Dielectric fluid in inhomogeneous pulsed electric field

Abstract

We consider the dynamics of a compressible fluid under the influence of electrostrictive ponderomotive forces in strong inhomogeneous nonstationary electric fields. It is shown that if the fronts of the voltage rise at a sharp, needle-like electrode are rather steep (less than or about nanoseconds), and the region of negative pressure arises, which can reach values at which the fluid loses its continuity with the formation of cavitation ruptures. If the voltage on the electrode is not large enough or the front is flatter, the cavitation in the liquid does not occur. However, a sudden shutdown of the field results in a reverse flow of liquid from the electrode, which leads to appearance of negative pressure, and, possibly, cavitation.

Initiation process and propagation mechanism of positive streamer discharge in water

“…11 In the case of nanosecond discharges, the formation of a low-density region by an electrostrictive force acting on a dielectric fluid in a non-uniform electric field has been assumed. [31][32][33][34][35] The electrostrictive force induces negative pressure regions in the vicinity of an electrode tip, resulting in the formation of liquid ruptures (or nanopores). 31,32 This electrostriction model is effective in liquid with high dielectric permittivity such as in water and is dominant in a nanosecond time scale of 10 ns, which is much shorter than the characteristic time scale of hydrodynamic processes.…”

“…[31][32][33][34][35] The electrostrictive force induces negative pressure regions in the vicinity of an electrode tip, resulting in the formation of liquid ruptures (or nanopores). 31,32 This electrostriction model is effective in liquid with high dielectric permittivity such as in water and is dominant in a nanosecond time scale of 10 ns, which is much shorter than the characteristic time scale of hydrodynamic processes. 1,33,34 Electrons can be accelerated in the ruptures to ionize water molecules, leading to electron avalanche that initially forms a plasma region.…”

Pre-breakdown cavitation nanopores in the dielectric fluid in the inhomogeneous, pulsed electric fields

“…As already noted, the negative electrostrictive pressure in the fluid, caused by the ponderomotive forces, induces the fluid influx, which not only compensates the electrostrictive stretching tension, but also leads to the appearance of the cavitation nanopores [18][19][20][21][22][23][24]. As can be seen from Figure 1, there are regions in the vicinity of the pores where the absolute values of the negative pressure are maximum, and therefore the cavitation development is most probable.…”

“…It is seen that the pore in water as well as in transformer oil is stretched along the electric field, since the pressure at its poles is greater than the pressure at the equator. In the approximation of a spherical pore, from the equations (1) and (2) and after averaging over the angle of all the forces acting on the surface, we obtain the equation for the radius of the expanding pores [28]: It can be seen that the rate of expansion of the pores reaches hundreds of meters per second, which on one hand they are almost by two orders of magnitude greater than the velocity of the fluid near the electrode [19], but on the other they are much smaller than the velocity of sound and reach the size of about 10 nanometers within a few hundreds of nanoseconds. Thus, for accepted values of "external" field 0 E , the potential difference at the poles of pores reaches or exceeds the ionization potential, at 5 ≈ R and at 14 nm, for water and oil, respectively.…”

“…formation of cavitation nanopores in which an electron can gain the energy necessary for ionization. The main difference between fast (nanosecond) pulses from slow (microsecond) is that in the time of a few nanoseconds, the liquid due to inertia does not have time to displace to the electrode and to compensate the negative pressure [19]. The work [18] has stimulated numerous experimental and theoretical studies on nanosecond breakdown in liquid dielectrics [20][21][22][23][24][25].…”