A combustor of a gas turbine combustion engine often includes several individual combustor cans. Within each can there are multiple swirlers which impart rotational movement to the air-fuel mixture flowing through it. A conventional configuration includes eight main swirlers and a central pilot swirler, where all swirlers have parallel axes. Compressed air flows, into each main swirler individually and into the central pilot swirler individually. Fuel is added to the air as it flows through the swirler, resulting in an air-fuel mixture flowing through each main swirler. Accordingly, in a configuration with eight main swirlers and a central pilot swirler, there are nine air-fuel mixture flows; one through each of the eight main swirlers, and one through the central pilot swirler. Each air-fuel mixture flows axially, centered on the same axis as the swirler through which it is flowing. A swirler then imparts a rotation to this axial flow, such that the air-fuel mixture exiting an individual swirler is flowing along the central axis of that swirler while simultaneously rotating around that central axis. Each of the main swirlers in this relevant configuration imparts a clockwise rotation to the air-fuel mixture flowing through it as viewed looking downstream, and the central pilot swirler imparts a counterclockwise rotation. Consequently, because each main swirler imparts a clockwise rotation to the air-fuel mixture flowing through it, the tangential velocities of the rotation of adjacent air-fuel flows will be opposite where the adjacent air-fuel flows meet. Friction in these areas where adjacent tangential fuel flows oppose each other results in shear and vortices.
The formation of oxides of nitrogen NOx is correlated to the temperature of combustion. Therefore, NOx emissions are reduced by reducing the temperature and size of the hot zones within the combustor. In the combustor configuration described above, the air-fuel flow through the pilot swirler runs relatively rich, i.e. a higher concentration of fuel in this mixture exists than exists in the main swirler flows. This provides a hot central flame to stabilize the overall combustor dynamics, which is necessary because the outer swirlers are unable to stabilize on their own due to the lean air-fuel mixture flowing through them. Thus, reducing NOx emissions in this configuration means reducing the size of the central pilot zone, and/or reducing the temperature of the air-fuel flow in the central pilot zone by reducing the amount of fuel in that air-fuel mixture. However, as the central pilot zone air-fuel flow (and associated temperature) is reduced, combustion dynamics (i.e. pressure oscillations) increase. These dynamic pressure oscillations can be harmful to the combustion chamber.
Dynamic pressure oscillations are associated with either the lean flammability limit of the air-fuel mixture, or fluctuations in the heat release rate of the combustion flame. Oscillations associated with the lean flammability limit are typically characterized by frequencies below 50 hertz. Oscillations associated with combustion flame heat release rate are typically associated with higher frequencies, and they and are often the limiting dynamic in the higher firing-temperature applications currently under development. High frequency pressure oscillations cause fluctuations in the heat release rate of the combustion flame, which is responsive to changes in pressure. A change in the heat release rate of the combustion flame produces pressure oscillations, and the feedback cycle repeats.
As a result, the ability to reduce NOx and CO emissions in the above described combustor configuration is limited by the need to minimize high frequency pressure oscillations. Accordingly, once the temperature of the central pilot zone is reduced to the point where combustion dynamics have reached a maximum safe level, NOx and CO emissions can not be reduced any more.
Conventional swirlers also have variable fuel-hole injection patterns to enable a center rich concentration of fuel in the air fuel mixture. Other patterns known in the art result in air-fuel mixtures where the fuel is either uniformly distributed throughout the air-fuel flow, or is concentrated in the outer portion of the air-fuel flow, result in high levels of combustion driven oscillations. However, because the fuel is concentrated in the center of the air-fuel flow, the peak temperature of the burn at the center of the flow is greater than the temperature of the burn of an evenly distributed air-fuel flow. This center-rich fuel configuration results in greater NOx and CO production, due to the exponential nature of NOx production with temperature.
Further, when main swirler flows have a center rich fuel configuration, complete combustion, which requires complete mixing of the central pilot flow and the main swirler flows, requires more time, resulting in a longer central flame, and yet further increased NOx and CO production.
Thus, there exists a need in the art to further reduce the temperature and/or size of the central pilot zone without increasing combustion dynamics, in an effort to reduce NOx and CO emissions.