Patent Application
20100180599
The invention relates to a fuel nozzle in a gas turbine including a fuel/air premixer and, more particularly, to a fuel nozzle including independently insertable pre-drilled swirl vanes.
Typical industrial gas turbine premixing fuel nozzles may employ a cast swirl vane assembly in which a circular array of shaped hollow vanes are used to swirl the incoming air. The hollow vanes also serve as fuel delivery passages, where each vane is provided with several gas port holes drilled into the sides, through which fuel is injected into the passing air stream.
Gas turbine manufacturers are currently involved in research and engineering programs to produce new gas turbines that will operate at high efficiency without producing undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen (NOx) is an exponential function of temperature. If the temperature of the combustion chamber hot gas is controlled to a sufficiently low level, thermal NOx will not be produced. An existing method of controlling the temperature of the reaction zone of a heat engine combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor reduces the peak temperatures in the reaction zone to minimize formation of thermal NOx.
There are numerous problems associated with the existing design. Flow testing is used to measure the effective opening area of the gas port orifices in the current configuration, and the flow testing is performed after these orifices have been drilled. If the gas port open area is too large, there is no process for repairing or modifying the part to correct this area, and an expensive part must thus be discarded. Additionally, current flow tests can assess the fuel circuit areas for the entire fuel nozzle, but cannot assess the vane-to-vane variation in fuel flow. Since NOx emissions are strongly influenced by local regions of high fuel-air ratio, it is desirable to provide as uniform a fuel-air mixture as possible.
Fuel nozzle gas port diameters are sized to accommodate a specific range of fuel compositions and temperatures. Changes in operating fuel temperature or fuel specific gravity can require a change in gas port orifice size to maintain proper operation of the gas turbine. As noted, however, such changes are not easily or. inexpensively effected. Current fuel nozzle designs employ uniform gas port diameters and locations in all swirl vanes. As combustion system analysis methods and system performance continue to evolve, benefits may be accrued from the use of varying gas port locations and/or diameters within a single fuel nozzle. With the prior design, it is not possible to vary the gas port locations and/or diameters without substantial nozzle reconstruction.
Flow field analysis has shown that the use of counter-rotating swirl in adjacent fuel nozzles can provide operational and/or performance benefits in the combustion system. The complexity of the prior fuel nozzle swirlers casting, however, requires expensive tooling, and thereby makes it very difficult to employ counter-rotating vane configurations.
Still further, prototype fuel nozzles in the prior design are costly and time-consuming to fabricate. In the prior design, the swirl vanes are integral to the swirl vane casting, which greatly limits the ability to modify prototype hardware for optimal fuel delivery and mixing.
It would be desirable to provide a design that addresses these drawbacks in the prior construction.
In an exemplary embodiment, a swirl vane is independently connectable to a central hub assembly of a gas turbine. The swirl vane includes a structural body, at least one fuel delivery passage including a corresponding at least one fuel port defined within the structural body, and at least one connecting tab cooperable with the structural body and connectable to the central hub assembly.
In another exemplary embodiment, a premixing fuel nozzle for a gas turbine includes a central hub assembly and a plurality of the independently connectable swirl vanes connected to the central hub assembly in a circular array.
In yet another exemplary embodiment, a method of assembling a premixing fuel nozzle for a gas turbine includes the steps of (a) preparing a plurality of swirl vanes to each include a structural body, at least one fuel delivery passage including a corresponding at least one fuel port defined within the structural body, and at least one connecting tab cooperable with the structural body; and (b) securing the plurality of swirl vanes independently to a central hub assembly of the premixing fuel nozzle.
Preferably, the structural body 36 includes a plurality of fuel delivery passages 38 and corresponding fuel ports 40. At least one connecting tab 42 is cooperable with the structural body 36 and is connectable to corresponding slots 44 in the central hub assembly 32.
The independently connectable swirl vane 34 has its fuel passages and ports 38, 40 drilled or machined prior to installation into the central hub assembly 32. By pre-forming fuel passages and ports 38, 40 prior to installation, each swirl vane 34 can be flow-tested individually to ensure that the fuel circuit effective flow area meets design intent. That is, flow-testing can be used to tune the individual vanes 34 for assurance of proper flow characteristics. The independently connectable swirl vanes 34 are then inserted into the central hub assembly 32 and brazed, swaged, welded or connected using any other known process into position via connecting tabs 42 and corresponding slots 44. With individually flow-tested vanes 34, the completed vane assembly would necessarily automatically meet design intent.
Because vanes 34 are individually flow-tested, rather than testing the entire assembly as a unit, vane-to-vane variation can be directly measured and easier to control. Moreover, insertable swirl vanes 34 provide a relatively easy way to re-size fuel nozzles for changes in fuel composition. In an exemplary application, the vanes 34 can be machined off the central hub assembly 32 and replaced with re-sized swirl vanes, rather than replacing an entire fuel nozzle.
The vanes 34 can be cast or machined in both a clockwise and counterclockwise rotational orientation to permit the easy use of counter-rotating swirl. Casting tooling for an individual vane is much less expensive and requires less manufacturing time. Still further, the insertable swirl vanes 34 facilitate the testing of alternate concepts since the vanes can be made more quickly and at a lower cost.
A clocking feature can be incorporated into the fuel nozzle base, so that the fuel nozzle can only be installed in one orientation relative to the combustor liner; this feature would insure that the position of each swirl vane was fixed, relative to the combustion flow field inside the liner. By incorporating a clocking feature on the base of the fuel nozzle (so that its orientation relative to the combustion liner is known), it is possible to use swirl vanes of varying orifice diameters and/or locations to deliver an improved fuel-air distribution into the combustor. Improved access to the gas port locations on an individual vane makes it easier and less expensive to vary gas port orientations among the vanes.
In the illustrated embodiment, the swirler assembly shroud may be a separate entity rather than an integral part of the swirler assembly.
The insertable swirl vanes 34 can serve to deliver fuel in a non-symmetric, preferentially-oriented manner by incorporating features in the fuel nozzle design that position the fuel nozzle in a unique orientation with respect to the combustor. Analysis of the combustor internal flow field may indicate fuel-rich or fuel-lean regions that can be made more uniform by fitting swirl vanes with larger or smaller ports in some sections of the premixing assembly. Because the flow areas of each swirl vane are known prior to assembly, the fuel-air distribution inside the premixing assembly can be tuned to deliver a richer or leaner mixture into different regions of the combustor.
With the independently connectable swirl vanes, fuel nozzle manufacturing time and prototype hardware procurement time can be reduced. Moreover, the independent vanes provide for greater design flexibility. Gas port holes can be drilled into individual vanes much more easily than in a single vane casting since access to the side of the vanes is unimpeded. Moreover, since tooling for drilling the holes is much less complex, there is greater flexibility to use varying gas port configurations among the different vanes in the assembly. This variation offers potential advantages in combustion system operability and emissions, which could yield gas turbine performance improvements.
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.
