High Pressure Thermal Plasma System

Abstract

A high pressure induction plasma system of the flowing type includes structure defining a plasma chamber, an electrical coil surrounding the plasma chamber for creating an intense electromagnetic field within the plasma chamber, means to supply a gas for flow through the chamber under at least 10 atmospheres of pressure and conversion to plasma condition under the influence of the electromagnetic field, a flow restriction structure spaced at least one chamber diameter downstream from the electrical coil and structure defining a stabilizing volume between the coil and the flow restriction. This stabilizing volume provides aerodynamic and electrical compensation and allows flowing operation of the system at pressures of 50 atmospheres and above.

Description

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of section 305 of the National Aeronautics and Apace Act of 1958, Public Law 85-568 (72 stat. 435; 42 USC 2457).

SUMMARY OF INVENTION

This invention relates to thermal plasma systems and more particularly to thermal plasma systems of the induction type.

Induction plasma generators create and maintain a thermal plasma by providing an intense electromagnetic field which produces an electrodeless discharge in an ionized gaseous medium. Such a thermal plasma has a temperature in the range of 8,000.degree.-11,000.degree.K and is useful for many purposes such as performing chemical reactions, the working of metallic and refractory materials, and other processes that utilize the high temperatures produced. A number of devices have been proposed for generating thermal plasmas in such manner including both closed and flowing systems in which the plasma has been stably maintained both in reduced pressure environments and in atmospheric pressure environments. A flowing type of system facilitates the performance of useful work, either within the plasma chamber or by the output from the plasma chamber. However, in such systems problems arise concerning the stabilization of the plasma both spatially and electrically, that is maintaining the plasma in a stable position within the chamber so that, for example, it does not move off axis and damage the integrity of the wall structure, and maintaining appropriate electrical characteristics so that the thermal plasma is not extinguished. While such spatial and electrical stability has been achieved in systems operating at atmospheric pressure and below, efforts to provide stable thermal plasmas in flowing systems operating at pressures substantially above atmospheric pressure have not been successful.

Stable operation of induction plasma generators at elevated pressures involves a complex of interdependent consideration including gas conductivity, temperature, pressure and electrical coupling factors. A stable flowing thermal plasma in a high pressure environment would be particularly useful in performing certain chemical processing and also in simulating different types of environments.

Accordingly it is an object of this invention to provide a novel and improved flowing type of induction plasma generation system which operates with improved stability at pressures substantially above atmospheric pressures.

Another object of this invention is to provide a novel and improved flowing type of induction plasma generation system which provides improved yields and which produces greater radiation.

We have discovered that stable operation of a high pressure flowing induction plasma system may be obtained if the location of the flow restriction structure is coordinated with the size of the plasma chamber and the position of the induction coil. A stabilizing volume should be provided between the flow restriction structure and the induction coil, the requisite size of this volume being in part a function of the spacing of the flow restriction structure from the induction coil. The flow restriction structure should be spaced at least one chamber diameter from the downstream end of the coil and with such spacing a volume at least three times the plasma chamber volume should be interposed between the coil and the flow restriction structure. The requisite size of the stabilizing volume decreases as the flow restriction structure is spaced greater distances from the coil. When the flow restriction structure is spaced three chamber diameters or more from the coil, adequate stabilizing volume is provided by a uniform dimensional extension of the plasma chamber.

In the above description the phrase "plasma chamber volume" means twice the volume of the chamber between the upstream and downstream ends of the induction coil; "chamber diameter" means the linear dimension of a plasma chamber of circular cross-section or the average of the two dimensions of a chamber of rectangular cross-section, for example as it will be obvious that plasma chambers of other cross-sectional configurations may be employed and the term "chamber diameter" is intended to describe only a cylindrical chamber; "flow restriction structure" means a structure which produces a pressure drop of at least 50 psi across it; and "stabilizing volume structure" means that part of the chamber disposed between the induction coil and the flow restriction structure.

In particular embodiments of the invention there is provided a plasma generator system that includes an elongated tubular plasma defining chamber with flow restriction structure at one end. An electrical coil surrounds a portion of this chamber and is connected to a power supply for creating an intense electromagnetic field within the chamber. The flow restriction is spaced downstream from the downstream end of the electrical coil a distance of at least three times the diameter of the plasma chamber to provide the stabilizing volume. An injector is provided for introducing material at the end of the chamber opposite the flow restriction for maintaining the plasma condition. The material also may be used as a transport medium for material to be treated within the chamber by exposure to the high temperature environment provided by the plasma. The material is introduced to flow in an annular flow sheath along the inner wall of the chamber to provide stabilization of the plasma. The requisite flow rate for adequate aerodynamic stabilization is a function of approximately the three-fourths power of the pressure and an inverse function of the cross-sectional area. For example, a minimum flow of 50 SCFH is required for operation of a particular system at atmospheric pressure while a minimum flow of 500 SCFH is required for operation of the same system at 500 psi. In such system the maximum flow is in the order of about 6 times the minimum flow. The uniform cross-sectional dimension of the chamber extends a substantial distance downstream from the plasma so that the stabilized flow condition is maintained downstream from the coil. Apparatus constructed in accordance with the invention provides a simple, reliable and relatively easy to operate stable induction plasma system of the flowing type operable at pressures up to 50 atmospheres and above.

Other objects, features and advantages of the invention will be seen as the following description of a particular embodiment progresses, in conjunction with the drawings, in which:

FIG. 1 is a diagram of an induction plasma generator apparatus constructed in accordance with the invention;

FIG. 2 is a graph indicating requisite stabilizing volume; and

FIG. 3 is a sectional view showing details of the induction plasma generator apparatus shown in FIG. 1.

DESCRIPTION OF PARTICULAR EMBODIMENT

The induction plasma generator structure shown in FIG. 1 includes a tubular plasma chamber defining structure 10 that is in this embodiment 2 inches in diameter. An injector structure 12 is disposed at one end of chamber structure 10 and a nozzle structure 14 is disposed at the opposite end. An electrical induction coil 16, six turn that has an inner diameter of 21/2 inches surrounds chamber 10 adjacent the injector structure so that the nozzle structure is spaced 15 inches (71/2 chamber diameters) from the downstream end of coil 16. A nominal 4 megahertz 190-kilowatt DC plate power power supply 18 is connected to energize coil 16. Gas from pressurized supply 20 is introduced through injector 12 to flow in a sheath 22 having generally laminar flow characteristics along the inner wall of tubular member 10. The power supply 18 is energized and plasma 24 is initiated by suitable means and stable operation is maintainable in this system at pressures up to 50 atmospheres and above at argon gas flow rates over 1,000 SCFH.

The graph in FIG. 2 indicates the requisite size of the stabilizing volume as a function of chamber diameter and chamber volume.

Additional details of the plasma generator may be had with reference to FIG. 3. As shown in that figure, the chamber defining structure 10 is a quartz cylinder that has an inner diameter of 2 inches and a length of 18 inches. Thin segments 30 of thermally reflective material such as gold preferably extend over its inner surface adjacent coil 16. The upstream end of 32 of cylinder 10 is disposed within recess 34 of housing structure 36 of the injector assembly 12 and sealed by O-ring 38.

The inner surface 40 of member 36 defines a cylindrical wall and cooperates with injector insert structure 42. That insert 42 has a set of six axially directed orifices 44 (each 0.028 inch in diameter), a set of six radially directed orifices 46 (each 0.028 inch in diameter), and a set of three generally tangentially directed swirl orifices 48 (each 0.033 inch in diameter). Conduits 50, 52 and 54 supply orifices 44, 46 and 48, respectively. A passage through insert 42 may receive an electrode 58 which may be used for starting purposes. End plate 60 is secured to injector housing 36 by thumb screw 62 and has a passage 64. A sight port structure 66 that includes a quartz sight window 68 is secured to end plate 60 by bolts 70.

A similar end plate 72 has a recess 74 which receives the downstream end 76 of cylinder 10 and is sealed by O-ring 78. Secured between end plate 72 and injector retainer housing 36 is an acrylic cylinder 80. The upstream end of cylinder 80 is secured in recess 82 of housing 36 and sealed by O-ring 84 while the downstream end 85 of cylinder 80 secured in recess 86 of end plate 72 and similarly sealed by O-ring 88. Coil 16 is disposed in the space between cylinders 10 and 80 and connected to terminals 90, 92. Each terminal structure includes a flow passage for coolant, which coolant is circulated in the space between cylinders 10 and 80 and exhausted through conduit 94. To reduce the radiant heat load on the acrylic cylinder 80, nigrosine, a water soluble dye, is added to the cooling water flowing in the space between cylinders 10 and 80. Four glass fiber fabric laminated epoxy rods 100 extend between end plates 60 and 72 and are secured by nuts 102 for clamping the assembly together.

Nozzle structure 14 is secured to end plate 72 and includes an orifice insert 104 and housing 106. In this embodiment a calorimeter structure 110 may be secured on nozzle structure 14 by bolts 112. That calorimeter structure includes end plate 114 and intermediate housing 116 that has flanges 118 which are bolted to orifice housing 106 and end plate 114. Two arrays of 1/8 inch O.D. copper tubes 122, 124 arranged in concentric circles extend between end plate 114 and insert 104. Heat transfer fluid is circulated from passage 130 through the inner array of tubes 122 to recess 132 in end plate 116 and then returned through the outer array 124 of tubes to annulus 134 and outlet passage 136 to provide a measure of the heat output passing through the nozzle 14. An exhaust passage 140 is provided in the side wall of the calorimeter and a downstream needle valve of passage 140 provides a control on the flow rate through the plasma generator assembly.

In a typical operation, the power supply 18 is energized and a plasma is initiated by suitable means, for example by temporary insertion of a graphite or tungsten rod 58 through either end plate. The gas flow rate and pressure is adjusted and preferably concurrent adjustment is made in the pressure of cooling water flowing in the channel between chamber structure 10 and housing 80 to control the pressure differential across quartz cylinder 10 so that that pressure differential never exceeds 300 psi. In usual operation, the sheath gas flow is supplied solely by the radial and swirl orifices 46, 48 with generally the same flow rates through those two sets of orifices. The following table indicates operating conditions in a series of operating runs of the apparatus:

HIGH PRESSURE OPERATION

DC Plate Plasma Gas Chamber Power kW SCFH Ar Pressure psia 46.0 427 215 52.2 277 215 120.0 226 270 147.7 535 317 157.5 472 370 156.0 472 433 169 472 565 128* 1040 620 __________________________________________________________________________

Comparison of visible spectra data obtained at one atmosphere and at 14.6 atmospheres with an argon plasma indicates that approximately 5 times as much radiation is produced when the system is operating at 14.6 atmospheres as is produced when the system is operating at one atmosphere.

While a particular embodiment of the invention has been shown and described, in various modifications will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

Claims

What is claimed is:

1. An induction plasma system comprising:

structure defining a plasma chamber,

an electrical coil surrounding said chamber and adapted to be connected to suitable power supply for creating an intense electromagnetic field within said chamber,

means for supplying a gas under at least 10 atmospheres of pressure for flow into said chamber and conversion to plasma condition under the influence of the electromagnetic field produced by said electrical coil,

flow restriction structure defining a port for flow of said gas from said chamber, said flow restriction structure being spaced at least one chamber diameter from the downstream end of said electrical coil, and structure defining a stabilizing volume between said coil and said flow restriction structure.

2. The system as claimed in claim 1 wherein said stabilizing volume is related to the spacing of said flow restriction structure from the downstream end of said coil, said volume being at least three times the plasma chamber volume with said flow restriction structure spaced one chamber diameter from said coil and being equal to a uniform dimensional extension of said plasma chamber when said flow restriction structure is spaced at least three chamber diameters from the downstream end of said coil.

3. The system as claimed in claim 1 wherein said gas flow through said chamber is a function of, approximately, the three-fourths power of the chamber pressure in atmospheres.

4. The system as claimed in claim 1 wherein said chamber defining structure includes a first tubular structure and further including a second tubular structure coaxially aligned with, surrounding and spaced from said first tubular structure such that an annular intermediate chamber is defined and means to supply fluid under pressure to said intermediate chamber, as to maintain the pressure in said intermediate chamber at least one-third the pressure in said plasma chamber.

5. The system as claimed in claim 1 and further including segmented electrically conductive material on the inner surface of said plasma chamber defining structure.

6. The system as claimed in claim 5 wherein said stabilizing volume is related to the spacing of said flow restriction structure from the downstream end of said coil, said volume being at least three times the plasma chamber volume with said flow restriction structure spaced one chamber diameter from said coil and being equal to a uniform dimensional extension of said plasma chamber when said flow restriction structure is spaced at least three chamber diameters from the downstream end of said coil.

7. The system as claimed in claim 6 wherein said chamber defining structure includes a first tubular structure and further including a second tubular structure coaxially aligned with, surrounding and spaced from said first tubular structure so that an annular intermediate chamber is defined and means to supply fluid under pressure to said intermediate chamber, to maintain the pressure in said intermediate chamber at least one-third the pressure in said plasma chamber.

8. The system as claimed in claim 7 wherein said gas flow through said chamber is correlated with the three-fourths power of the chamber pressure in atmospheres.