Patent Publication Number: US-2019170355-A1

Title: Fuel injector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from British Patent Application No. GB 1720167.4, filed on 4 Dec. 2017, the entire contents of which are incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure concerns a fuel injector and in particular to a lean burn fuel injector fora gas turbine engine combustion chamber. 
     Description of the Related Art 
     A current lean burn fuel injector comprises a fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector and the outer main air-blast fuel injector is arranged coaxially radially outwardly of the inner pilot air-blast fuel injector. The inner pilot air-blast fuel injector comprises a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage. The outer main air-blast fuel injector comprises a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage. A main fuel supply passage is arranged to supply main fuel to the main inner air swirler passage and a pilot fuel supply passage is arranged to supply pilot fuel into the pilot inner air swirler passage. 
     In an annular combustion chamber using lean burn fuel injectors of the current arrangement the pilot fuel interacts with the air in the pilot inner air swirler passage and the main fuel interacts with the air in the main inner air swirler passage. The pilot fuel and air mixture exiting from the pilot inner air swirler passage is shielded from the main fuel and air mixture exiting from the main inner air swirler passage by the air exiting the pilot outer air swirler passage. The fuel and air mixture from the pilot inner air swirler passage tends to flow directly downstream along the centre line of the annular combustion chamber. The fuel and air mixture from the main inner air swirler passage tends to flow radially outwardly and radially inwardly towards the radially outer and radially inner annular walls of the annular combustion chamber. The air from the pilot outer air swirler passage provides a recirculating feature to couple the flows from the pilot inner air swirler passage and the main inner air swirler passage. 
     The present disclosure seeks to provide an improved fuel injector. 
     SUMMARY 
     According to a first aspect of the present disclosure there is provided a fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a fuel supply passage extending there-through, the fuel injector head having an air-blast fuel injector, the air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of an inner air swirler passage and an outer air swirler passage, the inner air swirler passage and the outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being convergent in a downstream direction, the fuel supply passage being arranged to supply a first portion of the fuel into the inner air swirler passage and to supply a second portion of the fuel into the outer air swirler passage, the fuel supply passage being arranged to supply fuel onto a pre-filming surface in the inner air swirler passage, the fuel supply passage being arranged to supply fuel onto a pre-filming surface in the outer air swirler passage. 
     Note that the inner air swirler passage and the outer air swirler passage being immediately adjacent each other means that there are no other air swirler passages arranged coaxially between the inner air swirler passage and the outer air swirler passage. 
     The fuel injector may be a rich burn fuel injector, the fuel injector having an additional air swirler passage arranged coaxially around the outer air swirler passage. 
     The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage. 
     The pilot inner air swirler passage may have a pre-filming surface. The pilot outer air swirler passage may have a pre-filming surface. The main inner air swirler passage may have a pre-filming surface. The main outer air swirler passage may have a pre-filming surface. 
     The pilot inner air swirler passage and the pilot outer air swirler passage may be arranged immediately adjacent each other, e.g. there are no other air swirler passages arranged coaxially between the pilot inner air swirler passage and the pilot outer air swirler passage. The main inner air swirler passage and the main outer air swirler passage may be arranged immediately adjacent each other, e.g. there are no other air swirler passages arranged coaxially between the main inner air swirler passage and the main outer air swirler passage. 
     The single main fuel supply passage may be arranged to supply main fuel to the main inner air swirler passage or the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply a first portion of pilot fuel into the pilot inner air swirler passage and to supply a second portion of pilot fuel into the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage or the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     The single pilot fuel supply passage may be arranged to supply pilot fuel into the pilot inner air swirler passage or the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply a first portion of the main fuel into the main inner air swirler passage and to supply a second portion of the main fuel into the main outer air swirler passage the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage, the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage or the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     The single pilot fuel supply passage may be arranged to supply a first portion of pilot fuel into the pilot inner air swirler passage and to supply a second portion of pilot fuel into the pilot outer air swirler passage and the single main fuel supply passage being arranged to supply a first portion of the main fuel into the main inner air swirler passage and to supply a second portion of the main fuel into the main outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage, the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage and the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     A pilot fuel passage may be arranged coaxially between the pilot inner air swirler passage and the pilot outer air swirler passage. 
     The single pilot fuel supply passage may be arranged to supply pilot fuel to the pilot fuel passage. 
     The pilot fuel passage may be arranged to supply the pilot fuel into the pilot inner air swirler passage and into the pilot outer air swirler passage. 
     The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage. 
     The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage. 
     The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage. 
     A second pilot fuel passage may be arranged coaxially between the pilot outer air swirler passage and the main inner air swirler passage. 
     The single pilot fuel supply passage may be arranged to supply pilot fuel to the second pilot fuel passage. 
     The pilot fuel passage may arranged to supply the pilot fuel into the pilot inner air swirler passage and the second pilot fuel passage is arranged to supply pilot fuel into the pilot outer air swirler passage. 
     The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage. 
     The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage. 
     The second pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     The second pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage. 
     The pilot inner air swirler passage and the pilot outer air swirler passage may be arranged to swirl the air in opposite directions. 
     The fuel may be supplied into the outer air swirler passage through an annular slot. The fuel may be supplied into the inner air swirler passage through an annular slot. 
     The number of points at which the fuel is supplied into the outer air swirler passage may be the same as the number of swirl vanes in the outer air swirler passage. 
     The points at which the fuel is supplied into the outer air swirler passage are located adjacent the leading edge of the swirl vanes, adjacent the trailing edge of the swirl vanes or between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. 
     The points at which the fuel is supplied into the outer air swirler passage may be aligned circumferentially with the leading edge of the swirl vanes, aligned circumferentially with the trailing edge of the swirl vanes or arranged circumferentially between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. 
     The points at which the fuel is supplied into the outer air swirler passage may be equi-circumferentially spaced. 
     The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the outer air swirler passage may be between 10:1 and 1:10. 
     The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the outer air swirler passage may be 1:1. 
     A main fuel passage may be arranged coaxially between the main inner air swirler passage and the main outer air swirler passage. 
     An intermediate air swirler passage may be sandwiched between the pilot outer air swirler passage of the inner pilot air-blast fuel injector and the main inner air swirler passage of the outer main air-blast fuel injector. 
     The main inner air swirler passage and the main outer air swirler passage may be arranged to swirl the air in opposite directions. 
     According to a second aspect of the present disclosure there is provided a method of operating a fuel injector, the fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a fuel supply passage extending there-through, the fuel injector head having an air-blast fuel injector, the air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of an inner air swirler passage and an outer air swirler passage, the inner air swirler passage and the outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being convergent in a downstream direction, the fuel supply passage being arranged to supply a first portion of the fuel into the inner air swirler passage and to supply a second portion of the fuel into the outer air swirler passage, the method comprising supplying a first portion of the fuel to a pre-filming surface in the inner air swirler passage and supplying a second portion of the fuel to a pre-filming surface in the outer air swirler passage. 
     Note that the inner air swirler passage and the outer air swirler passage being immediately adjacent each other means there are no other air swirler passages arranged coaxially between the inner air swirler passage and the outer air swirler passage. 
     The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying a first portion of the pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying a second portion of the pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying a first portion of the pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying a second portion of the pilot fuel to the pre-filming surface in the pilot outer air swirler passage and supplying main fuel to a pre-filming surface in the main inner air swirler passage or to a pre-filming surface in the main outer air swirler passage in a second mode of operation. 
     The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying the pilot fuel to the a pre-filming surface in the pilot inner air swirler passage or to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying the pilot fuel to the pre-filming surface in the pilot inner air swirler passage or to the pre-filming surface in the pilot outer air swirler passage, supplying a first portion of the main fuel to a pre-filming surface in the main inner air swirler passage and supplying a second portion of the main fuel to a pre-filming surface in the outer air swirler passage in a second mode of operation. 
     The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying a first portion of the pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying a second portion of the pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying a first portion of the pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying a second portion of the pilot fuel to the pre-filming surface in the pilot outer air swirler passage, supplying a first portion of the main fuel to a pre-filming surface in the main inner air swirler passage and supplying a second portion of the main fuel to a pre-filming surface in the outer air swirler passage in a second mode of operation. 
     The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the outer air swirler passage may be between 10:1 and 1:10. 
     The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the pilot outer air swirler passage may be 1:1. 
     According to a third aspect of the present disclosure there is provided a lean burn fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the pilot inner air swirler passage and the pilot outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the pilot outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the pilot outer air swirler passage being convergent in a downstream direction, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the main inner air swirler passage and the main outer air swirler passage being arranged immediately adjacent each other, the single main fuel supply passage being arranged to supply main fuel to the main inner air swirler passage or the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel into the pilot inner air swirler passage and to the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage or the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage, the single pilot fuel passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     A pilot fuel passage may be arranged coaxially between the pilot inner air swirler passage and the pilot outer air swirler passage. 
     The single pilot fuel supply passage may be arranged to supply pilot fuel to the pilot fuel passage. 
     The pilot fuel passage may be arranged to supply the pilot fuel into the pilot inner air swirler passage and into the pilot outer air swirler passage. 
     The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage. 
     The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage. 
     The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage. 
     A second pilot fuel passage may be arranged coaxially between the pilot outer air swirler passage and the main inner air swirler passage. 
     The single pilot fuel supply passage may be arranged to supply pilot fuel to the second pilot fuel passage. 
     The pilot fuel passage may be arranged to supply the pilot fuel into the pilot inner air swirler passage and the second pilot fuel passage may be arranged to supply pilot fuel into the pilot outer air swirler passage. 
     The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage. 
     The second pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage. 
     The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage. 
     The second pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage. 
     The pilot inner air swirler passage and the pilot outer air swirler passage may be arranged to swirl the air in opposite directions. 
     The number of points at which the pilot fuel is supplied into the pilot outer air swirler passage may be the same as the number of swirl vanes in the pilot outer air swirler passage. 
     The pilot fuel may be supplied into the pilot outer air swirler passage through an annular slot. The pilot fuel may be supplied into the pilot inner air swirler passage through an annular slot. 
     The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be located adjacent the leading edge of the swirl vanes, adjacent the trailing edge of the swirl vanes or between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. 
     The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be aligned circumferentially with the leading edge of the swirl vanes, aligned circumferentially with the trailing edge of the swirl vanes or arranged circumferentially between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. 
     The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be equi-circumferentially spaced. 
     The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be between 10:1 and 1:10. 
     The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be 1:1. 
     The main fuel may be supplied into the main inner air swirler passage through an annular slot. The main fuel may be supplied into the main outer air swirler passage through an annular slot. 
     The main inner air swirler passage and the main outer air swirler passage may be arranged to swirl the air in opposite directions. 
     A main fuel passage may be arranged coaxially between the main inner air swirler passage and the main outer air swirler passage. 
     An intermediate air swirler passage may be sandwiched between the pilot outer air swirler passage of the inner pilot air-blast fuel injector and the main inner air swirler passage of the outer main air-blast fuel injector. 
     According to a fourth aspect of the present disclosure there is provided a method of operating a lean burn fuel injector, the lean burn fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the pilot inner air swirler passage and the pilot outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the pilot outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the pilot outer air swirler passage being convergent in a downstream direction, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the main inner air swirler passage and the main outer air swirler passage being arranged immediately adjacent each other, the single main fuel supply passage being arranged to supply main fuel to the main inner air swirler passage or the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel into the pilot inner air swirler passage and to the pilot outer air swirler passage, the method comprising supplying pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying pilot fuel to the pre-filming surface in the pilot outer air swirler passage and supplying main fuel to a pre-filming surface in the main inner air swirler passage or to a pre-filming surface in the main outer air swirler passage. 
     The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be between 10:1 and 1:10. 
     The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be 1:1. 
     The fuel injector may be provided in a combustion chamber. The fuel injector may be provided in a gas turbine engine. 
     The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a cross-sectional side view of a gas turbine engine. 
         FIG. 2  is an enlarged cross-sectional view of an annular combustion chamber of the gas turbine engine. 
         FIG. 3  is a further enlarged cross-section view of a lean burn fuel injector. 
         FIG. 4  is a further enlarged schematic cross-sectional view of the pilot fuel injector of the lean burn fuel injector shown in  FIG. 3 . 
         FIG. 5  is a schematic view of the pilot fuel injector shown in  FIG. 4 . 
         FIG. 6  is an alternative further enlarged schematic cross-sectional view of the pilot fuel injector of the lean burn fuel injector shown in  FIG. 3 . 
         FIG. 7  is another alternative further enlarged schematic cross-sectional view of the pilot fuel injector of the lean burn fuel injector shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a gas turbine engine is generally indicated at  10 , having a principal and rotational axis X. The engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , an intermediate pressure turbine  17 , a low-pressure turbine  18  and an exhaust nozzle  19 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  11  and the exhaust nozzle  19 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  13  compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbines  17 ,  18  and  19  respectively drive the high pressure compressor  15 , intermediate pressure compressor  14  and the fan  13  respectively, each by a suitable interconnecting shaft  26 ,  28  and  30  respectively. 
     Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. 
     The combustion chamber  15  is shown more clearly in  FIG. 2 . The combustion chamber  15  is an annular combustion chamber and comprises an inner annular wall  32 , an outer annular wall  34  and an upstream wall  36 . The upstream end wall  36  has a plurality of circumferentially spaced apertures, for example equi-circumferentially spaced apertures,  38 . The combustion chamber  15  is surrounded by a combustion chamber casing  40  and the combustion chamber casing  40  has a plurality of circumferentially spaced apertures  42 . The combustion chamber  15  also has a plurality of fuel injectors  44  and each fuel injector  40  extends radially through a corresponding one of the apertures  42  in the combustion chamber casing  40  and locates in a corresponding one of the apertures  38  in the upstream end wall  36  of the combustion chamber  15  to supply fuel into the combustion chamber  15 . 
     A fuel injector  44  according to the present disclosure is shown more clearly in  FIGS. 3 to 5 . The fuel injector  44  comprises a fuel feed arm  46  and a fuel injector head  48 . The fuel feed arm  46  has a single first internal fuel passage, a single pilot fuel supply passage,  50  for the supply of pilot fuel to the fuel injector head  48  and a single second internal fuel passage, a single main fuel supply passage,  52  for the supply of main fuel to the fuel injector head  48 . The fuel injector head  48  has an axis Y and the fuel feed arm  46  extends generally radially with respect to the axis Y of the fuel injector head  48  and also generally radially with respect to the axis X of the turbofan gas turbine engine  10 . The axis Y of each fuel injector head  48  is generally aligned with the axis of the corresponding aperture  38  in the upstream end wall  36  of the combustion chamber  15 . 
     The fuel injector head  48  has a coaxial arrangement of an inner pilot air-blast fuel injector  54  and an outer main air-blast fuel injector  56 . The inner pilot air-blast fuel injector  54  comprises, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage  60 , a pilot fuel passage  62  and a pilot outer air swirler passage  64 . The outer main air-blast fuel injector  56  comprises, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage  68 , a main fuel passage  70  and a main outer air swirler passage  72 . An intermediate air swirler passage  66  is sandwiched between the pilot outer air swirler passage  64  of the inner pilot air-blast fuel injector  54  and the main inner air swirler passage  68  of the outer main air-blast fuel injector  56 . 
     The fuel injector head  48  comprises a first generally cylindrical member  74 , a second generally annular member  76  spaced coaxially around the first member  74  and a third generally annular member  78  spaced coaxially around the second annular member  76 . A plurality of circumferentially spaced swirl vanes  80  extend radially between the first member  74  and the second annular member  76  to form a first air swirler  81 . The second annular member  76  has a greater axial length than the first member  74  and the first member  74  is positioned at an upstream end  76 A of the second annular member  76  and a generally annular duct  60 A is defined between the first member  74  and the second annular member  76  and the swirl vanes  80  extend radially across the annular duct  60 A. A generally cylindrical duct  60 B is defined radially within the second annular member  76  at a position downstream of the first member  74 . The pilot inner air swirler passage  60  comprises the annular duct  60 A and the cylindrical duct  60 B. 
     The second annular member  76  has one or more internal fuel passages  62  which are arranged to receive fuel from the single first internal fuel passage, the single pilot fuel supply passage,  50  in the fuel feed arm  46 . The one or more fuel passages  62  are arranged to supply fuel to a fuel swirler  84  which supplies a film of fuel onto a radially inner surface, a pre-filming surface,  86  at a downstream end  76 B of the second annular member  76 . A plurality of circumferentially spaced swirl vanes  88  extend radially between the second annular member  76  and the third annular member  78  to form a second air swirler  89 . The second annular member  76  has a greater axial length than the third annular member  78  and the third annular member  78  is positioned at the downstream end  76 B of the second annular member  76  and a generally annular duct  64 A is defined between the second annular member  76  and the third annular member  78  and the swirl vanes  88  extend across the annular duct  64 A. The pilot outer air swirler passage  64  comprises the annular duct  64 A. The one or more fuel passages  62  are also arranged to supply fuel to a fuel swirler  90  which supplies a film of fuel onto a radially outer surface, a pre-filming surface,  92  at a downstream end  76 B of the second annular member  76 . The pre-filming surface  92  is located axially downstream of the swirl vanes  88  of the second swirler  89 . The radially outer surface, the pre-filming surface,  92  is a frustoconical surface which converges in a downstream direction. 
     The downstream end  78 B of the third annular member  78  is conical and is convergent in a downstream direction. The downstream end  78 B of the third annular member  78  is downstream of the downstream end  76 B of the second annular member  76  and the downstream end  76 B of the second annular member  76  is downstream of the downstream end  74 B of the first member  74 . In operation the pilot fuel supplied by internal fuel passages  62  and fuel swirler  84  onto the radially inner surface  86  of the second annular member  76  is atomised by swirling flows of air from the swirl vanes  80  and  88  of the first and second air swirlers  81  and  89  respectively. In operation the pilot fuel supplied by internal fuel passages  62  and fuel swirler  90  onto the radially outer surface  92  of the second annular member  76  is atomised by swirling flows of air from the swirl vanes  80  and  88  of the first and second air swirlers  81  and  89  respectively. The pilot inner air swirler passage  60  and the pilot outer air swirler passage  64  are arranged to swirl the air in opposite directions. Alternatively, the pilot inner air swirler passage  60  and the pilot outer air swirler passage  64  may be arranged to swirl the air in the same direction. 
     The fuel injector head  48  also comprises a fourth generally annular member  94  spaced coaxially around the third annular member  78 , a fifth generally annular member  96  spaced coaxially around the fourth annular member  94  and a sixth generally annular member  98  spaced coaxially around the fifth annular member  96 . A plurality of circumferentially spaced swirl vanes  100  extend radially between the fourth annular member  94  and the fifth annular member  96  to form a third air swirler  101 . The fifth annular member  96  has a greater axial length than the fourth annular member  94  and the fourth annular member  94  is positioned at the downstream end  96 B of the fifth annular member  96  and a generally annular duct  68 A is defined between the fourth annular member  94  and the fifth annular member  96  and the swirl vanes  100  extend across the annular duct  68 A. The main inner air swirler passage  68  comprises the annular duct  68 A. The fifth annular member  96  has one or more internal fuel passages  70  which are arranged to receive fuel from the single second internal fuel passage  52  in the fuel feed arm  46 . The one or more fuel passages  70  are arranged to supply fuel to a fuel swirler (not shown) which supplies a film of fuel through outlet  70 A onto the radially inner surface  102  at the downstream end  96 B of the fifth annular member  96 . The radially inner surface  102  of the fifth annular member  96  is a pre-filming surface. A plurality of circumferentially spaced swirl vanes  104  extend radially between the fifth annular member  96  and the sixth annular member  98  to form a fourth air swirler  105 . A generally annular duct  72 A is defined between the downstream end  96 B of the fifth annular member  96  and the downstream end  98 B of the sixth annular member  98  and the swirl vanes  104  extend across the annular duct  72 A. The main outer air swirler passage  72  comprises the annular duct  72 A. The downstream end  94 B of the fourth annular member  94  is conical and is divergent in a downstream direction. In operation the main fuel supplied by internal fuel passages  70 , fuel swirler and outlet  70 A onto the radially inner surface  102  of the fifth annular member  96  is atomised by swirling flows of air from the swirl vanes  100  and  104  of the third and fourth air swirlers  101  and  105  respectively. The main inner air swirler passage  68  and the main outer air swirler passage  72  are arranged to swirl the air in opposite directions. Alternatively, the main inner air swirler passage  68  and the main outer air swirler passage  72  may be arranged to swirl the air in the same direction. 
     The fuel injector head  48  also comprises a plurality of circumferentially spaced swirl vanes which extend radially between the third annular member  78  and the fourth annular member  94  to form a fifth air swirler. An annular duct is defined between the third annular member  78  and the fourth annular member  94 . The intermediate air swirler passage  66  comprises the annular duct. The intermediate air swirler passage  66  is sandwiched between the pilot outer air swirler passage  64  of the inner pilot air-blast fuel injector  54  and the main inner air swirler passage  68  of the outer main air-blast fuel injector  56 . In operation the swirl vanes of the fifth air swirler provide a swirling flow of air over the radially inner surface of the fourth annular member  94 . 
     As discussed above, in operation, the pilot fuel supplied from the single first internal fuel passage  50  in the fuel feed arm  46  to the at least one internal fuel passage  62  is split and a first portion of the pilot fuel is supplied onto the radially inner surface  86  of the second annular member  76  and a second portion of the pilot fuel is supplied onto the radially outer surface  92  of the second annular member  76 . The pilot fuel and air mixture from the pilot inner air swirler passage  60  tends to flow directly downstream along the centre line of the annular combustion chamber  15 . The main fuel and air mixture from the main inner air swirler passage  68  tends to flow radially inwardly and radially outwardly towards the inner and outer annular walls  32  and  34  respectively of the annular combustion chamber  15 . 
     As shown in  FIG. 4  there are six points at which the pilot fuel is supplied into the pilot inner air swirler passage  62  and twelve points at which the pilot fuel is supplied into the pilot outer air swirler passage  64 . The number of points at which the pilot fuel is supplied into the pilot outer air swirler passage  64  may be the same as the number of swirl vanes in the pilot outer air swirler passage  64 . The points at which the pilot fuel is supplied into the pilot outer air swirler passage  64  may be located adjacent the leading edge of the swirl vanes, adjacent the trailing edge of the swirl vanes or between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be aligned circumferentially with the leading edge of the swirl vanes, aligned circumferentially with the trailing edge of the swirl vanes or arranged circumferentially between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. The points at which the pilot fuel is supplied into the pilot outer air swirler passage  64  may be equi-circumferentially spaced. The number of points at which, and their area, the pilot fuel is supplied into the pilot inner air swirler passage  60  and the number of points at which, and their area, the pilot fuel is supplied into the pilot outer air swirler passage  64  are selected such that the mass flow of the pilot fuel to the pilot inner air swirler passage  60  to the mass flow of the pilot fuel to the pilot outer air swirler passage  64  may be between 10:1 and 1:10 and may be 1:1. 
     The fuel and air mixture from the pilot outer air swirler passage  64  provides a recirculating feature to couple the flows from the pilot inner air swirler passage  60  and the main inner air swirler passage  68 . Thus, some of the pilot fuel is supplied into the recirculating feature and this pilot fuel takes part in the pilot combustion process and the main combustion process without compromising the original design function. The supply of some of the pilot fuel into the pilot outer air swirler passage  64  provides the possibility of greater interaction of the pilot fuel and the air to provide better mixing of the pilot fuel and air so that the atomised pilot fuel particles have a lower SMD (Sauter Mean Diameter) and so that there is a reduction in emissions, e.g. smoke and unburned hydrocarbons. Furthermore, the supply of some of the pilot fuel into the pilot outer air swirler passage  64  stabilises the pilot combustion zone where pilot combustion process occurs and the main combustion zone where main combustion process occurs. The supply of pilot fuel into the pilot outer air swirler passage  64  may be tailored to reduce circumferential variation in the fuel and air mixture. 
     An alternative pilot fuel injector  54 A of the lean burn fuel injector  44  is shown in  FIG. 6  and is substantially the same as that shown in  FIG. 4  and like parts are denoted by like numbers. The pilot fuel injector  54 A shown in  FIG. 6  differs in that the one or more fuel passages  62  are arranged to supply fuel to a fuel swirler  90 A which supplies a film of fuel onto a radially outer surface, a pre-filming surface,  92 A of the second annular member  76 . The pre-filming surface  92 A is located at a position axially upstream of that shown in  FIG. 4  and in general the pre-filming surface  92 A may be located at any suitable axial position in the pilot outer air swirler passage  64 . The pre-filming surface  92 A is located axially downstream of the swirl vanes  88  of the second swirler  89 . The radially outer surface, the pre-filming surface,  92 A is a cylindrical surface. 
     Another alternative pilot fuel injector  54 B of the lean burn fuel injector  44  is shown in  FIG. 7  and is substantially the same as that shown in  FIG. 4  and like parts are denoted by like numbers. The pilot fuel injector  54 B shown in  FIG. 7  differs in that the one or more fuel passages  62 B are arranged to supply fuel to a fuel swirler  90 B which supplies a film of fuel onto a radially inner surface, a pre-filming surface,  92 B of the third annular member  78 . The one or more fuel passages  62 B and the fuel swirler  90 B are arranged in the third annular member  78  and the single pilot fuel supply passage  50  in the fuel feed arm  46  is arranged to supply pilot fuel to the one or more fuel passages  62 B. 
     The pre-filming surface  92 B is located at a position axially upstream of that shown in  FIG. 4  and in general the pre-filming surface  92 B may be located at any suitable axial position in the pilot outer air swirler passage  64 . The pre-filming surface  92 B is located axially downstream of the swirl vanes  88  of the second swirler  89 . The radially inner surface, the pre-filming surface,  92 B is a cylindrical surface. 
     A further alternative fuel injector arrangement to the present disclosure is shown in  FIG. 3 . The main fuel passage  70  may have an outlet  70 B to supply fuel onto the radially outer surface  106  of the fifth annular member  96 . The radially outer surface  106  is a pre-filming surface. Thus, in operation, the pilot fuel supplied from the single first internal fuel passage  50  in the fuel feed arm  46  to the at least one internal fuel passage  62  is split and a first portion of the pilot fuel is supplied onto the radially inner surface  86  of the second annular member  76  and a second portion of the pilot fuel is supplied onto the radially outer surface  92  of the second annular member  76 . The main fuel supplied from the single second internal fuel passage  52  in the fuel feed arm  46  to the at least one internal fuel passage  70  is split and a first portion of the main fuel is supplied onto the radially inner surface  102  of the fifth annular member  96  and a second portion of the main fuel is onto the radially outer surface  106  of the fifth annular member  96 . The radially inner surface  102  and the radially outer surface  106  of the fifth annular member  96  are pre-filming surfaces. The pilot fuel and air mixture from the pilot inner air swirler passage  60  tends to flow directly downstream along the centre line of the annular combustion chamber  15 . The main fuel and air mixture from the main inner air swirler passage  68  tends to flow radially inwardly and radially outwardly towards the inner and outer annular walls  32  and  34  respectively of the annular combustion chamber  15 . 
     The supply of some of the pilot fuel into the main outer air swirler passage provides the possibility of greater interaction of the main fuel and the air to provide better mixing of the main fuel and air so that the atomised main fuel particles have a lower SMD (Sauter Mean Diameter) and so that there is a reduction in emissions, e.g. smoke and unburned hydrocarbons. 
     In each of the above lean burn fuel injector arrangement the pilot fuel may be supplied into the pilot outer air swirler passage through an annular slot. In each of the above lean burn fuel injector arrangements the pilot fuel may be supplied into the pilot inner air swirler passage through an annular slot. In each of the above lean burn fuel injector arrangements the main fuel may be supplied into the main inner air swirler passage through an annular slot. In each of the above lean burn fuel injector arrangements the main fuel may be supplied into the main outer air swirler passage through an annular slot. 
     The present disclosure is also applicable to a rich burn fuel injector which comprises an inner air swirler passage and an outer air swirler passage in which a first portion of the fuel is supplied into the inner air swirler passage and a second portion of the fuel is supplied into the outer air swirler passage and to a rich burn fuel injector which comprises an inner air swirler passage, an outer air swirler passage and an additional air swirler passage arranged coaxially around the outer air swirler passage in which a first portion of the fuel is supplied into the inner air swirler passage and a second portion of the fuel is supplied into the outer air swirler passage. The fuel supply passage is arranged to supply fuel onto a pre-filming surface in the inner air swirler passage, the fuel supply passage being arranged to supply fuel onto a pre-filming surface in the outer air swirler passage. The outer air swirler passage in this arrangement is an intermediate air swirler passage between to two coaxial air swirler passages. A downstream end of an annular member defining the radially outer extremity of the outer air swirler passage is frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage is convergent in a downstream direction. A downstream end of an annular member defining the radially outer extremity of the additional air swirler passage is frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage is convergent in a downstream direction. The supply of some of the fuel into the outer air swirler passage provides the possibility of greater interaction of the main fuel and the air to provide better mixing of the main fuel and air so that the atomised main fuel particles have a lower SMD (Sauter Mean Diameter) and so that there is a reduction in emissions, e.g. smoke and unburned hydrocarbons. 
     In each of the above rich burn fuel injector arrangements the fuel may be supplied into the inner air swirler passage through an annular slot. In each of the above rich burn fuel injector arrangements the fuel may be supplied into the outer air swirler passage through an annular slot. 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.