Gas turbine engine combustion chamber

A combustion chamber which has a primary combustion zone and a secondary combustion zone is provided with a plurality of secondary fuel and air mixing ducts arranged around the primary combustion zone. The secondary fuel and air mixing ducts are defined by a pair of annular walls and by a plurality of walls extending radially between the annular walls. Each secondary fuel and air mixing duct has an aperture to direct a fuel and air mixture into the secondary combustion zone. The apertures have the same flow area. Each secondary fuel and air mixing duct has one or more fuel injectors to inject fuel into the upstream end of the secondary fuel and air mixing duct. This arrangement ensures that the fuel/air ratio emitted from each aperture is within 3.0% of the mean fuel/air ratio of all the apertures even though the air flow to the secondary fuel and air mixing ducts is non-uniform

This application claims benefit of international application PCT/GB 
94/01135 filed May 24, 1994. 
FIELD OF THE INVENTION 
The present invention relates to a gas turbine engine combustion chamber. 
BACKGROUND OF THE INVENTION 
In order to meet the emission level requirements, for industrial low 
emission gas turbine engines, staged combustion is required in order to 
minimise the quantity of the oxides of nitrogen (NOx) produced. Currently 
the emission level requirement is for less than 25 volumetric parts per 
million of NOx for an industrial gas turbine exhaust. The fundamental way 
to reduce emissions of nitrogen oxides is to reduce the combustion 
reaction temperature, and this requires premixing of the fuel and all the 
combustion air before combustion takes place. The oxides of nitrogen (NOx) 
are commonly reduced by a method which uses two stages of fuel injection. 
Our UK patent no. 1489339 discloses two stages of fuel injection to reduce 
NOx. Our International patent application no. WO92/07221 discloses two and 
three stages of fuel injection. In staged combustion, all the stages of 
combustion seek to provide lean combustion and hence the low combustion 
temperatures required to minimise NOx. The term lean combustion means 
combustion of fuel in air where the fuel to air ratio is low i.e. less 
than the stoichiometric ratio. In order to achieve the required low 
emissions of NOx and CO it is essential to mix the fuel and air uniformly 
so that it has less than a 3.0% variation from the mean concentration 
before the combustion takes place. 
The industrial gas turbine engine disclosed in our International patent 
application no. WO92/07221 uses a plurality of tubular combustion 
chambers, whose longitudinal axes are arranged in generally radial 
directions. The inlets of the tubular combustion chambers are at their 
radially outer ends, and transition ducts connect the outlets of the 
tubular combustion chambers with a row of nozzle guide vanes to discharge 
the hot exhaust gases axially into the turbine sections of the gas turbine 
engine. Each of the tubular combustion chambers has an annular secondary 
fuel and air mixing duct which surrounds the primary combustion zone. A 
plurality of equi-spaced secondary fuel injectors are arranged to inject 
fuel into the upstream end of the annular secondary fuel and air mixing 
duct. The annular secondary fuel and air mixing duct has a plurality of 
equi-spaced outlet apertures to direct the fuel and air mixture into the 
secondary combustion zone. Each of the tubular combustion chambers of the 
three stage variant also has an annular tertiary fuel and air mixing duct 
which surrounds the secondary combustion zone. A plurality of equi-spaced 
tertiary fuel injectors are arranged to inject fuel into the upstream end 
of the annular tertiary fuel and air mixing duct. The annular tertiary 
fuel and air mixing duct has a plurality of outlet apertures to direct the 
fuel and air mixture into the tertiary fuel and air mixing zone. 
Unfortunately the flow of air into the tubular combustion chambers is not 
uniform, this is because of an asymmetric flow of air from a diffuser at 
the downstream end of the gas turbine engine compressor to the tubular 
combustion chambers. Each of the secondary fuel injectors passes identical 
fuel flows and therefore a non uniform fuel and air mixture is created at 
the points of injection due to the non uniform air flow. The fuel and air 
mixture directed from the outlet apertures into the secondary combustion 
zone is non uniform. Similarly the fuel and air mixture directed from the 
outlet apertures of the tertiary mixing duct into the tertiary combustion 
zone will be non uniform. This increases the emissions of NOx to above the 
acceptable levels. 
An initial solution for the problem was to redistribute the fuel to match 
the air mass flow distribution by adjusting the fuel hole sizes of the 
individual fuel injectors. This requires all of the fuel injectors to be 
unique in fuel hole diameters and position of the fuel holes to match the 
air mass flow to achieve the required uniformity of mixing. The air mass 
flow distribution also varies with the operating power range of the 
engine. However redistributing the fuel to match the air mass flow 
distribution would not achieve the required 3.0% variation in 
concentration uniformity at all powers and hence emissions of NOx would be 
above the acceptable levels. 
Another solution for the problem was to fit air guidance devices upstream 
of the secondary fuel and air mixing duct, and tertiary fuel and air 
mixing duct, to create a uniform air mass flow at the intakes of the 
secondary fuel and air mixing duct, and tertiary fuel and air mixing duct. 
Unfortunately any minor changes in the air guidance devices formed during 
the production processes result in relatively large changes in air mass 
flow distribution i.e. greater than the 3.0% variation in concentration 
uniformity. 
A further solution for the problem was to redistribute the air mass flow 
upstream of the intakes of the secondary fuel and air mixing duct, and 
tertiary fuel and air mixing duct, using a flow distributor which uses its 
pressure drop to create uniform flow through each of its flow routes. 
Unfortunately an increase in system pressure drop is not acceptable 
because this reduces the surge margin of the compressor and also reduces 
the thermal efficiency of the engine i.e. increases the engine fuel 
consumption. 
The only acceptable solution therefore must be tolerant to upstream air 
flow variations without increasing the system pressure loss. 
EPO388886A discloses a combustor for burning of fuel by premixing fuel with 
air in a number of premix flame forming nozzles which inject the premixed 
fuel and air into a secondary combustion zone. Fuel injectors are provided 
to inject fuel into the premix flame forming nozzles downstream of the 
intakes of the premix flame forming nozzles. 
The present invention seeks to provide a novel gas turbine engine 
combustion chamber which overcomes the above mentioned problem. 
Accordingly the present invention provides a gas a turbine engine 
combustion chamber comprising a primary combustion zone defined by at 
least one peripheral wall and an upstream end wall connected to the 
upstream end of the at least one peripheral wall, the upstream end wall 
has at least one aperture, primary air intake means and primary fuel 
injector means to supply air and fuel respectively through the at least 
one aperture into the primary combustion zone, a secondary combustion zone 
in the interior of the combustion chamber downstream of the primary 
combustion zone, means to define a plurality of secondary fuel and air 
mixing ducts, each secondary fuel and air mixing duct has an outlet at its 
downstream end for discharging the fuel and air mixture into the secondary 
combustion zone, each secondary fuel and air mixing duct has secondary air 
intake means at its upstream end to supply air into the secondary fuel and 
air mixing duct, each secondary fuel and air mixing duct has secondary 
fuel injector means arranged to supply fuel into the secondary fuel and 
air mixing duct, each secondary fuel injector means is located downstream 
of the secondary air intake means of the associated secondary fuel and air 
mixing duct, the outlets of the secondary fuel and air mixing ducts have 
substantially equal flow areas to produce substantially the same air flow 
rate through each of the secondary fuel and air mixing ducts, the 
secondary fuel injector means of each secondary fuel and air mixing duct 
is arranged to supply substantially the same flow rate of fuel so that the 
fuel to air ratio of the mixture leaving each of the secondary fuel and 
air mixing ducts is substantially the same. 
Preferably the secondary fuel and air mixing ducts radially inwardly of the 
primary combustion zone, the secondary fuel and air mixing ducts are 
defined at their radially inner extremity and radially outer extremity by 
a second pair of walls and a plurality of walls extending radially between 
the second pair of annular walls. 
Preferably at least one of the secondary fuel injector means comprises a 
hollow cylindrical member, the hollow cylindrical member has a plurality 
of apertures spaced apart axially along the cylindrical member to inject 
fuel into the secondary fuel and air mixing duct. 
The hollow cylindrical member may extend axially with respect to the axis 
of the combustion chamber. The hollow cylindrical member may extend 
radially with respect to the axis of the combustion chamber. The apertures 
in the hollow cylindrical member may be arranged to direct the fuel 
circumferentially. 
Preferably the walls extending radially between the annular walls are 
secured to both the annular walls. 
Preferably the secondary fuel injector means for at least one of the 
secondary fuel and air mixing ducts comprises two secondary fuel 
injectors. The two secondary fuel injectors may be spaced apart 
circumferentially relative to the axis of the combustion chamber. 
Preferably each secondary fuel injector is arranged to supply fuel to the 
upstream end of the associated secondary fuel and air mixing duct. 
Preferably the combustion chamber includes means to define a plurality of 
tertiary fuel and air mixing ducts, each tertiary fuel and air mixing duct 
is in fluid communication at its downstream end with a tertiary combustion 
zone in the interior of the combustion chamber downstream of the secondary 
combustion zone, each tertiary fuel and air mixing duct has tertiary air 
intake means at its upstream end to supply air into the tertiary fuel and 
air mixing duct, each tertiary fuel and air mixing duct has tertiary fuel 
injector means arranged to inject fuel into the tertiary fuel and air 
mixing duct, the tertiary fuel and air mixing ducts are arranged in an 
annulus outside the peripheral wall, each tertiary fuel injector means is 
located downstream of the tertiary air intake means of the associated 
tertiary fuel and air mixing duct, each tertiary fuel and air mixing duct 
has an outlet at its downstream end for discharging the fuel and air 
mixture into the tertiary combustion zone, the outlets of the tertiary 
fuel and air mixing ducts have substantially equal flow areas to produce 
substantially the same air flow rate through each of the tertiary fuel and 
air mixing ducts, the tertiary fuel injector means of each tertiary fuel 
and air mixing duct is arranged to supply substantially the same flow rate 
of fuel so that the fuel to air ratio of the mixture leaving each of the 
tertiary fuel and air mixing ducts is substantially the same. 
Preferably the tertiary fuel and air mixing ducts are defined by a radially 
inner annular wall, a radially outer annular wall and a plurality of walls 
extending radially between the pair of annular walls, the radially 
extending walls are secured to at least one of the pair of annular walls. 
Preferably the tertiary fuel and air mixing ducts are arranged around the 
combustion chamber. 
The combustion chamber may be tubular, the peripheral wall of the primary 
combustion zone is annular and the upstream end wall has a single 
aperture, the plurality of tertiary fuel and air mixing ducts are arranged 
circumferentially in an annulus radially outwardly of the secondary 
combustion zone. 
Preferably at least one of the tertiary fuel injector means comprises a 
hollow cylindrical member, the hollow cylindrical member has a plurality 
of apertures spaced apart axially along the cylindrical member to inject 
fuel into the tertiary fuel and air mixing duct. 
The hollow cylindrical member may extend axially with respect to the axis 
of the combustion chamber. The hollow cylindrical member may extend 
radially with respect to the axis of the combustion chamber. The apertures 
in the hollow cylindrical member may be arranged to direct the fuel 
circumferentially. 
Preferably the tertiary fuel injector means for at least one of the 
tertiary fuel and air mixing ducts comprises two tertiary fuel injectors. 
The two tertiary fuel injectors may be spaced apart axially relative to 
the axis of the combustion chamber. The two tertiary fuel injectors may be 
spaced apart circumferentially relative to the axis of the combustion 
chamber. 
The present invention also provides a gas turbine engine combustion chamber 
comprising a primary combustion zone defined by at least one peripheral 
wall and an upstream end wall connected to the upstream end of the at 
least one peripheral wall, the upstream end wall has at least one 
aperture, primary air intake means and primary fuel injector means to 
supply air and fuel respectively through the at least one aperture into 
the primary combustion zone, a secondary combustion zone defined by a 
downstream portion of the at least one peripheral wall, the secondary 
combustion zone is in the interior of the combustion chamber downstream of 
the primary combustion zone, secondary air intake means and secondary fuel 
injector means to supply air and fuel respectively into the secondary 
combustion zone, means to define a plurality of tertiary fuel and air 
mixing ducts, each tertiary fuel and air mixing duct is in fluid flow 
communication at its downstream end with a tertiary combustion zone in the 
interior of the combustion chamber downstream of the secondary combustion 
zone, each tertiary fuel and air mixing duct has tertiary air intake means 
at its upstream end to supply air into the tertiary fuel and air mixing 
duct, each tertiary fuel and air mixing duct has tertiary fuel injector 
means arranged to supply fuel into the tertiary fuel and air mixing duct, 
each tertiary fuel injector means is located downstream of the tertiary 
air intake means of the associated tertiary fuel and air mixing duct, each 
tertiary fuel and air mixing duct has an outlet at its downstream end for 
discharging the fuel and air mixture into the tertiary combustion zone, 
the outlets of the tertiary fuel and air mixing ducts have substantially 
equal flow areas to produce substantially the same air flow rate through 
each of the tertiary fuel and air mixing ducts, the tertiary fuel injector 
means of each fuel and air mixing duct is arranged to supply substantially 
the same flow rate of fuel so that the fuel to air ratio of the mixture 
leaving each of the tertiary fuel and air mixing ducts is substantially 
the same. 
Preferably the tertiary fuel and air mixing ducts are arranged around the 
combustion chamber. 
Preferably the tertiary fuel and air mixing ducts are arranged in an 
annulus outside the peripheral wall, the tertiary fuel and air mixing 
ducts are defined by a radially inner annular wall, a radially outer 
annular wall and a plurality of walls extending radially between the pair 
of annular walls, the radially extending walls are secured to at least one 
of the pair of annular walls.

An industrial gas turbine engine 10, shown in FIG. 1, comprises in axial 
flow series an inlet 12, a compressor section 14, a combustion chamber 
assembly 16, a turbine section 18, a power turbine section 20 and an 
exhaust 22. The turbine section 18 is arranged to drive the compressor 
section 14 via one or more shafts (not shown). The power turbine section 
20 is arranged to drive an electrical generator 26 via a shaft 24. 
However, the power turbine section 20 may be arranged to provide drive for 
other purposes. The operation of the gas turbine 10 is quite conventional, 
and will not be discussed further. 
The Combustion chamber assembly 16 is shown more clearly in FIGS. 2 to 5. A 
plurality of compressor outlet guide vanes 28 are provided at the axially 
downstream end of the compressor section 14, to which is secured at their 
radially inner ends an inner annular wall 30 which defines the inner 
surface of an annular chamber 32. A first passage 38 of a split diffuser 
is defined between an annular wall 34 and the upstream end of the inner 
annular wall 30 and a second passage 40 of the split diffuser is defined 
between the annular wall 34 and a further annular wall 36. The downstream 
end of the inner annular wall 30 is secured to the radially inner ends of 
a row of nozzle guide vanes 42 which direct hot gases from the combustion 
chamber assembly 16 into the turbine section 18. 
The combustion chamber assembly 16 comprises a plurality of, for example 
nine, equally circumferentially spaced tubular combustion chambers 44. The 
axes of the tubular combustion chambers 44 are arranged to extend in 
generally radial directions. The inlets of the tubular combustion chambers 
44 are at their radially outermost ends and their outlets are at their 
radially innermost ends. 
Each of the tubular combustion chambers 44 comprises an upstream wall 46 
secured to the upstream end of an annular wall 48. A first, upstream, 
portion 50 of the annular wall 48 defines a primary combustion zone 52, 
and a second, downstream, portion 54 of the annular wall 48 defines a 
secondary combustion zone 56. The second portion 54 of the annular wall 48 
has a greater diameter than the first portion 50. The downstream end of 
the first portion 50 has a frustoconical portion 58 which reduces in 
diameter to a throat 60. A third frustoconical portion 62 interconnects 
the throat 60 at the downstream end of the first portion 50 and the 
upstream end of the second portion 54. 
A plurality of equally circumferentially spaced transition ducts 64 are 
provided, and each of the transition ducts 64 has a circular cross-section 
at its upstream end. The upstream end of each of the transition ducts 64 
is located coaxially with the downstream end of a corresponding one of the 
tubular combustion chambers 44, and each of the transition ducts 64 
connects and seals with an angular section of the nozzle guide vanes 42. 
A plurality of cylindrical casings 66 are provided, and each cylindrical 
casing 66 is located coaxially around a respective one of the tubular 
combustion chambers 44. Each cylindrical casing 66 is secured to a 
respective boss 68 on an annular engine casing 70. A number of chambers 72 
are formed between each tubular combustion chamber 44 and its respective 
cylindrical casing 66. 
The upstream end of each transition duct 64 and the downstream end of a 
corresponding tubular combustion chamber 44 are located in a respective 
annular mounting structure 74 which is secured to one of the bosses 68 by 
one of the cylindrical casings 66. The annular mounting structure 74 is 
provided with apertures 76 to allow the flow of air from chamber 32 into 
the chambers 72. 
The upstream wall 46 of each of the tubular combustion chambers 44 has an 
aperture 78 to allow the supply of air and fuel into the primary 
combustion zone 52. A first radial flow swirler 80 is arranged coaxially 
with the aperture 78 in the upstream wall 46 and a second radial flow 
swirler 82 is arranged coaxially with the aperture 78 in the upstream wall 
46. The first radial flow swirler 80 is positioned axially downstream, 
with respect to the axis of the tubular combustion chamber, of the second 
radial flow swirler 82. The first radial flow swirler 80 has a plurality 
of fuel injectors 84, each of which is positioned in a passage formed 
between two vanes of the swirler. The second radial flow swirler 82 has a 
plurality of fuel injectors 86, each of which is positioned in a passage 
formed between two vanes of the swirler. The first and second radial flow 
swirlers 80 and 82 are arranged such they swirl the air in opposite 
directions. For a more detailed description of the use of the two radial 
flow swirlers and the fuel injectors positioned in the passages formed 
between the swirl vanes see our International Patent Application No 
WO92/07221. The primary fuel and air is mixed together in the passages 
between the vanes of the first and second radial flow swirlers 80 and 82. 
A plurality of secondary fuel and air mixing ducts 88 are provided for each 
of the tubular combustion chambers 44. The secondary fuel and air mixing 
ducts 88 are arranged circumferentially in an annulus around the primary 
combustion zone 52. Each of the secondary fuel and air mixing ducts is 
defined between a second annular wall 90, a third annular wall 92 and by 
walls 94 which extend radially between the second and third annular walls 
90 and 92. The second annular wall 90 defines the radially outer extremity 
of each of the secondary fuel and air mixing ducts 88 and the third 
annular wall 92 defines the radially inner extremity of each of the 
secondary fuel and air mixing ducts 88. The walls 94 separate the 
individual secondary fuel and air mixing ducts 88. The axially upstream 
end 96 of the third annular wall 92 is curved radially outwardly so that 
it is spaced axially from the upstream end of the second annular wall 90. 
The upstream end of the third annular wall 92 is secured to a side plate 
of the first radial flow swirler 80. Each of the secondary fuel and air 
mixing ducts 88 has a secondary air intake 98 defined axially between the 
upstream end of the second annular wall 90, the upstream end of the third 
annular wall 92 and the upstream ends of the walls 94 which also extend 
axially between the second and third annular walls 90 and 92 respectively 
at this position. For example sixteen secondary fuel and air mixing ducts 
88 are provided. 
A plurality of secondary fuel injectors 100 are provided, at least one 
secondary fuel injector 100 is provided per secondary fuel and air mixing 
duct 88. Each of the secondary fuel and air injectors 100 comprises a 
hollow cylindrical member which extends axially with respect to the 
tubular combustion chamber 44. Each of the hollow cylindrical members 100 
passes through the upstream end of the third annular wall 92 to supply 
fuel into the upstream end of the secondary fuel and air mixing duct 88. 
The hollow cylindrical member is provided with a plurality of apertures 
102 through which the fuel is injected into the secondary fuel and air 
mixing duct 88. The apertures 102 are of equal diameters and are spaced 
apart axially along the hollow cylindrical member at suitable positions, 
and the apertures 102 in the hollow cylindrical member are arranged at 
diametrically opposite sides of the hollow cylindrical member so that the 
fuel injectors 100 are arranged to inject the fuel circumferentially with 
respect to the axis of the tubular combustion chamber 44. In this example 
two fuel injectors 100 are provided for each secondary fuel and air mixing 
duct 88. The secondary fuel injectors are spaced apart circumferentially 
with respect to the axis of the tubular combustion chamber 44. 
Each second and third annular wall 90 and 92 is arranged coaxially around 
the first portion 50 of the annular wall 48. At the downstream end of each 
secondary fuel and air mixing duct 88, the second and third annular walls 
90 and 92 are secured to the respective third frustoconical portion 62, 
and each frustoconical portion 62 is provided with a plurality of 
equi-circumferentially spaced apertures 104 which are arranged to direct 
fuel and air into the secondary combustion zone 56 in the tubular 
combustion chamber 44, in a downstream direction towards the axis of the 
tubular combustion chamber 44. The apertures 104 may be circular or slots. 
Each of the apertures 104 is arranged to allow the fuel and air mixture 
from one of the secondary fuel and air mixing ducts 88 to flow into the 
secondary combustion zone 56. The apertures 104 are of equal flow area. 
The operation of the gas turbine combustion chamber is substantially as 
described in our International Patent Application No WO92/07221 and this 
should be consulted for a more complete description. 
The use of a single annular secondary fuel and air mixing duct in our 
International Patent Application No WO92/07221 results in an air and fuel 
mixture which has a variation in concentration of more than 3.0% from the 
mean concentration and this results in NOx levels greater than 25 volume 
parts per million (vppm). 
The use of a plurality of secondary fuel and air mixing ducts each of which 
has an aperture into the secondary combustion zone enables the air and 
fuel mixture to have a variation in concentration less than the 3.0% from 
the mean concentration and hence results in NOx less than 25 vppm. 
The mass flow rate through each secondary fuel and air mixing duct 88 is 
dominated by the aperture 104 exit area and the pressure drop across it. 
The exit areas of the apertures 104 are controlled to be within 1.0% more, 
or less of the required flow area and the upstream velocity/pressure 
variations are negligible compared to the pressure across the exit area of 
the aperture 104. This results in the air mass flow entering each 
secondary fuel and air mixing duct 88 being within 1.0% more, or less, of 
the mean mass flow through all of the fuel and air mixing ducts 88. Each 
duct 88 is supplied by two secondary fuel injectors 100, each of which is 
within 2.0% of the mean area, the overall resultant concentration is 
within 3.0% of the mean concentration. This arrangement ensures that the 
fuel/air ratio emitted from each aperture 104 is within 3.0% of the mean 
fuel/air ratio of all the apertures 104. The arrangement has been tested 
and has produced NOx and CO exhaust emissions of less that 10 vppm 
throughout its full operating power range, ie at temperatures in the 
secondary combustion zone of 1600.degree. K. to 1750.degree. K. 
A feature of the invention is that the adjacent mixing ducts share a common 
wall. The walls 94 separating the individual secondary fuel and air mixing 
ducts 88 extend from the secondary air intake 98 at their upstream ends 
all the way to the frustoconical portion 62 and the walls 94 are secured 
to the frustoconical portion 62. Also the walls 94 extend radially between 
and are secured to both the annular walls 90 and 92. Thus the secondary 
fuel and air mixing ducts 88 are completely separated mechanically by the 
walls 94. 
The use of the secondary annular mixing duct which is subdivided by 
radially extending walls 94 creates uniform fuel and air mixtures, 
independent of upstream air maldistributions. The fuel and air mixture is 
injected as discrete jets into the secondary combustion zone 52. The 
secondary annular mixing duct subdivided by the radially extending walls 
94 creates the minimum amount of blockage and flow disturbance to the 
airflow around the combustion chamber. This is of particular importance to 
the tubular combustion chambers whose axis are arranged in generally 
radial directions, because the air flow has to turn through 180.degree.. 
This arrangement of the secondary fuel and air mixing ducts 88 has a 
minimum diameter increase greater than the primary combustion zone 52, to 
create the maximum annular flow area between the outer annular wall 90 of 
the secondary fuel and air mixing duct 88 and the cylindrical casing 66 in 
the chambers 72. The air flow to the secondary fuel and air mixing ducts 
88 in the chamber 72 is counter to the flow in the secondary fuel and air 
mixing ducts 88, and the air flow in the chamber 72 is at a low velocity 
to create a high flow acceleration into the secondary fuel and air mixing 
ducts 88 in order to prevent flow separation as the air flow turns through 
180.degree.. 
The invention has been described with reference to staged combustion in 
tubular combustion chambers, it may also be applied to staged combustion 
in annular combustion chambers as shown in FIG. 6. An annular combustion 
chamber 110 has an annular primary combustion zone 52 and an annular 
secondary combustion zone 56 defined between a radially outer annular wall 
46 and a radially inner annular wall 146. A plurality of secondary fuel 
and air mixing ducts 88 are arranged in a first annulus radially outwardly 
of the annular primary combustion zone 52 and a plurality of secondary 
fuel and air mixing ducts 88 arranged in a second annulus radially 
inwardly of the annular primary combustion zone 52. The secondary fuel and 
air mixing ducts 88 are defined between two annular walls 90 and 92 and by 
walls 94 extending radially between the walls 90 and 92. A fuel injector 
100 is positioned at the upstream end of each secondary fuel and air 
mixing duct 88, and extends radially with respect to the axis of the 
combustion chamber 110. The secondary fuel and air mixing ducts 188 are 
defined between two annular walls 190 and 192 and by walls 194 extending 
radially between the walls 190 and 192. A fuel injector 200 is positioned 
at the upstream end of each secondary fuel and air mixing duct 188, and 
extends radially with respect to the axis of the combustion chamber 110. 
Each of the secondary fuel and air mixing ducts 88 communicates via a 
respective aperture 104 in the annular wall 46 to allow the fuel and air 
mixture to flow into the secondary combustion zone 56. The apertures 104 
are of equal flow area. Each of the secondary fuel and air mixing ducts 
188 communicates via a respective aperture 204 in the annular wall 146 to 
allow the fuel and air mixture to flow into the secondary combustion zone 
56. The apertures 204 are of equal flow area. 
The invention is also applicable to the tertiary stage of three stage 
combustion chamber as shown in FIG. 7. A tubular combustion chamber 210 
has a plurality of tertiary fuel and air mixing ducts 288 arranged in an 
annulus radially outwardly of a tertiary combustion zone 290. The tertiary 
fuel and air mixing ducts 288 are defined between two annular walls 290 
and 292 and by walls 294 extending radially between the walls 290 and 292. 
A fuel injector 300 is positioned at the upstream end of each tertiary 
fuel and air mixing duct 288, and extends axially with respect to the axis 
of the combustion chamber 210. Each of the tertiary fuel and air mixing 
ducts 288 communicates via a respective aperture 304 in the annular wall 
46 to allow the fuel and air mixture to flow into the tertiary combustion 
zone 290. The apertures 304 are of equal flow area. 
The invention has been described with reference to tubular and annular 
combustion chambers, but the invention is applicable to combustion 
chambers of other shapes. The secondary fuel and air mixing ducts need not 
be positioned around the primary combustion zone and the tertiary fuel and 
air mixing ducts need not be positioned around the secondary combustion 
zone. 
In a further embodiment, shown in FIG. 8, the walls 94 of the secondary 
fuel and air mixing ducts 88 do not extend the full distance to the 
frustoconical portion 62. Deflecting member 95 are secured to the annular 
walls 90 and 92 to direct the fuel and air mixture at the appropriate 
angle through the apertures 104 into the secondary combustion zone 56. The 
walls 94 extend a sufficient distance from the intakes 98 towards the 
members 95 to aerodynamically separate the airflows, such that there are 
no, or insignificant, mass flows between adjacent secondary fuel and air 
mixing ducts 88, ie the walls 94 must extend a sufficient distance to 
control the flow of air. Similarly the walls 94 do not extend the full 
radial distance between the annular walls 90 and 92. The walls 94 extend a 
sufficient distance from one of the annular walls 90 or 92 respectively 
towards the other annular wall 92 or 90 respectively to aerodynamically 
separate the airflows, such that there are no, or insignificant, mass 
flows between adjacent secondary fuel and air mixing ducts 88. FIG. 8 
shows one wall 94A secured to the annular wall 90 and one wall 94B secured 
to the other annular wall 92. The mass flow rate through the secondary 
fuel and air mixing ducts 88 is such that the air and fuel cannot turn 
through the gaps between the walls 94 and annular walls 90 and 92 or 
deflecting members 95. 
Also the fuel injectors 100 in FIG. 8 are located at a position spaced from 
the intake 98. The fuel injectors 100 may be located at any position along 
the secondary air and fuel mixing ducts 88 which produces acceptable 
mixing of the fuel and air. The fuel injectors 100 must be downstream of 
the intakes 98, and there must be a sufficient distance between the fuel 
injectors 100 and the apertures 104 to give the required mixing. The fuel 
injectors 100 must be downstream of the intakes 100 so that the fuel is 
supplied into the airflow after it has been divided into the individual 
secondary fuel and air mixing ducts 88 in order to obtain the required 
fuel to air ratio at the aperture 104 of each duct. 
Thus it can be seen that the invention provides a number of secondary fuel 
and air mixing ducts for premixing the fuel and air before it is supplied 
into the secondary combustion zone. The main feature of these premixing 
ducts is that their outlets into the secondary combustion zone are of 
substantially the same flow area, and thus each secondary fuel and air 
premixing duct has substantially the same flow rate of air therethrough. 
Furthermore the fuel injectors for each of the secondary fuel and air 
mixing ducts are arranged to supply substantially the same flow rate of 
fuel. Thus the fuel to air ratio of the mixture leaving each of the 
secondary fuel and air mixing ducts is substantially the same. Similarly 
each of the tertiary fuel and air mixing ducts have substantially the same 
outlet flow area, substantially the same air flow rate, and substantially 
the same flow rate of fuel supplied to it. 
The invention also provides that the outlets of the secondary fuel and air 
mixing ducts may have different flow areas and thus different air flow 
rates. In this case the secondary fuel injectors have their fuel flow 
rates adjusted so that the fuel to air ratio of the mixture leaving each 
of the secondary fuel and air mixing ducts is substantially the same.