HIGH -FREQUENCY IvanovUDC 622.01.013In a high-frequency electromagnetic field, pure ice behaves like a perfect dielectric: it heats up only slightly in rapidly alternating fields. Experiments at the Leningrad Mining Institute have revealed that ordinary polycrystalline ice can be fractured efficiently in strong high-frequency electromagnetic fields (HF SEMF). This property of ice is undoubtedly of practical interest, because it offers possibilities for breaking frozen rock and ice without converting the latter into water, which would require large amounts of energy owing to the latent heat of fusion.Let us consider the action of strong electromagnetic fields (SEME) on ice. We subjected polycrystalline ice, with salinity corresponding to natural ground ice, to SEMF. The ice specimens were rectangular plates 60 x 70 x 30 mm in size. We investigated the action of SEMF on ice at high (13 and 40.63 MHz) and very high (2375 MHz) frequencies. Below we will discuss only the action of the electromagnetic field at 40.68 MHz (with a field strength of H= 4 kV/cm), inasmuch as the ice behaved similarly at the other frequencies.At 1.5-2.0 sec after the action of the high-frequency electromagnetic field on the ice, we observe a loss of optical and mechanical homogeneity; after 3-4 see we see within the specimen nearly indistinguishable liquid inclusions and cavities; after 7-8 sec a stage of intense disintegration begins and is completed by partial breakup of the specimen under its own weight. Finally, the ice specimen is converted to a heap of separate crystals. The quantity of ice converted to water at this stage is slightly over 3% of the initial weight of the specimen.Owing to the action of SEMF on ice, the latter very quickly loses its original strength practically without melting. For example, after 2-2.5 see the short-term strength of ice under uniaxial compression with the load increasing at 200 ram/rain is reduced by a factor of 5-7 below its original value; after 10 sec the short-term crushing strength of the ice is reduced practically to zero. Thus the action of HF SEMF on polycrystalline ice is accompanied by catastrophic avalanche-like loss of mechanical strength.Pure ice and ice with high salinity behave in a similar manner. The behavior of ice in strong electromagnetic fields is at first sight inexplicable; however, it can be explained if we bear in mind the fact that polycrystalline ice with any degree of salinity, and even pure ice, at subzero temperatures contains quasiliquid inclusions. This is due to the general property of matter that a film phase has a lower melting point [1].The existence of quasiliquid layers on the surfaces of ice crystals down to -40"C follows from many experimental data and theoretical conclusions [2].The fact that in ice at subzero temperatures there is a certain amount of ice with molecules which are very much more mobile follows from nuclear magnetic resonance data [3]. Furthermore, direct optical observations [4] have shown that growth of ice crystallites continues at-40"C. Natura...
The use of the weakening action of powerful electromagnetic fields (PEMF) on rocks to intensify their breakage has been discussed in several scientific-technical papers [1][2].The recent creation of efficient methods of inducing electromagnetic fields in rocks enables us to proceed to the construction of a fundamentally new class of heading and earth-moving machines with electrothermomechanical (ETM) cutting heads.Radiating and rock-breaking elements of ETM heads can in principle be constructed in the form of spatially separated independent units [3], or their functions can be combined in a single construction [4].Input of electromagnetic energy into the rock by ETM heads of the separated type takes place from the surface of the rock. Owing to the comparatively shallow penetration of the electromagnetic field (especially in the superhigh-frequency range), this involves the construction of multistage heads which can minimize the expenditure of electromagnetic energy on working the rock.Inthis article we give a theoretical solution of the problem of optimization of the input of electromagnetic energy by multistage ETM heads. The solution is based on an idealized model of an ETM head, which creates a one-dimensional electromagnetic energy flux perpendicular to the direction of motion of the machine. Assuming that the electrical parameters of the medium do not alter under the influence of the electromagnetic field (i.e., using the averaged characteristics), neglecting the thermal conductivity, we succeed in obtaining simple analytical formulas and in estimating the influence of various factors on the optimization of the breakage process.Let us consider an ETM head consisting of n stages. Let us denote the power emitted by stage m (m = 1, 2 ..... n) by Pm, the length of the emitter by l m, and its width by a.The emitted electromagnetic field acts in the region x > Xm_ t ( Fig. 1: dashed arrows denote direction of flux of electromagnetic energy~ solid arrows denote action of mechanical forces).It is known [5] that in the one-dimensional ease the electric and magnetic field intensities areHere A is the amplitude of the electric field, to is the angular frequency, k = k0( eV )t/z is the wave number of the wave in the rock, k0= w(e0V0 )I/z is the wave number of the wave in air, e and ~ are the relative dielectric and magnetic permeabilities of the medium, e 0 and /a 0 are the absolute dielectric and magnetic constants, and w= (~/e) t/z is the wave impedance of the rock. Since electromagnetic energy is absorbed in the rock, the relative dielectric constant is a complex number: e = e'+ie'.Consequently, the wave number is also complex: k= B' + ia t. The attenuation factor [5] is o~=ko(e'/2) '/'[ 1W (l+tg ~ 6)'/,] ,/, tg 6 tg 6= e"/e'.For most rocks tan z 6 << 1, and the expression for the attenuation factor takes the simpler form (2) i G. V. Plekhanov Mining Institute, Leningrad.
Drtvlng vertical or steeply sloping mine workings in permafrost is well known to be one of the least mechanized and inefieleientoperatlons in the underground working of aUuvtal deposits and the fabrication of underground engineering structures, and It leads to marked Impairment of the economic Indices.With existing methods (drilling and steam-heating of predrilled holes) the rate of drivage of vertical workings tn frozen rocks for communication with the surface Is only 2.5-5.0 m/day. It ts therefore evident that new, more efficient methods must be found.At the Leningrad Mining Institute an electrothermal (ETM) machine for borehote dtiUfng has been constructed: it is the first practical specimen in the world [1]. It can give a tenfold Increase tn the rate of driving workings.The physical basis of the machine Is the discovery that frozen rock Is softened by a powerful high-frequency electromagnetic field [2], the energy of which is absorbed by the rock. After absorption of a sufficient quantity of energy the rock becomes suttable for mechanical working by rock-breaking cutter bits.Inasmuch as during widening of a borehole there are two surfaces accessible for working, it was possible to design a comparatively simple ETM cutting head with efficient combined action on the frozen rock -softening by a uhf eleetromagnetie field at the stdes of the hole and mechanical rock cutting tn a cavity perpendicular to its axis. A diagram of the cutting head Is shown tn Fig. 1 (the horizontal arrows denote the direction of the flux of electromagnetic energy, and the vertical arrows denote the direction of the mechanical forces). From this figure we see that the machtne makes use of a two-stage system of electromagnetic radiators and rock-cutting elements. We will show that this type of construction is more efficient than a one-stage machine for driving large boreholes (800 mm or more in diameter). The physical necessity of making two-stage (or tn principle even multistage) ETM cutting heads is evident from the exponential attenuation of an electromagnetic field In the rock.In this article we will attempt to calculate some parameters for such a machine. In the first place, we were Interested In the Influence of the electromagnetic field on the choice of optimum driUing conditions. Distribution of Energy Sources in the Rock. The electromagnetic field crea~ed by the ETM emitter must possess the same symmetry as the hole, i.e., it depends only on the coordinates p and z. Because the mechanical action Is exerted on a layer with a thickness which Is comparable with the original dimensions of the borehole, we can assume that within this layer the field depends practically only on p.In this case, Maxwell's equatiom for an electromagnetic field which changes with time as exp iwt are [3] as follow s:~' dp (pH~)-----ikeEz,l d dE = + ik~tH~, T" dp (pE~)=--ikpHz, Hp-----0. dp E(0. E @. E z) is the electric field, H(0, Hir Hz) is. the magnetic field strength, ~ Is the dielectric constant of G. V. Plekhanov Leningrad Mining Institute.
Belikov [1] and Vlasov et al. [2] report successful thawing of frozen loose rocks in the high-frequency electric field of a capacitor with single-side electrode positioning.With single-sided electrode placement, even in a homogeneous material there is a markedly inhomogeneons electric field, the intensity of which is maximal near the surface of the electrodes and fails off progressively with increasing distance from them [3]. Clearly, the relative power dissipated in the material will fall off even more rapidly [3, 4]. A few centimeters from the surface of the electrodes the heating will be insufficient for effective high-frequency electrothermal thawing of frozen rock.Besides this shortcoming which limits the use of the field of a capacitor with single-sided electrode positioning for thawing frozen rocks, there is an equally serious drawback: owing to the necessity of thawing the whole volume of frozen rock to be extracted with this electrode configuration, the energy consumption of the process will be high [2]. * Thus, the high energy consumption on the one hand, and the impossibility of intensifying the process of thawing owing to the slow heating rate on the other, make it inefficient to use the field of a capacitor with singlesided electrode configuration for high-frequency electrothermy of frozen rocks. To make use of high-frequency energy, it is thus necessary to secure a great increase in the relative power conveyed into the frozen rock so as to intensify the process of thawing it; thawing of the frozen rock must be carried out not over the whole bulk of the rock to be extracted, but only along the peripheries of individual pieces, which are thin in comparison with the volumes of the strata. Meeting this condition will enable us to reduce greatly the cost of breaking frozen rocks; the action of high-frequency currents transformed to heat inside the rock itself should be combined with mechanical action which secures penetration of the working tool and final separation of a definite volume of the thawed zone from the solid rock.To conform with the above requirements, the working tool (working capacitor) must possess such an electrode configuration as to ensure greater power injection into the rock and heating of the frozen rock in a thin zone.Thus, the working capacitor must be constructed so that it has reasonable mechanical strength and can be introduced without hindrance into the thawed zone.These requirements can in principle be satisfied by the field created by a capacitor with plane parallel electrodes (Fig. 1).The method of high-frequency electrothermal fracture of frozen rocks (the V~TM method), developed at the Leningrad Mining Institute, combines the operations of thawing in the high-frequency field of a plane-parallel capacitor and of separation of the contoured thawed layer of pieces of frozen rock from the mass. In essence, it is as follows.* In this case, the high energy consumption of high-frequency thawing is partly due to dissipation of energy by the edges and internal surfaces of the elec...
622.01.0131. At the Laboratory for Problems of Rock Breaking by Powerful High-Frequency Electromagnetic Fields (G. V. Plekhanov Mining Institute, Leningrad), we have developed a fundamentally new class of electrothermomechanical combined instruments which simultaneously combine the functions of rock-breaking tool and radiator of uhf electromagnetic energy. These instruments can minimize the loss of electromagnetic energy in the electrothermomechanical method of rock breaking.However, it is not clear whether in principle the power consumption is reduced by the use of combined ETM instruments in comparison with ordinary mechanical rock-breaking instruments. The answer to this question is far from obvious. The point is that intensification of the process of rock breaking by ETM ins=uments is due to additional energy consumption on softening up therock (the energy of the etectromagnetic field). A sharp rise in the cutting output (the technological efficiency) can be achieved at the expense of undue increase in power consumption.An estimate of the energy consumption of rock breaking by combined ETM instruments can hardly be obtained in general form, because it depends on the strength, moisture content, temperature, density, and thermal capacity of the rock, on the characteristics of the electromagnetic field-the frequency and the complex dielectric constant of the medium at this frequency -and on the characteristics of the rock-breaking tool-angle of cut, depth of cut, etc. Therefore in this article we have made a first attempt to study the process of cutting by the simplest model of a combined ETM instrument which emits an electromagnetic field in its direction of motion.For simplicity, we will assume that the electromagnetic field is uniform, with a Poynting vector directed atong the z axis.Then the density of the energy absorbed in unit time by the rock is known [1] to bewhere ~ ~ 2k0 (e) ''~ [ ( 1 -~-t g2 6) 's: 1 ] :" is twice the coefficient o f attenuation of the electromagnetic field, k 0 is the wave number of the electromagnetic field in air, ~ = e'-ie" is the complex dielectric constant of the medium, and tan 6 = e'/e' Let Pern be the electromagnetic power of the radiator, ~. its area, ~, a coefficient representing the fraction of the energy which is radiated by the emitter into a cylinder with generators parallel to the z axis and with a crosssectional area equal to r., Then obviously ~Pem: ~ .! qo exp (--az) dz, 0 whence qo= ~gerrlxlY. 9('2)
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