Gas turbine reheat combustor including a fuel injector for delivering fuel into a gas mixture together with cooling air previously used for convectively cooling the reheat combustor

A reheat combustor for a gas turbine engine includes a fuel/gas mixer for mixing fuel, air and combustion gases produced by a primary combustor and expanded through a high pressure turbine. Fuel injectors inject fuel into the mixer together with spent cooling air previously used for convectively cooling the reheat combustor. The fuel mixture is burnt in an annular reheat combustion chamber prior to expansion through low pressure turbine inlet guide vanes. The fuel/gas mixer and optionally the combustion chamber define cooling paths through which cooling air flows to convectively cool their walls. The fuel injectors are also convectively cooled by the cooling air after it has passed through the fuel/gas mixer cooling paths. The low pressure turbine inlet guide vanes may also define convective cooling paths in series with the combustion chamber cooling paths.

FIELD

The present disclosure relates to a reheat combustor for a gas turbine engine, to a gas turbine engine including a reheat combustor, and with cooling of a reheat combustor for a gas turbine engine to increase engine efficiency and optimize combustion within the reheat combustor.

BACKGROUND INFORMATION

FIG. 1is a diagrammatic longitudinal sectional view of part of a reheated or afterburning gas turbine engine10above the turbine rotational axis X-X. The gas turbine engine10includes a low pressure compressor12, a high pressure compressor14, a combustion system16, a high pressure turbine18and a low pressure turbine20. The combustion system16can operate on the reheat or afterburning principle and includes a primary combustor22and a reheat combustor24located downstream of the primary combustor22. Both the primary and reheat combustors22,24are annular and extend circumferentially around the turbine axis. The fuel burnt in the combustors can be, for example, oil, or a gas such as natural gas or methane.

In operation, air entering the gas turbine engine10is compressed initially by the low pressure compressor12and then by the high pressure compressor14before the compressed air is delivered to the primary combustor22. Fuel is injected into the primary combustor22by a suitable fuel injector or lance26, where it mixes with the compressed air. Alternatively, the fuel and air may be at least partially premixed together before the fuel/air mixture is injected into the combustion chamber. A plurality of circumferentially spaced burners28then ignite the fuel/air mixture to create hot combustion gases, which are expanded through, and thereby drive, the high pressure turbine18.

Referring toFIG. 2, which shows a configuration of a known reheat combustor24in more detail, the expanded combustion gases are delivered through high pressure turbine outlet guide vanes (HP OGV's)27and vortex generators29to the reheat combustor24for reheating. The reheated combustion gases are directed through low pressure turbine inlet guide vanes (LP IGV's)35into the low pressure turbine20and exhausted from the engine. Both the high pressure and low pressure turbines18,20are drivingly connected, via suitable connecting shafts, respectively to the high pressure and low pressure compressors14,12which are, thus, driven in a known manner by the high pressure and low pressure turbines18,20.

The temperature of the hot combustion gases produced by the primary combustor22decreases as those hot combustion gases are expanded through the high pressure turbine18. Because the power output of a gas turbine engine can be, proportional to the temperature of the combustion gases, it is desirable to reheat the combustion gases that have been expanded through the single-stage high pressure turbine18before they are expanded further through the multi-stage low pressure turbine20. Although a single-stage HP turbine has been described, an HP turbine can have two or more stages if the combustion gases generated by the primary combustor have sufficient energy.

Referring again toFIG. 2, the reheat combustor24includes a fuel/gas mixer30, which can be substantially annular but is segmented into a number of discrete mixing zones25. The area referenced as30is not a continuous annulus but can include individual mixing zones25whose circumferential extents are defined by angularly spaced-apart side walls. However, the walls44,46, which define the radially inner and outer boundaries of the fuel/gas mixer30, can be circumferentially continuous, though this is not essential. Each mixing zone25has an upstream inlet end41to receive the combustion gases43that have been expanded through the high pressure turbine and its annular array of outlet guide vanes27. At the inlet ends41, the combustion gases43pass through vortex generators29before fuel is injected into them by a fuel injector32. The vortex generators29aid mixing of the injected fuel with the combustion gases43in the fuel/gas mixer30. The mixture is delivered into an annular combustion chamber34through outlets45of the mixing zones and the mixture can spontaneously combust due to the heat of the combustion gases.

The number and spacing of the fuel injectors employed should be sufficient to ensure that the circumferential distribution of fuel, air and combustion gases around the mixing zones25is sufficiently uniform to enable adequate mixing before combustion occurs. It is desirable if there is one fuel injector per mixing zone of the fuel/gas mixer30but this is not an essential characteristic of the fuel/air mixer30. For example, if each mixing zone has a sufficient circumferential extent, a more even distribution of fuel can be obtained if there are two or more fuel injectors per mixing zone. Assuming one fuel injector per mixing zone, it has been found that a suitable number of fuel injectors and mixing zones in a large heavy duty gas turbine engine can be twenty-four.

As the flame temperature in the reheat combustor24increases, the cooling requirements of the walls of the combustion chamber34and the fuel/gas mixer30can increase, as do the cooling requirements of the HP OGV's27and the LP IGV's35(FIG. 1). At the same time, the level of undesirable NOx emissions and the danger of premature ignition of the fuel/oxidant mixture can also increase. Hence, to control the level of NOx emissions and generally ensure efficient and reliable operation of the reheat combustor24, it is desirable to provide suitable cooling for the reheat combustor24and associated components.

The HP OGV's27and the LP IGV's35can be cooled by convective and/or effusion and/or film cooling techniques, the cooling air being supplied from different sources, usually the high pressure and low pressure compressors, respectively. The annular combustion chamber34of the known reheat combustor24has walls including radially inner and radially outer annular double-walled combustion liners40,42, respectively, which can be convectively cooled by a supply of cooling air, which can be drawn from the low pressure compressor12. The cooling air flows through radially inner and outer cooling paths36,38defined between the double walls of the radially inner and radially outer combustion liners40,42. In contrast, the walls of the fuel/gas mixer30can be effusion-cooled. Specifically, radially inner and radially outer walls44,46of the fuel/gas mixer30both can include a large number of holes having a small diameter (for example, about 0.7 to 0.8 mm) through which cooling air47effuses. Furthermore, the dividing walls between adjacent mixing zones25of the fuel/gas mixer can also be effusion cooled. The air for effusion cooling can be supplied from the combustion liner flow paths36,38, which exhaust into annular plenum chambers adjacent the radially inner and outer fuel/gas mixer walls44,46. Due to the acute inclination of the holes relative to the interior surfaces of the radially inner and radially outer fuel/gas mixer walls44,46, and the low momentum of the jets of effusion air47, the effusion air remains close to the interior surfaces of the fuel/gas mixer walls44,46, thus keeping them suitably cool. Despite being efficient and reliable, there can be some issues associated with effusion cooling of the fuel/gas mixer30.

One is that the effusion air47may not mix properly with the fuel injected into the mixing zones25of the fuel/gas mixer30via the fuel injectors32, whose outlets are located generally centrally between the radially inner and radially outer walls44,46of each individual mixing zone25. The effusion air does not, therefore, make much contribution to reducing the flame temperature in the annular combustion chamber34and thus to reducing the level of undesirable NOx emissions.

To provide cooling for the fuel injectors32, to reduce the flame temperature and furthermore to ensure that the fuel emerging from the fuel injectors32does not combust prematurely in the presence of the relatively high temperature combustion gases, it may be necessary to provide a supply of carrier air. The carrier air is injected into the mixing zones25of the fuel/gas mixer30with the fuel, through the fuel injectors32, and can include re-cooled air from the high pressure compressor14but the provision of such carrier air is undesirable and can result in loss of efficiency and power.

There is, therefore, a desire for an improved reheat combustor for a gas turbine engine, and for a reheat combustor with improved cooling which provides for the reduction in flame temperature to reduce the level of undesirable NOx emissions and which also minimizes power and efficiency losses within the gas turbine engine.

SUMMARY

A reheat combustor for a gas turbine engine is disclosed comprising a fuel/gas mixer for mixing fuel with combustion gases that have been produced by a primary combustor and expanded through a high pressure turbine, a plurality of fuel injectors for injecting fuel into the fuel/gas mixer and an annular combustion chamber downstream of the fuel/gas mixer, in which the mixture of injected fuel and combustion gases is combusted prior to expansion through a low pressure turbine, a wall of the fuel/gas mixer defining at least one convective cooling path through which cooling air flows, in use, for convectively cooling the fuel/gas mixer, and when the fuel injectors are arranged to inject the cooling air previously used for convective cooling of the fuel/gas mixer into mixing zones of the fuel/gas mixer together with the fuel.

A gas turbine engine is disclosed comprising a low pressure compressor, a high pressure compressor, a primary combustor, a high pressure turbine for expanding combustion gases produced by the primary combustor, a reheat combustor for reheating the combustion gases following expansion through the high pressure turbine; and a low pressure turbine for expanding the reheated combustion gases wherein the reheat combustor includes a fuel/gas mixer for mixing fuel with combustion gases that have been produced by the primary combustor and expanded through the high pressure turbine, a plurality of fuel injectors for injecting fuel into the fuel/gas mixer, an annular combustion chamber downstream of the fuel/gas mixer, in which the mixture of injected fuel and combustion gases is combusted prior to expansion through a low pressure turbine, wherein a wall of the fuel/gas mixer defines at least one convective cooling path through which cooling air flows, in use, to convectively cool the fuel/gas mixer; and the fuel injectors are arranged to inject the cooling air previously used for convective cooling of the fuel/gas mixer into mixing zones of the fuel/gas mixer together with the fuel.

A method of cooling a reheat combustor in a gas turbine engine is disclosed, including injecting cooling air previously used for convectively cooling at least a part of the reheat combustor by fuel injectors into mixing zones of the reheat combustor together with fuel.

The drawings are all diagrammatic in character and are not to scale.

DETAILED DESCRIPTION

Exemplary embodiments of the disclosure provide an apparatus and a method of cooling a reheat combustor in a gas turbine engine, in which cooling air previously used for convectively cooling at least a part of the reheat combustor is injected by fuel injectors into mixing zones of the reheat combustor together with fuel. The mixing zones, and a reheat combustion chamber downstream of the mixing zones, can include the parts of the reheat combustor that are convectively cooled, cooling air from the combustion chamber being used to convectively cool the mixing zones. The fuel injectors can also be convectively cooled by the cooling air before it is injected into the mixing zones with the fuel.

A method of an exemplary embodiment of the disclosure can further include convectively cooling low pressure turbine inlet guide vanes (LP IGV's) downstream of the combustion chamber, cooling air therefrom then being used to convectively cool the reheat combustion chamber. The cooling air may be supplied from a single source, for example, a low pressure compressor of the gas turbine engine.

Exemplary embodiments of the present disclosure also provide a reheat combustor for a gas turbine engine, the reheat combustor including a fuel/gas mixer for mixing fuel with combustion gases that have been produced by a primary combustor and expanded through a high pressure turbine, a plurality of fuel injectors for injecting fuel into the fuel/gas mixer, and an annular combustion chamber downstream of the fuel/gas mixer, in which the mixture of injected fuel and combustion gases is combusted prior to expansion through a low pressure turbine, wherein a wall of the fuel/gas mixer defines at least one convective cooling path through which cooling air flows, in use, to convectively cool the fuel/gas mixer and the fuel injectors are arranged to inject the cooling air previously used for convective cooling of the fuel/gas mixer into mixing zones of the fuel/gas mixer together with the fuel.

An exemplary embodiment of the present disclosure provides a gas turbine engine including a primary combustor, a high pressure turbine for expanding combustion gases produced by the primary combustor, a reheat combustor for reheating the combustion gases following expansion through the high pressure turbine, and a low pressure turbine for expanding the reheated combustion gases. The reheat combustor includes a fuel/gas mixer for mixing fuel with combustion gases that have been produced by a primary combustor and expanded through a high pressure turbine, a plurality of fuel injectors for injecting fuel into the fuel/gas mixer, and an annular combustion chamber downstream of the fuel/gas mixer, in which the mixture of injected fuel and combustion gases is combusted prior to expansion through a low pressure turbine, wherein a wall of the fuel/gas mixer defines at least one convective cooling path through which cooling air flows, in use, to convectively cool the fuel/gas mixer and the fuel injectors are arranged to inject the cooling air previously used for convective cooling of the fuel/gas mixer into mixing zones of the fuel/gas mixer together with the fuel.

In an exemplary embodiment of the disclosure, the fuel injectors can also be convectively cooled, and to this end walls of each fuel injector can define a fuel injector convective cooling path and the fuel injector convective cooling path can be connected to receive cooling air from the at least one convective cooling path of the fuel/gas mixer.

In an exemplary embodiment of the disclosure, the fuel/gas mixer can include an overall annular structure that is segmented into a plurality of discrete mixing zones that are angularly spaced apart around the annulus. The circumferential extent of individual mixing zones can be defined by angularly spaced-apart side walls and their radial extent can be defined by radially inner and radially outer walls of the fuel/gas mixer. The side walls and/or at least one of the radially inner and outer walls can define fuel/gas mixer cooling paths through which the cooling air flows, in use, to convectively cool the fuel/gas mixer.

By convectively cooling the fuel/gas mixer walls and thereafter injecting the cooling air that has been used for convective cooling into the fuel/gas mixer together with the fuel, greater mixing of the cooling air and the injected fuel can be achieved than in the effusion-cooled fuel/gas mixer of the known reheat combustor described above. The cooling air can therefore be put to better use than in the effusion cooled fuel/gas mixer where it provides mostly for cooling of the walls of the fuel/gas mixer. Exemplary embodiments of the disclosure can enable the same cooling air to perform the duties of providing not only effective cooling of the fuel/gas mixer walls but also a reduction in the flame temperature in the combustion chamber, and thus a resultant reduction in undesirable NOx emissions.

In exemplary embodiments according to the disclosure, the side walls of the fuel/gas mixer and both of the radially inner and radially outer walls define fuel/gas mixer cooling paths. In this manner, all the fuel/gas mixer walls can be protected from the heating effects of the hot combustion gases, thus reducing the thermal stresses on the fuel/gas mixer structure and increasing the life of the reheat combustor.

The reheat combustion chamber according to an exemplary embodiment of the disclosure can include a wall defining at least one combustion chamber cooling path through which the cooling air flows, in use, to convectively cool the combustion chamber. The combustion chamber can be defined by radially inner and radially outer combustion chamber walls, either or both of which define a combustion chamber cooling path. Each cooling path can protect a combustion chamber wall from overheating by the hot combustion gases, so reducing the thermal stresses on the walls of the combustion chamber and increasing the life of the reheat combustor.

In an exemplary embodiment of the disclosure, the combustion chamber cooling paths and the fuel/gas mixer cooling paths can be arranged so that the cooling air flows through a combustion chamber cooling path and then through a fuel/gas mixer cooling path. The cooling air may thus not only be used for convectively cooling the combustion chamber but additionally for convectively cooling the fuel/gas mixer. The overall efficiency of the gas turbine engine can thereby be improved.

In an exemplary embodiment of the disclosure, the radially inner combustion chamber cooling path and the radially inner fuel/gas mixer cooling path can communicate to define a common radially inner cooling path through which the cooling air may flow to convectively cool the inner walls of both the annular combustion chamber and the fuel/gas mixer. Similarly, the radially outer combustion chamber cooling path and the radially outer fuel/gas mixer path can communicate to define a common radially outer cooling path through which the cooling air may flow to convectively cool the outer walls of both the annular combustion chamber and the fuel/gas mixer.

To simplify construction of the reheat combustor and maximize efficiency, all the convectively cooled cooling paths, i.e., both radially inner and radially outer cooling paths, can share a common supply of cooling air.

Injection of the cooling air into the fuel/gas mixer together with the fuel can bring about the advantage that a separate source of carrier air, such as that required for the effusion-cooled fuel/gas mixer of the known reheat combustor described above, is not needed. The loss of efficiency associated with the provision of the carrier air can be eliminated.

There can be one or more fuel injectors per discrete mixing zone of the fuel/gas mixer. Fuel injectors that extend radially into the fuel/gas mixer from an outer wall can be used to inject the fuel and cooling air, each fuel injector including a plurality of fuel injector tubes arranged to inject the fuel into the fuel/gas mixer in the downstream direction. This arrangement can make it possible to eliminate the high pressure turbine outlet guide vanes (HP OGV's) and the vortex generators that are provided in the known gas turbine engine described above. Elimination of the HP OGV's and the vortex generators is possible because injector tubes, or the jets of fuel expelled from them, can present the same profile to the flow coming from the high pressure turbine, no matter from which upstream direction the flow approaches the injectors. The cross-sectional area of the fuel/gas mixer can therefore be reduced, thereby increasing the velocity of the flow through it without any increase in pressure drop, due to the absence of the outlet guide vanes and the vortex generators.

Because the fuel is injected into the fuel/gas mixer together with cooling air that has been used for convective cooling of at least the fuel/gas mixer, there can be a significant mass flow rate of low pressure air through the fuel/gas mixer, and the size and number of the fuel injectors can be greater than in the known reheat combustor described with respect toFIGS. 1 and 2.

The fuel injectors can be located near the inlets of the mixing zones, or at points intermediate their inlets and outlets. Furthermore, either the entire length of the fuel/gas mixer walls can be convectively cooled before the cooling air is injected into the fuel/gas mixer with the fuel, or only the parts of the fuel/gas mixer walls that are downstream of each fuel injector can be convectively cooled. In the latter case, the parts of the fuel/gas mixer upstream of the fuel injector may be effusion cooled or film cooled.

The fuel injectors can be in the form of struts or the like that extend radially into or across the mixing zones. The above-mentioned plurality of fuel injector tubes that form part of each fuel injector can enable more even distribution of injected fuel and air within the mixing zones. In an exemplary embodiment of the disclosure, the convective cooling path in each fuel injector can be defined between an inner fuel passage and an outer wall of each fuel injector and the plurality of radially spaced fuel injector tubes extend from the fuel passage through the outer wall, thereby to inject jets of fuel into the mixing zones. In this arrangement, each injector tube projects through a corresponding hole in the outer wall, the holes being of larger cross-section than the tubes so that cooling air can exit from the fuel injector cooling path into the fuel/gas mixer as jets of air, whereby in use each fuel jet is surrounded by an annular air jet.

Whereas the above described fuel injector can inject only one type of fuel, e.g., either gaseous or liquid, many gas turbine engine fuel systems make provision for the injection of two different types of fuel, where the two different fuels may be injected either simultaneously, or during different parts of the engine operating cycle. These are known as “dual fuel” systems. In an exemplary embodiment of the disclosure, therefore, the fuel injectors can be constructed as dual fuel injectors. Each fuel injector includes an outer wall, a first fuel passage for a first fuel and second fuel passage for a second fuel. The second fuel passage is located inside the first fuel passage. The fuel injector convective cooling paths are defined between the first fuel passage and the outer wall of each fuel injector. A first fuel is injectable into the mixing zones through a plurality of radially spaced first injector tubes that extend from the first fuel passage through the outer wall of the fuel injector. A second fuel is injectable into the mixing zones through a plurality of radially spaced second injector tubes that extend from the second fuel passage through a wall of the first fuel passage and the outer wall of the fuel injector. The second injector tubes are of smaller cross-section than the first injector tubes and extend concentrically through the first injector tubes. Each first injector tube projects through a corresponding hole in the outer wall of the fuel injector, the holes being of larger cross-section than the first injector tubes. In use cooling air exits from the fuel injector cooling path into the mixing zones as annular jets of air surrounding jets of the first and/or second fuel.

The first fuel passage can be for gaseous fuel and the second fuel passage can be for liquid fuel.

An annular array of low pressure turbine inlet guide vanes (LP IGV's) can be provided at the exit of the reheat combustion chamber to direct the reheated combustion gases into the low pressure turbine. In an exemplary embodiment of the disclosure, the LP IGV's can be convectively cooled by the same air used for convective cooling of the reheat combustor, i.e., a convective cooling path in each LP IGV communicates with at least one convective cooling path in the reheat combustion chamber. It will therefore be appreciated that a single source of cooling air can be used to successively cool the LP IGV's, the annular combustion chamber, the fuel/gas mixer and the fuel injectors, before the fuel injectors finally inject the cooling air into the fuel/gas mixer with the fuel. This can achieve an increase in efficiency relative to the known gas turbine engine described above, in which cooling air used for effusion or film cooling of the LP IGV's is simply released into the main flow and one or more separate sources of cooling air are employed for cooling other parts of the reheat combustor and the HP OGV's. The cooling air for the above convective cooling duty can be supplied by the low pressure compressor of the gas turbine engine in which the reheat combustor is located. Although in this exemplary embodiment the cooling air has absorbed heat from the LP IGV's, the reheat combustion chamber, the fuel/gas mixer and the fuel injectors, before it is injected into the fuel/gas mixer, it can still have a significant cooling and shielding effect when injected coaxially with the fuel and can therefore contribute towards a reduction in the reheat flame temperature, thus reducing the level of undesirable NOx emissions.

FIG. 3Aillustrates an exemplary embodiment of a reheat combustor50for a gas turbine engine. Except for certain aspects of the reheat combustor50to be described below, the engine of which the reheat combustor is a part is a similar construction to the known reheated gas turbine engine10described previously with respect toFIGS. 1 and 2. The reheat combustor50includes a fuel/gas mixer51of substantially annular form. As indicated inFIG. 3C, which is a view the direction of arrow C inFIG. 3A, the upstream end of the combustor is segmented into an annular array of circumferentially spaced mixing zones52, defined by side walls52A. Each mixing zone52has an inlet53receiving combustion gases that have been produced by a primary combustor and then expanded through a high pressure turbine. The reheat combustor50also includes an annular combustion chamber58located adjacent to and downstream from the fuel/gas mixer51. Fuel/air/gas mixture flows through outlets56of the individual mixing zones52and expands into the annular combustion chamber58through its inlet60.

The reheat combustor50includes an annular array of circumferentially spaced-apart fuel injectors63, only one of which is shown inFIG. 3A, though several are shown inFIG. 3C. Each fuel injector injects fuel and air into a mixing zone52of the fuel/gas mixer51. The number and angular spacing of the mixing zones and fuel injectors employed should be sufficient to ensure that the circumferential distribution of mixed fuel, air and combustion gases around the annular combustion chamber58enables efficient combustion. For example, if a mixing zone52is of a sufficiently large angular extent between its circumferentially spaced-apart side walls52A, it can be necessary for it to have more than one fuel injector in order to ensure adequate circumferential distribution of mixed fuel, air and combustion gases.

The velocity of the fuel mixture in the downstream direction slows abruptly because of its expansion into the larger cross-sectional area of the annular combustion chamber58, whereupon the fuel in the mixture can spontaneously combust or auto-ignite in the combustion chamber due to the presence of the hot combustion gases. Mixing of the injected fuel and expanded combustion gases mainly occurs in the mixing zones52and combustion of the mixture mainly occurs in the combustion chamber58but it should be appreciated that combustion processes can begin in the fuel/gas mixer51and that mixing will continue in the combustion chamber58.

The annular combustion chamber58has walls of a double-skinned construction including radially inner and radially outer combustion liners64,66, which define respective radially inner and radially outer combustion chamber cooling paths68,70, through which cooling air flows to thereby convectively cool the combustion chamber walls. The mixing zones52also have walls of a double-skinned construction, thereby defining respective radially inner and radially outer fuel/gas mixer cooling paths76,78, for convective cooling. It is preferred that the side walls52A of the mixing zones52are also double-skinned to provide further convective cooling paths in the fuel/gas mixer structure.

In the exemplary embodiment, the radially inner fuel/gas mixer cooling path76is in series flow communication with the radially inner combustion chamber cooling path68, thereby defining a common radially inner convective cooling path for the reheat combustor. Likewise, the radially outer fuel/gas mixer cooling path78is in series flow communication with the radially outer combustion chamber cooling path70, thereby defining a common radially outer convective cooling path for the reheat combustor. These cooling combustion chamber and fuel/gas mixer cooling paths can receive their supply of cooling air from a common source, for example, a low pressure compressor of the gas turbine engine.

InFIGS. 3A and 3C, the circumferentially spaced side walls52A of each mixing zone52can have internal cooling flow paths and are in flow communication with either or both of the radially inner and radially outer combustion chamber cooling paths. Alternatively, to enable a simpler design of the reheat combustor and its cooling system, it can be arranged that the cooling air from the combustion chamber liners (i.e., the radially inner and outer combustion chamber cooling paths68,70), flows into a plenum chamber surrounding the fuel/air mixer, and that all the cooling paths in the fuel/gas mixer are connected to receive their supply of cooling air from the plenum chamber.

The fuel injectors63can be in the form of hollow struts80that extend across the inlet53of the fuel/gas mixer51. The struts80can be of circular, elliptical or similar cross-section. Each strut has a cooling air path84defined between an outer wall and an inner wall of the strut to enable convective cooling of the fuel injectors63. The fuel injectors63can be configured so that after the cooling air has been used for convective cooling of the annular combustion chamber58, the fuel/gas mixer51, and the fuel injectors63, the spent cooling air is exhausted from the fuel injectors63into the fuel/gas mixing zones52with the fuel, as denoted by the reference numeral86. The spent cooling air thus facilitates injection of the fuel and mixes with it, thus reducing the temperature of the resulting mixture of fuel and expanded combustion gases that are created inside the mixing zones52.

The structure of the fuel injector63is illustrated in more detail inFIG. 3B, which is a view of the part within box B inFIG. 3A.FIGS. 3A and 3Btogether show that fuel82flows into a tube54that is blind-ended at its radially inner end. Tube54thus defines a fuel passage83within strut80. Jets of fuel82issue from passage83into the mixing zone52through a number of radially spaced-apart fuel injector tubes85that are securely fixed in the wall of the tube54and that penetrate both the tube wall and the outer skin87of the strut80, which forms the outer wall of the injector cooling air path84. Air that has been used to convectively cool the injector63exits from the fuel injector cooling path84into mixing zone52through air exit holes88provided in the outer skin87of each strut80. The distal or free end of each injector tube85projects through a corresponding one of the air exit holes88, the holes88being of larger diameter than the external diameter of the tubes85, so that each jet of fuel issuing from the tubes85is surrounded by a coaxial annular jet of cooling air. The air therefore has a cooling and shielding effect, so helping to reduce the reheat flame temperature and hence NOx emissions.

To provide the reheat combustor with “dual fuel” capability, the fuel injectors63can be constructed to inject two types of fuel, for example, gas fuel and liquid fuel. This is diagrammatically illustrated inFIG. 3Bby dashed lines. In an exemplary embodiment, each fuel injector strut80includes an outer wall87, a first tube54defining a first fuel passage83and a second tube100, located inside the first tube54, defining a second fuel passage102. The fuel82in passage83can be gaseous, for example, natural gas, and the fuel104in passage102can be liquid, for example, diesel or fuel oil. In addition to the radially spaced injector tubes85that extend from fuel passage83through the wall of tube54and the outer wall87of the fuel injector strut80, a second set of radially spaced injector tubes106can be provided to inject fuel104into the mixing zone52of the fuel/gas mixer51. Injector tubes106are of smaller cross-section than injector tubes85and extend from the second or inner fuel passage102through its wall as defined by tube100and then concentrically through the injector tubes85. Hence, if it is desired to burn both fuels simultaneously within the reheat combustor, jets of the second fuel can be injected into the fuel/gas mixer51concentrically within jets of the first fuel. Furthermore, as previously described, because injector tubes85project through holes88in the outer wall87of the fuel injector strut, cooling air exits from the fuel injector cooling path84into the fuel/gas mixer51as annular jets of air. Each such air jet therefore surrounds and is coaxial with a jet of the first fuel and/or a jet of the second fuel, according to an operating mode of the reheat combustor.

FIG. 3Ashows the coaxial jets86of fuel and cooling air issuing from the fuel injectors63in a direction aligned with the downstream direction, and this is an orientation of the injector tubes and their surrounding air exit holes88.

The relative dimensions of the tubes85,106and the holes88can be chosen as required to obtain the desired fuel mixing and combustion characteristics and will depend on a variety of factors but can be ascertained by the use of computerized fluid flow modeling and rig tests. If necessary or desirable for correct functioning of the mixing zones52and the combustion chamber58, the number of air holes88can be greater than the number of injector tubes85, those air holes that are not paired with corresponding injector tubes being located, for example, in between adjacent injector tubes, or near the walls of the mixing zone52and radially spaced-apart.

The temperature of the cooling air can increase by the time it is injected into the mixing zones52, because it has been used to convectively cool multiple component parts of the reheat combustor50. However, its temperature can still be sufficiently low (relative to the temperature of the expanded combustion gases that have flowed into the mixing zones52from the high pressure turbine18) to have a significant cooling effect. This cooling effect can be further enhanced by the fact that the cooling air has a high mass flow rate, for example, of the order of twice the mass flow rate of the carrier air injected with the fuel in the known reheat combustor24described with reference toFIG. 1. The reduction in the temperature of the mixture of the injected fuel and the expanded combustion gases can bring about a reduction in the flame temperature when the mixture is combusted in the annular combustion chamber58and a consequent reduction in the level of undesirable NOx emissions.

Unlike the known gas turbine engine described with reference toFIG. 1, injection of the convective cooling air into the fuel/gas mixer51together with the fuel can render it unnecessary to provide the fuel injectors62with carrier air from a separate source. A gas turbine engine including the reheat combustor50can therefore be more efficient than the known gas turbine engine10.

Use of the convectively cooled tube-type fuel injectors63can enable the high pressure turbine outlet guide vanes27and the vortex generators29that are required in the known gas turbine engine10ofFIG. 1to be eliminated because injector tubes, or the fuel jets that issue from them, can present the same profile to the downstream flow of combustion gases no matter what transverse velocity components are present in the flow. This can result in a further increase in the efficiency and power output of a gas turbine engine that includes the reheat combustor50, because the pressure drop through the fuel/gas mixer51is reduced. The absence of the high pressure turbine outlet guide vanes27and the vortex generators29also enables the cross-sections of the mixing zones52to be reduced without any increase in pressure drop, thereby increasing the velocity of the main flow of combustion gases through the reheat combustor50. This can be advantageous as it enables fuels, such as syngas and dry oil, to be combusted in the reheat combustor50without flashback, due to the reduced residence time in the mixing zones52and the annular combustion chamber58.

Referring now toFIG. 4, there is shown an exemplary embodiment of a reheat combustor90according to the disclosure. The reheat combustor90is similar in construction and operation to the reheat combustor50described above. Corresponding components are thus designated using the same reference numerals and will not be described again.

The outlet62of reheat combustor90exhausts into the low pressure turbine through an array of circumferentially spaced inlet guide vanes (LP IGV's), one of which is shown schematically at the reference numeral92. Each of the LP IGV's92includes a vane cooling path94through which cooling air flows for convective cooling of the vanes92. In an exemplary embodiment, the same cooling air performs multiple cooling duties. It is supplied by the low pressure compressor and flows initially through the guide vane cooling path94before it divides to flow through two parallel flow paths, i.e., the radially inner cooling paths68,76and the radially outer cooling paths70,78, inside the walls of the combustion chamber58and the mixing zones52of the fuel/gas mixer51. The radially inner and outer flow paths are then merged to convectively cool the fuel injectors63, which then inject the spent cooling air into the mixing zones52together with the fuel.

It will be understood from the above that because a separate supply of cooling air is not required to provide for effusion cooling or film cooling of the LP IGV's92, a further increase in efficiency compared with known gas turbine engines can be obtained with a gas turbine engine employing the reheat combustor90.

Embodiments have been described above purely by way of example, and modifications can be made within the scope of the disclosure. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. For example, it is possible that convective cooling could be employed only for the fuel/gas mixer51before the cooling air is injected by the fuel injectors63into the mixing zones52with the fuel, the annular combustion chamber58being cooled other than by convection cooling.

Although radially inner and radially outer double-skinned walls64,66,72,74are provided to define respective radially inner and radially outer convective cooling paths68,70,76,78to cool the combustion chamber58and the fuel/gas mixer51, it can be possible to substitute effusion cooled walls for either the inner or the outer convectively cooled walls, thereby defining only a radially inner or a radially outer combustion chamber-fuel/gas mixer cooling path.

Due to eliminating the need for HP OGV's and vortex generators, the above description has focused on the use of fuel injectors63including multiple injector tubes for the injection of fuel together with spent cooling air into the mixing zones. However, other known types of fuel injectors could alternatively be used, provided that such injectors could be modified to inject the fuel together with the spent cooling air.

It should be understood that fuel injectors63can be located axially at any suitable position at or downstream of inlet53within the mixing zones52, as necessary to obtain desired fuel mixing and ignition characteristics for the combustion process. Moreover, the entire lengths of the mixing zones52can be convectively cooled, as shown inFIGS. 3A and 4, or only the parts of the mixing zones52that are downstream of the fuel injectors63can be convectively cooled.

Note that each feature disclosed in the specification, including the claims and drawings, can be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise. Unless the context clearly requires otherwise, throughout the description the disclosure is to be construed in an inclusive as opposed to an exclusive or exhaustive sense.