Patent Application
20130180248
The invention relates to premixing fuel and air prior to combustion in a gas turbine engine and, more particularly, to a combustor nozzle/premixer including curved sections to improve mixing.
Gas turbine engines generally include a compressor for compressing an incoming airflow. The airflow is mixed with fuel and ignited in a combustor for generating hot combustion gases. The combustion gases in turn flow to a turbine. The turbine extracts energy from the gases for driving a shaft. The shaft powers the compressor and generally another element such as an electrical generator.
Cold air from the flow sleeve enters into the combustor headend region and is distributed among a plurality of nozzles. Generally, air passes through an inlet flow conditioner (IFC) and becomes uniform in circumferential direction. Subsequently, it is rotated by axially placed vanes, and fuel is injected into the flow through holes in the vanes for premixing of fuel and air.
The exhaust emissions from the combustion gases generally are a concern and may be subject to mandated limits. Certain types of gas turbine engines are designed for low exhaust emissions operation, and in particular, for low NOx (nitrogen oxides) operation with minimal combustion dynamics, ample auto-ignition, and flameholding margins.
Low NOx combustors typically include a number of combustion cans circumferentially adjoining each other around the circumference of the engine. Each can may have one or more fuel air mixers or nozzles positioned therein. Nozzles use swirling air to mix fuel and air, and hence are referred to as “swirlers.” The swirlers may have a number of circumferentially spaced apart vanes for swirling and mixing the compressed airflow and the fuel as they pass therethrough.
The combustor nozzle serves to provide fuel air premixing with minimum pressure losses. It would be desirable to perform the premixing function over a shorter distance and more efficiently. With more fuel-air mixing over a shorter distance, combustion emissions can be reduced. Additionally, existing swirler vanes typically include internal passages for fuel, and the swirler vanes are thus larger in size than desired and more expensive to manufacture. It would be desirable to simplify the construction and eliminate the internal passages in the swirler vanes.
An apparatus for premixing fuel and air prior to combustion in a gas turbine engine includes an annular fuel passage receiving fuel from a fuel source, an annular air passage surrounding the annular fuel passage, and a plurality of swirler vanes disposed in the annular air passage adjacent a fuel plenum section. The annular fuel passage has an upstream passage, a neck passage narrower than the upstream passage, and the fuel plenum section that curves from the neck passage to a wider passage. The annular air passage receives air to be mixed with the fuel in the fuel passage for downstream combustion.
In another exemplary embodiment, a combustor includes a casing, and a plurality of nozzles disposed in the casing. Each of the nozzles includes the structure of the apparatus for premixing fuel and air prior to combustion in a gas turbine.
In still another exemplary embodiment, an apparatus for premixing fuel and air prior to combustion in a gas turbine engine includes an annular fuel passage receiving fuel from a fuel source. The annular fuel passage includes a fuel plenum and has a wave shape with a peak section surrounded by trough sections. An annular air passage surrounds the annular fuel passage. The annular air passage receives air to be mixed with the fuel in the fuel passage for downstream combustion. A plurality of swirler vanes are disposed in the annular air passage adjacent the fuel plenum.
The fuel plenum section 68 includes fuel holes in the annular fuel passage 62. In an exemplary embodiment, the nozzle includes at least 40 fuel holes 74. Preferably, the fuel holes 74 are disposed in the wider passage (widest section) of the fuel plenum section 68. As shown in
By virtue of the curved fuel passage 62 and correspondingly curved air passage 70, an exterior surface of the annular air passage is similarly curved substantially corresponding to the fuel plenum section 68. The curved exterior surface 76 provides for slight compression/diffusion with the casing 55 and thereby reduces incoming circumferential non-uniformities. That is, along with the surface of the casing 55, the nozzle 50 forms a convergent/divergent passage that acts a flow conditioner. Non-uniformities from the compressor due to the 180° turn of the air may be smoothed out efficiently. Overall, air will be distributed without significant non-uniformities via the outside curvature and rounded entrance.
In contrast with the prior designs, instead of an axial swirler, the structure of the described embodiment is curved or wavy. Air from the headend passes through the rounded entrance 71 to maintain circumferential uniformity. The air in the annular air passage 70 flows up the curved section, which further serves to remove non-uniformities. Subsequently, the air passes through the vanes 72 and is mixed with fuel entering the air path via the fuel holes 74. Since the vanes do not include passages for fuel, the vanes can be made highly aerodynamic. Additionally, the vanes can be made about 50% shorter than existing vanes, which provides extra fuel/air premixing length or alternatively, the nozzle can be shortened. Additionally, the vanes can be made thinner in the absence of the fuel passages, which serves to decrease the overall weight of the swirler. The curved or wavy shape enables air to travel radially upward and to come down at fuel injection. The resulting radial current facilitates fuel and air mixing (radial current is not present in an axial swirler).
Fuel is injected from the downstream curvature of the nozzle but still at a high radius such that premixing is efficient. The fuel plenum beneath can be made diffusive to decrease pressure variations. Multiple fuel hole locations between two vanes or downstream of the vanes can be provided. Hole locations should be such that fuel from multiple holes does not mix, and the fuel stream is not directed towards the trailing edge of the vanes. As noted, by placing the fuel holes in the region of slightly high axial velocity, a risk of flameholding is reduced. The structure effects a slight radial current, resulting in early mixing. Moreover, there is no flow conditioning device needed, and the overall pressure drop reduces to a greater extent.
The shorter and aerodynamic vanes along with the rounded entrance make the nozzle highly efficient in terms of pressure drop. The only pressure drop will be due to the rotation of the flow in the burner tube. Moreover, since the swirler is independent of the vane core and cavity, it can be made shorter, providing opportunities to reduce the length of the swirler or provide for better mixing with the same length. On the exterior of the nozzle, with the convergent-divergent passage, the design filters out non-uniformities in CDC air after flow sleeve exit. Since the compression-diffusion is smooth, associated pressure losses are minimized.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
