A fuel injector and igniter assembly

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to an assembly for catalytic ignition and a method FOR operating a gas turbine engine applying said assembly according to claim 1 and claim 11.

BRIEF DESCRIPTION OF RELATED ART

Commercial aircraft gas turbine combustors utilize combustor wall mounted igniters, typically a spark plug, for combustor light-off. This requires the presence of fuel close to the wall. Inasmuch as combustion of fuel near the wall during full power operation tends to raise the wall temperature, combustor designs tend to be a compromise between ignition and operational requirements. Thus, there have been numerous attempts to achieve igntion away from the wall.

Ideally, ignition should be achieved right at the fuel source so that ignition of the initial fuel flow is possible. This avoids the necessity to dump unburned fuel into the combustor prior to ignition (creating the potential for hot starts of explosive detonations with consequent damage to the turbine) and enables the use of spray patterns which keep fuel away from the combustor walls.

From JP-A-59 180220 a combustor equiped with an ordinary noble metal bearing honeycomb catalyst is known. This assembly for catalytic ignition comprises a honeycomb catalyst, which is arranged on the front stage side of the catalyst. The main component of which is noble metal bearing lanthanum chromite. Further, it comprises a secondary fuel nozzle and a primary fuel nozzle. The catalyst generates heat by energizing. The fuel jetted from the nozzle mixes with combustion air passes through the catalyst. At this time, catalytic combustion occurs regardless of the temperature of said mixture of the fuel and the combustion air being low. Once the catalytic combustion occurs, the temperature of the catalyst is maintained by the electric power and heat of combustion. The total quantity or a part of the primary fuel is burnt at the catalyst, resulting in heating the secondary fuel and combustion air up to a high temperature necessary for starting catalytic combustion at the catalyst. The heated mixture of the fuel and the air is catalytically burnt at the catalyst, resulting in establishing low NO_x and highly efficient combustion.

There has been interest in integrating the ignition source into the fuel injector. For example, U.S. patent 4,938,019 describes a fuel nozzle with an integrated spark plug igniter assembly and U.S. patent 4,825,658 describes a fuel nozzle with a catalytic glow plug igniter assembly. Such designs have major drawbacks which limit utility. For example, a spark plug integrated into an injector is subject to fouling if wetted by liquid turbine fuel, rendering it inoperative. In addition, size limitations reduce spark plug life. On the other hand, although the glow plug of U.S. Patent 4,825,658 eliminates the fouling problem of spark plugs, the glow plug is designed such that the return flow of the recirculating flow downstream of the injector contacts the hot glow plug surface resulting in ignition of the downstream recirculating gases. Inasmuch as the initial direction of the incoming fuel-air flow from the swirler is away from the glow plug considerable fuel can travel downstream before sufficient fuel is injected to increase the recirculation zone fuel concentration high enough at the glow plug to allow ignition. Thus explosive detonation is possible as is the case with conventional spark igniters presently used in aircraft gas turbine engines. Advantageously, fuel should be ignited immediately as it enters the combustor.

Injectors for injection of fuel into combustors for gas turbine engines are typically either of the air blast type or pressure atomizers. Both types have advantages and disadvantages. The former requires little pumping power but gives poor atomization at low air flows. Pressure atomizers are simpler but have a high power requirement and with some fuels are subject to plugging. A typical air blast nozzle for atomization of fuel for use in the combustor of a gas turbine engine is described in U.S. Pat. No. 3,684,186 issued Aug. 15. Air blast fuel nozzles utilize compressor discharge air to provide atomization of fuel and mixing of fuel and air discharged from the nozzle whereas pressure atomizers use a high pressure to force flow through a small orifice at a high velocity with fuel droplet formation on contact with the combustion air.

Thus, the object of the present invention is to improve the catalytic ignition and to overcome the above mentioned disadvantages.

This object is solved by the features of the subject matter of claims 1 and 11.

SUMMARY OF THE INVENTION

In the present invention atomized fuel entering a gas turbine combustor is ignited as it enters the combustor by contact with a hot surface igniter. Advantageously, the igniter surfaces are catalytic for oxidation of fuel so that the igniter remains at a temperature high enough for continuous ignition of entering fuel, thus providing assurance against combustor flameout. For combustor light-off, the igniter is heated electrically by passage of a current. Although a smooth surface igniter may be used, it has been found that igniters with flow through channels, as for example slots, are advantageous particularly at higher air flow velocities in an air blast fuel injector. In the present invention, fuel may be injected and atomized with any of a variety of fuel injectors including for example air blast atomizers, pressure atomizers, high shear atomizers, and electrostatic atomizers.

In operation of an igniter/injector of the present invention, fuel is atomized and mixed with air and passed into a combustion chamber. At least a portion of the entering fuel contacts a hot ignition catalyst surface, thus providing continuous catalytic ignition of entering fuel. Such continuous ignition allows operation of the combustor much closer to its lean limit than would otherwise be prudent because of the danger of a flameout. With electrical preheating of the catalyst prior to initial introduction of fuel, even combustor light-off can be achieved at the combustor lean limit.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic cross-sectional view of a preferred embodiment assembly of the invention.

Figure 2 is a graphical representation of the improvement in combustion limits demonstrated in tests.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

As shown in the drawing of Figure 1, assembly 10 has an electrically heated igniter 1 placed in the center of axial line A-A of a conventional air blast fuel atomizing nozzle having concentric fuel passage 2 placed between concentric air flow passages 3 and 4, replacing the start-up pressure atomizer otherwise in this location. Swirlers 5 and 6 in air passages 3 and 4 respectively provide swirl to the air flowing through passages 3 and 4 thus generating the shear forces necessary to atomize and mix with fuel exiting fuel passage 2 through nozzle tip 7. In operation, atomized fuel is brought into contact with the hot surface of igniter 1 by air flowing through passages 3 and 4 resulting in ignition of fuel during passage into the combustion chamber. During starting of a gas turbine engine, typically the igniter is heated to a temperature above the minimum temperature required for ignition at the given air flow condition prior to introduction of fuel thus assuring a rapid light-off. After light-off, electrical power to the igniter may be controlled to maintain the igniter temperature below a safe value for the materials used. Typically, electrical heating is discontinued after light-off though continued controlled heating is utilized in the present invention to provide near instantaneous relight in those situations where aircraft operation for example could result in engine flame-out,such as by ingestion of water into the engine. Although this invention has been described in terms of an air blast fuel nozzle, gas turbine fuel nozzles such as pressure atomizers or high shear nozzles can be utilized in the present invention.

Catalytic igniters of the present invention may employ any ignition catalyst known in the art which is reactive for the fuels to be used.

Platinum containing catalysts, however, are preferred because of their high activity with a wide variety of fuels and resistance to the effects of sulfur in the fuel. The temperature of the igniter element may be monitored and controlled using temperature sensing devices such as resistive elements, thermocouples, infrared detectors and laser beam sensors.

EXAMPLE

Using an engineering development laboratory combustor test rig, an air-blast fuel nozzle with a catalytic igniter in place of the bill of material start up center line pressure atomizer was mounted in the dome and liner assembly of an AGT 1500 gas turbine. For ignition testing, air flow was first established, next electrical power was applied to the catalytic igniter, and finally fuel flow was established. Full combustion was achieved in less than 1/15 of a second with initial flame propagation within 1/30 of a second of fuel flow. Ignition was not only achieved at normal lean limit fuel flow for the bill of material-spark ignition system but also at the lower combustor lean limit. The test results are shown Figure 2. A graphical representation showing the improved ignition limits of the assembly of the invention compared to the bill of material (production configuration).