Abstract:
The igniter for the combustor in a gas turbine engine includes a tubular member extending beyond the igniter tip, wherein the tubular member is a porous ceramic or high temperature nickel alloy. Fuel is fed to the bore of the tubular member by capillary action through the porous material of the tubular member and air passes through the porous tubular member to the bore.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ignition system, and more particularly, to an injector for such ignition systems. 
     2. Description of the Prior Art 
     It has been found that dependable ignition using conventional aircraft engine ignition systems is not possible under certain situations. Of particular difficulty is to attempt to ignite a small engine at very low speed using large flow number or air blast fuel injectors. Small engines cannot produce a significant combustor pressure drop during cranking conditions and, therefore, atomization of fuel by the main fuel injector is poor. 
     Large flow number injectors are sized for optimum performance at full power. Fuel flows during the starting mode are not sufficient to pressurize the injectors at the starting mode flow rates and thus atomization of the fuel is inadequate, resulting in difficulty. 
     So-called hot starts can result if the fuel flow rate is increased at such low speed conditions. In order to solve the above problem, very small pressure atomizers, known as primary injectors, are used in association with larger secondary injectors. The small size of these primary injectors produces a greater fuel pressure at low speed, allowing for better atomization of the fuel. 
     The disadvantage with these small primary injectors is their tendency to become contaminated owing to the very small orifice sizes. This requires increased maintenance in the field. In addition, a second fuel manifold and fuel flow divider valves add to the complexity of the system and to the cost. At high altitude and in conditions where flame-out of the combustor might be more prevalent, requires that the primary injectors be continuously operating to inject fuel into the combustion chamber which, in fact, diverts fuel from the secondary fuel injectors to thereby hamper the cooling of the secondary injectors. 
     So-called torch igniters utilize a small primary injector in close proximity to the igniter, thus eliminating the requirement for a large number of small injectors. However, these primary injectors still have the problem of contamination in view of their very small orifice sizes. In order to keep the injector cool, it must be operated throughout the entire engine cycle. 
     Although torch igniters can solve some problems, particularly of ignition during low speed cranking conditions, their performance can still suffer at high altitudes when it is required to reignite after a flame-out, because the air flow rates and the combustor pressure drops are much greater. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide an igniter system that can ignite injectors at low speeds during a cranking mode or at high altitude conditions when fuel flow rates are low. 
     It is a further aim of the present invention to eliminate the need for small orifice injectors. 
     It is a still further aim of the present invention to provide a continuous controllable source in order that burning may occur as soon as a flammable mixture of air and fuel is supplied. 
     It is a yet further aim of the present invention to provide an improved fuel distribution system for an igniter that reduces the occurrence of carbon buildup and coking since small orifices or small dead spots in the conduits are not present. 
     A construction in accordance with the present invention comprises a fuel and air distribution means for use with an igniter in a combustor. The distribution means includes a tubular member having a bore with a first end near the igniter such that the igniter tip is within the bore at the first end and the second end projects into the combustor characterized in that the tubular member is porous material chosen from a material having high thermal tolerance whereby liquid fuel and air are fed to the tubular porous device such that the liquid fuel is retained and distributed by capillary action toward the bore of the device where the liquid fuel will vaporize and form an atomized mixture with the air. 
     In a more specific embodiment of the present invention, the igniter is a plasma igniter of the type described in U.S. Pat. No. 5,587,630, Dooley, issued Dec. 24, 1996. 
     In a still further embodiment of the present invention, the conduits supplying fuel to the porous tubular member are relatively large bore conduits, thus reducing the risks of coking. 
     It is a still further embodiment of the present invention whereby the tubular porous member is a circular cylinder, and the porosity of the cylinder ranges between 60 pores per inch and 200 pores per inch. 
     It is also contemplated that the tubular device might be spherical or frusto-conical. 
     A method for distributing atomized fuel to an igniter in a combustion chamber in accordance with the present invention comprises the steps of placing a tubular member having a bore with a first end near the igniter such that the igniter tip is within the bore at the first end and the second end projects into the combustor characterized in the steps of choosing the tubular member from a porous material having high thermal resistance, feeding liquid fuel to the tubular porous member such that the liquid fuel is retained and distributed by capillary action toward the bore of the device, passing air through the tubular porous member to carry the liquid fuel and vaporize the fuel and form an atomized mixture with the air. 
     Thus, as might be contemplated, the tubular porous member is installed to the combustor with the igniter tip just within the bore of the tubular device, and the liquid fuel is supplied to the porous tubular device where, by capillary action, the fuel will soak the porous member, but the pressurized air, also being fed to the porous tubular member, will atomize the fuel as it carries the fuel into the bore portion of the tubular device. 
     An advantage of the present invention is the ability to use pure air blast injectors in the combustor at low cranking speeds and high altitude conditions. 
     Another advantage of the present invention is the formation of a combustion cavity fed by controlled fuel and air flow rates independent of the conditions in the combustor. 
     Furthermore, the plasma igniter may be cooled by the air flow through the porous tube. 
     Flow number is defined as the fuel mass flow divided by the square of the pressure drop across the nozzle to drive that flow. The smaller the flow number, the greater the pressure drop required to flow a certain rate of fuel. It is a measure of the orifice size of the nozzle. Small flow numbers are anywhere from 0.5 to 1.5 while large flow numbers are greater than 10. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration, a preferred embodiment thereof, and in which: 
     FIG. 1 is a fragmentary, axial cross-section showing a combustor of a gas turbine engine incorporating the present invention; 
     FIG. 2 is an enlarged axial cross-section of a torch igniter in accordance with the present invention; 
     FIG. 3 is a radial cross-section taken along line  3 — 3  of FIG. 2; 
     FIG. 4 a  is a schematic view of the torch igniter shown in FIG.  2  and showing some detail of the plasma electrode; and 
     FIG. 4 b  is a schematic view of another embodiment of the igniter showing a different plasma electrode configuration. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown schematically a torch igniter  10  mounted to a combustor  13 . In fact, the torch igniter includes a plasma igniter  12  in axial alignment with a cavity defined by the tubular member  18  in the housing  16  in FIG. 1. A fuel injector  34  is shown schematically next to the torch igniter  10 . 
     Referring now to FIG. 2, the plasma igniter  12  is shown schematically. However, the preferred plasma igniter is in accordance with U.S. Pat. No. 5,587,630, issued Dec. 24, 1996 to Kevin A. Dooley, and assigned to the present assignee. In that patent, the plasma igniter  12  provides a continuous gaseous plasma arc across an igniter gap at the igniter tip. The description in the above-mentioned patent is incorporated herein by reference. 
     A tubular porous member  18  has a circular cylindrical shape in the present embodiment. The porous cylinder  18  defines an axial bore  20  defined by an inner surface  22 . The cylinder has an outer recessed surface  24 . The cylinder  18  is mounted in the housing  16  mounted to the exterior of the combustor wall  14 . The bore  20  defines an exit opening  20   a  at the combustor wall  14 . 
     Cylinder  18  is made of a porous ceramic or metallic material having a high thermal tolerance. The ceramic version of the cylindrical tube  18  is a high temperature silicon carbide. In the case of a metal tube, Inco 718™ may be utilized. High temperature nickel alloys are generally contemplated. A preferred range of the porous material is 100 pores per inch to 200 pores per inch. The maximum porosity would be material with 60 pores per inch. It is contemplated that the cylinder could have an increased density nearer the inner surface  22  in order to increase the capillary action. 
     The cylinder  18  would have a maximum length of 4 inches and a minimum length of 2 inches. A preferred cylinder  18  would have an inside diameter of no more than ½ inch and an overall axial length of 2 inches and an outside diameter of 1 inch or less. Referring to FIGS. 2 and 3, the cylinder is shown as having an outer diameter (recessed) D and the bore  20  inner diameter is d and L is the length. The thickness of the recessed cylinder wall is t. Thus, L/D˜3 to 8 and preferably 4, and d/D˜&lt;0.5, preferably 0.5; also t/d˜1. 
     Liquid fuel may be applied to the tubular cylinder  18  at inlet  30 . The fuel is soaked up by capillary action within the wall of the tubular cylinder  18 . Pressurized P 3  air from the engine can enter the housing  16  through openings  32 , thus sweeping through the wall of the tubular cylinder  18  into the cavity formed by the bore  20  while carrying fuel and atomizing it through the porous material of the wall. 
     Thus, it is not necessary to swirl the fuel and air mixture in order to atomize it, but it is naturally atomized as it passes through the porous material. Thus, the air entering the plenum formed by the recessed outer wall  24  and the housing  16  percolates through the porous wall of the tubular cylinder  18  and emerges into the cavity at a low velocity and laden with a quantity of vaporized fuel picked up as the air moves through the porous material. 
     The plasma igniter  12  is located at the end  20   b  of the tubular cylinder  18  to the housing  16  as shown. The plasma igniter  12  provides an intense local source of heat which ignites the fuel/air mixture in the cavity formed by bore  20 . The expanding combustion gases escape into the combustor  13  providing a much greater source of heat for ignition of the injector  34  than would be available from the plasma igniter alone. It has been seen that such an arrangement produces ignition with pure air blast fuel injectors at very low fuel pressure. 
     Although continuous plasma igniters are preferred, the arrangement would also provide successful ignition with conventional intermittent igniters. 
     It has also been noted that once heat from the gases in the cavity is imposed on the inner surface  22 , the fuel begins to evaporate at an increased rate. The evaporation from the surface  22  pulls more fuel from the porous wall to the surface  22  by capillary action while air flow continues to percolate through the porous material. There is a tendency for the fuel and air to premix and result in a continuous blue flame which continues to burn even after the plasma source is shut off, and it stops once the fuel is exhausted. 
     A continuous flow of air through the tubular cylinder  18  keeps the porous material cool despite the presence of the flame. As the air temperature increases, the remainder of the fuel is evaporated, thus completely drying the tube for the remainder of the cycle thereof. The continuous air flow in the remote location of the igniter helps to protect the igniter from the harsh conditions of the combustion chamber. Low air flow rates prevent a major disruption to the main combustor gas path. 
     A conical cavity  26  is formed with conical wall  28  in the base of the housing, terminating at the end  20   b  of bore  20 , and is included to prevent the submergence of the igniter with liquid fuel. Air injected tangentially into the cavity  26  blows fuel out of the base. The swirling action helps keep liquid fuel away from the plasma surface while attracting vapor into the recirculation zone formed by bore  20 . This can aid in ignition and in stabilizing the flame in the area. Air from the auxiliary external air supply is preferable in controlling the processes in the base cavity. 
     FIGS. 4 a  and  4   b  illustrate in more detail the various arrangements that can be made to maximize the performance of the igniters. For instance, in FIG. 4 a , the air and fuel is injected below the surface of the igniter central electrode  40  and is swirled to produce a recirculation zone Z within the bore  20  and over the igniter electrode. The plasma occurs between the casing  42 , of the electrode  12 , and the central electrode  40 . 
     The reference numerals in FIG. 4 b  correspond to similar elements in FIG. 4 a  but have been increased by  100 . In this embodiment, the opening  144 , formed by the base, has been reduced, thereby producing a step  142 . The air and fuel, in this case, entered the recirculation zone defined by the bore  120  through the opening  144 . Swirling and mixing was, therefore, induced on the so-formed step  142 . The plasma is observed between the electrode disc  140  and the wall  128  of the base. 
     The capillary pressure developed in the porous material is controlled by the pore size. The smaller the pore size, the higher the capillary pressure. The capillary pressure determines the fuel feed rate developed during the ignition sequences as well as controlling the quantity of air flowing through the porous material. Typically, the capillary pressure is very nearly the same as the pressure drop across the combustor during the start sequence. This helps restrict air flow prior to ignition while allowing it to flow more freely once ignition is achieved. 
     It is contemplated that fuel channels can be drilled in the porous material for rapid delivery of fuel during starts. Fuel flows through these channels and would quickly saturate the entire porous wall. Another improvement which has been contemplated is to heat the porous material in order to preheat the fuel retained in the porous material to promote faster ignition over a wider range. Additionally, catalytic surface materials can be applied to enhance combustion reactions.