Combustion device
A combustor (1) has a swirler (22). The swirler has mixing channels (40), the mixing channels having a height/width aspect ratio of less than (2). A secondary fuel inlet (32) is located downstream of mixing channel (40) and may be in a zone (65) of separated flow. Primary fuel admission may be through a threaded or push fit circular movable plug (100). The plug (100) may be conveniently removed for calibration purposes and may have a bell-mouthed entrance.
The present invention relates to a combustion device, and more particularly but not exclusively to a combustion device for a gas turbine engine, and furthermore to the assembly of components forming a combustion device. A combustor is commonly used in a gas turbine engine to burn fuel in compressed air to produce exhaust gas for exhaust to a turbine. Recent combustor designs have aimed at reducing emissions of nitrogen oxide (NO) and nitrogen dioxide (NO2)— collectively known as NOx. Running the combustor to produce well-mixed fuel and air in a mixing channel can decrease NOx emissions. However, this level of mixing can create flame instability at low flow rates, leading to flame blow-out. Consequently, it is known to use a secondary or pilot fuel inlet to prevent the blow-out from occurring. One combustor design has a stub tube having several fuel inlets arranged to admit fuel into a mixing channel to increase mixing and reduce NOx emissions. However, this arrangement is prone to flame instability and potential flame blow-out. A stub tube creates a wake, which reduces the effective area of the mixing channel and reduces the flow rate of air and fuel through the mixing channel. Each of these multiple inlet designs has the problem that each inlet must be calibrated at every position individually. This is time consuming and fiddly, and can also result in costly downtime. An object of the present invention is to relieve the problems of the prior art in a simple and effective manner at no major expense.
Furthermore, radial swirlers for industrial gas turbine combustors typically involve a series of rectangular mixing ducts issuing tangentially into a closed end cylindrical chamber. This arrangement results in the formation of a stable solid body rotation of sufficient strength for the formation of a recirculation zone to provide for flame stability. The highly swirling flow within the chamber also provides for additional mixing. The use of rectangular ducts does, however, have several disadvantages. The need to achieve a uniform fuel distribution within each duct for low emission performance can necessitate a complex injection arrangement, such as fuel rods projecting across each duct. The inter duct discharge coefficient may also show some variation, due to slight variations in duct aspect ratio, impacting on mixture uniformity within the combustion chamber. Finally, the use of rectangular ducts dictates certain manufacturing methods which may not be conducive with low cost or ease of production.
Various aspects of the present invention are set out in the independent claims. A number of optional features are set out in the dependent claims. The optional features are applicable to each aspect of the invention and the invention envisages and extends to any combination of the aspects and optional features hereof which is not specifically recited herein.
SUMMARY OF THE INVENTIONWhen an insert channel portion/plug is provided, this may be removably attached to the body, such as by threaded engagement or a push fit, or permanently attached, such as by braising.
According to another aspect of the present invention, there is provided an apparatus for mixing compressed air with fuel, comprising a body having a mixing channel for mixing fuel and air, a primary fuel inlet and a secondary fuel inlet, wherein the secondary fuel inlet is adapted to admit fuel into a zone of separated flow on the body. In a further aspect of the present invention, the secondary fuel inlet is positioned outside of the mixing channel. The advantage of this is that a relatively small amount of fuel may be admitted to a zone in which there is little mixing of air and fuel, therefore providing flame stability and avoiding flame blow-out, whilst the majority of fuel is admitted through a primary fuel inlet in the mixing channel to produce a well mixed mixture of air and fuel.
According to another aspect of the invention, the mixing channel has a rectangular cross section, having a width dimension defined in the axis substantially parallel to the plane of the swirler body, and a depth dimension defined in the access substantially orthogonal to the plane of the body, wherein the mixing channel aspect ratio is such that its depth-to-width ratio is less than or equal to 2. In yet another aspect of the invention, the same depth-to-width is less than or equal to 1.5, preferably ≦1.25 or ≦1.0. This ratio is preferably ≧0.7. The aspect ratio may be ≦2, ≦1.5, ≦1.2, ≧0.5 and/or ≧0.7. This ratio may be from 0.5 and 2, preferably from 0.7 to 1.5, e.g., from 0.8 to 1.2, one example being 1.0. Accordingly, better mixing of the air and fuel is achieved than would be obtained by a taller mixing channel having a higher depth-to-width ratio.
According to a further aspect of the invention, the apparatus has a fuel metering means having an elliptic or otherwise curved or partly curved cross section such as circular, oval or racetrack-shaped. In another aspect of the invention, the fuel metering means is removable and may also be of elliptic cross section.
A number of further optional features, which may be applicable to any one or more of the above aspects of the invention, will now be described.
Preferably, the mixing channel comprises a bore formed in the body of the apparatus, the bore having an elliptical cross section, which may be circular. The advantage of an elliptic cross section channel is that fuel may be conveniently admitted into the channel from around the channel, so as to increase mixing of the fuel with air entering the channel, and so the channel flow does not suffer from the reduced effect of area that can occur in the corners of rectangular channel flows.
Preferably, the zone of separated flow is outside of the mixing channel so as to avoid reducing the mixing channel effective area. The secondary fuel inlet is more preferably positioned downstream of the mixing channel. The advantage of this is that air and fuel are well mixed prior to secondary fuel being added, thus minimising NOx emissions, and yet the pilot (or secondary) fuel is available when the mixture is vulnerable to flame blow-out. The body may be a swirler. The mixing channels may be equi-spaced around the circumference of the swirler, and the secondary fuel inlets may be equi-spaced around the center of the swirler to facilitate good mixing of air and fuel. In other cases, the secondary fuel inlets may not be equally spaced. Preferably, the secondary fuel inlets are positioned on axes, each axis being aligned with the longitudinal axis of a mixing channel. Hence, fuel may be admitted into multiple streams of air and the mixture is able to enter the air/fuel chamber from several directions simultaneously, creating a circumferentially uniform influx of the mixture into the air/fuel chamber. In one aspect of the invention, there may be an equal number of secondary fuel inlets and primary fuel inlets, and in another aspect of the invention, there may be fewer secondary fuel inlets than there are primary fuel inlets. Preferably, the mixing channels are oriented so as to impart a swirl component of motion to the air/fuel mixture exiting the mixing channels, such that a vortex is formed in the air/fuel chamber producing a low pressure core in the air/fuel chamber flow. The low-pressure core will induce mixture in the air/fuel chamber to recirculate back up the chamber such that any excess fuel in the mixture can be burnt.
The removable means for metering of the fuel preferably comprises a fuel-metering insert. An advantage of a removable fuel metering means is that the means may be calibrated outside of the apparatus and then installed on the apparatus. This is not only more convenient for the calibrator, but also avoids having to stop the apparatus in order to perform the task, reducing costly downtime. Preferably, the fuel-metering insert comprises a bell mouth entrance and a main insert bore, the bell mouth reducing pressure losses as intake air enters the insert. Preferably, the fuel-metering insert is insertable into the mixing channel insert bore and may be threadably insertable (or a push fit) into the mixing channel insert bore for ease of installation and removal thereof.
When installed onto the apparatus, the insert's bell-mouth entrance may define the mixing channel entrance. Preferably, when the insert is installed on the apparatus, the mixing channel then comprises the bell mouth entrance through which mixing air enters the insert, the main insert bore, the body channel main bore and exit. Preferably, the mixing channel body bore has a cross section graduating from being elliptical adjacent the mixing channel insert bore to rectangular at the exit, such that the flow exiting the mixing channel is tangential to the body, producing a uniform vortex in the air/fuel chamber. The insert's main bore may comprise an elliptical cross section having a similar cross section to that of the mixing channel body bore, thus ensuring a smooth transition between the two bore sections. This will allow the air/fuel mixture to flow undisturbed to the channel exit. The elliptical cross section is preferably a circular cross section to obtain the advantage already mentioned.
The insert may comprise an opening for the admission of fuel into the main bore of the insert, and the opening may be in fluid communication with a primary fuel manifold. There is preferably a plurality of openings spaced around the perimeter of the fuel-metering insert to facilitate good mixing in the mixing channel. The fuel-metering insert may include an annular manifold in fluid communication with the primary fuel inlet at one end thereof and with the primary fuel manifold at the other end. The opening may be the primary fuel inlet. Preferably, the fuel metering means is positioned so that the insert bore is tilted upwards towards the center of the body.
The air/fuel mixing apparatus is preferably a combustor, which may have a body, an air/fuel chamber, a casing and an exhaust. The casing, body and air/fuel chamber may be adapted so that intake air, preferably from a compressor, flows through the gap between the casing and the air/fuel chamber prior to entering the mixing channel. The body may be a swirler adapted to induce a low-pressure core in the air/fuel chamber. The low-pressure core caused by the swirler may induce a recirculation zone in the air/fuel chamber. The swirler may include a surface in fluid communication with the air/fuel chamber, the surface preferably having a boundary layer adjacent to it that includes an attached portion and a separated zone downstream of the attached portion. The swirler may comprise a fuel manifold having a primary fuel manifold and a secondary fuel manifold for admission of fuel into the swirler. The fuel may be a fluid, and is more preferably a gas.
According to another aspect of the invention, a method of mixing air and fuel comprises the admitting of fuel through a primary inlet and a secondary inlet into a body, wherein the fuel admitted by a secondary fuel inlet is admitted to a flow separation zone on the body.
According to an alternative aspect of the invention, a method of calibrating a fuel metering means comprises calibration of the fuel metering means and then installation of the fuel metering means on an apparatus for mixing air and fuel.
In a preferred construction, an improved method of fuel injection for a lean, well-mixed system for use in a low NOx combustor is provided in combination with a radial inflow swirler/mixing apparatus, employed for establishing a flame stabilizing zone. A preferably movable fuel injector or plug gives advantages over conventional methods in terms of mixing length, flow prediction, ease of calibration, clean aerodynamics and pressure loss. The injector/removable plug/portion of the mixing apparatus is essentially a bell-mouthed air metering orifice of high discharge coefficient, which preferably screws or push-fits into the body of the swirler/mixing apparatus which is preferably of the radial inflow type. The injector is preferably surrounded by a gas fuel gallery from which the fuel is injected into the mixing channel through one or more fuel metering holes. The size and number of the holes may be selected to control the quantity of fuel injected. Mixing of fuel and air may be achieved in a relatively short distance from the point of injection. In contrast, other conventional injection systems are believed to incorporate stub tubes and wall injectors needing long passage lengths to achieve low standard deviation of mixing and low NOx production in the subsequent flame zone. In addition, these known injectors often upset the aerodynamics and discharge coefficient of the mixing channel, calling for individual calibration and higher pressure losses. Additional benefits from improved injection/mixing of fuel may be the option of selecting a lower combustion zone airflow and therefore wider flame stability margin at a given level of NOx production and the benefit of such a change may be the allocation of more air for cooling other parts of a combustor on which the mixing apparatus may be employed.
The mixing apparatus may be employed for gaseous fuel although applications for liquid fuels are also envisaged.
BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will now be explained in more detail by the following non-limiting description of preferred embodiments and with reference to the accompanying drawings, in which:
A primary fuel inlet 46 for admission of fuel into a channel 40 is positioned upstream of the channel 40 although it may be positioned inside the channel, preferably towards the annulus than towards the channel exit 48 as defined by the dashed circular line in
A secondary fuel connection 32 is located within the top flange 14 such that it may be accessed at one end 33. Its other end 37 is in fluid communication with the secondary fuel distributor 58 which in turn is also in fluid communication with the secondary fuel inlet 50. A plurality of fuel distributor seals 54 surround the secondary fuel distributor 58. A secondary fuel connection 32, distributor 58 and inlet 50 are aligned with each other in the present embodiment and are positioned downstream of the channel exit 48.
The wedge sections 36 each have a through-bore 29 extending through the plate such that clamping studs 24 may extend therethrough. Each clamping stud 24 holds together the swirler 22, top flange 14 and a flange 60 of the air/fuel chamber or liner 4. Each clamping stud is inserted through a bore 61 in the flange 60, then into the swirler bore 29 before it is inserted into the top flange 14. A nut 62 locks the clamping stud in place.
The mixing channel cross section may be as in
According to a second embodiment of the invention, as shown in FIGS. 4 to 6, the secondary fuel inlets 50 are positioned further downstream of the channel 40 and there are fewer secondary fuel inlets 50 than there are primary fuel inlets 46. The secondary fuel inlets are placed so as to admit fuel into a zone of separated flow 65 at the swirler wall 64 adjacent the pre-chamber 16. The secondary fuel connection 32 and secondary fuel distributor 58 are also positioned in alignment with the secondary fuel inlets 50. Thus the secondary fuel inlets are positioned closer to the point of ignition 31 as shown in
During operation of the gas turbine engine, compressed air enters the combustor 1 through a gap 11 between the internal insulation 12 of the casing 10 and the liner or combustion chamber 4. The mixing airflow path as shown by the arrows in
As the vortex forms in the liner (or combustion chamber) 4, a low-pressure region is formed at the vortex core. A portion of the flow at the vortex is induced into the core such that flow reverses direction at the central axis of the vortex near to the swirler. Thus, any excess fuel that was not burnt at the separation zone 65 is returned to the flame front at the point of ignition to be burnt. The remaining air/fuel mixture flows through the liner or combustion chamber 4 in the vortex and is exhausted to a turbine.
The channels 40 may be of rectangular cross section and may have an aspect ratio as shown in
A further embodiment of the invention is shown in FIGS. 8 to 11.
The fuel metering insert 100, through-bore 116 and the channel 40 have identical cross sections at the junction between them, and once installed on the apparatus, the bore 116 is in exact fluid communication with the channel 40. Thus, the channel section downstream of the insert in this embodiment of the invention is elliptical, preferably circular in accordance with the cross section of the insert through-bore 116. The channel 40 may be elliptical at the junction with the inserts 100 and graduates to a generally rectangular cross section at the channel exit 48.
The openings 122 are, at the insert outer surface 120, in fluid communication with an annular fuel manifold 125, which is in turn into fluid communication with the primary fuel distributor 52. Manifold sealing ring 54 surrounds the primary fuel distributor 52. Manifold 125 is shown in further detail in
The installed insert is tilted upwards towards the fuel manifold, such that the channel 40 subsequently bends to be integral with the generally horizontal plane of the swirler upstream of the channel exits 48.
In use, gas fuel is administered into the air/fuel mixing apparatus through the primary and secondary fuel connections 30,32. Fuel from the primary fuel connection passes through the primary fuel distributor 52 from which it enters the annular fuel manifold 125. Fuel spreads throughout the annular manifold 125 and enters the insert through-bore 116 at the openings 122. Inner insert seal 121 and outer insert seal 123 ensure that fuel does not escape from the insert other than via openings 122. Meanwhile, air from the compressor flows through the gap between the casing 10 and the air/fuel chamber 4 and enters the insert bell-mouth entrance 113 where it passes through the bell-mouth and into the through-bore. Mixing of air and fuel occurs in the through-bore 116 and channels 40 before the mixture exits the channels. Secondary fuel may be added, as with embodiments 1 and 2, before the mixture enters the pre-chamber in the same manner as therein described. In the secondary manifold, it is preferred that the majority of fuel entering the swirler 22 is admitted via primary fuel inlet 46 and that a much smaller amount of fuel is admitted via secondary fuel inlet 50. As shown for example in
The fuel is a fluid, most preferably a gas such as propane or natural gas. However, it may be a liquid such as diesel.
The secondary inlets may be arranged for proportionate or on/off flow. In one embodiment, when secondary inlets are on, 70% of flow may be through primary inlets and 30% through secondary inlets for pilot fuel, although this may be variable and may be different in other embodiments such as a 60:40 split.
As shown in
The swirler block 522 incorporates eight channels 500, although 10 or 12 may also be used or other numbers if desired. Each duct issues tangentially into the toroidal shaped chamber 510 which essentially has part-toroidal side surfaces 512 a flat back surface 514 and an open exit 518 leading into the chamber 516. Each premixing channel 500 incorporates a bell mouth 504 so that a repeatable discharge coefficient can be achieved across all of the channels 500 to ensure an even flow distribution. Located in each bell mouth are either three or four equi-spaced fuel injection points to provide for a uniform injection of fuel into the premix duct, although the number of injection points may be varied. The injection points are preferably equi-spaced peripherally around the bell mouth, as in the embodiment of
Various modifications may be made to the embodiments described without departing from the scope of the invention as defined by the following claims, as interpreted under patent law.
Claims
1-78. (canceled)
79. A mixing apparatus for mixing fuel and air for combustion in a gas turbine, the mixing apparatus comprising:
- a body having a mixing channel for mixing fuel and air for combustion, the mixing channel having a main channel portion and a distinct insert channel portion, a fuel inlet being located on the insert channel portion.
80. A mixing apparatus as claimed in claim 79, wherein:
- the fuel inlet is located in a portion of the insert channel portion having a curved cross section.
81. A mixing apparatus as claimed in claim 79, wherein:
- the insert channel portion comprises a plug attached to one end of the main channel portion, the plug being removable from the body.
82. A mixing apparatus as claimed in claim 79, wherein:
- the insert channel portion comprises a pre-calibrated insert of the mixing channel.
83. A mixing apparatus as claimed in claim 79, wherein:
- the mixing channel has a curved cross section upstream portion thereof and a transition portion merging to an exit portion with a rectangular cross section, the upstream portion of the mixing channel being tilted relative to the exit portion thereof.
84. A mixing apparatus as claimed in claim 79, wherein:
- the insert channel portion comprises a plug having several primary inlets spaced therearound.
85. A mixing apparatus as claimed in claim 79, wherein:
- the mixing channel has a bell-mouth entrance.
86. A mixing apparatus for mixing fuel and air for combustion in a gas turbine, the mixing apparatus comprising:
- a body having a mixing channel for mixing air and fuel, the mixing channel in one portion thereof having an at least partly curved cross section.
87. A mixing apparatus as claimed in claim 86, wherein:
- the one portion of the mixing channel has an elliptic cross section.
88. A mixing apparatus for mixing fuel and air for combustion in a gas turbine, the mixing apparatus comprising:
- a body having a mixing channel for mixing air and fuel, the mixing channel having a fuel inlet section which has a plurality of fuel inlets spaced around a periphery thereof.
89. A mixing apparatus as claimed in claim 79, wherein:
- the mixing channel has a height/width aspect ratio≦2.
90. An apparatus as claimed claim 86, wherein:
- the body includes a plurality of said mixing channels, the mixing channels being regularly spaced about a dominant axis of the body.
91. An apparatus as claimed claim 86, wherein:
- each mixing channel comprises a bore formed in the body of the apparatus.
92. An apparatus as claimed in claim 86, wherein:
- a plurality of primary fuel inlets are provided.
93. An apparatus as claimed in claim 86, wherein:
- a plurality of secondary fuel inlets are provided.
94. An apparatus as claimed in claim 93, wherein:
- the secondary fuel inlets have a shield for providing shielded pilot fuel injection.
95. An apparatus as claimed in claim 94, wherein:
- a configuration of the shield conforms to an outflow direction of a mixing channel.
96. An apparatus as claimed in claim 94, wherein:
- the shield comprises a circular plate for providing shielded flow in a radially inward direction from under the side plate.
97. An apparatus as claimed in claim 96, wherein:
- the plate includes at least one hole therethrough enabling pilot fuel to flow in an axial direction through said plate.
98. An apparatus as claimed in claim 86, wherein:
- the body has a back plate and each mixing channel is formed in a portion of the body upstanding from the back plate on a fuel side thereof.
99. An apparatus as claimed in claim 93, wherein:
- the secondary fuel inlets are adapted to admit fuel at a location outside the mixing channel part.
100. An apparatus as claimed in claim 93, wherein:
- the secondary fuel inlets are adapted to admit fuel into a zone of separated flow on the body.
101. A radial flow swirler for mixing air and fuel for combustion, the swirler comprising:
- a body having a primary fuel inlet and a secondary fuel inlet, the secondary fuel inlet being configured for direct injection of pilot fuel.
102. A swirler for mixing air and fuel for combustion, the swirler comprising:
- a body having a series of mixing channels, the mixing channels having a bell-mouthed entrance, the mixing channels being generally co-planar with one another and radially inwardly angled in order to induce swirl.
103. A swirler as claimed in claim 102, further including:
- a backplate adjacent the mixing channels.
104. A swirler as claimed in claim 102, wherein:
- each channel is an elliptical bore.
105. A combustor for burning fuel and air in a gas turbine engine, the combustor incorporating the mixing apparatus as claimed in claim 79.
106. A combustor as claimed in claim 105, wherein:
- the combustor has a cylindrical outer casing wall with an end plate, the mixing apparatus being located centrally on the end plate.
107. A method of calibrating a fuel mixer for mixing fuel and air in a gas turbine, the method comprising:
- providing a fuel/air mixing channel having a fuel inlet device formed with a fuel inlet;
- calibrating the fuel inlet device; and
- then installing the fuel inlet device on to the mixer.
108. A method as claimed in claim 107, wherein:
- calibrating the fuel inlet device includes calibrating the device with respect to fuel flow characteristics thereof.
109. An apparatus as claimed in claim 86, wherein:
- one or more secondary fuel inlets are provided shielded by an annular ring coaxial with a central axis of the apparatus discharging pilot fuel in a radially inward direction onto a back wall of the apparatus.
110. A swirler as claimed in claim 102, wherein:
- the mixing channel leads to a toroidal chamber.
111. A swirler for mixing fuel and air for combusting, the swirler comprising:
- a body having at least one mixing channel, wherein the mixing channel leads to a toroidal chamber.
112. A swirler as claimed in claim 111, wherein:
- the toroidal chamber has a same height as a height of the mixing channel.
Type: Application
Filed: Dec 22, 2003
Publication Date: Nov 16, 2006
Inventors: Robert Hicks (Hampshire), Eric Norster (Nottinghamshire)
Application Number: 10/540,372
International Classification: F23D 14/62 (20060101);