a t e goal o f t h i s e s t i m a t i o n may be one o r several among t h e f o l l o w i n g :

- m i n i m i s i n g . a dimension,

. a t o t a l mass,

. a t o t a l volume,

. some c o s t / e f f e c t i v e n e s s r a t i o ,

. some f a i l u r e frequency,

. some t r i a l and e r r o r process expenses,

. e t c ...

- compromising w i t h a non-explosive p r o p e r t y such as thermomechanical behaviour, etc.. .

Thinking o f these p r a c t i c a l issues . ( t h e economical weight o f which must be k e p t i n mind) makes me uncomfortable d u r i n g these days because such a work- .shop does show which TOOL each o f us has and what he doing w i t h h i s t o o l ,

b u t does n o t show which t o o l ( s ) i s ( a r e ) a p p r o p r i a t e f o r a given PRACTICAL ISSUE.

L e t us consider an example : t h e problem o f t h e steady detonation v e l o c i t y D i n a l o n g plastic-bonded s o l i d e x p l o s i v e rod, surrounded by a g i v e n medium m.

This v e l o c i t y D depends on t h e diameter B o f t h e rod, on t h e i n i t i a l temper- ature, on some parameters o f t h e aggregate (mean g r a i n diameter

f o r example) and some parameters o f t h e surrounding medium. Yodeling D r e q u i r e s many t o o l s .

- continuum mechanics a t 0 l e v e l ,

- continuum thermodynamics a t

l e v e l ,

- quantum s t a t i s t i c a l mechanics a t molecular l e v e l .

A l l these t o o l s have been separately discussed d u r i n g t h i s workshop; b u t t h e use o f modern methods o f MULTIPLE SCALING which enable t o combine the t o o l s has n o t even been mentioned.

I n o t h e r terms and i n conclusion, I would l i k e t o p o i n t o u t t h a t much

r e c e n t progress i n t h e f i e l d o f combustion and explosions has o r i g i n a t e d from

t h e use o f t h e FlULTIPLE SCALING methods, and t h a t much more i s s t i l l t o come.

VOIES DE RECHERCHES

1 .

I n t e r v e n t i o n d e J.

Les suggestions pour des voies de recherches p o r t e r o n t s u r t r o i s p o i n t s : l e s e x p l o s i f s primaires, c e r t a i n s melanges, l e s oolymeres e x p l o s i f s .

1 - E x p l o s i f s primaires. DiamPtres c r i t i q u e s .

L ' o b j e c t i f e s t de chercher d comprendre ce qui. se passe e n t r e l e nanometre e t l e m i l l i m e t r e , e t donc d l & t u d i e r l e s e x o l o s i f s p r i m a i r e s e t l a faqon dont s ' y developpe e t s ' y propage 1 a detonation.

- Y a - t - i l pour l e s e x p l o s i f s p r i m a i r e s un oassaqe d i r e c t de l ' i n i t i a - t i o n ?I l a detonation ou y a - t - i l t r a n s i t i o n p a r d e f l a g r a t i o n i n t e r m e d i a i r e ? S i o u i , comment l ' o b s e r v e r , comment mesurer l a longueur de l a zone de reac- t i o n ? e s t - e l l e f o n c t i o n de l a nature e t de l l i n t e n s i t @ de l ' i n i t i a t i o n ?

- Si l a mesure d i r e c t e e s t t r o p d i f f i c i l e , que se p a s s e - t - i l s i l ' o n melange 1 'e x p l o s i f p r i m a i r e avec a u t r e chose ( e x p l o s i f secondaire, d i l uant i n e r t e s o l i d e , d i l u a n t i n e r t e l i q u i d e , s o l v a n t ) ?

- Y - a - t - i l deux s o r t e s d ' e x p l o s i f s p r i m a i r e s , ceux q u i sont " s t a b l e s "

e t pour l e s q u e l s une i n i t i a t i o n d ' i n t e n s i t e non n e g l i g e a b l e e s t necessaire (par ex. azide de plomb) e t ceux q u i sont t e l l e m e n t i n s t a b l e s que c e t t e

~ n i t i a t i o n peut & r e extremement f a i b l e pour provoquer l e u r d e t o n a t i o n ( p a r ex. N H, ou HNF2 s o l i d e ...). O n t - i l s des courbes de p o t e n t i e l s d ' e t a t fondamenta? d i f f e r e n t e s , ou b i e n ne s ' a g i t - i l que d ' une v a l e u r d i f f e r e n t e des s e u i l s d ' e x c i t a t i o n ?

2 - Melanges.

On peut certainement o b t e n i r des renseignements i n t e r e s s a n t s p a r t i r de me1 anges.

- Melanges homogPnes, p a r exemple de l i q u i d e s , d'eutectiques, de c r i s - t a u x s y n c r i s t a l l i s ~ s ; comparaison de deux e x p l o s i f s l i q u i d e s , l ' u n forme de molecules d'un seul p r o d u i t , l ' a u t r e de deux l i q u i d e s m i s c i b l e s , l ' u n oxydant, l ' a u t r e combustible, l e melange ayant une composition g l o b a l e v o i - s i n e de c e l l e du premier ~ r o d u i t . Etude du nitromethane s e n s i b i l i s e par une ami ne.

- Melanges h@t&rogenes avec une phase continue e t une phase g r a n u l a i r e , en f o n c t i o n du taux de l a phase g r a n u l a i r e au voisinage du taux de percola- t i o n ; en f o n c t i o n de l a t a i l l e des grains, descendant j u s q u ' i i quelques nano- metres.

3 - Polymeres e x p l o s i f s .

U t i l i s a t i o n de polymeres e x p l o s i f s homogenes e t b i e n c a r a c t e r i s @ s , comme materiau d'etude : avantages, d i f f i c u l t e s ? On Dourra prendre comme e x p l o s i f de base l e n i t r a t e de p o l y v i n y l e . On oourra f a i r e v a r i e r sa s t r u c t u r e ( n a t u r e des enchainements), sa composition ii p a r t i r des copolymeres avec des carbonates de v i n y l e n e ou de m@thylvinyl@ne. On peut f a i r e v a r i e r sa s t r u c - t u r e de facon continue, f a i r e une etude f i n e autour,des p o i n t s de t r a n s i - t i o n ; on peut mPme c r e e r des s t r u c t u r e s p l u s ou moins ordonnees, par modi- f i c a t i o n des polymeres t e l l e s que des i n s e r t i o n s de grouoes mesogenes ( s t r u c t u r e s nematiques ou smectiques).

I 1 f a u t n o t e r e t s o u l i g n e r e n f i n une p r o p o s i t i o n de recherche f a i t e par

M. de L o n g u e v i l l e : m o d i f i c a t i o n d'une onde de detonation en regime s t a t i o n -

n a i r e par une p e r t u r b a t i o n .

2.

I n t e r v e n t i o n d e A.

"Simple" shock-induced chemistry i n non-expl osives.

- Are shocks thermal ?

- How does mechanical shock promote r e a c t i o n ? I d e a l e x p l o s i v e systems.. . 1 iq u i d , s i n g l e c r y s t a l

- more exoerimental methods a r e u s e f u l

- Results may be modeled Granular "pure" explosives

- m a t e r i a l v a r i a b l e s

- i n i t i a t i o n surface

- s u b - i n i t i a t i o n t h r e s h o l d

- steady-state detonation.

3.

F u t u r e of C h e m i c a Z K i n e t i c s in D a t o n a t i o n

A - Coupling o f Chemical Reactions w i t h F l u i d Mechanics.

B - D i s t i n g u i s h i n g the d i f f e r e n t Reaction Stages.

I ) Decomposition 2 ) Heat Release

C - Coupling Experiment and Theory.

1) Species I d e n t i - f i c a t i o n

2 ) Regimes : Thermal Decomposition, D e f l a g r a t i o n , Detonation.

4.

M o d e Z Z i n g I n i t i a t i o n / Detonation. " C r i t i c a Z I s s u e s "

J. N U N Z I A T O Hot Spots

.. Mechanism o f Formation

Pore c o l l a p s e vs. shear band Mechanism f o r Hot Spot decomposition

Thermally a c t i v a t e d vs. bond s i s s i o n Role o f Hot Spots i n Detonation

D e t a i l e d Chemical K i n e t i c s

- 0

Best estimates o f r e a c t i o n oaths

Rate oarameters

- *

Role o f phase changes (me1 t i n g ) Endothermic r e a c t i o n s

Pressure ( d e n s i t y ) e f f e c t s n n condensed phase r e a c t i o n s

- Numerical M o d e l l i n g (continuum) o f I n i t i a t i o n / Oetonatiorl

Provides answers, i d e n t i f i e s s e n s i t i v e parameters, guides exoeriments, t i e s experiments t o g e t h e r

.. F l u i d / S o l i d mechanics. "Hot Spots"

Chemical K i n e t i c s

- ZND/CJ Theory

D e t a i l e d k i n e t i c s Products EOS

- I n s t a b i l i t i e s

- Boundary Conditions Computational Methods (Hardware)

Remarks o f D . T S A I a f t e r t h e Megeve W o r k s h o p

(July,

A C e n t r a l Q u e s t i o n : R a t e o f E n e r g y T r a n s f e r i n a n E n e r g e t i c S y s t e m .

Some o f the, t o n i c s discussed a t t h e workshop may be broadly d i v i d e d i n t o t h r e e groups : (1) hydrodynamic ~henomenology, ( 2 ) chemistry o f detonation, ( 3 ) mole- c u l a r dynamics o f energy t r a n s f e r processes.

1. Dn the hydrodynamic approach, i t i s b e l i e v e d t h a t t h e equations f o r d e s c r i b - i n g detonation a r e complete, b u t t h a t i n o r d e r t o describe t h e phenomenology more accurately, b e t t e r chemical data are r e q u i r e d .

2. On t h e chemistry o f detonation, t h e main concern i s i n s t u d i e s o f bond energies, r e a c t i o n path ways and r e a c t i o n r a t e s o f e n e r g e t i c molecules i n the gas phase. Reactions i n dense systems a r e n o t emphasized.

3. The molecular dynamical s t u d i e s have emphasized energy t r a n s p o r t processes i n h y p o t h e t i c a l ( s i m p l i f i e d ) chemical models i n t h e condensed phase capable o f undergoing sustained exothermic r e a c t i o n s l e a d i n g t o detonation.

All t h r e e approaches a r e concerned w i t h t h e c e n t r a l question o f t h e r a t e o f energy t r a n s f e r i n an e n e r g e t i c system s u f f i c i e n t t o cause i n i t i a t i o n and s u s t a i n - ed detonation.

Since each has i t s own l i m i t a t i o n s , these approaches must be pursued i n a comple- mentary way i n o r d e r t o ensure progress i n our understanding. However, t h e languages o f t h e t h r e e groups were s u f f i c i e n t l y d i f f e r e n t as t o h i n d e r f r e e ex- change o f ideas, For example, i t was thought t h a t t h e d i f f e r e n c e i n s c a l e between hydrodynamics and molecular chemistry was much t o o g r e a t f o r t h e detonation process t o be modeled by molecular dynamics. On t h i s respect, t h e workshop has served a p a r t i c u l a r l y u s e f u l f u n c t i o n i n e n f o r c i n g exposure t o new areas and new approaches, w i t h r a t h e r good r e s u l t s . Such exposure should be broadened i n o r d e r t o achieve continued progress.

On a d d i t i o n t o exposure, t h e p o s s i b i l i t y o f j o i n t i n v e s t i g a t i o n s combining t h e

t h r e e approaches were a l s o discussed.. . and have t o be promoted.

Chacune des quatre premieres journees de l'atelier a fait l'objet d'un chapitre oh se trouvent retranscrites les conf6rences donn6es dans l'ordre du programme. A la suite de chacune d'elles figurent, s'il y a lieu, les seules interventions manuscrites r6dig6es sur place. Elles ne temoignent que bien partiellement des discussions passionnees dont la diversit6 allait de pair avec celle des participants. La t2che du redacteur d'une synthkse objective de la journb n'en est que plus mkritoire dans la mesure oii "le participant lecteur" s'y retrouve un peu dans sa propre pensee.

Le cinquikme chapitre, memoire de la derniere journee, bilan d'ensemble de nos reflexions, s'il ouvre des perspectives d'avenir certaines, doit dtre percu aussi dans sa di- mension restrictive. En effet, nous avons bien compris ce que chacun peut r6aliser dans son domaine B l'kchelle microscopique ou B 1'6chelle macroscopique, dans son applica- tion au problhme dktonique, mais nous mesurons davantage combien chacun d'entre nous doit rester critique vis-B-vis des corr6lations trouvbes dans les deux approches et r6serv6 en ce qui concerne les indispensables relations th6orie-expkrience.

Cette perception a eu le mkrite de nous entrainer naturellement dans une dy- namique de regroupement de nos potentialites compl6mentaires, puisque maintenant nous nous connaissons, nous nous comprenons et nous rkalisons mieux le chemin B par- courir ensemble. Des kchos venant des Etats Unis, d'Europe, me sont parvenus dans ce sens et 12int6rbt pour une seconde rencontre s'est clairement manifest6.

Ainsi est atteint l'un de nos objectifs (cf. introduction) : "entrainer, au delb de cette rencontre, une collaboration cordiale et efficace" ... mais dans quelles directions ? Le problhme posk par les transferts d'energie, leur vitesse, dans les syst&mes mol6culaires 6nerg6tiques est, et reste au centre des preocupations des exp6rimentateurs et des thkoriciens. Quel que soit le type d'approche d'ktude des mecanismes, dy- namique mol6culaire, hydrodynamique, iI veut repondre B la question essentielIe qui concerne le bilan d'6nergie dans un systeme pour initier la detonation et en assurer le d6veloppement.

Au niveau thkorique, I'ordre de grandeur du nombre de particules pris en compte N, N4, ou N ' ~ , (2

N

lo), impose le choix d'une technique et d'une applica- tion physique B la dktonique soigneusement d6liitke. On ne saurait assez regret- ter l'absence B l'atelier, de physiciens de la mkcanique quantique statistique pour l'ktude, par exemple, des m6canismes engag6s dans la phase liquide et l'analyse de son comportement vis-&vis de la phase solide. Tant du point de vue expkrimental que thkorique, on a beaucoup Bvoqu6 les "points chauds" comme support de ce transfert d'energie, leur formation et leur mode d'action. Leur r6le dans la detonation qui passe par un processus de decomposition molkculaire n'est cependant pas clair.

Dans chacune des voies d'exploration adoptbs, microscopique ou macroscopique, experimentale ou theorique, nous nous trouvons confrontks B la moMcule, c'est-bdire B l'ordre de grandeur No, B la fois le plus simple pour le theoricien et le plus complexe pour l'expkrimentateur.

C'est en effet la dBcomposition de la mol6cule en fragments reactifs qui entraine les

processus physico-chimiques derriere le front de detonation et l'entretien du ph6nomkne

par l'knergie chimique degag6e.

C4-430

DE

Les Btapes chimiques, leurs couplages avec la mBcanique des fluides, phBnom&nes essentieis non encore maitrises, la cinetique chimique, passent par 1'Btude de la structure du front de l'onde de detonation, de la reaction de la molBcule au stimulus (modifica- tions internes : excitation Blectronique, polarisation...). Si son aptitude B la diisocia- tion peut dtre evaluBe thhriquement, la d6finition des produits de d6composition de la molBcule, des mecanismes de sa fragmentation, la reconnaissance des espgces chimiques prBsentes dans le front de dktonation et leur Bvolution en fonction de la temperature et de la pression dBj& abordBe par certaines experiences dBcrites ci-dessus, restent un champ d'exploration de haute technicit6 compte-tenu des Bchelles de temps concern6es.

Lors de ces Bchanges de nos connaisances et de nos savoir-faire, libre cours a 6t6 donne B l'imagination creatrice des groupes informels que nous avons pu constituer, en reaction aux questions posees. Les idees qui germent s'Bchappent d6jB du cadre de ce livre.

Nous tenons d remercier Monsieur Seigneur, directeur de l'Ofice du Tourisme, et Monsieur Tourneur, directeur du Palais des Congrds de Megdoe, qui, toujours disponibles, n'ont pas manque' de transmettre d ce se'minaire le message de plinitude et d'harmonie de la montagne dont ils sont porteurs.

Simone Odiot

Each of the first four days of the workshop is the subject of a chapter in which is found, following the order of the programme, the transcribed conferences which were presented. Following each of these appear, if they arose, the only manuscripts written on site. They are only partially witness to the passionate discussions whose diversity goes hand in hand with that of the participants. The task of writing an objective synthesis of the day is all the more praiseworthy in that the "reader participant" is able to find his own thoughts.

The fifth chapter, report of the last day, appraisal of our reflections, while open- ing up certain future perspectives, must also be perceived in its restrictive dimension.

Indeed we have well understood what each of us can achieve in his domain on the microscopic or macroscopic scale in its application to the detonation, but we are now capable of judging to what degree each of us must remain critical vis B vis the corre- lations found in the two approaches and reserved with respect to all that concerns the indispensable theory-experiment relationships.

This perception had the merit of leading us naturally t o regroup our comple- mentary potentials, since we now know each other, we understand each other and we will better accomplish the path to be covered together. The echoes from the United States and Europe were in this spirit and the interest in a second meeting was clearly manifested.

Thus was achieved one of our objectives (cf. introduction) : "to achieve through this meeting a cordial and efficient collaboration".

The problem posed by energy transfers, their speed, in energetic molecular sys- tems is, and remains, the central preoccupation of experimentalists and theoreticians.

Whatever the type of approach to studying molecular or hydrodynamic mechanisms, he wants to reply to the essential question which concerns the assessment of energy in a system necessary for initiating the detonation and ensuring its development.

At the theoretical level, the range in the number of particles taken into account N, N4 or N1* (2 5 N 5 10) imposes the choice of a technique and a carefully defined physical application to detonics. We will never regret enough the absence of statistical quantum mechanic physicists at the workshop who could have contributed to the study, for example, of the mechanisms involved in the liquid phase and the analysis of its behaviour vis B vis the solid phase.

As much from the experimental point of view as from the theoretical, we have often raised the "hot point" as a basis of this energy transfer , their formation and their mode of action. Their role in the detonation which passes by a molecular decomposition process is not however clear.

In each of the adopted channels of exploration, microscopic or macroscopic, exper- imental or theoretical, we find ourselves confronted by the molecule i.e. range No, at the same time simpler for the theoretician and more complex for the experimentalist.

It is in fact the decomposition of the molecule into reactive fragments which leads to the physico-chemical processes behiid the detonation barrier and the upkeep of the phenomena through released chemical energy.

The chemical stages, their coupling with fluid mechanics, essential phenomena not

yet mastered, chemical kinetics, require the study of the structure of the detonic wave front, by the reaction of the molecule to stimulus (internal modifications : electronic excitation, polarization ...) if its aptitude to dissociation can be theoretically valued, the definition of the decomposition products of the molecule, its fragmentation mech- anisms, the recognition of the chemical species present in the detonation front and their evolution with respect to temperature and pressure already dealt with by certain experiments described earlier, remain a field of exploration of high technicality, taking into account the time scales concerned.

During these exchanges of our knowledge and know-how, free rein was given to the creative imagination of the informal groups which we were able to set up, in reac- tion to the questions raised. The ideas which are germinating already lie outwith the framework of this book.

We would like to thank Mr. Seigneur, the Director of the Tourist Ofice, and Mr.

Tourneur Director of the Palais des CongrCs at Megive who, always at hand, did not fail t o pass on to this seminar the mountains' message of richness and harmony.

Simone Odiot