Patent Application
20100050647
The present invention relates to swirlers for the fuel injectors of combustors of gas turbine engines and in particular to swirlers for the fuel injectors of lean burn combustors of a gas turbine engine.
Fuel injection systems deliver fuel to the combustion chamber of an engine, where the fuel is thoroughly mixed with air before combustion. One form of fuel injection system well-known in the art is a fuel spray nozzle. Fuel spray nozzles atomise the fuel to ensure its rapid evaporation and burning when mixed with air.
An airblast atomiser nozzle is a type of fuel spray nozzle in which fuel delivered to the combustion chamber by a fuel injector is aerated by swirlers to ensure rapid mixing of fuel and air, and to create a finely atomised fuel spray. The swirlers impart a swirling motion to air entering the combustion chamber, so as to create a high level of shear in the fuel flow.
Typically, an airblast atomiser nozzle will have a number of swirlers. An annular fuel passage between a pair of swirlers feeds fuel onto a pre-filming lip. Thus a sheet of fuel is formed that breaks down into ligaments. These ligaments are then broken up into droplets within the shear layers of the surrounding highly swirling air, to form the fuel spray stream that is emitted from the fuel injection system.
One type of fuel injection system that may have airblast atomiser nozzles is a lean burn fuel injector. A typical lean burn injector 100 is shown in
An annular mains airblast fuel injector 110 is located radially outwards of the pilot fuel injector 102. Inner and outer mains swirlers 112,114 are located radially inwards and radially outwards of the mains fuel injector 110 respectively, and provide a swirling airflow for atomisation of the fuel injected at the mains fuel outlet 116.
When the gas turbine engine is operating at low power, only the pilot fuel injector is activated. As engine power is increased, some of the mains fuel supplies are activated, until at high power the mains and pilot fuel injectors are both fully active.
The configuration of the inner and outer swirlers of an airblast fuel injector plays a role in controlling the efficient mixing of air and fuel and so influences the smoke emissions and NOx emissions from the burning fuel.
There is a continuing need to enhance the performance of gas turbine engines of the type used to power jet aircraft or generate electricity, and thus it is desirable to improve the design of fuel injection systems, such as airblast atomiser nozzles, to achieve improved control of smoke and NOx emissions from the combustion chamber.
In general terms, the present invention provides a swirler for a fuel injector of a combustor of a gas turbine engine, which is configured so that the angle and/or angular direction of the swirling motion imparted by the swirler to the air entering the combustion chamber may be selectively varied.
In a first aspect, the present invention provides a swirler arrangement for a fuel injector of a gas turbine engine, the swirler arrangement having a plurality of passages for channelling airflow through the swirler and into a combustor, each of the passages having an inlet, the arrangement having a plug movable between a first position in which the inlet to one of said passages is open and a further position where another of said passages is open, wherein the arrangement has a pressurisable chamber for receiving a fluid, the pressure of the fluid in the chamber in use providing motive force to move the plug between the first and further position.
The role of a swirler is to atomise the fuel injected into a combustor, so as to provide efficient mixing of fuel and air. By providing a variable swirler that can direct airflow into the combustor in such a way that the swirl angle and/or direction of rotation of the airflow exiting the swirler may be selectively varied, it is possible to control the process of mixing air and fuel within the combustor more closely, and thus to maintain smoke and/or NOx emissions within desirable limits.
Typically, each of the one or more passages follows a helical path relative to the swirl axis of the swirler. In this case, the path followed by the passage or passages may have an axial component that is aligned with the swirl axis and a further component that is tangential relative to the swirl axis. In general, the helical path followed by each of the one or more passages corresponds to less than one complete turn around the swirl axis.
Airflow exiting one of these passages generally follows a helical path having a helical angle that corresponds to the exit swirl angle and an angular direction that corresponds to the direction of rotation.
Typically, the swirler of the present invention is part of a fuel injection system and provides one of a pair of swirlers for swirling air past a fuel injector. For example, the swirler of the present invention may be an inner swirler for a pilot fuel injector, the pilot fuel injector also having an outer swirler. This outer swirler may itself be a variable swirler according to the present invention or a conventional, fixed-angle swirler.
By pairing the variable swirler of the present invention with a second swirler, it may be possible to vary the interaction of the airstreams exiting each swirler, so as to vary the parameters of the shear flow around the fuel injection point of the fuel injector. Thus the atomisation of the fuel and the mixing of fuel and air may be controlled more closely and smoke and/or NOx emissions maintained within desirable limits.
Typically, the variable swirler of the present invention is configured so that the direction of rotation of the airflow into the combustor may be varied from clockwise to anti-clockwise. That is, the angular direction of the path followed by the airflow exiting the swirler may be varied between a first angular direction and a second angular direction that is opposed to the first angular direction.
In this case, the variable swirler may be paired with a second swirler, so that the operation of the two swirlers may be selectively varied from co-swirl (in which both swirlers direct air in the same angular direction) to counter-swirl (in which the swirlers direct air in opposing angular directions).
It has been found that co-swirl operation of a pair of swirlers, e.g. the inner and outer pilot swirlers, may result in low smoke emissions at low power (e.g. when only the pilot fuel injector is active), but high NOx emissions at high power (e.g. when both the pilot and mains fuel injectors are active).
By contrast, it has been found that counter-swirl operation of a pair of swirlers, e.g. the inner and outer pilot swirlers, may result in low NOx emissions at high power, but high smoke emissions at low power.
By pairing a variable swirler according to the present invention with another swirler, it may be possible to operate the swirlers in co-swirl mode at low power and counter-swirl mode at high power, in order to maintain smoke and NOx emissions at desirably low levels throughout a broad operational range of the gas turbine engine.
It is thought that the variation in smoke emissions and NOx emissions during co-swirl and counter-swirl operation is related to the different shapes of the fuel spray stream emitted by the pilot fuel injector during these two modes of operation.
It is thought that during co-swirl operation, the pilot fuel spray stream has a significant radially outward flare in the downstream direction. This provides the pilot fuel spray stream with a frustoconical shape having a relatively large cone angle. The shape of the fuel spray stream promotes efficient combustion of the fuel, resulting in low smoke emission. However, at high power settings, when the mains fuel injector is also active, the pilot fuel spray stream tends to interact strongly with the air flow from the mains fuel injector, resulting in high NOx emissions.
By contrast, it is thought that during counter-swirl operation, the pilot fuel spray stream is narrower than that produced during co-swirl operation, i.e. the fuel spray stream has a frustoconical shape having a relatively small cone angle. The shape of the fuel spray stream results in less efficient combustion of the fuel spray stream than is the case for co-swirl operation, resulting in higher smoke emission. At high power levels, the pilot fuel spray stream interacts less strongly with the mains fuel spray stream than is the case for co-swirl operation, resulting in lower NOx emissions.
In order to provide a selectively variable path for the airflow into the combustor, the swirler of the present invention may include a first and a second passage. The two passages may be configured such that the swirl angle and/or direction of rotation of air exiting each passage is different. The exit swirl angle and/or direction of rotation of the airflow may then be controlled by directing the airflow through a selected one of the first and second passages. Typically, in this case, the first and second passages are configured to rotate the airflow in opposing angular directions.
The selection of the first or second passage for directing the airflow into the combustor may be effected by a plug that is movable between a first position, in which it blocks the entrance to the second passage, so that the airflow is channelled through the first passage and a second position, in which the plug blocks the entrance to the first passage, so that the airflow is channelled through the second passage.
Movement of the plug is preferably effected by the pressure of fluid within a pressurisable chamber. The fluid is preferably a hydrocarbon mixture that is used for fuel in the gas turbine engine. The plug may be biased into a first position by a resilient member with the bias being overcome by the pressure of the fuel.
The variable swirler of the invention may include a control mechanism for varying the exit swirl angle and/or direction of rotation of the airflow in response to the power generated by the combustor. When the variable swirler is paired with a second swirler (variable or conventional), this control mechanism may allow the mode of operation of the two swirlers to be changed from e.g. co-swirl at low power to e.g. counter-swirl at high power.
Typically, the control mechanism is configured to respond to changes in the pressure of said combustor.
In a second aspect, the present invention may provide a fuel injection system including a fuel injector and the swirler of the first aspect of the invention, for swirling air past the fuel injector. Typically, the fuel injector is an airblast fuel injector, and in general, the swirler is concentric with the fuel injector. The swirler may be integral with the fuel injector, that is, the passages of the swirler may pass through the body of the fuel injector.
The fuel injection system may comprise a pilot and a mains fuel injector, the swirler of the first aspect of the invention being for swirling air past the pilot fuel injector. By varying the angle and/or angular direction of swirl of a swirler associated with a pilot fuel injector, it may be possible to maintain smoke emissions and NOx emissions within desirable limits during both low power and high power operation of a gas turbine engine.
The swirler of the first aspect of the invention may be disposed radially inwardly or radially outwardly of the fuel injector, but is preferably disposed radially inwardly. In particular, the swirler may be disposed radially inwardly of a pilot fuel injector.
Typically, the swirler is one of a pair of swirlers, both swirlers being for directing a respective airflow past the fuel injector. The pair of swirlers may both be variable swirlers according to the first aspect of the invention. Alternatively, one swirler of the pair of swirlers may be a conventional swirler for directing airflow according to a fixed exit swirl angle and direction of rotation.
In a third aspect, the present invention may provide a lean burn combustor having a fuel injection system according to the second aspect of the invention.
In a fourth aspect, the present invention may provide a gas turbine engine having a fuel injection system according to the second aspect of the invention.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:
a) shows a schematic longitudinal cross-sectional view of a fuel injector having a swirler according to an embodiment of the invention, and
a) shows a pilot airblast fuel injector 10 having a swirler according to an embodiment of the present invention. The pilot airblast fuel injector 10 typically forms part of a fuel injection system that is in general circularly symmetrical about a central axis 12. The pilot airblast fuel injector typically lies along this central axis 12. The fuel injection system generally also includes a mains airblast fuel injector (not shown), situated radially outwards from the pilot fuel injector 10.
The fuel injection system is generally mounted on the upstream wall of a combustor, i.e. at the head of a flame tube of the combustion chamber of a gas turbine engine.
The pilot fuel injector 10 has an annular fuel outlet (not shown) at the downstream end 14 of the injector. Liquid fuel is provided to this outlet and forms a film across the downstream end 14. An inner swirler 16 and an outer swirler (not shown) are provided on the radially inward and outward sides of the fuel injector 10, respectively. In this embodiment, the inner swirler 16 is integrally formed with the body of the pilot fuel injector 10, and the swirl axis of the inner swirler coincides with the central axis 12. However, in other embodiments, the inner swirler and the fuel injector may be separate compoments.
The inner and outer swirlers direct an airflow into the combustor and supply rapidly moving and swirling air to the region adjacent the downstream end 14 of the fuel injector 10. These air streams cause the annular film of liquid fuel to be atomised into small droplets, thus creating a fuel spray stream.
The inner swirler 16 includes swirl passages 18a,18b for channelling an airflow into the combustor. Each of the swirl passages 18a,18b has an entrance that extends radially relative to the central axis 12, while the remainder of the passage follows a helical path relative to the central axis 12 of the fuel injection system. Since the inner swirler 16 is integrally formed with the body of the pilot fuel injector 10, the passages 18a,18b pass through the pilot fuel injector.
Similarly, the outer swirler, disposed radially outwards of the pilot fuel injector 10 has swirl passages that channel an airflow into the combustor. Each of the swirl passages follows a helical path relative to the central axis 12 of the fuel injection system.
The inner and outer swirlers channel the respective airflow of each swirler along a helical path. This helical path has an axial component that is aligned with the central axis 12 and a further component that is tangential relative to the central axis 12. Thus, the inner and outer swirlers condition the motion of the airflow into the combustor to generate helical flow.
Thus, when exiting the swirler, the airflow follows a path having an exit swirl angle and direction of rotation that have been imparted by the swirler.
The swirl passages of the inner swirler are divided into two sets. A first set of swirl passages 18a follow a right-handed (i.e. clockwise) helical path, while a second set of swirl passages 18b follow a left-handed (i.e. anti-clockwise) helical path. That is, the helical angle of the first set of swirl passages 18a is equal in magnitude but opposite in sign to the helical angle of the second set of swirl passages 18b, i.e. the first and second sets of swirl passages 18a, 18b have opposing angular directions.
More specifically, as shown in
The upstream entrance to the first set of swirl passages 18a is axially displaced from the upstream entrance to the second set of swirl passages 18b.
A sliding cylindrical plug 20 provided within the inner swirler 16 has a main channel 22 extending along the central axis 12 of the fuel injection system. The main channel 22 is in fluid communication with further channels 24a,24b provided within the plug that extend from the main channel 22 in a radial direction. The radially-extending channels 24a,b include three channels 24a that extend in the same radial directions as the entrances to the first set of swirl passages 18a, and three channels 24b that extend in the same radial directions as the entrances to the second set of swirl passages 18b.
The plug 20 has a first axial position in which the first set of channels 24a is in fluid communication with the first set of swirl passages 18a, while the entrance to the second set of swirl passages 18b is blocked. The plug 20 also has a second axial position, in which it is axially displaced from the first axial position and in which the second set of channels 24b is in fluid communication with the second set of swirl passages 18b. In this case, the entrance to the first set of swirl passages 18a is blocked by the plug.
When the plug 20 is in its first axial position, airflow into the inner swirler 16 is directed along the main channel 22 of the plug, through the channels 24a and along the first set of swirl passages 18a. Thus, the airflow is directed along a right-handed (i.e. clockwise) helical path, and exits the swirler with a clockwise direction of rotation.
Similarly, when the plug 20 is in its second axial position (shown in
The plug 20 is resiliently biased to the second axial position by means of a spring 26 disposed at the upstream end of the plug 20.
The resilient bias of the spring is countered by the pressure within a pressure compartment 28 provided within the fuel injector 10 at the downstream end of the plug 20. The pressure compartment 28 is in fluid communication with a signal line connected to the fuel supply system.
When the signal line is above a predetermined pressure, typically while the combustor is operating at high power, the plug 20 will tend to be located at the first axial position and the airflow into the inner swirler 16 will be directed along a right-handed (i.e. clockwise) helical path.
By contrast, when the signal line is lower than a predetermined pressure, the plug 20 will tend to be located at the second axial position and the airflow into the inner swirler 16 will be directed along a left-handed (i.e. anti-clockwise) helical path.
The predetermined pressure that overcomes the bias of the spring is preferably similar to the pressure that is required to open the valves which supply the fuel to the main injector. Thus, as the fuel begins to flow to the main injector the cylindrical plug will be moved to position 1 by the pressure of the fuel to vary the exit swirl angle and rotation direction of the air through the pilot injector.
It is alternatively possible to use the fuel supply to the pilot injector to pressurise the pressure compartment since both the pilot fuel pressure and main fuel pressure increases with power demand from the engine.
Thus the sliding cylindrical plug 20, spring 26, pressure compartment 28 and conduit 30 provide a passive control mechanism responsive to changes in the pressure of the fuel supply for varying the exit swirl angle and rotation direction.
The helical angle and angular direction of the helical path followed by the passages of the outer swirler may be fixed. In this case, the fuel injection system may be operated such that the inner and outer swirlers operate in co-swirl mode, i.e. both swirlers direct airflow into the combustor along a clockwise helical path or both swirlers direct airflow along an anti-clockwise helical path, such that the angular direction of the helical flow generated by the two swirlers is the same. Alternatively, by changing the axial position of the plug 20 in the inner swirler 16, the fuel injection system may be operated such that the inner and outer swirlers operate in counter-swirl mode, e.g. the inner swirler directs airflow along a clockwise helical path, while the outer swirler directs airflow along an anti-clockwise helical path, such that the angular direction of the helical flow generated by the inner swirler is opposite to that generated by the outer swirler.
Co-swirl operation of the inner and outer swirlers tends to result in relatively low smoke emissions at low power, but high NOx emissions at high power. By contrast, counter-swirl operation of the inner and outer swirlers tends to result in high smoke emissions at low power, but low NOx emissions at high power.
In this embodiment of the invention, the direction of rotation of the airflow exiting the inner swirler of the pilot fuel injector may be varied from clockwise to anti-clockwise. Thus, during operation of a fuel injection system according to this embodiment of the invention, the mode of operation of the inner and outer swirlers of the pilot fuel injector may be changed from co-swirl to counter-swirl, according to the need to reduce either smoke or NOx emissions. Typically the mode of operation will be changed from co-swirl to counter-swirl as the combustor changes from a low-power state to a high-power state.
While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention as claimed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0815761.2 | Sep 2008 | GB | national |
