Explosion

EXPLOSION (EN: explosion, blast; DE: Explosion, Abschuß; FR: explosion; ES: explosion; RU: взрыв) is the process of the rapid physical-chemical transformation of the substance, during which the energy is released, and the work is performed. There most often serve as the source of the energy for the explosion the exothermic chemical (the chemical explosion) and nuclear (the nuclear explosion) reactions.

(Figure) The massive explosion within the open-pit mine

The release of the potential chemical energy from the charge of the explosive substances as the result of its detonation or rapid combustion (the powder charge) leads to the rapid increase of the pressure within the volume of this charge, which event causes the characteristic movement of the environment, and of the products of the chemical transformation. The cavity, which was initially occupied by the charge, expands, the environment is deformed and destructed, the individual parts of this environment acquire the significant kinetic energy, and so on. The characteristic peculiarity of the movement of the environment during the explosion is the forming of the blast wave, which propagates through the medium with the speed, which is exceeding or equal to the speed of sound, thanks to which fact there are involved during the short time into the movement the large amounts of the medium.

In the broader sense, they understand as the explosion the combination of the chemical and mechanical effects, which are caused by the rapid release of the energy within the limited volume, but which are not obligatorily associated with the usage of the explosive substances.

The mechanical effects of the explosion are caused by the work, which is performed during the expansion of the products from the chemical transformation of the initial substance. These effects are conventionally divided into the brisant (local) form, and the pushing (general) form. The brisant action of the explosion manifests itself within the immediate locality of the charge, when the explosion proceeds within the hard medium, or near the surface of the hard body, while the pushing (general) action manifests itself at the distances, which are much larger than the dimensions of the charge, and is limited to the action of the blast wave. For the brisant action (the near zone of the explosion), there is characteristic the intense deformation and fragmentation of the medium, while the general effect (the size of the crater of the ejection, the degree of the fragmentation, and so on) is determined by the momentum, which is imparted onto the medium by the products of the detonation, t hat is by the initial pressure within the cavity of the explosion and by the dimensions of this cavity.

The pushing action of the explosion depends only on the energy of the charge, because there are determined by this energy the parameters of the blast wave (the intensity and duration). The destructive action of the explosion at the large distances from the charge is completely associated with the parameters of the blast wave. The maximal work of the explosion depends on the total reserve of the energy within the charge of the explosive substance (the heat of the explosion), on the properties of the products of the detonation, on the shape of the charge, and on the properties of the medium. The detonation characteristics and shape of the charge substantially influence only the brisant actions of the explosion; the pushing action is associated in the significant degree with those properties of the medium, on which there depends the total work of the explosion.

The energy, which is imparted onto the medium, is equal to the work of the expansion of the gaseous products from the reaction within the cavity of the explosion, because the expansion of the chemical transformation of the explosive substance (the products of the detonation) proceeds without the heat exchange with the environment. They most often evaluate this work according to the Cheltsov - Belyaev formula, that is as the work of the isoentropic expansion of the products, from the Pn pressure of the gas bubble, to the Pk final pressure:

where Q is the heat of the explosion;

g is the index of the isoentrope.

For the air, Pk = 0.1 megapascals, and (if the condition Pk << Pn is true) almost all the potential energy of the charge transitions into the energy of the blast wave. During the explosion of the charge within the rocks, the final pressure depends on the sturdiness of the medium, and may amount to the noticeable share of the initial pressure.

During the explosion of the charge within the vacuum (or within the atmosphere at the high altitude), the potential energy of the products of the detonation is consumed onto the imparting of the kinetic energy to them, and the products may expand without the obstacles. During this process, the speed of the movement of the products is maximal at the periphery of the cloud, while the pressure is maximal at the centre. Within the central part of the charge with the arbitrary shape, there always exist the multitude of the points, within which the speed of the movement is equal to zero. The maximal speed for the dispersion of the products within the vacuum depends on the heat of the explosion, and during the detonation of the powerful explosive substances, this speed amounts to 10-15 kilometres per second.

During the explosion of the charge within the certain medium, the expansion of the products from the detonation will proceed differently. At the moment of the explosion of the charge within the medium, there propagates along all the directions the shock wave, the parameters of which are determined by the shock adiabate of the medium, and by the initial pressure on the boundary between the charge and the medium. If the acoustic stiffness of the products from the detonation is greater, than the acoustic stiffness of the medium, then there propagates into these products the wave of the rarefaction, but in the opposite case, also the shock (reflected) wave. In the first case, the pressure within the products from the detonation at their boundary of the division with the medium decreases continuously, but in the second case, the pressure initially abruptly increases, and later decreases.

During the explosion of the charge within the air (the acoustic stiffness of which is always less, than the acoustic stiffness of the products from the detonation), there emerges the shock wave, behind the front of which the pressure decreases according to the determined law. If the explosion of the charge proceeds within the non-limited medium (for example, at the sufficient height above the surface of the Earth), then the expansion of the products from the detonation will proceed till such time, when they will occupy the certain limiting volume Vpp, within which their pressure will be balanced by the pressure of the surrounding air. For the typical explosive substances, the volume Vpp exceeds the volume of the initial charge by approximately 800-1600 times, thus the products from the explosion may cause the immediate action at the distances with the order of magnitude of the tens of the radiuses of the charge Zo. At the large distances, the action of the explosion is wholly determined by the blast wave, which has been formed, into which there transitions 80-90% of the energy of the charge.

During the explosion within the water, the emergence of the detonation wave onto the surface of the charge leads to the forming of the intensive shock wave, which is propagating within the water, to the movement of the boundary of the division, to the oscillations of the gas bubble, which floats up to the surface (see the "Underwater explosion" article).

There have the greatest national economic significance the explosions within the soils and rocks. The real rocks, unlike the air and water, represent by themselves the multi-component media, which are comprising the hard particles, liquids (petroleum, water), and bubbles of the air. Besides these facts, under the conditions of the natural embedment, the rocks manifest the clearly expressed layered and blocked structure, crackedness, and so on. The properties of the individual blocks may vary from one place to another place. This fact makes the study and prediction of the results of the explosion the difficult and not always solvable problem. Nevertheless, certain general regularities of the explosion within the soils may be identified.

During the conducting of the industrial explosions using the chemical explosive substances, the shock waves practically never emerge, because the pressure of the products from the detonation of the industrial explosive substances Rd turns out to be lesser, than the characteristic product of the multiplication r*v2zv for the medium (r is the density of the medium, vzv is the speed of the sound within this medium). The powerful shock waves are formed only during the underground nuclear explosions at the not very large distances from the charge. Because the ratio Rd/r*v2zv characterizes the compressibility of the medium, which in this case turns out to be small, therefore in many cases it is permissible to consider the soil as the non-compressible fluid. The sturdiness of the medium is, as a rule, much less than the pressure of the detonation Rd, and manifests itself only during the last stages of the expansion of the explosive cavity, because there depends on the value of this sturdiness the final pressure of the produ cts from the explosion at the instant of the termination of the movement of the medium. The general picture of the motion of the soil during the explosion is much more complicated in comparison with the explosions within the air and water, because the expansion of the cavity is accompanied by the intense deformation and destruction of the rock, which is adjacent to the charge (see the "Explosive destruction" article).

After the explosion, the portion of its energy remains within the medium in the form of the elastic energy of the residual stresses. Because the shock waves do not emerge during the underground explosion, therefore the major portion of the energy is consumed for the non-reversible deformations of the medium within the near zone, and the energy of the elastic waves, which are radiated, amounts to the small share within the total balance of the energy.

During the underground explosions, there may be distinguished around the cavity, which is filled with the expanded products of the detonation, the zone of the crushing, following which are the zone of the radial cracks, and the zone of the elastic deformations, out of the boundaries of which there is radiated the seismic wave. During the explosion near the surface of the Earth (the explosion for the ejection), the movement of the soil is complicated by the influence of the force of gravity, and by the absence of the symmetry within the general picture of the movement. The processes of the forming of the crater of the ejection have been studied experimentally for the series of the soils, for both the chemical and nuclear charges, there have been determined the speeds and displacements of the medium at various distances from the charge.

If the explosion of the charge is conducted at the great depth in comparison with the radius of the charge ro, then there emerge onto the surface only the blast waves, and there are not observed any visible changes of this surface (see the "Camouflet blasting" article). With the decreasing of the depth for the embedment of the charge, the intensity of the blast waves grows, and, at the certain depth, there does not proceed the ejection of the soil, but the soil above the cavity of the explosion is destructed, and collapses into the cavity, thus there is formed the collapse crater. With the further decreasing of the depth, there increases the speed of the ejection of the particles of the soil, and, at the certain optimal value of the depth, the volume of the crater of the ejection reaches the maximum. This volume becomes smaller with the further decreasing of the depth for the embedment of the charge.

The explosions of the large scale, which are conducted for the national economic purposes, often are directional (see the "Directional explosion" article). The forecasting of the results of the explosion under such conditions is extremely important, and the theoretical calculation is difficult. Therefore, there have acquired the great importance the methods for the modeling, which are allowing to verify the initial prerequisites during the design of the large explosions at the models of the significantly smaller scale.

The major principle for the modeling of the explosion within the arbitrary medium is such, that in case of the compliance with the geometric similarity during the increasing of the linear dimensions of the charge by n times, the field of the blast wave remains the same, if the distances and time are measured in the units, which are by n times more than the initial units. The limiting case of this principle is the energetic similarity of the explosion, which is obtained under the condition, that the dimensions of the charge are small in comparison with the remaining linear dimensions.

According to the law of the energetic similarity, the single parameter for the problem is the normalized distance r0 = r/E1/3,

where r is the distance from the centre of the explosion to the point for the measurement of the parameters from the blast wave,

E is the energy of the explosion.

During the studying of the explosion within the non-bounded medium, there has been identified, that any of the characteristics of the field of the blast wave pi may be expressed as the dependence of the type of pi = f(r0).

In the particular case of the explosion within the soils, the determining of the laws for the transfer from one scale to another turns out to be the more complicated problem, because the major quantity of the parameters is determined not by the field of the blast wave (for which there is preserved the energetic similarity), but by such values, as the force of gravity, the sturdiness of the medium, and so on. Therefore, during the change of the scale of the explosion, it is necessary not only to comply with the geometric similarity, but also to correctly model the rocks, and also to take into account the influence of the force of gravity. According to these causes, within the method, which has been proposed by M. A. Sadovsky and V. N. Rodionov, for the modeling of the explosion for the ejection, for which the force of gravity plays the substantial role, and the sturdiness of the rock s is much less, than the pressure of the products from the detonation, there may be used as the roc k the loosely bound sand, and the charge of the explosive substance is imitated by the gas bubble with the low pressure of the gas. Under these conditions, during the model experiments, the dimensionless relationships R/sgW and R/s are preserved the same, as during the large-scale explosions within the rock (here R is pressure of the products from the explosion, g is the acceleration because of the force of gravity, W is the depth for the embedment of the charge).

The crushing of the rock by the explosion turns out to disobey the simple relations of the similarity, because the initial crackedness of the medium causes the strongest influence onto the effects of the grinding of this medium.

The effects, which are characteristic for the explosion, are observed during the destruction of the vessels, which are containing the gases under the high pressure (steam boilers, "Hydrox" cartridges, and so on), during the electrical breakdown of the dielectrics (spark, lightning), and during the series of other physical processes. The impact of the meteorite onto the surface of the planet leads to the explosion-like forming of the crater (the cone of the explosion), and to the emergence of the blast wave; the jump-like change of the stressed state of the rock is accompanied by the manifestation of the seismic wave (mine bumps, earthquakes).

About the scientific researches in the field of the explosion, see within the "Physics of the explosion" article.