US3706894A - Electro-aerodynamic generator - Google Patents

Abstract

This invention relates to an electrodynamic generator of the type comprising a passage for the flow of a fluid at least partially at supersonic speed from an upstream portion constituted by a convergent-divergent region in which is disposed ionization means substantially in the neck of the said region, to a downstream region at high potential in the neighborhood of the outlet from the passage. The invention is characterized in that the passage is constituted of a ceramic material or vitrified material and in that the internal wall is very smooth. Moreover, filters are provided in order to obtain an air supply substantially free from impurities. Such a generator which provides low currents at high voltages (several tens of kilovolts) is particularly suitable for the electrostatic deposition of coating products.

Description

United States Patent Tholome i541 ELECTRO-AERODYNAMIC' v GENERATOR [72] Inventor: Roger Tholome, 38 Meylan, France [73] Assignee: Tunzinl-Sames, Grenoble, France [22] Filed: July 31, 1970.

[21] App1.No.: 59,910

[52] US. Cl ..310/10, 310/6 [51] Int. Cl. ..1102n 3/00 [58] Field of Search ..310/5, 6, 10

[56] References Cited UNITED STATES PATENTS 3,439,197 7 4/1969 Ohain et al..' ..310/ll 2,308,884 1/1943 Lindenblad ..310/5 3,225,225 12/1965 Wattendorf et al. ....310/6 3,417,267 12/1968 Marks ..3l0/6 Primary Examiner-D. X. Sliney Aztorney-l-lolcombe, Wetherill and Brisebois ABSTRACT This invention relates to an electrodynamic generator of the type comprising a passage for the flow of a fluid at least partially at supersonic speed from an upstream portion constituted by a convergent-divergent region in which is disposed ionization means substantially in the neck of the said region, to a downstream region at high potential in the neighborhood of the outlet from the passage. The invention is, characterized in that the passage is constituted of a ceramic material or'vitrified material and in that the internal wall is very smooth. Moreover, filters are provided in order to obtain an air supply substantially free from impurities. Such a generator which provides low currents at high voltages (several tens of kilovolt s) is particularly suitable for the electrostatic deposition of coating products.

3 Claims, 2 Drawing Figures PATENTEUnEc 19 I972 Ixweni-or 7 l ELECTRO-AERODYNAMIC GENERATOR The present invention relates to an electro-aero dynamic generator of the type comprising a passage for the flow of a fluid, at least partially at supersonic speed, from an upstreamportion, constituted by a convergentdivergent region having ionization means disposed substantially in the'neck of said region, to a downstream zone at high potential in the neighborhood of the outlet from the said passage. Such generators effect the delivery of electrical energy at high potential by converting a part of the thermodynamic energy of a gas, for example air, into electrical energy by making it goup in an electric field with electrical charges transported by this air. The charge current transported is, to a certain degree, a direct function of the speed of the air, and it is therefore advantageous that the speed of flow of the gaseous fluid is as high as possible. This is why it is advantageous to use supersonic speeds.

Experience has shown that such generators will permit the formation of supersonic currents, but that the supersonic flow presents certain anomalies. One of the causes of these anomalies is constituted by deposits of viscous produces on the intemal wall of the flow passage located downstream of the convergent divergent region, such that they create turbulence and possibly local short circuits leading to considerable reductions of the output voltage of such a generator.

The present invention has for an object to reduce these disadvantages and to provide a generator which effects the formation of a particularly homogenous flow, substantially without turbulence and this during periods of operation which are particularly prolonged.

According to a first characteristic of the invention, the flow passage for the supersonic fluid disposed downstream of the convergent-divergent region is constituted by a ceramic material or a vitrified mineral and its internal wall is smooth. Equally so far as concerns the internal wall of the convergent-divergent region, the latter must be produced of a material enabling one to obtain a very smooth surface and moreover resistant to electrical erosion.

According to another feature of the invention, the generator is supplied with air substantially free from dust and hydrocarbons, but however with a hygro-metrical degree greater than 50 percent. These arrangements enable the production of generators whose performances and durations of operation are clearly superior than those of known types of electro-aero dynamic generators.

The invention will now be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic sectional view of one embodiment of generator according to the invention, and

FIG. 2 is a" sectional view showing a detail of the generator in FIG. 1.

Referring to the drawings the generator essentially comprises a flow passage 1 having at its upstream end a convergent-divergent region 2 of which the internal wall is made of metal and connected to earth potential. An ionization needle 3 is positioned in the neck of the said convergent-divergent region. This region 2 leads to a coaxial passage 4 having a length several times greater than that of the region 2. For example the convergent-divergent region 2 may have a length of the order of one centimeter while the insulating passage 4 has a length of the order of 10 centimeters. With this arrangement there can be obtained from an excitation potential of the order of 4 to 5 kV fed over the cable 5 and with a flow at least partially'supersonic in the interior of the passage 4, an output potential of the order of 40 to kV at a current of several tens of micro amperes. The air is admitted to the generator through a pipe 6 in which are disposed a filter 7 and apre-filter 8 with a drain 9 for the evacuation of condensates at 10.

In order to obtain a suitable flow within the passage 1 of the generator and particularly a flow at least partially supersonic at the interior of the passage 4, it is preferable to employ the following arrangements. So far as concerns the passage 4 for the flow .at least partially at supersonic speed which is necessarily constructed of an insulating material and of which the diameter is of the order of some millimeters only, it must be ensured that the internalwall of this passage is extremely smooth. If a very high degree of polish is not obtained, then the supersonic flow cannot be entirely assured. Amongst the materials capable of ensuring this polish, it is necessary to eliminate those which are not capable of presenting a resistance to electrical erosion, particularly oxidation to the intense ionization which exists within the passage. It has been determined that ceramic materials or other mineral materials can be used with advantage. In order to obtain a satisfactory polish, it is preferred to use such'vitrified materials, for example raw tubes of molten glass (rough fused glass) particularly with 'a high silicon content, such as for example those produced under the trade name PYREX without any subsequent polishing.

So far as concerns the convergenbdivergent region, this must be produced of a conductive material presenting a high resistance to oxidation by reason of the presence of ozone given off in the ionization discharge at the region of the needle 3. This material must moreover be subjected to a very fine polish in order to disturb as little as possible the high speed flow. For example, brass has a sufficient polish, is rapidly degraded by ionization, while stainless steel of the 18/8 type better resists oxidation, but is difficult to machine, thus making the polishing operation long and costly. The use of a material which can easily be machined and polished, such as cut-out brass, covered with an'alloy resistant to corrosion, such as chromium, seems to provide a good solution, but the electrolytic coating at the interior of a hole which is of the order of 2 millimeters diameter and 5 millimeters in length necessitates an electrode and it is a very delicate industrial operation to produce. it has been shown that a convenient and economic procedure is' to efiect a chemical nickel plating, without employing an electric current, according to the process known as the Kanigen process, which enables a very fine deposit to be obtained of a nickelphosphorus alloy by the simple immersion in a bath of nickel salts. The deposit thus obtained which is of a very uniform thickness, with differences of thickness less than 10 percent, presents a good hardness greater than 500 vickers and its structure is completely amorphous and presents an absence of porosity, even for a deposit of the order of some 10 microns thick. The uniformity and homogeneity of the deposit enables the obtention of a particularly polished surface state by 1 nnnr I "(00 a simple brightening. This method of production which utilizes the chemical coating according to the Kanigen process presents'the advantage that it does not employ an electric current, such that the support of the nickelphosphorus coating, according to the process, may be constituted by an insulating material such as a thermoplastic or thermosetting material, which enables simplification and to reduce the price of producing the support. It is thought that another economic process would be the metallization by vaporization in a vacuum of a metal presenting the required anti-corrosive qualities (such as gold, silver, nickel), the support being either insulating or metallic. There is shown in FIG. 2 a composite nozzle 2 forming the convergent-divergent region and constituted by an insulating support 2a and a metallic covering 2b.

In the case of the convergent-divergent nozzle, or the supersonic flow passage, it has been shown that it is convenient to retain on all sides of a certain roughness and by way of example, there are given the following experimental results. Two measurements were made, one of the total roughness (RT) which is the difference between the highest peak and the deepest hollow along a particular length of the passage, while one measures the arithmetic difference (RA) which is the arithmetic mean of the absolute value of the differences from a neutral line along this same passage, and it has been shown that with a convergent-divergent nozzle of brass covered with a nickel-phosphorus alloy according to the Kanigen process and with a final brightening, and also with a passage formed of a tube of Pyrex, the total roughness (RT) is between 0.5 and 0.7 microns, while the mean difference (RA) is 0.09 microns, while these same components, but made of polished stainless steel for the nozzle and a plastics tube for the passage, according to usual techniques, present a roughness about ten times greater and performances, from the point of view of electrical energy obtained at the output, of the order of to 40% less than that of the nozzle and passage according to the invention. Another improvement according to the invention relates to the means of feedingthe gaseous fluid to an electro-aero dynamic generator and-this is shown in FIG. 1. In effect, the compressed air in the passage 3 which is generally used to convey charges is provided by pistontype compressors which are lubricated with oil and this air generally contains, amongst other impurities such as metal or metal oxide particles, traces of oil which is more or less oxidized. These impurities create in the nozzle 2 and in the passage 4 deposits which, in the long term are prejudicial not only to the flow of gaseous fluid but which may lead also to the establishment of leakage currents in the passage and a large decrease in the output potential, as well as a reduced flow of electric current. It has been shown that it is convenient therefore to feed such generators with very pure air and one has been led to provide, upstream of the generator, filters 8 and 7 which are particularly efficient, such as absolus filters (reference 8). These absolus filters enable the trapping of particles in a proportion of 99.98 percent of the particles having a diameter greater than 0.3 microns. Such filters are therefore generally indispensable and may sometimes be sufficient. However, by reason of the increase of temperature resulting from the compression of the air in the compressor which is situated upstream of the filter 7 and 8, it can produce local vaporizations of the hydrocarbons which are not stopped by the filter 8, even if it is of excellent quality, so that these vapors penetrate into the generator, where the air, subjected to a drop, has its temperature considerably reduced. These hydrocarbon vapors may then be condensed within the generatorand may be precipated on the walls by ionization action. Thus, even with a very fine filter, the operating conditions of the generator are such that it is very'difficult to stop the hydrocarbon vapors and the invention recognizes therefore to abstain from feeding the generator with air containing condensable oil vapors. To this end the invention provides the use, in compressors, of lubricating oil with a very low vapor tension and in all cases free of all volatile components such as certain additives or anti-oxidizing agents.

Advantageously there is placed between the compressor and the generator a filter containinga substance on which hydrocarbon molecules have a ten dency to deposit or be absorbed. These substances are advantageously used in a form offering a very large contactsurface with the gas to be purified, for example in granular or pulverulent form. But it is also convenient to choose, amongst the numerous substances which are capable of stopping oil vapors, those which have the smallest affinity with water, for on one hand there would be rapid saturation by water of the filters and on the other hand it is convenient to maintain a hygrometric degree suitable in the air, since the operation of the generator involves the formation of small particles of condensed water, which serve in fact to carry the electric charges within the generator. Amongst these products one may mention activated carbons.

By way of example a generator of 60 kV and of 10 micro-amperes consuming 13 N cubic meters per hour, under an absolute pressure'of 5 bars, fed with air by an industrial compressor positioned upstream of an absolu filter of the type described above may function about a hundred hours until the moment where the walls of the passage are covered with a viscous deposit of hydrocarbons either oxidized and/or non-oxidized. This deposit makes the voltage and current of the generator drop to one-fifth of their initial values. The same generator connected downstream of a cartridge containing 200 grams of active carbon doesnot have its performance substantially reduced after 1,300 hours of operation, the passage having practically no deposit.

During this time of operation, the filter cartridge which has been traversed by 17,000 Nm of air containing.

several grams of water by Nm (that is to say a total of several tens of kilograms of water) is only increased by ll grams, almost entirely constituted by hydrocarbons or hydrocarbon derivatives.

I claim:

1. In a device comprising an electro-aerodynamic generator of the type having a passage for the flowof a fluid at least partially at supersonic speed from an upstream region constituted by a convergent-divergent region with ionization means substantially in the neck of said region, to a downstream zone at high potential in the neighborhood of the outlet from the said passage, the improvement which comprises a very fine filter positioned upstream of said generator and stopping at l060l l 0600 UNiTE-D STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.-. 3,706,89 Dated December 19, 1972 Inventor(s) ROGER THOLOME It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[30] Foreign Application Priority Data July 31, 1969 France 69-26381 Signed and sealed this 8th day of May 1973.

(SEAL) Attest:

EDE-JARD ELFLETCHERJR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM 5 0-1050 (10-69) USCOMM-DC 6O376-F'69 us. GOVERNMENT PRINTING OFFICE: I969 0-366-334

Claims (3)

1. In a device comprising an electro-aerodynamic generator of the type having a passage for the flow of a fluid at least partially at supersonic speed from an upstream region constituted by a convergent-divergent region with ionization means substantially in the neck of said region, to a downstream zone at high potential in the neighborhood of the outlet from the said passage, the improvement which comprises a very fine filter positioned upstream of said generator and stopping at least 99 percent of those particles of the fluid having a diameter greater than 0.3 microns.

2. Device according to claim 1, in which said filter contains substantially non-hygroscopic absorbants for hydrocarbons.

3. Device according to claim 2, in which said absorbants are superactive carbons.

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