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Glowing liquid in thin dielectric channels
Herzenshteyn S.Ya., Monkhov A.A.
In preliminary experimental studies of the flow of a weakly conducting fluid in thin dielectric channels, a phenomenon has been discovered - the glow of a fluid [1, 2]. Glow can be observed with the naked eye in daylight. The description of this phenomenon both in the domestic and foreign literature has not been found.
This paper presents the results of a study of fluid flow in a channel with a diameter of 0.1 cm and a length of 5 cm . The movement of the fluid is given by the pressure differential, while the Re numbers did 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 fluoroplast brand F4MB and its terminal part of 2 cm . with the same diameter of plexiglass. Fluoroplastic of this brand has a specific resistance of 1017 V / m , and organic glass is 7 orders less. As a liquid, technical oil with a viscosity of 75 cSt was used .
In the second variant, a brass insert with a thickness of 2 mm was inserted between the fluoroplast 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 part of the pipe, where the velocity profile is formed from rectangular to parabolic. Here the main acceleration of the flow core and a significant pressure drop occur .
Studies for the channel of the first type (without a brass insert) showed the occurrence of liquid glow from the interface of dielectrics in the direction of flow at a speed of about 15 m / s ( Fig.2 ).
Here (1) is a channel of fluoroplast, (2) is a continuation of a channel of organic glass, (3) is the region of glow of a liquid at the interface of dielectrics. The fluid moves upwards.
Fig. 2 Fluorescence in the composite channel PTFE
With increasing flow velocity, the glow region increases. When registering the glow with a photomultiplier, 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 band. There is a good correlation in time of the flash of light with electromagnetic interference. With a sharp increase in the flow rate, the brightness of the glow and increases.
The cause of luminescence is associated with the electrification of the channel wall and liquid. In the initial part of the channel at a length of 5-10 calibers, the main acceleration of the flow core and pressure drop occur. This leads to small 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. Fluoroplast (polytetrafluoroethylene (CF2 - CF2) n ) is a good insulator, the electron work function is ∆ (eφ) = 10.1 eV . This parameter is often determined by the occurrence of emission current from the surface of the material at a certain value of the electric field intensity ( the Shotki effect ).
∆ (еφ) = е 3 Е 1/2
For fluoroplast Ecr = 7 * 108 V / cm . Fluoroplastic, like many fluorine-containing materials, has a high electron affinity. This is due to the highest electronegativity of fluorine. It should be noted that fluoroplast is not only a hydrophobic material, but also oleophobic. And in this case, in the initial part of the channel, slippage of the liquid can occur relative to the channel walls .
Fig. 3. The glow of the fluid in the channel for the brass ring.
When a fluid moves, an electrical double layer is formed with a negative potential on the channel wall and positive in the fluid. At a flow rate of 15 m / s, its intensity is still small for the occurrence of field emission on the channel wall of the fluoroplastic, but sufficient for the occurrence of emission on the walls of the channel of organic glass. As a result, the emission current excites a part of the liquid molecules with the last emission of light quanta in the form of the observed emission ..
In the experiments with the placement of the brass insert between the fluoroplastic and plexiglass, a luminescence was observed. As in the first embodiment of the channel, here a double electric layer is formed on the wall of fluoroplastic. 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 rate of 15 m / s . ( Fig. 3 ).
Here (1) is the fluoroplast channel, (2) is the brass ring, (3) is the region of glow of the liquid behind the brass ring, (4) is the continuation of the channel from organic glass. The fluid moves upwards.
The brightest glow region is observed above the brass ring, where the field emission of electrons and the excitation of fluid molecules occur. Further downstream, the recombination of liquid molecules occurs, which is observed in the form of a bluish glow.
Intense luminescence in the channel leads to an increase in the temperature of the liquid. Measurements have shown that the temperature of the fluid at the exit of the channel rises by 10 degrees. The process of field emission is characterized not only by heating the surface of the channel and the 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 start of the experiment and after 30 minutes .
Registration of luminescence with a photomultiplier showed that luminescence in the form of flares occurs at constant pressure. However, the intensity of the glow increases with sharp pulsations of speed.
Fig. 5. Oscillogram of the intensity of the glow (3), electromagnetic
In Fig. 5 shows the oscillogram of the intensity of the glow (3) of the electromagnetic background (2) with a quasi-static pressure change (1) in front of the channel entrance edge. There is a good correlation between the flash of light and electromagnetic interference.
In the course of the experimental studies, it was found that the electrical conductivity of the fluid significantly affects electrification and, accordingly, the intensity of the glow. Similar results were obtained in calculations .
A small film about the glow of a liquid in a dielectric channel with a brass insert can be viewed here .
Thus, according to the experimental studies of the flow of a weakly conducting fluid in a channel with varying electrophysical properties, a new phenomenon has been discovered - the glow of a fluid. Areas with a high electric field intensity are established. It is shown that luminescence occurs at the boundary of changes in the electrophysical properties of the channel material and is a consequence of the fluorescence of the liquid. The glow is discrete in nature and is accompanied by electromagnetic interference.
Baranov D.S., Bukharin N.S., Herzenshteyn S.Ya., Monkhov A.A. Electrification of a weakly conducting fluid in a thin dielectric channel // Abstracts of the reports of the 13th school seminar "Modern problems of aerohydrodynamics". September 5-15, 2005 Sochi, Petrel, Moscow State University. M .: MGU Publishing House, 2005. p.14.
A.A. Monkhov Electrification of 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. Area boarding house Office of the President of the Russian Federation "Forest Dali". Moscow State University. M .: Publishing House of Moscow State University, 2006.p.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. Formation of strong electric fields when the fluid flows in narrow channels // Reports of the Russian Academy of Sciences. 2005. T.403. №5. Pp. 619-622.
Author: Gertsenshtein S.Ya., Monkhov A.A.
Institute of Mechanics, Moscow State University Mv Lomonosova, Moscow
PS Material is protected.
Date of publication: 30.11.2006