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CURRENT LIQUID IN THIN DIELECTRIC CHANNELS
Gertsenshtein S.Ya., Monakhov A.A.
In preliminary experimental studies of the flow of a weakly conducting liquid in thin dielectric channels, a phenomenon has been observed-liquid luminescence [1, 2]. Glow can be observed with the naked eye in daylight. The description of this phenomenon both in the domestic literature, and in the foreign, we have not been found.
In this paper, we present the results of an investigation of the flow of a liquid in a channel 0.1 cm in diameter and 5 cm in length . The movement of the liquid is determined by the pressure drop, the Re numbers do not exceed 600 .
Two types of a composite channel with different materials along its axis were considered ( Fig . 1 ). In the first variant, the initial channel region is 3 cm long. was made of PTFE grade F4MB and its final part is 2 cm . with the same diameter of organic glass. The fluoroplast of this brand has a resistivity of 1017 V / m , and the organic glass is 7 orders of magnitude smaller. As a fluid, a technical oil with a viscosity of 75 cSt was used .
In the second variant, a brass insert 2 mm thick was inserted between the PTFE and the organic glass. with the same diameter. In both cases, the geometrical dimensions of the channels were the same. A channel with such data represents the initial section of the pipe, where the velocity profile is formed from a rectangular to a parabolic profile. Here, the main acceleration of the core of the flow occurs and a significant pressure drop .
The conducted investigations for the first type channel (without the brass insert) showed the appearance of liquid glow from the dielectric interface in the direction of flow at a velocity of about 15 m / s ( Fig . 2 ).
Here (1) is the fluoroplastic channel, (2) is the continuation of the channel from the organic glass, and (3) is the liquid emission region at the dielectric interface. The fluid moves from bottom to top.
Fig. 2 Fluorescence in the composite channel of fluoroplastic-plexiglass
As the flow velocity increases, the luminescence region increases. When the luminescence is recorded by a photoelectric multiplier, its discreteness is established in the form of individual flashes with a frequency of up to 50 kHz , accompanied by electromagnetic interference in the radio range. There is a good correlation with the time of the flash of light with electromagnetic interference. With a sharp increase in the flow velocity, the brightness of the glow increases.
The cause of the glow is associated with the electrification of the channel and liquid walls. In the initial section of the channel, at a length of 5-10 calibers, the core of the current flows and the pressure drop. This results in a finely bubbling effervescence of the dissolved gases in the liquid and the formation of a charge on the channel wall and in the liquid. The second factor in the formation of charges on the wall is the manifestation of the electrophysical properties of the channel material. Fluoroplastic (polytetrafluoroethylene (CF2-CF2) n ) is a good insulator, the work function of the electrons is Δ (eφ) = 10.1 eV . This parameter is often determined by the appearance of the emission current from the surface of the material at a certain value of the electric field strength ( Schottky effect ).
Δ (еφ) = е 3 Е 1/2
For fluoroplastic, Ecr = 7 * 108 V / cm . Fluoroplastic, like many fluorine-containing materials, has a large affinity for the electron. This is explained by the largest value of electronegativity in fluorine. It should be noted that fluoroplastic is not only a hydrophobic material, but also an oleophobic material. And in this case, in the initial section of the channel, liquid can be perforated relative to the walls of the channel .
Fig. 3. Fluorescence in the channel behind the brass ring.
When the liquid moves, a double electric layer with a negative potential on the channel wall and a positive one in the liquid is formed. At a flow rate of 15 m / s, its strength is still low for the occurrence of field emission on the wall of the fluoroplastic channel, but it is sufficient to cause emission on the walls of the channel from the organic glass. As a result, the emission current excites some of the liquid molecules with the last emission of light quanta in the form of an observed luminescence.
In experiments with the placement of a brass insert between PTFE and plexiglas, a glow was observed. As in the first version of the channel, a double electric layer is formed on the wall of fluoroplastic. Its intensity increases with increasing flow velocity. As is known, the work function of electrons at a metal is much less than for a dielectric and here the emission is more intense than in a channel without a metal insert at the same flow rate of 15 m / s . ( Figure 3 ).
Here, (1) is a fluoroplastic channel, (2) is a brass ring, (3) is the region of liquid glow behind the brass ring, (4) is the continuation of the channel from the organic glass. The fluid moves from bottom to top.
The brightest region of luminescence is observed over the brass ring, where the field emission of electrons occurs and the molecules of the liquid are excited. Further downstream is a recombination of the molecules of the liquid, which is observed in the form of a bluish glow.
Intensive glow in the channel leads to an increase in the temperature of the liquid. Measurements have shown that the temperature of the liquid at the outlet of the channel rises by 10 degrees. The process of field emission is characterized not only by the heating of the surface of the channel and the liquid, but also by the destruction of the walls of the channel due to the movement of positive ions to it. Destruction occurs as the edges of the channel, and the walls of the organic glass ( Figure 4 a, b )
Fig. 4 a
Fig. 4 b
The end of the canal before the start of the experiment and after 30 minutes .
The registration of the luminescence by a photoelectric multiplier showed that the glow in the form of flares occurs also at constant pressure. However, the intensity of the luminescence increases with sharp velocity pulsations.
Fig. 5. Oscillogram of the intensity of the glow (3), electromagnetic
In Fig. 5 shows an oscillogram of the luminescence intensity (3), the electromagnetic background (2) for a quasistatic change in pressure (1) in front of the entrance edge of the channel. There is a good correlation between the flash of light and electromagnetic interference.
During the experimental studies it was established that the electrical conductivity of a liquid has a significant effect on the electrification and, accordingly, on the intensity of the luminescence. Similar results were obtained in the calculations of .
A small film about the glow of liquid in a dielectric channel with a brass insert can be seen here .
Thus, according to the experimental studies of the flow of a weakly conducting liquid in a channel with varying electrophysical properties, a new phenomenon has been discovered-liquid glow. Areas with a large electric field strength are established. It is shown that the luminescence arises at the boundary of the change in the electrophysical properties of the channel material and is a consequence of fluorescence of the liquid. The glow has a discrete character and is accompanied by electromagnetic interference.
Baranov DS, Bukharin NS, Gertsenstein S.Ya., Monakhov AA Electrification of a weakly conducting liquid in a thin dielectric channel. / / Abstracts of the XIII school seminar "Modern problems of aerohydrodynamics." September 5-15, 2005 Sochi, "Burevestnik" of the Moscow State University. Moscow: Moscow State University, 2005. p.14.
Monakhov AA Electrification during the flow of a dielectric liquid in a dielectric channel. / / Abstracts of the international conference "Nonlinear Problems of the Theory of Hydrodynamic Stability and Turbulence". February 26-March 5, 2006 Moscow. Of the Ob. boarding house Administration of affairs of the President of the Russian Federation "Forest Distances". Moscow State University. Moscow: Izd-vo MGU, 2006.c.76.
G. Schlichting. Boundary layer theory. Publishing house "Science", M. 1974.
SM Dammer and D. Lohse, Phys. Rev. Lett. 96, 206101 (2006).
Pankratieva IL, Polyansky VA Formation of strong electric fields at liquid flow in narrow channels // Doklady RAN. 2005. T.403. №5. Pp. 619-622.
Author: Gertsenshtein S.Ya., Monakhov AA
Institute of Mechanics, Moscow State University. M.V. Lomonosov Moscow
PS The material is protected.
Date of publication 11/30/2006