Patent Application
20140366551
1. Field of the Invention
The present invention relates to combustion, and more particularly to ignition systems such as in gas turbine engines.
2. Description of Related Art
A variety of devices are known for initiating combustion, for example in a gas turbine engine. Many gas turbine engines use spark igniters for ignition. One or more spark igniters are positioned to ignite a fuel and air mixture to initiate the flame in the combustor. These typical igniters provide ignition energy intermittently, and the spark event must coincide with a flammable mixture local to the igniter in order for engine ignition to occur. Often this means fuel will be sprayed toward the combustor wall near the igniter to improve the chances of ignition. This increased concentration of fuel can wet the igniter, making it more difficult to light and can lead to carbon formations which will also make ignition more difficult. Although the igniter is used for a very minute portion of the life of the engine, a great deal of care must be devoted to it such that it does not oxidize or melt in the course of the mission when it is not functioning. Typical igniters can fail instantaneously and without warning, which also requires special design considerations in anticipation of failure. The high voltages that are used to generate the spark can often find alternate paths in the circuit leading to the spark surface across which they can discharge and in such cases, the igniters can fail to provide an adequate spark for engine ignition. The high voltage transformers required to generate the arc are heavy and require heavy electrical cables and connectors. The sparks have trouble generating enough heat to vaporize cold fuel in cold conditions. Fuel must be in vapor form before it will ignite and burn. High velocity air, as may occur in altitude flameout situations can quench the spark out before it ignites significant fuel. The ignition process can interfere with electronic device functions through stray electromagnetic interference (EMI). Sparking systems have difficulty in maintaining a lit combustor under very low power or other unstable or transient mode of operation. Often, pilots might choose to leave the igniters on for an extended period of the mission to prevent flameout, such as during bad weather. Leaving the spark plugs on for the entire mission can lead to early igniter deterioration and failure.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for systems and methods that allow for improved ignition. There also remains a need in the art for such systems and methods that are easy to make and use. This disclosure provides a solution for these needs.
A new and useful ignition system includes a housing defining an interior and an exhaust outlet. The housing is configured and adapted to be mounted to a combustor case to issue flame from the exhaust outlet into the combustor for ignition and flame stabilization within the combustor. A fuel injector is mounted to the housing with an outlet of the fuel injector directed to issue a spray of fuel into the interior of the housing. An igniter is mounted to the housing with an ignition point of the igniter proximate the outlet of the fuel injector for ignition within the interior of the housing.
In certain embodiments, an inner wall is mounted in the interior of the housing, spaced apart inward from the housing to define an air plenum between the inner wall and the housing and to define a combustion chamber within the inner wall. An air swirler can provide fluid communication from the air plenum into the combustion chamber, wherein the air swirler is configured to impart swirl onto a flow of air entering the combustion chamber. For example, a spaced apart pair of air swirlers can be provided, one of the swirlers being proximate a first end of the inner wall, and another of the swirlers being proximate a second end of the inner wall. Each air swirler can be configured to impart swirl onto a flow of air entering the combustion chamber.
An elbow can be included with an elbow inlet operatively connected to receive combustion products from the combustion chamber along a longitudinal axis and with an elbow outlet in fluid communication with the inlet. The elbow outlet can be aligned along an angle relative to the longitudinal axis. An exhaust tube can be included in fluid communication with the elbow outlet for issuing combustion gases from the exhaust tube. The housing and the inner wall can be slidingly engaged to one another. The inner wall and the elbow can be slidingly engaged to one another. The exhaust tube and the elbow can be slidingly engaged to one another. The exhaust tube and the housing can be slidingly engaged to one another. These sliding engagements can accommodate relative thermal expansion and contraction. An axial spring can bias the elbow toward the inner wall, and a radially oriented spring can bias the exhaust tube toward the elbow.
The axial length of the combustion chamber can be about twice the interior diameter of the combustion chamber in length. The inlet diameter of the elbow inlet can be between about 25% and 75% of the interior diameter of the combustion chamber. For example, the inlet diameter of the elbow inlet can be about 50% of the interior diameter of the combustion chamber. The elbow inlet diameter can be about equal to the elbow outlet diameter in length. It is also contemplated that the outlet diameter of the exhaust tube can be about 0.5 to 0.6 times the inlet diameter of the elbow inlet.
In another aspect, the housing can define an air inlet configured and adapted to issue air for combustion into the interior of the housing. The air inlet and the exhaust outlet can be aligned to accommodate attachment of the housing to a combustor to issue flame from the exhaust outlet into the combustor and to take in compressor discharge air through the air inlet from a high pressure casing outboard of the combustor. It is also contemplated that the air inlet can be radially oriented relative to a longitudinal axis defined by the housing, and the exhaust outlet can be aligned with the longitudinal axis.
A new and useful method of ignition for a combustor in a gas turbine engine includes initiating a fuel and air flow through the fuel injector of an ignition system as described above. The method also includes igniting the fuel and air flow with the igniter and igniting a fuel and air flow in a combustor with a flame from the exhaust outlet of the ignition system.
Also disclosed is a new and useful method of combustion stabilization for a combustor in a gas turbine engine. The method includes detecting a combustion instability in a combustor and issuing a flame from the exhaust outlet of an ignition system as described above into the combustor to stabilize combustion in the combustor. The method can further include increasing flame strength from the exhaust outlet of the ignition system in response to weak flame conditions in the combustor, and decreasing flame strength from the exhaust outlet of the ignition system in response to stable flame conditions in the combustor.
These and other features of the systems and methods of the subject invention will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings.
So that those skilled in the art to which the subject invention appertains will readily understand how to make and use the devices and methods of the subject invention without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein:
Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject invention. For purposes of explanation and illustration, and not limitation, a partial view of an exemplary embodiment of an ignition system is shown in
In
Ignition system 100 includes a housing 108 in the form of a pressure case defining an interior. Ignition system 100 also includes an exhaust outlet 110. Housing 108 is mounted to a combustor 104 to issue flame from exhaust outlet 110 into combustor 104 for ignition and flame stabilization within combustor 104.
Referring now to
A cylindrical inner wall 116 is mounted in the interior of housing 108, spaced apart inward from housing 108 to define an air plenum 118 between inner wall 116 and housing 108. The inside of inner wall 116 defines a combustion chamber. A spaced apart pair of air swirlers 120 and 122 are provided. Swirler 120 proximate a first end of inner wall 120 proximate fuel injector 112 and igniter 114. Swirler 122 is proximate the opposite end of inner wall 116. Air swirlers 120 and 122 provide fluid communication from air plenum 118 into the combustion chamber inside inner wall 116. Each of the air swirlers 120 and 122 is a radial swirler configured to meter and impart swirl onto a flow of air entering the combustion chamber. Cool swirling air clings to the inner surface of inner wall 116, and spreads both ways along longitudinal axis A. The two swirling flows engage in the interior of inner wall 116. This provides a stable, flame holding flow while providing cooling flow to the surface of inner wall 116, since the flame can be maintained without attaching to inner wall 116.
Inner wall 116 can be of ceramic or ceramic composite material, and swirlers 120 and 122 can be made of similar materials or metallic since they are cooled by the air flow into the combustion chamber. Those skilled in the art will readily appreciate that any other suitable high temperature materials can be used, and that these components can be formed separately or integrally as appropriate for given applications. Provision of two swirlers encourages some of the air to flow on the outer or backside of the combustion chamber, helping to cool wall 116 from the backside.
Swirlers 120 and 122 each have three or more integral tabs 121 as shown in
An elbow 124 is included with an elbow inlet operatively connected to receive combustion products from the combustion chamber along a longitudinal axis A. The inlet diameter d can be between about 25% and 75% of the combustion chamber diameter D. In certain applications, the inlet diameter d is preferably about 50% of the diameter D. Elbow 124 has an elbow outlet in fluid communication with the elbow inlet. The elbow outlet is aligned along a radial angle relative to longitudinal axis A. In system 100, the length of the combustion chamber is about twice the diameter D.
An exhaust tube 126 is connected in fluid communication with the outlet of elbow 124 for issuing combustion gases from exhaust outlet 110 of exhaust tube 124. The diameter dl of the outlet passage through exhaust tube 126 can be in a range of about 0.5 to 0.6 times the diameter d of the elbow inlet. All of the wall surfaces in contact with combustion products can be made from high temperature materials which can be metallic, but can preferably be ceramic or ceramic composite materials in certain applications. While elbow 124 has an inlet diameter and an outlet diameter smaller than d,
In
In order to accommodate thermal expansion and contraction gradients, many of the components of ignition system 100 are slidingly engaged to one another. Swirlers 120 and 122 are not seated, but centralized by outer tabs. Swirlers 120 and 122 seat the cylindrical flow elements in a sliding fashion to prevent or minimize any bending moments being transmitted to the cylinder. Exhaust tube 126 and elbow 124 are slidingly engaged to one another for relative movement in the direction of longitudinal axis A. Exhaust tube 126 and housing 108 are slidingly engaged to one another for relative movement in the radial direction relative to longitudinal axis A.
An axial spring 128 biases elbow 124 toward inner wall 116 to keep elbow 124, inner wall 116, and swirlers 120 and 122 assembled to housing 108. A radially oriented spring 130 biases exhaust tube 126 toward elbow 124 to keep the inlet flange of exhaust tube 126 engaged to the outlet of elbow 124. It is contemplated that assembled in compression in this manner, housing 108, inner wall 116, elbow 124, and exhaust tube 126 can all be made of ceramic or ceramic composite materials. However, those skilled in the art will readily appreciate that any other suitable materials can be used without departing from the scope of this disclosure. Housing 108 includes an air inlet 132 for issuing air for combustion into the interior of the housing 108. Air inlet 130 and exhaust outlet 110 are aligned to accommodate attachment of housing 108 to the walls of combustor 104 and high pressure casing 102 to issue flame from exhaust outlet 110 into combustor 104 and to take in compressor discharge air through air inlet 132 from high pressure casing 102 outboard of combustor 104. Ignition system 100 can be retrofitted onto a gas turbine engine to replace a traditional igniter by removing the traditional igniter and connecting air inlet 132 with a modified air passage of the high pressure casing, and by connecting exhaust tube 126 to issue into the combustor.
Ignition systems as described above are based around a small combustion volume relative to the main combustor, and remote from the main combustion chamber. The housing, e.g., housing 108, is secured to the exterior of the engine to allow routine maintenance similar to conventional igniters. The orientation of the internal conduits containing high temperature combustion gases are such as to permit the axis of the main combustion element, e.g., the axial length of housing 108, to lay parallel to the engine axis, reducing the overall diameter of the engine envelope. The elbow, e.g., elbow 124, and exhaust tube whose axis is normal to the engine axis, allow the engagement with the engine combustor to be similar to conventional ignition devices. Those skilled in the art will recognize that any suitable modification of this orientation can also be used, for example to allow for improved ignition performance as needed for specific applications.
A relatively, small amount of metered air enters the combustion volume, e.g., inside housing 108, fed from the pressure of the main engine air supply. With the use of air swirlers, e.g. air swirler 120, to admit the air into the combustor of the ignition system, an air flow pattern is developed which enhances stable combustion while a small amount of fuel is injected in the air through an appropriate fuel injector, e.g., injector 112. The atomized fuel is ignited by the heat of an electric element or glow plug igniter, e.g., igniter 114, which is fed by low voltage DC electric current. The fuel ignites to produce a continuous stream of heat in the small combustor. The heat is of sufficient intensity to be able to ignite the fuel nozzle in the main combustor.
Once engine ignition has occurred, the electric element can be shut off. The flame in the small combustor can be left on continuously for the duration of the mission, supplying heat and radicals present in the combustion products to the main combustor at all times. Because the supply of fuel is small, the temperature produced by the ignition system does not overwhelm the temperature from the main fuel injectors when stable combustion is achieved. Only under very low power condition or during ignition processes does the energy from the ignition system rival the energy derived from the main combustor nozzles. As such, the impact from the ignition system is diminished at higher engine power and dominates at low engine power. This decoupled phasing and continuous duty helps the ignition system extend the flammability limits over that of a conventional combustor.
The hot gases from the ignition system can be projected deeply into the main combustor volume. This allows the spray pattern from the main nozzles to be optimized for durability and emissions compared to conventional situations where fuel must be sprayed towards the wall in order to approach a traditional igniter.
The continuous injection of heat into the main combustor allows for faster, higher quality main combustor ignition at lower, more adverse ignition conditions. Conventional fuel injectors require substantial fuel flow at low power to be able to form an atomized spray of sufficient quality to ignite. Aerated injectors require substantial air pressure to atomize fuel. At low starting speeds, air flows are low and the relatively high fuel flows are required for atomization produce relatively hot ignition situations when they finally ignite. This is exemplified by torching seen at the exhaust and large quantities of white smoke seen in cold weather starts. Within the ignition system, e.g., ignition system 100, the ignition of the nozzle, e.g., of injector 112, can be optimized for low flow conditions. The resulting flame is capable of igniting low quality sprays in the main combustor, speeding up engine ignition and reducing the overall temperature experienced during the main ignition sequence. This can prolong the life of the engine hot end components.
The ignition system can remain on continuously during a mission, protecting the main combustor from flame out. Its power can be controlled to vary with engine conditions through the fuel flow delivered to the ignition system. As such, it is capable of withstanding large excursions in engine conditions thereby assisting the main combustor.
The ignition system can utilize relatively low, DC power electric elements for ignition. These igniter devices are not prone to contamination from carbon deposits and are not prone to wetting or icing. They do not require high voltage cables and connectors, allowing for a lighter, more dependable delivery of ignition energy compared to higher voltage traditional igniters. They also emit significantly less electromagnetic interference to neighboring electronic equipment.
The size of the combustion chamber should be compact enough to easily be accommodated in an engine envelope and to utilize a small amount of fuel but be large enough to support a strong, stable flame. It has been found that using a cylindrical geometry with an approximate diameter of 1.5 inches (3.81 cm) can meet these objectives for certain typical applications.
Low emissions, lean burn type systems, present greater difficulty to ignition and flameout situations. The decoupled nature of the ignition systems described herein allow them to optimize the conditions for ignition within a confined volume away from the main nozzles allowing them to burn more cleanly while maintaining adequate ignition and re-light capability.
An exemplary method of ignition for a combustor in a gas turbine engine includes initiating a fuel and air flow through the fuel injector of an ignition system as described above. The method also includes igniting the fuel and air flow with the igniter, e.g., igniter 112, and igniting a fuel and air flow in a combustor with the flame from the exhaust outlet of the ignition system. An exemplary method of combustion stabilization for a combustor in a gas turbine engine includes detecting a combustion instability in a combustor and issuing a flame from the exhaust outlet of an ignition system as described above into the combustor to stabilize combustion in the combustor. The method can further include increasing flame strength from the exhaust outlet of the ignition system in response to weak flame conditions in the combustor, and decreasing flame strength from the exhaust outlet of the ignition system in response to stable flame conditions in the combustor. While shown and described in the exemplary context of gas turbine engines, those skilled in the art will readily appreciate that ignition systems in accordance with this disclosure can be used in any other suitable application without departing from the scope of this disclosure.
The methods and systems of the present invention, as described above and shown in the drawings, provide for ignition with superior properties including easier startup, continuous operation, and enhanced reliability. While the apparatus and methods of the subject invention have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the spirit and scope of the subject invention.
