WO1992020913A1

WO1992020913A1 - Plasma ignition apparatus and method for enhanced combustion and flameholding in engine combustion chambers

Info

Publication number: WO1992020913A1
Application number: PCT/US1992/002850
Authority: WO
Inventors: Steven C. Knowles
Original assignee: Olin Corporation
Priority date: 1991-05-15
Filing date: 1992-04-02
Publication date: 1992-11-26
Also published as: AU1977992A

Abstract

An apparatus and method for igniting a fuel and air mixture in a gas turbine engine combustor is disclosed. The apparatus comprises a plasma injector or ignitor (10) exhausting preferably an ammonia derived plasma (44) into the combustion chamber (16) of the turbine although any inorganic gas containing nitrogen and a predominance of hydrogen may be used. The ignitor comprises an annular conductive anode (24) forming a convergent nozzle therethrough and an elongated cathode (34) spaced from the anode by a gap. An electrical arc is generated between the cathode (34) and the anode (24). A high pressure flow of ammonia gas (40) is passed through the arc and through the nozzle to produce a plasma (44) therein rich in hydrogen atoms and ions to ignite the fuel and air mixture contained in the combustion chamber.

Description

"PLASMA IGNITION APPARATUS AND METHOD FOR ENHANCED COMBUSTION AND FLAMEHOLDING IN ENGINE COMBUSTION CHAMBERS"

This invention generally relates to ignition in combustion devices and more particularly to plasma ignition of turbine combustion engines.

Jet aircraft typically utilize spark igniters to ignite the fuel-air mixture flowing into and through the jet engine combustor. If flameout occurs, these aircraft turbine engine combustors have a serious limitation in the altitude at which they can be relit using conventional spark ignitors. Typically a commercial aircraft must drop below about 28,000 feet before attempting to relight the engine. Increasing this altitude can provide a greater safety margin for the pilot.

Various approaches to enhancing combustion and flame stabilization using plasma generation devices have been experimentally evaluated in the literature. These approaches involve the use of gases such as nitrogen, argon, air, or argon/fuel/air mixtures as the plasma feed stock. These studies include Harrison and Weinberg, "Flame Stabilization by Plasma Jets", Proc. Roy. Soc. Lond. A. 321, 95-103 (1971); Chen et al, "Augmenting Flames With Electric Discharges", Tenth International Symposium on Combustion. 1965; Weinberg, "Plasma Jets In Combustion", I. Mechanical Engineering, pp 65-71 (1983); Warris et al, "Ignition and Flame Stabilization by Plasma Jets in Fast Gas Streams", Twentieth International Sym osium QH Combustion, pp

1825-1831 (1984); and Boston et al, "Flame Initiation in Lean, Quiescent and Turbulent Mixtures With Various igniters", Twentieth international symposium on

Combustion, pp 141-149 (1984). The capability to meet emissions requirements for gas turbine engines which power high speed commercial transport vehicles may also be provided by operating below the current lean flammability limit.

Ground based combustion systems have their own unique set of ignition and stability problems. For example, industrial gas turbines for power generation have to meet stringent emissions requirements. Of particular concern is the removal of NO compounds from the exhaust gases. Lowering the flame temperature through very lean operation is one possible method to achieve compliance with these standards. However, severe pressure oscillations generally occur during very lean operation. Thus an improved flameholding capability would increase combustion stability. Attempts at improved flameholding in these applications have been primarily directed to nitrogen plasma jet devices. Studies addressing NO removal are described in Behbahani et al, "The Destruction of Nitric Oxide by Nitrogen Atoms From Plasma Jets: Designing for Thermal Stratification", Combustion Science and Technology. Vol. 30, pp 289-302 (1983); J. C. Hilliard et al, "Effect of Nitrogen-containing Plasmas on Stability, NO Formation and Sooting of Flames", Nature, Vol.259, February. 1976; and Behbahani

SUBSTITUTE SHEET et al, "The Destruction of Nitric Oxide by Nitrogen

Atoms From plasma Jets", Combustion Science and

Technology. Vol. 27, pp 123-132 (1982).

Advanced aircraft engines currently under development could benefit not only from an enhanced ignition capability but also improved stability margins. Flame stabilization during afterburner operation or during non-optimized flight regimes is also an important technology issue currently being investigated.

Finally, there are significant combustion difficulties to be overcome with ignition and flameholding in advanced scramjet engines such as are envisioned for the National Aerospace Plane. Various methods of combustion enhancement in scra jets have been studied and reported in the literature such as utilizing plasma jets and electrical discharges. Sato et al in "Effectiveness of Plasma Torches For Ignition and Flameholding in Scramjet", 25th Joint Propulsion Conference. AIAA 89-2564 (1989) discloses using an air or Oxygen plasma torch to ignite supersonic air-fuel mixtures in a scramjet combustor. In Northam et al, "Development and evaluation of a Plasma Jet Flameholder for Scramjets, 20th Joint Propulsion Conference. AIAA-84-1408 (1984), the authors discuss using a mixture of argon and molecular hydrogen as the feedstock. This study suggested that hydrogen atoms play a major role in the ignition process.

The current approaches discussed in these papers include pulsed spark ignitors, physical flameholders, and chemical additives. Others have also reported investigating use of inert gases such as argon, and argon in conjunction with nitrogen, oxygen, water vapor, and fuel-air mixtures to ignite fuel-air

SUBSTITUTE SHEET mixtures in supersonic combustors, in particular, scramjets. These papers include: Kimura, "Promotion of Combustion by Electric Discharges", JSME International Journal Vol.31, No.3, 1988; Masuya et al, "Some Governing Parameters Of Plasma Torch Igniter/Flameholder In A Scramjet", 26th Joint Propulsion Conference. AIAA 90-2098 (1990); Kimura et al, "The Use Of A Plasma Jet For Flame Stabilization And Promotion Of Combustion in Supersonic Air Flows", Combustion and Flame. 42:297-305 (1981); Wagner et al, "Plasma Torch Igniter for Scramjets", 23rd JANNAF Combustion Meeting. Volume I. NASA-Langley Research Center, 1986.

It is, therefore, an object of the present invention to provide an apparatus and method to enhance the ignition in combustors.

It is another object of the invention to provide an improved method for relighting aircraft turbine engines at all operational pressures. It is another object of the invention to provide an apparatus and method for improving ignition limits of fuel rich and fuel lean mixtures.

It is a still further object of the invention to provide a method for enhancing the stability of the combustion process in combustors.

None of the reported studies discloses or suggests the following method of enhancing turbine combustor ignition. We have found that the operational capability of such engines can be markedly improved by injecting an inorganic gas plasma rich in hydrogen atoms and ion species via a continuous plasma ignitor or injector against the substantial back pressure of the fuel-air mixture in a turbine engine combustor. To my knowledge, use of a plasma ignitor to discharge into

SUBSTITUTE SHEET a high backpressure for ignition has not been previously attempted. The gas utilized is preferably ammonia (NH_). Hydrazine (N_?*¹^ could also be used if a catalyst bed is provided in the feedstock flowstream.

The method of fuel and air mixture ignition in a turbine engine combustor chamber in accordance with the present invention includes the following steps:

1) providing a plasma ignitor having an electrically conductive anode body having an annular shape defining a convergent nozzle along a central axis therethrough extending into the combustor chamber and an elongated electrically conductive cathode coaxially disposed in the nozzle and spaced from the anode by a gap,

2) generating an electrical arc between the cathode and the anode across the gap,

3) injecting a flow of ammonia gas (NH_) into and through the nozzle and through the arc producing a plasma containing a predominence of hydrogen atoms in the nozzle, and

4) exhausting the plasma into the combustor chamber to ignite the fuel and air mixture therein.

The apparatus for carrying out the method in accordance with the invention comprises a plasma ignitor attached to the turbine engine combustor and extending into the combustion chamber preferably downstream of the fuel inlet to the combustion chamber. The plasma ignitor is adapted to inject an inorganic gas plasma containing an abundance of hydrogen atoms and ions into the fuel and air mixture inside the combustion chamber. This plasma ignitor must operate at a gas pressure substantially higher than the pressure inside the combustion chamber to

SUBSTITUTE SHEET maintain arc stability. The combustion chamber pressure is usually less than atmospheric at high altitudes but is usually above 3 psia.

More particularly, the plasma ignitor apparatus of the invention has an annular electrically conductive anode body directly attached to and may be supported by the combustor. Its discharge end is directed into the combustion chamber. The anode body has an upstream interior conical surface merging into a downstream interior cylindrical surface. Both surfaces are symmetrical about a common axis through the anode. Although not required, this axis is generally normal to the wall of the combustor. The surfaces define a nozzle discharging into the combustion chamber. The upstream and downstream interior surfaces merge at a location of minimum diameter defining the throat of the nozzle and the cylindrical surface of the nozzle forms a constrictor. The throat and constrictor constitute an arc chamber. An elongated electrically conductive cathode member having a generally conical tip is axially disposed adjacent to the upstream interior conical surface and spaced axially upstream of said throat by a small gap. A source of inorganic gas is connected to the arc chamber to preferably supply a flow of this gas radially and tangentially into the arc chamber. An external high voltage power supply is connected to the cathode and to the anode. This power supply is used to generate an electrical arc between the cathode tip and the anode body in the arc chamber.

The improved fuel-air mixture ignition in accordance with the present invention is primarily due to the gas used to create a plasma as well as providing a continuous source of that plasma. The gas provides a

SUBSTITUTE SHEET pressurized source of inorganic gas containing nitrogen and a predominance of hydrogen atoms, such as ammonia. This gas flows through the nozzle throat and the constrictor into the combustion chamber concurrently with the generation of the arc. The gas flow pushes the downstream arc foot through the throat so that it attaches to the anode at a location in the constrictor. Passing the gas through the arc causes dissociation and ionization of the gas to form a plasma. This plasma exits the nozzle and disperses into the combustion chamber to instantaneously ignite the air-fuel mixture. In addition, the plasma ignitor of the invention is capable of continuous operation without signifigant electrode erosion. It has been found that a fuel-air mixture can be predictably lit and relit at high altitude conditions virtually simultaneously with plasma discharge into the chamber in accordance with the invention. In addition, the potential for relight capability at subatmospheric pressures about 20% lower than those achievable with spark ignitors has been demonstrated. This indicates that jet engines ignited in accordance with the present invention can be relit after flameout at much higher altitudes than currently can be achieved. This can provide a much greater margin of safety for these aircraft.

Figure 1 is a schematic sectional view of a turbine engine combustion apparatus in accordance with the present invention. Figure 2 is a longitudinal sectional view of one preferred embodiment of the apparatus in accordance with the invention.

The combustion apparatus in accordance with the invention is shown in Figure 1. The apparatus 10

SUBSTITUTE SHEET comprises an plasma ignitor 12 coupled to a turbine engine combustor 14 and extends into its combustion chamber 16 near the fuel inlet 18. An electrical power and control unit 20 and a pressurized supply of feedstock gas 22 are in turn connected to the ignitor 12.

The power and control unit 20 is either AC or DC and is sized and configured in a conventional manner and may be designed to operate from 100W to over 10KW. The higher power levels can provide a larger flow rate of chemically reactive atomic and ionic species to promote greater mixing and reaction. The higher powers would also tend to be required for steady state flameholding of large gas turbines. For jet aircraft engines, however, power levels of about 1KW are expected to be appropriate.

Several different feedstock gases 22 may be used in accordance with the invention. These gases are inorganic gases containing nitrogen and a predominance of hydrogen. Inorganic gases such as ammonia (NH_) and hydrazine (N₂H.) are preferable since they provide the required abundance of hydrogen and do not create unwanted deposits on the electrodes or harmful residues. If hydrazine is utilized, however, a catalyst bed must be added in the flow path upstream of the ignitor 12 to convert the liquid into gas.

The ignitor 12 is basically a plasma torch. A preferred arrangement of the apparatus of the invention is shown in Figure 2. The ignitor 12 includes an annular electrically conductive anode body 24 directly attached to the combustor 14 and having its discharge end 26 directed into the combustion chamber 16. The anode body 24 has an upstream interior conical surface 28 merging into a downstream interior cylindrical

SUBSTITUTE SHEET surface 30. Both surfaces are symmetrical about a common axis "A" generally normal to the wall of the combustor chamber 16 and define a nozzle discharging into the combustion chamber 16. The interior surfaces of the nozzle form tandemly arranged throat and constrictor regions which together define an arc chamber 32. The upstream and downstream interior surfaces merge at a location of minimum diameter defining the throat of the nozzle. An elongated electrically conductive cathode member 34 having a generally conical tip 36 is coaxially disposed adjacent to the upstream interior conical surface 28 of the anode 24 and is spaced axially upstream of the throat by a small gap. The cathode 34 is preferably doped with a material such as thorium to reduce the work function of the metal to increase the electron production.

An insulating sleeve 38 surrounds the cathode 34 and spaces the cathode 34 laterally from the anode 24. The sleeve 38 is typically made of boron nitride or other high temperature ceramic material.

The anode is typically made of tungsten. The anode can also be made of copper, tungsten copper alloys, molybdenum alloys such as molybdenum-rhenium alloy, and other tungsten alloys. The materials chosen depend particularly on the feedstock gas utilized to provide the plasma species.

The external power and control unit 20 is of conventional design and is electrically connected to the cathode 34 and to the anode 24. This power supply 20 is used to generate and control an electrical arc between the cathode tip 36 and the anode body in the arc chamber. The power supply may be either AC or DC.

SUBSTITUTE SHEET Also connected to the ignitor 12 is a pressurized supply of inorganic feedstock gas 40 which has its flow controlled also by the power and control unit 20. The gas 40 flows along at least a portion of the insulating sleeve 38 through passage 42 and is injected into the arc chamber 32 at the tip 36 of the cathode 34 via tangential holes 44 through the insulator sleeve 38 so as to form a vortex as the gas flows through the throat into the constrictor 30. Thus the feedstock is fed radially and tangentially into the arc chamber to enhance arc stability.

This gas 40 flows through the nozzle throat and the constrictor into the combustion chamber 16 concurrently with the generation of an electrical arc from the cathode 36 to the anode 26. The gas flow pushes the downstream end of the arc through the throat so that it attaches to the anode at a location in the constrictor 30. The vortex flow also stabilizes the arc drawn between the cathode and the anode. Passing the gas 40 through the arc causes intense heating, dissociation, and ionization of the gas 40 into a plasma 44 rich in hydrogen atom and ion species. This plasma 44 exits the nozzle and disperses into the combustion chamber 16 to cause almost instantaneous localized ignition of the fuel-air mixture.

Referring again to Figure 1, the ignitor 12 can be placed in a number of different locations within the combustor. It can be placed upstream or downstream of the fuel inlet. It can be placed in the free streaming flow or behind a physical flameholder. The particular placement depends upon the optimization of the particular combustor shape and to optimize the effect of the particular injected gas radicals on the ignition

SUBSTITUTE SHEET process. In conventional jet engines, the ignitor apparatus of the invention may be utilized as shown as a direct replacement for conventional spark ignitors. A less substantial improvement in plasma ignition as compared to spark ignition has been observed with gases such as nitrogen and argon. However, plasma ignition with a hydrogen rich gas such as ammonia in accordance with the invention has unexpectedly been demonstrated at substantially lower pressures for the same air flow rates than can be achieved with spark ignition. A relight pressure at a given flow rate corresponds to an operational altitude. For example, a decrease of about 20% in the pressure at which relight can be achieved utilizing ammonia gas at a given air-fuel mixture flow rate when compared to spark ignition has been observed. Consequently this means that there can potentially be a greater margin of safety for pilots of jet aircraft against flameout.

The feedstock gas, preferably ammonia (NH₃), produces an abundance of monatomic nitrogen and hydrogen atoms when passing through the arc. In addition, the presence of the hydrogen atoms and ions contributes to the unexpected increase in ignition capability under extremely low pressure conditions. This is in part due to the mobility of these atoms and ions into the air-fuel mixture.

The method of ignition of a fuel and air mixture in a turbine engine combustion chamber 16 in accordance with the present invention includes the following steps: 1) providing a plasma ignitor 12 which has an electrically conductive anode body 24 having an annular shape defining a convergent nozzle therethrough in the combustor 14 projecting into the combustion chamber 16 and an elongated electrically conductive cathode 34

SUBSTITUTE SHEET coaxially disposed in the nozzle and spaced from the anode by a gap,

2) generating an electrical arc between the cathode 34 and the anode 26 across the gap, 3) simultaneously with generation of the arc injecting a flow of ammonia gas 40 into and through the nozzle and through the arc producing a plasma 44 in the nozzle, and

4) exhausting the plasma 44 into the combustor chamber 16 to ignite the fuel and air mixture therein.

Hydrazine may also be used as the feedstock gas.

However, as this material is liquid under normal conditions, a catalyst bed must be included in the flow path to vaporize the hydrazine prior to injection through the plasma ignitor 12.

While the invention has been described above with reference to specific embodiments thereof, it is apparent that many changes, modifications and variations can be made without departing from the inventive concept disclosed herein. For example, the plasma ignition apparatus as above described using a hydrogen rich feedstock such as ammonia or hydrazine should also be directly applicable to scramjet engines with corresponding results. Accordingly, it is intended to embrace all such changes, modifications and variations that fall within the spirit and broad scope of the appended claims.

SUBSTITUTE SHEET

Claims

WHAT IS CLAIMED IS:

1. An apparatus for igniting a fuel-air mixture in an engine combustion chamber 16 characterized by: means for defining an arc chamber 32 and for producing an electrical arc therein; a source 22 of inorganic gas 40 containing nitrogen and a predominance of hydrogen; means 42 for injecting said gas 40 from said source into said arc chamber 16 concurrently with generation of said arc so as to create a plasma 44 in said arc chamber 32; and nozzle means 24 in fluid communication with said arc chamber 32 and said combustion chamber 16 for directing said plasma from said arc chamber 32 into said combustion chamber 16 to ignite said mixture therein.

2. In a turbine engine combustor 14 having a hollow combustion chamber 16 into which a flow of air and fuel to be combusted is directed, a plasma ignition apparatus 12 for enhanced combustion characterized by: an annular electrically conductive anode body 24 attached to said combustor 14 and penetrating said chamber 16, said anode body 24 having an upstream interior conical surface 28 and a downstream surface 30 about a common axis defining a nozzle discharging into said combustion chamber 16 , said nozzle having tandemly arranged throat and constrictor regions which together define an arc chamber 32, said upstream and downstream interior surfaces merging at a location of minimum diameter defining said throat of said nozzle;

SUBSTITUTE SHEET an elongated electrically conductive cathode member 34 having a tip 36 disposed adjacent to said upstream interior conical surface 28 and spaced axially upstream of said throat by a gap; means 20 for applying an electrical potential to said anode body 24 and said cathode member 34 so as to generate an arc in said arc chamber 32 extending from said cathode member tip to said anode body; a supply source of an inorganic gas 40 containing nitrogen and a predominance of hydrogen; and means for injecting said gas 40 into said arc chamber 32 through said nozzle throat and said constrictor 30 into said combustion chamber 16 concurrently with the generation of said arc so as to produce a plasma 44 exiting said nozzle into said chamber 16, said plasma 44 containing a substantial number of hydrogen atoms and ions.

3. The apparatus according to claim 2 characterized in that said gas is ammonia.

4. A method of igniting a fuel-air mixture in a combustion chamber 16 of a turbine engine combustor 14 characterized by the steps of: a) providing an electrically conductive anode body 24 attached to said combustor 14 having an annular shape defining a nozzle therethrough discharging into said chamber 16; b) providing an electrically conductive cathode 34 coaxially disposed in said nozzle and spaced from said anode 24 by a gap c) providing a pressurized supply 40 of an inorganic gas containing nitrogen and a predominance of hydrogen connected to said nozzle; d) applying an electrical potential to said anode 24 and said cathode 34 to produce an arc therebetween; and e) injecting said gas into said nozzle and through said arc producing a plasma 44 therein containing an abundance of hydrogen atoms and ions, said plasma 44 exhausting from said nozzle into said combustion chamber 16 to ignite said fuel-air mixture.

5. A method of igniting a fuel and air mixture in a combustion chamber 16 of a turbine engine combustor 14 wherein said mixture is at a pressure of at least 3 PSIA characterized by the steps of: providing a plasma ignitor 12 having an electrically conductive anode body 24 having an annular shape defining a convergent nozzle along a central axis A therethrough attached to said combustor 14, said nozzle communicating with said chamber 16; said ignitor further having an elongated electrically conductive cathode 34 coaxially disposed in said nozzle and spaced from said anode 24 by a gap; providing a pressurized supply of ammonia gas 40 connected to said nozzle; generating an electrical arc between said cathode 34 and said anode 24 across said gap; injecting a flow of said ammonia gas 40 from said supply into and through said nozzle and through said arc producing a plasma 44 in said nozzle; and exhausting said plasma 44 into said combustion chamber to ignite said fuel and air mixture therein.

SUBSTITU

6. The method according to claim 5 characterized in that said ammonia gas 40 is injected into said combustion chamber 16 at a pressure substantially greater than the pressure of said mixture in said chamber 16.

7. The method according to claim 6 characterized in that said step of injecting further includes pushing an attachment point of _said arc through a throat portion of said nozzle into a constrictor portion 30 of said nozzle.