APPLICATION OF ELECTRICAL DISCHARGES IN DIELECTRICS FOR BLASTING OPERATIONS

A. A. VOROB'EV

An explosion is a chemical reaction as a result of which heat is released and the explosive substance is converted into gases. During the explosion of a one-kilogram TNT block, lasting one thousandth of a second, the gases do not have time to expand and occupy a volume practically equal to the original volume of the block. This volume is thousands of times smaller than that which the explosion gases would occupy at atmospheric pressure. The gases formed have a high temperature, owing to which a considerable increase in pressure occurs. Therefore the gases formed during the explosion exert great pressure on the surrounding bodies. In the explosion of TNT the pressure on the surrounding surfaces reaches 190,000 atmospheres. Owing to the short duration of the action of the explosion, this pressure is perceived by the surrounding bodies as an impact. Under impact action even liquid bodies behave as brittle ones. Solid bodies under the action of an impact load undergo brittle fracture. The fraction of plastic deformation of bodies decreases as the duration of the action decreases, and an increase in the power of the explosion takes place. The effectiveness of an explosion is determined by the great power of the process, which is ultimately defined by its short duration.

Electrical breakdown of dielectrics is likewise characterized by a high power of the process because of its short duration. These external analogies suggest the possibility of the same technical applications of electrical discharges as of an explosion.

In the breakdown of dielectrics in a condensed electrical discharge, the energy density exceeds the energy density in the detonation of explosive substances, which can be applied to the performance of blasting operations, using for this purpose the processes accompanying breakdown. The phenomenon of breakdown of a liquid dielectric is accompanied by the occurrence in it of an explosive wave, which causes a hydraulic impact on the surface surrounding the liquid dielectric. By passing through a liquid dielectric a large amount of energy in the discharge, one can cause significant or complete evaporation of the liquid. Depending on the energy released in the discharge, the vapor obtained is superheated to a high temperature. Striving to increase its volume, the vapor will exert pressure on the walls surrounding it. In this case one should not seek a high temperature of superheating of the vapor, since at temperatures of the order of tens of thousands of degrees significant ionization of the gas molecules will begin. The magnetic field of the current flowing in the discharge will act on

ions, attracting them to the discharge axis and reducing the pressure on the vessel walls.

In an electric discharge, vapor molecules—for example, water vapor—will decompose. This process is accompanied by an increase in the number of atoms and a rise in pressure. The resulting mixture of oxygen and hydrogen has the ability to explode when they combine. Such an explosion will occur when the gas temperature is lowered. By varying the rates at which the listed physicochemical processes proceed, one can select the conditions most favorable for the greatest explosive effect.

Several methods can be proposed for regulating the rate of the processes during a discharge in a liquid dielectric or semiconductor, by applying methods of braking the discharge and changing the duration of its various stages.

The leader stage of a discharge in a liquid can be bypassed if the following method is used. The gap between the electrodes in the liquid is bridged by a thin conductor. After the electrode circuit is closed to the pulse-current generator, the conductor evaporates, forming a strongly ionized channel. Along this channel an arc electric discharge will develop. Depending on the pulse parameters and the magnitude of the resistance in the discharge circuit, the duration of the discharge and the course of the processes associated with heat release and heat removal change.

As an example, we point out that the flow of pulse currents along wood fibers causes violent vapor formation and the fragmentation of the wood into pieces.

It is known that, for older designs of oil circuit breakers, it was necessary to take into account the danger of explosion during emergency interruptions and the prolongation of the electric arc. In some cases, the explosion of oil circuit breakers is accompanied by considerable destruction.

The application of the phenomena under consideration is possible in carrying out blasting operations on a small scale and, in some cases, may have advantages over the use of explosive substances. A major advantage of the method under consideration is the absence of an explosive substance and of its residues after the operation. Cartridges filled with water may be used as the material for producing the explosion.

By expending an energy of 100 kilojoules to evaporate 10 g of water, we obtain vapor at a temperature of 4000°, which develops a pressure of 190000 atmospheres (equal to the pressure developed in the explosion of one kilogram of TNT). In an explosion using water vapor, the energy is supplied from the electric network. Thus, the technology for preparing explosive substances, their storage and transport, and the costs and hazards associated with these processes are eliminated. The overall economic effect of using an “electric explosion” in the national economy, according to the cycle described, has obvious advantages over the use of explosive substances.

Let us note that the mining industry consumes a large quantity of explosive substances. For each ton of coal mined, more than 10 g of explosive substances is used. Worldwide, the coal-mining industry alone consumes over one hundred thousand tons of explosive substances.

The propagation velocity of an explosion for various explosive substances varies within the limits \((1 \div 8.5)\,10^5\ \text{cm/sec}\). For TNT it is equal—

by \(6.7 \cdot 10^{5}\) cm/sec and is still ten times less than the velocity of electrical breakdown in a solid dielectric.

The high velocity of electrical breakdown of dielectrics entails an increase in the power of this process.

The duration of the breakdown process may amount to hundredths and thousandths of a second, i.e., be a thousand and ten thousand times shorter than the duration of explosive processes. Owing to the low thermal conductivity of dielectrics, well-known difficulties arise in the rapid heating and evaporation of the entire dielectric. It is necessary to develop conditions under which breakdown of the dielectric will occur throughout its entire volume, and not only along some weakened direction. Technically this is not an easy task. If one takes a volume of water on the order of 10 g or more, then the heat from the breakdown zone during the short time of breakdown will not have time to spread throughout the entire volume. In this case the volume of water should be divided into small parts, within which the processes under consideration will have time to spread during the calculated process time.

One may consider, for example, the following device of an explosive cartridge. In the cartridge, from end to end, a system of parallel wires is stretched, through which a current pulse is passed. One of the wires, having the lowest resistance, burns out first. An electric arc is ignited between the ends of the wire. Under the action of high temperature, and also owing to the reduction in the remaining cross-section of the wires, one or several subsequent wires burn out. A discharge is likewise ignited between the ends of the burned-out wires, and so on. With a successful choice of the development of this process, in a short time one may expect to obtain filling of the entire volume of the cartridge with a zone of electric discharge and to heat the water vapor to a high temperature. For low-power discharges the spark gap is placed in a paper moisture-proof cartridge. After water is poured into the cartridge, the cartridge is hermetically sealed. Breakdown of the spark gap will be accompanied by the formation of a high-temperature zone throughout the small volume of the cartridge and by the corresponding heating of the water vapor.

In an explosion, the heating of the explosive substance that causes the reaction is transmitted not by thermal conductivity, but as a result of compression caused by an elastic wave. The propagation velocity of the elastic wave reaches several kilometers per second and considerably exceeds the propagation of the thermal wave due to thermal conductivity.

The high temperature and high pressure attained during compression determine the rapid course of the chemical reaction—the explosion.

As is known, to create a shock wave in an explosive substance, initiating substances are used, for example mercury fulminate, the explosion of which causes the explosion of the secondary charge, for example TNT.

The explosion of the initiating substance is caused by heating to a high temperature, or by impact, or by heating, for example, with a wire brought to incandescence by an electric current. By reducing the time of propagation of the reaction in the explosive substance, one can correspondingly increase the power of the explosion.

Electrical breakdown propagates in a substance 10,000 times faster than an elastic wave. If electrical breakdown is produced in a block of explosive substance, then the chemical reaction will propagate along the path of breakdown with the velocity of this electrical process, which considerably exceeds the velocity of the elastic wave.

A reduction in the explosion time will result in an increase in its power. Thus, with a smaller reserve of explosive substance, one can

but the same power can be obtained if electrical detonation is used, producing breakdown in the explosive.

For breakdown of the explosive, impulse-voltage generators may be used, producing a voltage wave with a short front.