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Pressure Desensitization of a Gassed Emulsion Explosive in Comparison

with Micro-balloon Sensitized Emulsion Explosives

Shulin NieSwedish Rock Engineering Research/SveBeFo,

Box 491 53, S-100 29 Stockholm, Sweden

ABSTRACT

The detonability of a chemically gassed emulsion explosive has been studied in recent

tests, after the study of three micro-balloon sensitized emulsion explosives. Blasting

experiments in steel pipes and computer simulation have been carried out. This study

showed that the gassed emulsion explosive and the micro-balloon sensitized emulsion

explosives have different desensitization features. Namely, the gassed emulsion explosive

can recover its detonability very rapidly after the pressure vanishes, in contrary to themicro-balloon sensitized explosives.

The experiments determined that the tested emulsion explosive can tolerate a pressure of

as high as 17 MPa before it is dead-pressed. It also showed that the explosive can recover

its detonability rapidly. The computer simulation indicated that the dead-pressing and the

detonability recovery are very rapid processes, approximately 2 ms for the dead-pressing

and 50 ns for the detonability recovery. These simulation results are consistent with the

experimental evidences. This paper describes the study of the chemically gassed emulsion

explosive. The properties of the explosive, experimental design and procedure, simulation

principle and algorithm as well as the final results are outlined.

INTRODUCTION

The pressure desensitization of emulsion explosives is a well-known phenomenon. Many

research results can be obtained from the earlier proceedings of this conference in the

recent years. However, experience and experiments /1/ indicated that the desensitization

characteristics of a gassed emulsion can be quite different from those of a micro-balloon

sensitized emulsion. Therefore, after the studies of three micro-balloon sensitized

emulsions at SveBeFo /2/,a further study has been carried out on a chemically gassed

emulsion explosive. This study will be described in this paper and in detail in a SveBeFo

report /3/. The experiments, computer simulations and results achieved are outlined.

EXPERIMENTAL TECHNIQUES AND RESULTS

The Chemically Gassed Emulsion Explosive

The tested explosive is the fourth emulsion explosive in our testing series /3/. The main

difference between this explosive and the other three explosives is the different sensitizer

used. Namely, this explosive is sensitized by the means of chemical gassing instead of

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the dead-pressing and the detonability recovery after the dead-pressing are estimated.

Mechanism of Initiation and Dead-pressing

Initiation mechanism: Several initiation mechanisms can be involved in the initiation of a

high explosive, e.g. the hydrodynamic mechanism, adiabatic gas compression in thecavities and viscoplastic heating in the vicinity of cavities /6/. However, the viscous

heating of matrix in the vicinity of micro-balloons or gas bubbles is the dominant

mechanism responsible for the shock initiation of an emulsion explosive, as can be

concluded from reference /6/.

In the simulation, the initiation mechanism is assumed to be the viscous heating of the

matrix and the isentropic compression of the gas bubbles. The viscous heating and the gas

compression will heat up the matrix surrounding the gas bubbles. When the matrix reaches

a critical temperature, it starts to burn and an initiation results. This critical temperature is

assigned to be 300 C /3,7/.

Dead-pressing mechanism: The dead-pressing is a failure of initiation by the

above-mentioned initiation mechanisms. During the pre-compression, the matrix flows

inwards towards the centres of the gas bubbles. This flow results in a viscous heating in the

matrix itself and a compression in the bubbles. The bubble volume decreases. However,

the bubble temperature increases accordingly, determined by the equation of state of that

gas, and will be higher than the temperature in the surrounding matrix. Therefore, heat

dissipation from the gas bubbles to the matrix takes place. This heat loss will lower the

bubble temperature and further decrease the bubble volume, provided that the pressure is

kept constant.

If the explosive is re-shocked by a primer while the gas bubbles are cold and small, theexplosive may not be initiated because of insufficient viscous heating and gas compression.

Mechanism of detonability recovery: After the pre-compression vanishes, the gas bubbles

expand. Meanwhile, as the bubble volume increases, the temperature decreases

correspondingly. When the temperature in the bubbles is lower than the matrix

temperature, the bubbles will regain heat from the matrix. Gradually, the bubble

temperature and the bubble size will return to such a degree that the explosive is detonable

again.

Calculation Algorithm

Geometry and element layout: A spherical symmetry was assumed and the calculations are

performed in the spherical co-ordinates, of which the origin is located at the centre of the

gas bubbles. The matrix surrounding each gas bubble is divided into elements of spherical

shells, see Fig. 3.

Pressure profile for pre-compression, initiation and detonability recovery: The

pre-compression pressure has a simplified profile; it increases from 1 atm to 17 MPa in 0.1

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ms and then keeps constant at 17 MPa, see Fig. 4. This is an idealised profile based on the

measurements from the experiments /3/. The initiation pressure is a shock wave with an

amplitude of 24.8 GPa, which is the detonation pressure of the primer used in the

experiments /3/.

To simulate the recovery of detonability, the pre-compression is suddenly released from 17MPa to 1 atm after 20 ms. As will be described later in this paper, after a waiting time of

20 ms, the pre-compression can no longer harm the explosive significantly, in terms of

decreases in the bubble volume and temperature. However, this pressure release is an

artificial case. In the reality, there always exists a decaying time.

Program flow chart: The program flow chart is shown below. Four major calculations are

carried out in each time step as marked by ,,, and), in the flow chart. The

governing equations for the calculations are described in the following.

Viscous flow of matrix and the resultant heating: Compared with the gas bubbles, the

compressibility of the matrix is negligible. Therefore, the matrix is assumed to beincompressible in the simulation. For a viscous and incompressible fluid, the flow is

governed by the Navier-Stoke equation (Eq. 1) and the continuity equation (Eq. 2) below

/8/.

Where: ur= radial velocity, (m/s)

t = time, (s)

u = velocity vector ={ur, 0, 0}in a spherically symmetric flow

m = density of the matrix, (kg/m)

p = pressure in the matrix, (Pa)

r = radius, (m)

= viscosity of the matrix, (Pa s)


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Thermal conduction constants:

Coefficient of heat transfer between N2gas and the matrix: = 4190 W/m/K/9/

Thermal conductivity of the matrix: ~ = 0.209 W/m/K /9/

Results of Simulation

Dead-pressing: Simulation results are presented in Figs. 5 through 10. Figs. 5 and 6 show

how the radius respective the temperature of the N2gas bubbles changes during the

pre-compression. The radius decreases very rapidly in the beginning but more slowly lateron, while the temperature first increases due to the compression and then decreases due to

the heat transmission to the matrix. It can be seen that after 1 ms of compression, the

radius and the temperature decrease very slowly. After 20 ms, they no longer decrease

significantly.

Fig. 7 shows the temperature in the matrix element adjacent to the N2bubble first

increases due to the viscous heating and the heat gained from the N2bubble but then


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decreases due to the heat conduction.

If, at different waiting times during the compression, the explosive is initiated by the

primer, different temperature will be induced in the matrix by the shock. Fig. 8 shows how

this shock-induced temperature in the matrix element adjacent to the N2bubble decreases

with the waiting time. According to this figure, the explosive is dead-pressed at waitingtimes longer than 2 ms.

Recovery of detonability: When the pre-compression vanishes, the gas bubble will expand,

as shown in Fig. 9. At this time, if the explosive is initiated by the primer, a new

temperature will be induced in the matrix. Fig. 10 shows such shock-induced temperature

in the matrix element adjacent to the gas bubble after the compression vanishes.

According to Fig. 10, the explosive is detonable again 50 ns after the compression is

suddenly released.

CONCLUSIONS

A chemically gassed emulsion explosive has been studied by experiments and

simulations

The simulation resulted in good agreement with the experimental work.

The explosive studied can tolerate a pressure of 17 MPa, which is higher than some

micro-balloon sensitized emulsions.

The dead-pressing occurs very rapidly. It takes less than 2 ms as indicated by the

simulation.

According to the experiments, the detonability of the gassed explosive can be

recovered very rapidly. According to the simulation, the recovery takes about 50 ns,

should the pre-compression release abruptly. This rapid recovery is a unique feature

for the gassed emulsions, which the micro-balloon sensitized emulsions do not have.

REFERENCES

1. Huidobro J and Austin M: " Shock Sensitivity of Various Permissible Explosives", 8th

Annual Symposium on Explosives and Blasting Research, International Society of

Explosives Engineers, Orlando, Florida, USA. January 22-23, 1992.

2. Nie S: "Dynamic Dead-pressing of Three Emulsion Explosives", SveBeFo Report 2,

1993.

3. Nie S and Chen L: "Dynamic Dead-pressing of a Chemically Gassed EmulsionExplosive", SveBeFo Report 29, 1996.

4. Nie S: "Dead-pressing Phenomenon in Emulsion Explosives", Proceedings of the 9th

Annual Symposium on Explosives and Blasting Research, International Society of

Explosives Engineers, Jan. 31 - Feb. 4, 1993. San Diego, California, USA.

5. Nie S, Persson A and Deng J: "Development of a Pressure Gage Based on Piezo


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Ceramic Material", Experimental Techniques, pp 13-16, May/June, 1993.

6. Frey R B: "Cavity Collapse in Energetic Materials", 8th International Symposium on

Detonation, Albuquerque, New Mexico, USA. July 15-19, 1985.

Engsbriten B: Private communication. Nitro Nobel AB, 1995.

8. Acheson D J: "Elementary Fluid Dynamics", Clarendon Press, Oxford, 1990

9. Hanasaki K and Terada M: "Studies on the Sensitivity of Dead Pressed Slurry

Explosives in Delay Blasting", Rock Fragmentation by Blasting - FRAGBLAST-4.

Vienna, Austria. July 5-8, 1993.

10. Atkins P W: "Physical Chemistry", 4th Edition. Oxford, Melbourne, Tokyo, Oxford

University Press. 1990.


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