U.S. patent application number 10/540372 was filed with the patent office on 2006-11-16 for combustion device.
Invention is credited to Robert Hicks, Eric Norster.
Application Number | 20060257807 10/540372 |
Document ID | / |
Family ID | 9950357 |
Filed Date | 2006-11-16 |
United States Patent
Application |
20060257807 |
Kind Code |
A1 |
Hicks; Robert ; et
al. |
November 16, 2006 |
Combustion device
Abstract
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.
Inventors: |
Hicks; Robert; (Hampshire,
GB) ; Norster; Eric; (Nottinghamshire, GB) |
Correspondence
Address: |
Marcus P Dolce;Price Heneveld Cooper DeWitt & Litton
695 Kenmoor SE
Post Office Box 2567
Grand Rapids
MI
49501
US
|
Family ID: |
9950357 |
Appl. No.: |
10/540372 |
Filed: |
December 22, 2003 |
PCT Filed: |
December 22, 2003 |
PCT NO: |
PCT/GB03/05624 |
371 Date: |
May 10, 2006 |
Current U.S.
Class: |
431/354 |
Current CPC
Class: |
F23D 2900/14701
20130101; F23R 3/286 20130101; F23D 2900/14021 20130101 |
Class at
Publication: |
431/354 |
International
Class: |
F23D 14/62 20060101
F23D014/62 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2002 |
GB |
0230070.5 |
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.ltoreq.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.
Description
BACKGROUND OF THE INVENTION
[0001] 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
(NO.sub.2)-- 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.
[0002] 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.
[0003] 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 INVENTION
[0004] When 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.
[0005] 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.
[0006] 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
.ltoreq.1.25 or .ltoreq.1.0. This ratio is preferably .gtoreq.0.7.
The aspect ratio may be .ltoreq.2, .ltoreq.1.5, .ltoreq.1.2,
.gtoreq.0.5 and/or .gtoreq.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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] The mixing apparatus may be employed for gaseous fuel
although applications for liquid fuels are also envisaged.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The 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:
[0020] FIG. 1 is a view through a section of a combustor apparatus
according to a first preferred embodiment of the invention;
[0021] FIG. 2 is a detail of a portion of the apparatus of FIG.
1;
[0022] FIG. 3 is a plan view of a swirler of the apparatus of FIG.
1;
[0023] FIG. 4 is a sectional elevation of a combustor apparatus
according to a second preferred embodiment of the invention;
[0024] FIG. 5 is a detail of a portion of the apparatus of FIG.
5;
[0025] FIG. 6 is a plan view of a swirler of the apparatus of FIGS.
4 and 5;
[0026] FIG. 7A is a cross section of a mixing channel of the
apparatus of FIG. 2;
[0027] FIG. 7B is a cross section of a mixing channel according to
a modified embodiment of the invention;
[0028] FIG. 8 is a sectional elevation of a combustor apparatus
according to another embodiment of the invention;
[0029] FIG. 9 is a detail of parts of the apparatus of FIG. 8;
[0030] FIG. 10 shows a sectioned plan view and a side view of a
swirler of the apparatus of FIG. 8;
[0031] FIG. 11 shows a fuel metering means of the apparatus of FIG.
8.
[0032] FIG. 12 shows a partial perspective cross section through
the swirler plate of the embodiment of FIGS. 8 to 11;
[0033] FIG. 13 is a schematic view showing the various combustors
in accordance with the embodiments of the present invention which
may be incorporated in a gas turbine engine;
[0034] FIG. 14 shows a modified secondary/pilot inlet system,
having an annular recess and ring;
[0035] FIG. 15 is a cross section through mixing channels of a
swirler plate of a further embodiment of a combustor in accordance
with the invention;
[0036] FIG. 16 shows the swirler plate of FIG. 15 secured on a
combustion liner and with back plates for secondary fuel inlets and
an ignitor secured thereto;
[0037] FIGS. 17A to 17D show a modification of the embodiment of
FIG. 1 in which secondary/pilot fuel inlets are shielded;
[0038] FIGS. 18A to 18H show a different modification to the
embodiment of FIG. 1 in which a radial pilot/secondary fuel
arrangement is provided; and
[0039] FIGS. 19A to 19C show a modification to the embodiment of
FIG. 16 in which fully circular mixing ducts are used issuing
tangentially into a toroidal chamber.
[0040] FIG. 1 shows a sectional elevation through a combustor in
accordance with the invention. The combustor 1 comprises a casing
2, a liner (or combustion chamber) 4, a fuel manifold 6 and an
exhaust 8. The casing consists of an outer casing 10 having an
internal insulation 12, and a top flange 14. The casing 10 is
generally cylindrical with a rectangular shape in side section. The
combustion chamber 4 comprises a pre-chamber 16 expanding to the
main chamber 18. At an end of the combustion chamber opposite the
pre-chamber 16 is a contraction 20 that narrows to an exhaust 8.
The fuel manifold 6 comprises a swirler 22, fixedly attached to the
top flange 14 by one or more clamping studs 24 as shown in FIG.
2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] FIG. 1 and FIG. 3 show the swirler 22. In the embodiment
described, the swirler 22 consists of a circular plate 23 although
other shapes may be used. FIG. 3 shows a plan view of the swirler
22. The swirler comprises an outer annulus 34 inside of which is
located generally triangular wedge sections 36. The wedge sections
36 are, in the present embodiment, of a generally wedged shape,
having a longer side 42 and two shorter sides 38 and 44 in which a
shorter side 38 is curved to the same curvature as an inner
circumference 35 of the annulus 34. The wedge sections 36 are
fixedly attached to the annulus 34 at the curved side 38, and
arranged inside the annulus 34 so as to form channels 40 between
the straight longer side 42 of one wedge 36 and the straight
shorter side 44 of another. The channels 40 are thus inclined at an
angle to the swirler radius such that the longitudinal axis of a
channel 40 does not pass through the origin of the circular swirler
22. The channels 40 may be substantially straight or they may be
curved. The channels 40 exit into the pre-chamber 16. Ignitor 31 is
positioned off-center as in FIG. 8, although it may appear central
when viewed from certain directions e.g., FIG. 4. The ignitor 31 is
off-center since this will be cooler than a central location.
[0042] 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 FIG. 3. A secondary fuel inlet 50 for further admission of fuel
is located outside of the channel just beyond the exit 48. In this
embodiment, there is a secondary fuel inlet for every primary fuel
inlet. The primary fuel inlets 46 and secondary fuel inlets 50 are
located along the longitudinal axes of the channels 40 although
they could be positioned off axis. FIG. 1 shows a primary fuel
connection 30 and a secondary fuel connection 32 that are adapted
to admit fuel into the swirler 22. From FIG. 2, it is seen that the
primary fuel connection 30 is located within the top flange 14 such
that it may be accessed at one end 31. Its other end is in fluid
communication with a primary fuel distributor 52, which in turn is
also in fluid communication with the primary fuel inlet 46. A
plurality of fuel distributor seals 54 surround the primary fuel
distributor 52. The primary fuel connection 30, distributor 52 and
inlet 46 are aligned with each other in the present embodiment and
are positioned upstream of the channel 40.
[0043] 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.
[0044] 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.
[0045] The mixing channel cross section may be as in FIG. 7A or
FIG. 7B. The ratio of mixing channel height L along the combustor
axis to width W may in preferred embodiments be such that L/W is
more than or equal to 0.7 and less than or equal to 2.
[0046] 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 FIG. 4.
[0047] 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 FIG. 1 and
FIG. 4 passes through the gap towards the top flange 14, near which
it then enters the channel 40. Fuel is added to the air via primary
fuel inlet 46 in a predetermined and precalibrated amount so as to
produce a well-mixed mixture of air and fuel in the mixing channel
40. A boundary layer 13 forms at the mixing channel walls (FIG.
7A). The air/fuel mixture flows through the channel exit and enters
the pre-chamber 16. Upon immediately exiting the channel 40, the
boundary layer remains attached to a swirler/pre-chamber surface
(or back plate) 64. The orientation of the channels 40 imparts a
swirl motion to the mixture, causing the flow exiting the channels
to form a vortex as it is drawn into the pre-chamber 16. As the
vortex is formed, the boundary layer at the surface 64 separates,
creating a separation zone 65 on the surface 64. The secondary fuel
inlet 50 is placed outside of the channel 40 so as to provide a
pilot to avoid flame blow-out. In some embodiments of the
invention, the secondary fuel inlet 50 is placed in the separation
zone 65. It is believed that little mixing of air and fuel takes
place in the separation zone, hence the zone may have a stable
flame that is far less likely to blow out than the flame of the
well mixed air/fuel mixture. The introduction of secondary fuel at
this region 65 also acts as a pilot to reduce the chance of flame
blow-out elsewhere in the pre-chamber.
[0048] 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.
[0049] The channels 40 may be of rectangular cross section and may
have an aspect ratio as shown in FIG. 7A, in which the channel
height (L) 66 to width (W) 68 ratio is 1.25 to 1. However, in
another embodiment of the invention, the channel height 66' to
width 68' ratio is less than or equal to 1.0 as shown in FIG. 7B.
FIGS. 7A and 7B are views along the longitudinal axis of the mixing
channel. These low aspect ratios enable good mixing since fuel may
be injected substantially right across the channel to the wall
opposite inlet 46.
[0050] A further embodiment of the invention is shown in FIGS. 8 to
11. FIG. 11 shows a schematic of a fuel-metering insert 100. The
insert 100 comprises an annular plug of elliptical cross section,
and preferably of circular cross section. The insert 100 has a
bell-mouth flange 110 at one end thereof. The flange protrudes
outwardly away from the longitudinal axis 112 and defines a
bell-mouth entrance 113. The bore wall 114 (FIG. 9) has a threaded
section 118 around its peripheral surface 120. In the remaining
section of the bore wall 114 is an opening 122 (FIG. 11) providing
a through-bore from the inner wall 117 to the outer wall 120 of the
bore wall 114. In the present embodiment there are four such
openings. The fuel-metering insert 100 is insertable in and
removable from the air/fuel mixing apparatus 1. The channel 40
includes an inlet 125. In this embodiment of the invention, the
channel 40 includes a portion 130 at the inlet end thereof, which
is threaded according to the opposite of the insert threaded
portion 118. The fuel-metering insert 100 is threadably insertable
into the channel inlet 125. The channel 40 is profiled
substantially according to the female of the male insert outer
surface 120 profile such that a close fit between the two
components is ensured upon threadably inserting the insert 100 into
the channel 40 of the air/fuel mixing apparatus 1. The insert is
removed by unscrewing the insert from the channel 40. The insert
openings 122 may thus be calibrated for fuel admission in a
convenient manner whilst the insert is removed from the apparatus.
Whilst it is removed, another calibrated insert may replace it,
allowing the continued use of the apparatus and minimizing the
downtime during which the apparatus cannot be used.
[0051] 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.
[0052] 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 FIG. 12
[0053] 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.
[0054] 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 FIG. 9, the secondary inlet 50 is
configured at an angle to inject fuel with a component along the
back wall 150 of the swirler. This component is aligned with a
corresponding mixing channel's direction and central axis. This
allows the secondary or pilot fuel to remain near the back wall as
a source of rich mixture which assists in preventing unwanted flame
out. As shown by dotted lines in FIGS. 9 and 10A, one or more of
the secondary inlets 50 may (in addition to or as an alternative to
having the angled configuration) be provided by a shield 152 having
an outlet 154 facing away from a co-operating mixing channel 40,
the shield 152 thus conforming to an exit direction of the channel
40. The shield serves to improve resistance to unwanted flame out
and is useful for hot starts. The shield may be welded in position
or cast as part of the back wall 260. In one embodiment, four
shielded pilots (secondary inlets 50) are used out of a total of
eight pilots. As a further alternative, the back wall 260 may be
provided with a recessed ring for all pilots, e.g., all eight
pilots when eight are used, as indicated schematically in FIG. 14
in which an annular ring 160 coaxial with the swirler central axis
162 is fitted in the region of eight secondary, pilot inlets which
are located in an annular recess 164. This provides a radially
inward flow of pilot fuel along the back wall 260.
[0055] The fuel is a fluid, most preferably a gas such as propane
or natural gas. However, it may be a liquid such as diesel.
[0056] FIG. 13 shows schematically the configuration of the
combustor 1 in a gas turbine engine 200' having a main air
compressor 202' connected to a shaft 204' to a turbine 206' and an
alternator 208'. A gas boost compressor 210' is provided between
gas fuel 212' and the combustor 1. The compressor 202' is fed by an
air inlet 214', and the turbine exhausts and exhaust conduit 216'.
The gas turbine engine may be recuperated in a known way.
[0057] FIGS. 15 and 16 show a revised version of the embodiment of
FIGS. 8 to 12. As shown by the cross-sectional view of FIG. 15,
eight mixing channels 40 are shown in a swirler casting 200. The
mixing panels each have an upstream circular section 202 merging
into a rectangular downstream section 204. Inserted into the
entrance 206 of each mixing channel 40 is a bell-mouthed circular
insert 208 having four primary inlets for fuel 210 spaced equally
around the inside thereof, three of which are shown in the
cross-sectional view of FIG. 16. The primary inlets 210 of the
eight mixing channels 40 are connected via manifold channels 212,
cross sections of which are shown in FIG. 15, the manifold channels
212 being supplied by a supply port 214, shown schematically in
FIG. 16. A secondary or pilot fuel supply plate 216 and a further
back plate 218 are fixed to the swirler casting 200 by bolts 220.
One of several secondary fuel inlets 222 is shown schematically in
FIG. 16, this secondary fuel inlet having an angled injection
conduit 224 like the one shown in FIG. 9. In addition to or
alternatively to this, one or more secondary inlets may be provided
with a shield or may be located in an annular ring-type shield. An
off-center ignitor 228 is secured in position on the back plate
218. The supply plate 216 includes a fuel supply gallery/manifold
(not shown) for the various secondary/pilot fuel inlets 222. The
swirler casting 200, back plate 216 and back plate 218 effectively
replace the top flange 14 and swirler 22 shown in FIG. 8 and FIG.
16 shows how these components are fitted on the liner 4. In the
embodiment of FIGS. 15 and 16, the mixing channels 40 and the
bell-mouthed circular inserts 208 are arranged perpendicular to the
central axis of the swirler casting 200 and this can be contrasted
with the arrangement in FIG. 8 in which the mixing channel inlets
are tilted. However, in other embodiments, it is envisaged that a
cast swirler plate like the one shown in FIGS. 15 and 16 could have
tilted mixing channel inlets.
[0058] 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.
[0059] As shown in FIG. 17A to 17E, the swirler 22 may be replaced
with a swirler 322 in which each or some of the secondary fuel
inlets are shielded by shields 300, perspective and cross-sectional
views of each shield 300 being shown in FIGS. 17D and 17E
respectively. In the embodiment shown, eight secondary/pilot fuel
inlets 350 are provided, with four of the inlets 350 being shielded
by shields 300. Each shield is aligned with an axis of a
corresponding mixing channel 340 with a shield fuel exit aperture
facing away from the direction of incoming flow through the
corresponding mixing channel 340.
[0060] FIGS. 18A to 18H show a different modification to the
swirler 22 of FIG. 1. In this case, the swirler 422 is modified as
shown in FIGS. 18G and 18H by providing a deflector plate 424 over
a series of eight pilot outlets 450. Eight pilot outlets 450 are
provided. The deflector plate 424 is circular as shown in FIGS. 18E
and 18D. The plate 424 is provided with a slight lip 426 which
provides a gap between outlets 450 and a rear face 428 of the plate
424. The deflector plate forces pilot flow radially inwards towards
the center of the swirler 422. A possible extension to this
arrangement is to provide one or more relief holes in the plate 424
so that pilot flow is split between radial and axially fuel flow to
achieve best performance. In this case, a small relief hole may be
drilled or otherwise provided through the plate 424 at each
location axial aligned with or near to one or more or all of the
secondary/pilot fuel outlets 450. The shields 300 or deflector
plate 424 provide improved performance and a stable flame.
[0061] FIGS. 19A to 19C show a modification to the embodiment of
FIG. 16 in which completely circularly cylindrical mixing channels
500 are provided. The circular mixing channels or premixing ducts
500 exit tangentially into a toroidal space 502. The mixing
channels 500 can readily accommodate bell mouth shaped entrances
504 like the bell mouth shown in FIG. 16. For clarity, the bell
mouth inserts are shown removed from FIG. 19A but in practice would
be similar to those shown in FIG. 16. The schematic views of FIGS.
19D and 19C show bell mouths 504. Curved lines 506 in FIG. 19B
indicate the line of contact 508 between circular mixing channels
500 and toroidal surface 510, lines 507 schematically providing a
similar impression in side elevation in FIG. 19C. Circular ducts
500 can be manufactured by simple drilling and reaming operations.
Fuel placement is easier in a circular duct than a rectangular duct
due to the absence of corners where excessive fuel can get trapped.
The toroidal geometry provided by the surface 510 provides a smooth
transition from circular premix channels 500 into toroidal chamber
510. The swirling flow generated in the toroidal chamber 510 is
then accelerated into a cylindrical pre-chamber 516. The toroidal
chamber prevents significant flow separations and recirculations at
the duct/chamber interface and therefore prevents unsteadiness in
the bulk swirling flow within the chamber 510/516 and unsteadiness
leading to the generation of unacceptable combustion oscillations
and possible flashbacks, as well as the presence dead-zone-induced
auto-ignition as fuel trapped within such regions may experience
excessively long residence times.
[0062] 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 FIG. 16. The height of the
toroidal chamber is set to be equal to the diameter of the premix
ducts so that a smooth transition from duct 500 to chamber 510 is
achieved without flow separation. The arrangement of tangential
ducts and toroid chamber results in the formation of a stable
swirling motion within the chamber 510. The swirling motion is
sufficiently strong that a recirculation zone is established within
the combustor to provide a stabilizing mechanism for the flame. The
swirling flow accelerates out of the toroidal chamber 510 into the
cylindrical pre-chamber 516 prior to issuing into the main
combustor. Accordingly, it will be appreciated that the swirler
plate 522 may essentially replace the equivalent plate shown in
FIG. 16 in the overall combustor arrangement. A shielded or radial
deflector arrangement may be used as desired with the embodiment
shown in FIGS. 19A to 19C.
[0063] 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.
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