IPPT: On the low-power-consuming technology of electrohydroimpulse processing of melt in the ladle at the precasting stage

On the low-power-consuming technology of electrohydroimpulse processing of melt in the ladle at the precasting stage

Electrohydroimpulse processing (EHIP) concerns pulse methods of physical effect on the melt. The processing is realized by iterative pressure pulses, generated at high-voltage liquid breakdown in the electric-discharge generator of elastic oscillations (EDGEO). Pressure impulse transmission in the melt through waveguide EDGEO. Characteristic feature of EHIP are high instantaneous peak power and wide frequency spectrum of unit impulse (from 0,1 to 100000 Hz).

Installation consists of three main units: power, technological and control one. As an energetic unit (power source) pulse current generator (PCG) with capacitive energy storage is used. The main components of PCG are the charging circuit, cumulative capacity, switching device (discharger), operative interval, serving as a load for PCG. In a number of cases the regulating transformer RT - 100 / 0,5 УХЛ4 380 В ТУ 16 - 517. 739.76 is used in the scheme. It guarantees pulses frequency smooth variation - from 0,1 to 16 Hz. The main element of technological unit is EDGEO, equipped with a special waveguide, as a rule, made of steel Ст3сп. EDGEO is characterized by simple design, small size, does not have revolving parts, being arranged in process flowsheets of metallurgical production rather easily. The waveguide is fed to the melt by means of cranes or special devices. Equipment control is realized from the remote control console. Control system has power panel and control console of low-voltage installation and high-voltage equipment.

(PCG - pulse current generator, W - power, U - voltage, t - processing time

W - power, stored in condensers PCG)

Electric energy during(1 – 100) mks is generated in the discharge filament – area between anode and inner wave guide face EDGEO. As a result of electric energy conversion into thermal one, discharge filament substance turns into low-temperature plasma (4*10⁴ К). Discharge filament pressure reaches 10³ МPа. Discharge filament expands at speed, running up to tens and hundreds metres per second. It causes the appearance of intense pressure wave. Pulse repetition rate is limited by lifetime of gas-vapor cavity, although if may be essentially increased in EDGEO of multielectrode type. That media, in which high-voltage discharge is produces has a great influence upon the forming of electric discharge channel in discharge gap EDGEO. This is usually industrial water: conductivity range (1-100)·10^-2(ohm·m)^-1. At low electroconductivity energy is mainly spent on water heating near the electrode space by currents of ionic conduction and on internal energy of leaders froming at the discharge prebreakdown stage. Ctritical tension of electric field, securing the speed of water heating (more than 10⁷ К/s) enough for guaranteeing the explosive character of boiling, changes in a relatively range: from 10⁶ to 10⁷ V/m. Four pressure types are realized in water at high-voltage electric discharge. These are the pressure in the discharge filament, pressure at the front of pressure wave, and the one generated by hyfroflow braking and hydrostatic. At EDGEO discharge perpendicular to mobile element (see the general scheme) it's possibel to point out three limitative factors: plasma channel pressure on waveguidew EDGEO, pressure generated by expanding channel in liquid and effecting on the elements of reradiative system EDGEO, pressure from quasistatic water compression. In the end, the effect on technological media come to creatrion of power nonstationary acoustic field in it. Acoustic field causes cavitation. At that gas solute in the melt educes in resulting discontinuity flaws, gas inclusions coagulation takes place, their removal, melt fining from injurous additives, intensification of hydro- and mass exchanged processes in the melt voulme and its blending.

Effect mechanism is described by hydraulic theory EFGEO, which includes the model of pulse source (on the basis of energy balance equation and equation of processes in discharge circuit), description of dynamics of pressure transfer by a waveguide, description of pressure fields in the melt, interaction of pressure waves with gas bubbles (on the basis of gas bubble pulsation equation and expression, determining the pressure field, generated to the acoustic medium by pulsatile bubble). Bjerkness effect is the determinant at the influence of pressure waves, radiated by the waveguide to the melt. The heart of the problem lies in the fact that interaction arises at pulsation of closely set spheres (in the melt - gas inclusions). At that in-phase oscillations lead to interattraction, and antiphase – to repulsion of pulsatile spheres. The stated effect becomes apparent at harmonic oscillations and is practically proportional to the amplitude of oscillation. It's expected that linear (harmonic) oscillations of gas inclusions excited in the melt, lead to their interattraction, rapprochement and confluence. As a result, gas bubbles become larger and rise to the surface. At nonlinear oscillations of a gas bubble (peculiar to certain process conditions), in the phase, corresponding to the minimum bubble radius, its walls acceleration appears to be very high. It results in radiation of pressure pulse of small duration and high amplitude, which may exceed impinging pressure wave amplitude in two orders. Such an impulse leads to discontinuity flaws fragmentation and their quantity increase. It's natural to suggest that discharge pusle following this pressure pulse leads to the local pressure decrease in the melt, as a result of high flow velocities: (from 2,5 up to 30 m/s) wihin 0,5 m from the waveguide EFGEO axle, and rise of hydrodynamic cavitation. Hence, it's necessary to wait for the gas bubbles splitting up and increase of their quantity in the zone of nonlinear oscillations. Taking into account the heterogeneity of processing media - melt, sizes of arising bubbles will be different. the presence of developed effect of hydrodynamic cavitation will be defined by resonance phenomena, which in this case are limited by spectrum width of oscillations perturbing the medium. A wide oscillation spectrum is exactly that EHIP distinguishing feature.