US 3725737 A
A corona electrode discharge system, such as used in electrofluid dynamic generators, having corona electrode and an attractor electrode forming an annular converging-diverging nozzle, with the corona electrode having a step adjacent the diverging portion of the nozzle. A high pressure fluent material is supplied to the nozzle to provide a supersonic flow through the nozzle and to create a constant density high pressure region with a corresponding high dielectric breakdown strength between the corona and attractor electrodes and a constant density low pressure region adjacent the step with a corresponding low dielectric breakdown strength.
Claims available in
Description (OCR text may contain errors)
Fl-F8592 KR 51 7251737 ilnited States Patent [191 [111 3,725,737 Lawson et al. [4 1 Apr. 3, 1973  CORONA DISCHARGE ELECTRODE 3,573,512 4 1971 Lawson et al. ..310/10 STRUCTURE FOR ELECTROFLUID DYNAMIC GENERATOR Primary ExaminerGerald Goldberg  Inventors: Maurice 0. Lawson, Dayton; j a z g g fl i Theodore L. Willke, Columbus, both my f 0h 57 ABSTRACT  Assignee: The United States of America as represented by the Secretary of the v A corona electrode discharge system, such as used 1n Air Force electrofluid dynamic generators, having corona electrode and an attractor electrode forming an annular Filedl Nov-8,1971 converging-diverging nozzle, with the corona elec- 2 1 9 2 4 trode having a step adjacent the diverging portion of the nozzle. A high pressure fluent material is supplied to the nozzle to provide a supersonic flow through the [5%] US. Cl]. ..3l7/3, 310/5, 310/11 nozzle and to create a constant density high pressure ..'.7........:l002k region a corresponding dielectric breakdown 1 le 0 can 3 11 strength between the corona and attractor electrodes and a constant density low pressure region adjacent  References cued the step with a corresponding low dielectric break- UNITED STATES PATENTS down Strength- 3,439,197 4/1969 Von Ohain et al.- ..3l0/11 5 Claims, 3 Drawing Figures s gs rave fdMP SGFIEK rat SHEET 1 [1F 2 TEUAPR3 1973 PATH CORONA DISCHARGE ELECTRODE STRUCTURE FOR ELECTROFLUID DYNAMIC GENERATOR BACKGROUND OF THE INVENTION The trend in electrofluid dynamic generators is toward conversion sections of smaller diameters leading to higher current densities and lower voltages. The current per unit mass flow varies inversely with the spacing between the corona discharge electrode and the attractor electrode; therefore, very close spacings are desirable. However, for very close spacings it is difficult to produce corona as breakdown usually occurs before corona is established.
In the corona discharge gaps for unipolar charge production, there are two sources for the total electric field structure, one being the voltage across the gap and the other being the field due to the unipolar charges.
In the conversion section of electrofluid dynamic generators, the field associated with charge density should approach one-half of the breakdown strength of the gas. However, this is highly deleterious with field conditions for corona discharge as it adds to the field at the attractor electrode. In prior art systems, lower charge densities in the conversion section were accepted by permitting the total limitation by the space charge at the attractor electrode to be the limiting factor.
BRIEF SUMMARY OF THE INVENTION According to this invention, the geometry of the corona discharge system is chosen to provide supersonic flow at the corona edge such that the first strong expansion wave travels downstream toward the attractor nozzle tip to provide a region of high pressure, constant gas density and a corresponding high electric breakdown strength.
In order to make it possible then to establish corona, a step is provided on the corona electrode so that as the gas expands over the edge a loml region of low pressure, substantially constant gas density is created to provide a region of low breakdown strength favorable for corona.
IN THE DRAWING FIG. 1 is a partially schematic sectional view of an electrofluid dynamic generator according to the inventron;
FIG. 2 is a sectional view of the device of FIG. 1 along the line 2-2.
FIG. 3 is an enlarged partially cut away sectional view of the corona discharge electrodes for the device of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION Reference is now made to FIGS. 1 and 2 of the drawing which show an electrofluid dynamic generator having a pressure chamber 12. A corona electrode 14 is positioned within an annular attractor electrode 16 and is spaced therefrom by an insulating spacer 18. The spacer 18 has a plurality of channels 20 for the passage of a high pressure gas to the nozzle formed between electrodes 14 and 16. The space 22 between electrodes 14 and 16, as shown in greater detail in FIG. 3, is shaped to form an annular converging-diverging nozzle. With a high pressure gas supplied to the nozzle, a
supersonic flow is produced in the slightly diverging nozzle passage 23. The step 25 is provided around the corona electrode 14 so that the first strong expansion wave is carried downstream by the flow and reaches the attractor electrode at 26. Thus, there is a substantially constant gas density; high pressure region behind the line 28 of high dielectric breakdown strength. The gas expanding over the edge 30 travels toward the corona electrode to a point 31, leaving a low pressure, substantially constant gas density region 32, behind a line 33, of a low breakdown strength, wherein a corona discharge is readily established.
The collector 34 and field shaping electrode 36 perform essentially the same function as in the prior art, for example, as in the Lawson et al. US. Pat. No., 3,573,512. However, the insulator walls are not replaced by aerodynamic walls, as in the referenced patent, but rather the insulators are spaced from the conversion section with the electrode support being arranged to increase the length of insulator wall between the field shaping electrode and the collector, as shown in FIG. 1.
The mercury vapor and a low-molecular weight gas, such as hydrogen, are recycled through the heat exchanger and separator 38, injection pump 39 and mercury supply 40 as in the Lawson et a1. patent referenced above, with the low molecular weight gas being supplied at input 41 and the mercury vapor at a pressure from 20 to 400 atmospheres, depending upon the system geometry, being supplied at input 42. Working materials other than mercury vapor and hydrogen can be used, for example, water vapor with air or freon with helium could be used. Also, some systems have been operated with a single working medium, such as air, being used for both the primary and the secondary flow. With a single working medium the separator is not needed.
While an annular discharge gap has been described, the use of the particular corona electrode geometry with the step in the corona electrode need not be limited to this particular electrode configuration, but elongated corona and attractor electrodes could be shaped for use in a system such as shown in the referenced patent.
Also the corona electrode discharge system could be used in apparatus other than electrofluid dynamic generators, for example, it could be used in particle accelerators.
In one device built with water vapor and air used as the working mediums, the spacing between the corona electrode and the attractor electrode, in the narrow region of the nozzle, was 0.060 inch. The corona electrode was at ground potential with the attractor at +30,000 volts. The field shaping electrode was held at +l2,000 volts. Air with 20 percent relative humidity water vapor was supplied to input 42 at atmospheres pressure. With this system, the output power was 560 watts with 405,000 volts at the collector.
There is, thus, provided an electrofluid dynamic generator which provides a region of high electric breakdown strength between the corona electrode and the attractor electrode and which provides a local region of low breakdown strength favorable for corona.
1. An apparatus for providing a corona discharge into a fluent medium, comprising: a corona discharge electrode and an attractor including means for forming a converging-diverging nozzle therebetween; means for providing a supersonic flow of said fluent medium through said nozzle; means, on said corona electrode, for providing a high pressure substantially constant gas density region of high electric breakdown strength between the corona electrode and the attractor electrode and a region of low electric breakdown strength adjacent the corona electrode.
2. The device as recited in claim 1 wherein said means for providing the region of high electric breakdown strength between the corona electrode and the attractor electrode and a region of low electric breakdown strength adjacent the corona electrode including a step in the corona electrode in the diverging region of said nozzle.
3. In combination with the corona discharge apparatus of claim 1, a pressure chamber surrounding said corona discharge apparatus, a field shaping electrode, within said chamber spaced from the attractor electrode and surrounding the fluent medium flow leaving said nozzle; a collector electrode within said chamber spaced from said field shaping electrode; output means connected to said collector.
4. The device as recited in claim 3 including means for providing a secondary flow of gas between the attractor electrode and the field shaping electrode and around the fluent material leaving the nozzle.
5. The device as recited in claim 4 wherein said pressure chamber'comprises an insulator wall with the collector and field shaping electrode supported by the wall and with the supports for the electrodes adjacent the wall being spaced apart a greater distance than the distance between the electrodes adjacent the fluent medium flow.