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LIQUID LIQUID IN THIN DIELECTRIC CHANNELS
Herzenstein S.Ya., Monakhov A.A.
In preliminary experimental studies of the flow of a weakly conducting liquid in thin dielectric channels, the phenomenon of liquid glow was discovered [1,2]. Glow can be observed with the naked eye in daylight. A description of this phenomenon in both domestic and foreign literature was not found by us.
In this paper, we present the results of a study of the fluid flow in a channel 0.1 cm in diameter and 5 cm long. The fluid motion is determined by the pressure drop; the Re numbers do not exceed 600 .
Two types of composite channel with different materials along its axis were considered ( Fig . 1 ). In the first embodiment, the initial region of the channel is 3 cm long. was made of fluoroplastic brand F4MB and its terminal part 2 cm . with the same diameter of organic glass. Ftoroplast of this brand has a specific resistance of 1017 V / m , and organic glass is 7 orders of magnitude smaller. As a liquid, technical oil with a viscosity of 75 cSt was used .
In the second embodiment, a 2 mm thick brass insert was inserted between the fluoroplastic and organic glass. with the same diameter. In both cases, the geometric dimensions of the channels were the same. A channel with such data represents the initial section of the pipe, where a velocity profile is formed from rectangular to parabolic. Here, the main acceleration of the flow core and a significant pressure drop occur .
The studies for the channel of the first type (without a brass insert) showed the appearance of a glow of the liquid from the interface of the dielectrics in the direction of flow at a speed of about 15 m / s ( Fig . 2 ).
Here (1) is the fluoroplastic channel, (2) is the continuation of the organic glass channel, (3) is the region of the glow of the liquid at the interface of the dielectrics. The fluid moves from bottom to top.
Fig. 2 Fluorescence in the composite channel ftoroplast-plexiglass
With increasing flow velocity, the luminescence region increases. When registering the glow with a photomultiplier, its discreteness was established in the form of separate flashes with a frequency of up to 50 KHz , accompanied by electromagnetic interference in the radio range. There is a good correlation in the time of a flash of light with electromagnetic interference. With a sharp increase in the flow velocity, the brightness of the glow increases.
The reason for the glow is due to the electrification of the channel wall and the liquid. In the initial section of the channel over a length of 5-10 gauges, the main acceleration of the flow core and pressure drop occur. This leads to a fine bubble boiling of 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 electron work function is ∆ (еφ) = 10.1 eV . This parameter is often determined by the appearance of an emission current from the surface of the material at a certain value of the electric field strength ( Schottky effect ).
∆ (eφ) = e 3 E 1/2
For fluorine plastic Ekr = 7 * 108 V / cm . Fluoroplast, like many fluorine-containing materials, has a large electron affinity. This is due to the highest value of electronegativity in fluorine. It should be noted that fluoroplast is not only a hydrophobic material, but also oleophobic. And in this case, in the initial section of the channel, fluid slip can occur relative to the channel walls .
Fig. 3. The glow of the liquid in the channel behind the brass ring.
When the liquid moves, a double electric layer is formed with a negative potential on the channel wall and positive in the liquid. At a flow velocity of 15 m / s, its intensity is still small for the occurrence of field emission on the channel wall of fluoroplastic, but sufficient for the occurrence of emission on the channel walls of organic glass. As a result, the emission current excites a part of the liquid molecules with the emission of the last light quanta in the form of the observed glow.
In experiments with the placement of a brass insert between the fluoroplast and plexiglass, a glow was observed. As in the first version of the channel, a double electric layer is formed here on the fluoroplastic wall. Its intensity increases with increasing flow rate. As is known, the electron work function of a metal is much less than that of a dielectric, and here the luminescence is more intense than in a channel without a metal insert at the same flow velocity of 15 m / s . ( Fig. 3 ).
Here (1) is the fluoroplastic channel, (2) is the brass ring, (3) is the fluorescence region behind the brass ring, (4) is the continuation of the organic glass channel. The fluid moves from bottom to top.
The brightest glow region is observed above the brass ring, where field emission of electrons and excitation of liquid molecules occur. Further downstream, the liquid molecules recombine, which is observed as a bluish glow.
Intense glow in the channel leads to an increase in the temperature of the liquid. Measurements showed 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 heating the surface of the channel and liquid, but also by the destruction of the channel walls due to the movement of positive ions to it. Destruction occurs both the edges of the channel and the walls of organic glass ( Fig. 4 a, b )
Fig. 4 a
Fig. 4 b
The end of the channel before the experiment and after 30 minutes
The registration of the emission by a photomultiplier showed that emission in the form of flashes also occurs at constant pressure. However, the intensity of the glow increases with sharp pulsations of speed.
Fig. 5. The oscillogram of the intensity of the glow (3), electromagnetic
In Fig. Figure 5 shows an oscillogram of the glow intensity (3), the electromagnetic background (2) with a quasistatic change in pressure (1) in front of the channel input edge. There is a good correlation between a flash of light and electromagnetic interference.
In the course of experimental studies, it was found that the electrical conductivity of the liquid significantly affects the electrification and, accordingly, the luminous intensity. Similar results were obtained in the calculations .
A short film about the glow of liquid in a dielectric channel with a brass insert can be seen here .
Thus, according to experimental studies of the flow of a weakly conducting fluid in a channel with changing electrophysical properties, a new phenomenon was discovered - the glow of a fluid. Areas with high electric field strengths are identified. It was shown that 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 D.S., Bukharin N.S., Herzenstein S.Ya., Monakhov A.A. 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, “Petrel” of Moscow State University. M .: Publishing house of Moscow State University, 2005.S. 14.
Monakhov A.A. Electrification during the flow of a dielectric fluid in a dielectric channel. // Abstracts of the international conference "Nonlinear problems of the theory of hydrodynamic stability and turbulence." February 26 – March 5, 2006 Mosk. Region boarding house Office of the President of the Russian Federation "Forest Dali". Moscow State University. M.: Publishing House of Moscow State University, 2006.S. 76.
G. Schlichting. Theory of the boundary layer. Publishing House "Science", M. 1974.
SM Dammer and D. Lohse, Phys. Rev. Lett. 96, 206101 (2006).
Pankratieva I.L., Polyansky V.A. The formation of strong electric fields during fluid flow in narrow channels // Doklady RAS. 2005.V. 403. No. 5. S. 619-622.
Author: Herzenstein S.Ya., Monakhov A.A.
Institute of Mechanics, Moscow State University M.V. Lomonosov Moscow
PS Material is protected.
Publication date 11/30/2006