1. Field of the Invention
The subject invention is directed to gas turbines, and more particularly, to a system for delivering fuel to the combustion chamber of a gas turbine engine by lean direct injection.
2. Background of the Related Art
With increased regulation of pollutants from gas turbine engines, a number of concepts have been developed to reduce engine emissions while improving engine efficiency and overall operability. One such concept is the use of staged combustion. Here, the combustion process is divided into two or more stages or zones, which are generally separated from each other, either radially or axially, but still permitted some measure of interaction. For example, the combustion process may be divided into a pilot combustion stage and a main combustion stage. Each stage is designed to provide a certain range of operability, while maintaining control over the levels of pollutant formation. For low power operation, only the pilot stage is active. For higher power conditions, both the pilot and main stages may be active. In this way, proper fuel-to-air ratios can be controlled for efficient combustion, reduced emissions, and good stability.
In addition to staged combustion, providing a thoroughly blended fuel-air mixture prior to combustion, wherein the fuel-to-air ratio is below the stoichiometric level so that combustion occurs at lean conditions, can significantly reduce engine emissions. Lean burning results in lower flame temperatures than would occur during stoichiometric combustion. Since the production of NOx is a strong function of temperature, a reduced flame temperature results in lower levels of NOx. The concept of directly injecting liquid fuel into the combustion chamber of a gas turbine and enabling rapid mixing with air at lean fuel-to-air ratios is called lean direct injection (LDI).
The prior art is replete with example of LDI systems. For example, U.S. Pat. No. 6,389,815 Hura et al. discloses a lean direct injection system, which utilizes radially staged combustion within a single injector. The pilot fuel delivery stage includes a pressure swirl atomizer that sprays liquid fuel onto a filming surface. The liquid film is then stripped off into droplets by the action of compressor discharge air. The main fuel delivery system includes a series of discrete atomizers that spray fuel radially outward into a swirling cross-flow of air. The main fuel delivery system is staged radially outboard of the pilot fuel delivery system, and operates in the fuel-lean mode. Radial separation as well as an air jet located radially between the two stages achieves separation of the pilot combustion zone and the main combustion zone.
U.S. Pat. No. 6,272,840 Crocker et al. discloses a lean direct injection system, which also utilizes radially staged combustion within a single injector. The pilot fuel delivery is either a simplex air-blast type atomizer or a prefilming air-blast type atomizer, and the main fuel delivery system is a prefilming air-blast type atomizer. Separation of the pilot and main combustion zones is achieved by providing an air splitter between the pilot outer air swirler and the main inner air swirler. The air splitter develops a bifurcated recirculation zone that separates the axially aft flow of the pilot injector from the axially aft flow of the main injector. The bifurcated recirculation zone aerodynamically isolates the pilot flame from the main flame, and ensures that the pilot combustion zone remains on-axis with no central recirculation zone. A converging wall of the pilot air cap, which essentially acts as a flame holder to anchor the flame, defines the air splitter. Acting in this manner, the pilot air cap will likely suffer thermal distress (i.e., oxidation, melting), and require some form of thermal management. In this regard, Crocker et al. disclose the use of small cooling holes in the air cap to improve durability.
European Patent Application EP 1413830 A2 discloses a lean direct injection system, which also utilizes radially staged combustion. In this case, an air splitter with an aft end cone angled radially outward assists in creating a bifurcated recirculation zone. The additional function of the splitter is to prevent the inner main air stream from modulating with combustor pressure fluctuations, thus reducing combustion instability. This air splitter has a larger radial extent than the air splitter disclosed in U.S. Pat. No. 6,272,840 to Crocker et al., and acts as an even larger flame-holder, requiring thermal management to avoid distress.
While the concept of the LDI system is sound, achieving the required levels of performance can be difficult. Lean-burning systems are prone to localized flame extinction and re-ignition. This results in combustion instability that can damage the combustion chamber. Limitations in atomization, vaporization, and fuel-air mixing can result in heterogeneous stoichiometric burning, which yield higher than desired levels of NOx. Also, for these self-contained radially staged LDI systems, control over the level of mixing between the pilot combustion zone and the main combustion zone can be difficult. The negative effects can include reduced margin for lean blowout, and possibly increased levels of smoke.
Accordingly, there is a continuing need in the art to provide a lean direct injection system which can achieve low levels of combustion instability, enhanced atomization quality, increased fuel-air mixing rates, low pollutant formation, low smoke and improved lean blow-out margin.