Abstract:
A three-stage lean burn combustion chamber ( 28 ) comprises a primary combustion zone ( 36 ), a secondary combustion zone ( 40 ) and a tertiary combustion zone ( 44 ). Each of the combustion zones ( 36,40,44 ) is supplied with premixed fuel and air by respective fuel and air mixing ducts ( 54,70,92 ). The fuel and air mixing ducts ( 54,70,92 ) have a plurality of air injections apertures ( 62,64,76,98 ) spaced apart in the direction of flow through the fuel and air mixing ducts ( 54,70,92 ). The apertures ( 62,64,76,98 ) reduce the magnitude of the fluctuations in the fuel to air ratio of the fuel and air mixture supplied into the at least one combustion zone ( 36,40,44 ). This reduces the generation of harmful vibrations in the combustion chamber ( 28 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a combustion chamber, particularly 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 oxide 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 a large proportion, preferably all, of the combustion air before combustion occurs. The oxides of nitrogen (NOx) are commonly reduced by a method, which uses two stages of fuel injection. Our UK patent no. GB1489339 discloses two stages of fuel injection. 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. 
     The industrial gas turbine engine disclosed in our International patent application no. WO92/07221 uses a plurality of tubular combustion chambers, whose 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 gases axially into the turbine sections of the gas turbine engine. Each of the tubular combustion chambers has two coaxial radial flow swirlers, which supply a mixture of fuel and air into a primary combustion zone. An annular secondary fuel and air mixing duct surrounds the primary combustion zone and supplies a mixture of fuel and air into a secondary combustion zone. 
     One problem associated with gas turbine engines is caused by pressure fluctuations in the air, or gas, flow through the gas turbine engine. Pressure fluctuations in the air, or gas, flow through the gas turbine engine may lead to severe damage, or failure, of components if the frequency of the pressure fluctuations coincides with the natural frequency of a vibration mode of one or more of the components. These pressure fluctuations may be amplified by the combustion process and under adverse conditions a resonant frequency may achieve sufficient amplitude to cause severe damage to the combustion chamber and the gas turbine engine. 
     It has been found that gas turbine engines, which have lean combustion, are particularly susceptible to this problem. Furthermore it has been found that as gas turbine engines which have lean combustion reduce emissions to lower levels by achieving more uniform mixing of the fuel and the air, the amplitude of the resonant frequency becomes greater. 
     It is believed that the pressure fluctuations in the gas turbine engine produce fluctuations in the fuel to air ratio at the exit of the fuel and air mixing ducts. 
     SUMMARY OF THE INVENTION 
     Accordingly the present invention seeks to provide a combustion chamber which reduces or minimises the above-mentioned problem. 
     Accordingly the present invention provides a combustion chamber comprising at least one combustion zone defined by at least one peripheral wall, at least one fuel and air mixing duct for supplying a fuel and air mixture to the at least one combustion zone, the at least one fuel and air mixing duct having an upstream end and a downstream end, fuel injection means for supplying fuel into the at least one fuel and air mixing duct, air injection means for supplying air into the at least one fuel and air mixing duct, the pressure of the air supplied to the at least one fuel and air mixing duct fluctuating, the air injection means comprising a plurality of air injectors spaced apart in the direction of flow through the at least one fuel and air mixing duct to reduce the magnitude of the fluctuations in the fuel to air ratio of the fuel and air mixture supplied into the at least one combustion zone. 
     Preferably the at least one fuel and air mixing duct comprises at least one wall, the air injectors comprise a plurality of apertures extending through the wall. 
     Preferably the combustion chamber comprises a primary combustion zone and a secondary combustion zone downstream of the primary combustion zone. 
     Preferably the combustion chamber comprises a primary combustion zone, a secondary combustion zone downstream of the primary combustion zone and a tertiary combustion zone downstream of the secondary combustion zone. 
     The at least one fuel and air mixing duct may supply fuel and air into the primary combustion zone. The at least one fuel and air mixing duct may supply fuel and air into the secondary combustion zone. The at least one fuel and air mixing duct may supply fuel and air into the tertiary combustion zone. 
     The at least one fuel and air mixing duct may comprise a single annular fuel and air mixing duct, the air injection means being axially spaced apart. The annular fuel and air mixing duct may comprise an inner annular wall and an outer annular wall, the air injector means being provided in at least one of the inner and outer annular walls. The air injector means may be arranged in the inner and outer annular walls. 
     Preferably the fuel and air mixing duct comprises a radial fuel and air mixing duct, the air injection means being radially spaced apart. Preferably the radial fuel and air mixing duct comprises a first radial wall and a second radial wall, the air injector means being provided in at least one of the first and second radial walls. Preferably the air injector means are provided in the first and second radial walls. 
     Alternatively the fuel and air mixing duct comprises a tubular fuel and air mixing duct, the air injector means being axially spaced apart. 
     Preferably the fuel injector means is arranged at the upstream end of the fuel and air mixing duct and the air injector means are arranged downstream of the fuel injector means. 
     Alternatively the fuel injector means is arranged between the upstream end and the downstream end of the at least one fuel and air mixing duct, some of the air injector means are arranged upstream of the fuel injector means and some of the air injector means are arranged downstream of the fuel injector means. 
     Preferably each air injector means at the downstream end of the fuel and air mixing duct is arranged to supply more air into the fuel and air mixing duct than each air injector means at the upstream end of the fuel and air mixing duct. 
     Preferably each air injector means at a first position in the direction of flow through the fuel and air mixing duct is arranged to supply more air into the fuel and air mixing duct than each air injector means upstream of the first position in the fuel and air mixing duct. 
     Preferably each air injector means at the first position in the fuel and air mixing duct is arranged to supply less air into the fuel and air mixing duct than each air injector means downstream of the first position in the fuel and air mixing duct. 
     Preferably the volume of the fuel and air mixing duct being arranged such that the average travel time from the fuel injection means to the downstream end of the fuel and air mixing duct is greater than the time period of the fluctuation. 
     Preferably the volume of the fuel and air mixing duct being arranged such that the length of the fuel and air mixing duct multiplied by the frequency of the fluctuations divided by the velocity of the fuel and air leaving the downstream end of the fuel and air mixing duct is at least two. 
     Preferably the plurality of air injectors are spaced apart in the direction of flow through the at least one fuel and air mixing duct over a length equal to half the wavelength of the fluctuations of the air supplied to the at least one fuel and air mixing duct. 
     Preferably the at least one fuel and air mixing duct comprises a swirler. Preferably the swirler is a radial flow swirler. 
     The present invention also provides a fuel and air mixing duct for a combustion chamber, the fuel and air mixing duct comprising fuel injection means for supplying fuel into the fuel and air mixing duct, air injection means for supplying air into the fuel and air mixing duct, the air injection means comprising a plurality of air injectors spaced apart in the direction of flow through the fuel and air mixing duct. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is a view of a gas turbine engine having a combustion chamber according to the present invention. 
     FIG. 2 is an enlarged longitudinal cross-sectional view through the combustion chamber shown in FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view of part of the primary fuel and air mixing duct shown in FIG.  2 . 
     FIG. 4 is an enlarged cross-sectional view of part of the secondary fuel and air mixing duct shown in FIG.  2 . 
     FIG. 5 is a cross-sectional view of an alternative fuel and air mixing duct. 
     FIG. 6 is a cross-sectional view in the direction of arrows W—W in FIG.  5 . 
     FIG. 7 is a cross-sectional view in the direction of arrows X—X in FIG.  5 . 
     FIG. 8 is a cross-sectional view of an alternative fuel and air mixing duct. 
     FIG. 9 is a cross-sectional view in the direction of arrows Y—Y in FIG.  8 . 
     FIG. 10 is a cross-sectional view in the direction of arrows Z—Z in FIG.  8 . 
     FIG. 11 is a graph comparing the fuel to air ratio fluctuation with radial distance in a radial flow fuel and air mixing duct according to the present invention and a radial flow fuel and air mixing duct according to the prior art. 
     FIG. 12 is a graph of the fuel to air ratio of a fuel and air mixing duct according to the present invention divided by the fuel to air ratio of a fuel and air mixing duct according to the prior art against the frequency of fluctuation multiplied by the length of the fuel and air mixing duct divided by the velocity of the fuel and air mixture leaving the fuel and air mixing duct. 
     FIG. 13 is a cross-sectional view of an alternative fuel and air mixing duct. 
     FIG. 14 is a cross-sectional view of a further fuel and air mixing duct. 
     FIG. 15 is a graph of the fuel to air ratio of fuel and air mixing ducts according to the present invention against the frequency of the fluctuation multiplied by the length of the fuel and air mixing duct divided by the velocity of the fuel and air mixture leaving the fuel and air mixing duct. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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  20  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 engine  10  is quite conventional, and will not be discussed further. 
     The combustion chamber assembly  16  is shown more clearly in FIGS. 2,  3  and  4 . The combustion chamber assembly  16  comprises a plurality of, for example nine, equally circumferentially spaced tubular combustion chambers  28 . The axes of the tubular combustion chambers  28  are arranged to extend in generally radial directions. The inlets of the tubular combustion chambers  28  are at their radially outermost ends and their outlets are at their radially innermost ends. 
     Each of the tubular combustion chambers  28  comprises an upstream wall  30  secured to the upstream end of an annular wall  32 . A first, upstream, portion  34  of the annular wall  32  defines a primary combustion zone  36 , a second, intermediate, portion  38  of the annular wall  32  defines a secondary combustion zone  40  and a third, downstream, portion  42  of the annular wall  32  defines a tertiary combustion zone  44 . The second portion  38  of the annular wall  32  has a greater diameter than the first portion  34  of the annular wall  32  and similarly the third portion  42  of the annular wall  32  has a greater diameter than the second portion  38  of the annular wall  32 . 
     A plurality of equally circumferentially spaced transition ducts  46  are provided, and each of the transition ducts  46  has a circular cross-section at its upstream end  48 . The upstream end  48  of each of the transition ducts  46  is located coaxially with the downstream end of a corresponding one of the tubular combustion chambers  28 , and each of the transition ducts  46  connects and seals with an angular section of the nozzle guide vanes. 
     The upstream wall  30  of each of the tubular combustion chambers  28  has an aperture  50  to allow the supply of air and fuel into the primary combustion zone  36 . A radial flow swirler  52  is arranged coaxially with the aperture  50  in the upstream wall  30 . 
     A plurality of fuel injectors  56  are positioned in a primary fuel and air mixing duct  54  formed upstream of the radial flow swirler  52 . The walls  58  and  60  of the primary fuel and air mixing duct  54  are provided with a plurality of radially, and circumferentially, spaced apertures  62  and  64  respectively which form a primary air intake to supply air into the primary fuel and air mixing duct  54 . The radially spaced apertures  62  and  64  are thus spaced apart longitudinally, in the direction of flow, of the primary fuel and air mixing duct  54  over a distance D. The apertures  62  may be circular or slots. 
     A central pilot igniter  66  is positioned coaxially with the aperture  50 . The pilot igniter  66  defines a downstream portion of the primary fuel and air mixing duct  54  for the flow of the fuel and air mixture from the radial flow swirler  52  into the primary combustion zone  36 . The pilot igniter  66  turns the fuel and air mixture flowing from the radial flow swirler  52  from a radial direction to an axial direction. The primary fuel and air is mixed together in the primary fuel and air mixing duct  54 . 
     The fuel injectors  56  are supplied with fuel from a primary fuel manifold  68 . 
     An annular secondary fuel and air mixing duct  70  is provided for each of the tubular combustion chambers  28 . Each secondary fuel and air mixing duct  70  is arranged circumferentially around the primary combustion zone  36  of the corresponding tubular combustion chamber  28 . Each of the secondary fuel and air mixing ducts  70  is defined between a second annular wall  72  and a third annular wall  74 . The second annular wall  72  defines the inner extremity of the secondary fuel and air mixing duct  70  and the third annular wall  74  defines the outer extremity of the secondary fuel and air mixing duct  70 . The second annular wall  72  of the secondary fuel and air mixing duct  70  has a plurality of axially and circumferentially spaced apertures  76  which form a secondary air intake to the secondary fuel and air mixing duct  70 . The apertures  76  are spaced apart axially, longitudinally in the direction of flow, of the secondary fuel and air mixing duct  70 . The apertures  76  may be circular or slots. 
     At the downstream end of the secondary fuel and air mixing duct  70 , the second and third annular walls  72  and  74  respectively are secured to a frustoconical wall portion  78  interconnecting the wall portions  34  and  38 . The frustoconical wall portion  78  is provided with a plurality of apertures  80 . The apertures  80  are arranged to direct the fuel and air mixture into the secondary combustion zone  40  in a downstream direction towards the axis of the tubular combustion chamber  28 . The apertures  80  may be circular or slots and are of equal flow area. 
     The secondary fuel and air mixing duct  70  reduces in cross-sectional area from the intake  76  at its upstream end to the apertures  80  at its downstream end. The shape of the secondary fuel and air mixing duct  70  produces a constantly accelerating flow through the duct  70 . 
     A plurality of secondary fuel systems  82  are provided, to supply fuel to the secondary fuel and air mixing ducts  70  of each of the tubular combustion chambers  28 . The secondary fuel system  82  for each tubular combustion chamber  28  comprises an annular secondary fuel manifold  84  arranged coaxially with the tubular combustion chamber  28  at the upstream end of the secondary fuel and air mixing duct  70  of the tubular combustion chamber  28 . Each secondary fuel manifold  84  has a plurality, for example thirty two, of equi-circumferentially-spaced secondary fuel apertures  86 . Each of the secondary fuel apertures  86  directs the fuel axially of the tubular combustion chamber  28  onto an annular splash plate  88 . The fuel flows from the splash plate  88  through an annular passage  90  in a downstream direction into the secondary fuel and air mixing duct  70  as an annular sheet of fuel. 
     An annular tertiary fuel and air mixing duct  92  is provided for each of the tubular combustion chambers  28 . Each tertiary fuel and air mixing duct  92  is arranged circumferentially around the secondary combustion zone  40  of the corresponding tubular combustion chamber  28 . Each of the tertiary fuel and air mixing ducts  92  is defined between a fourth annular wall  94  and a fifth annular wall  96 . The fourth annular wall  94  defines the inner extremity of the tertiary fuel and air mixing duct  92  and the fifth annular wall  96  defines the outer extremity of the tertiary fuel and air mixing duct  92 . The tertiary fuel and air mixing duct  92  has a plurality of axially and circumferentially spaced apertures  98  which form a tertiary air intake to the tertiary fuel and air mixing duct  92 . The apertures  98  are spaced apart axially, longitudinally in the direction of flow, of the tertiary fuel and air mixing duct  92  in the fourth annular wall  94 . The apertures  98  may be circular or slots. 
     At the downstream end of the tertiary fuel and air mixing duct  92 , the fourth and fifth annular walls  94  and  96  respectively are secured to a frustoconical wall portion  100  interconnecting the wall portions  38  and  42 . The frustoconical wall portion  100  is provided with a plurality of apertures  102 . The apertures  102  are arranged to direct the fuel and air mixture into the tertiary combustion zone  44  in a downstream direction towards the axis of the tubular combustion chamber  28 . The apertures  102  may be circular or slots and are of equal flow area. 
     The tertiary fuel and air mixing duct  92  reduces in cross-sectional area from the intake  98  at its upstream end to the apertures  102  at its downstream end. The shape of the tertiary fuel and air mixing duct  92  produces a constantly accelerating flow through the duct  92 . 
     A plurality of tertiary fuel systems  104  are provided, to supply fuel to the tertiary fuel and air mixing ducts  92  of each of the tubular combustion chambers  28 . The tertiary fuel system  104  for each tubular combustion chamber  28  comprises an annular tertiary fuel manifold  106  positioned at the upstream end of the tertiary fuel and air mixing duct  92 . Each tertiary fuel manifold  106  has a plurality, for example thirty two, of equi-circumferentially spaced tertiary fuel apertures  108 . Each of the tertiary fuel apertures  108  directs the fuel axially of the tubular combustion chamber  28  onto an annular splash plate  110 . The fuel flows from the splash plate  110  through the annular passage  112  in a downstream direction into the tertiary fuel and air mixing duct  92  as an annular sheet of fuel. 
     As discussed previously the fuel and air supplied to the combustion zones is premixed and each of the combustion zones  36 ,  40  and  44  is arranged to provide lean combustion to minimise NOx. The products of combustion from the primary combustion zone  36  flow into the secondary combustion zone  40  and the products of combustion from the secondary combustion zone  40  flow into the tertiary combustion zone  44 . 
     Some of the air, indicated by arrow A, for primary combustion flows to a chamber  114  and this flow through the apertures  62  in wall  58  into the primary fuel and air mixing duct  54 . The remainder of the air, indicated by arrow B, for primary combustion flows to a chamber  116  and this flow through the apertures  60  in wall  56  into the primary fuel and air mixing duct  54 . The air, indicated by arrow C, for secondary combustion flows to the chamber  116  and this flow through the apertures  76  in wall  72  into the secondary fuel and air mixing duct  70 . The air, indicated by arrow E, for tertiary combustion flows to the chamber  118  and this flow through the apertures  98  in wall  94  into the tertiary fuel and air mixing duct  92 . 
     The combustion process amplifies the pressure fluctuations for the reasons discussed previously and may cause components of the gas turbine engine to become damaged if they have a natural frequency of a vibration mode coinciding with the frequency of the pressure fluctuations. 
     The pressure fluctuations, or pressure waves, in the combustion chamber produce fluctuations in the fuel to air ratio at the exit of the fuel and air mixing ducts. The pressure fluctuations in the airflow and the constant supply of fuel into the fuel and air mixing ducts of the tubular combustion chambers results in the fluctuating fuel to air ratio at the exit of the fuel and air mixing ducts. 
     Consider the equation: 
     
       
           Δu/U= 1/ M×Δp/P    
       
     
     Where U is the velocity of the air, M is the mass, P is the pressure, Δu is the change in velocity, Δp is the change in pressure, FAR is the fuel to air ratio and Δ(FAR) is the change in the fuel to air ratio. 
     Thus in a typical fuel and air mixing duct, if Δp/P is about 1%, then Δu/U is about 30% and hence the Δ(FAR)/FAR is about 30% into the combustion chamber. 
     The present invention seeks to provide a fuel and air mixing duct which supplies a mixture of fuel and air into the combustion chamber at a more constant fuel to air ratio. The present invention provides at least one point of fuel injection into the fuel and air mixing duct and a plurality of points of air injection into the fuel and air mixing duct. The air injection points are spaced apart longitudinally in the direction of flow of the fuel and air mixing duct. The pressure of the air at the longitudinally spaced air injection points at any instant in time is different. Thus as the fuel and air mixture flows along the fuel and air mixing duct the fuel and air mixture becomes weaker due to the additional air. More importantly the maximum difference between the actual fuel to air ratio and the average fuel to air ratio becomes relatively low, see line F in FIG.  11 . However for a single fuel injection point and a single air injection point the maximum difference between the actual fuel to air ratio and the average fuel to air ratio remains relatively high, see line G in FIG.  11 . 
     Calculations show, see FIG. 12, that the variation in the fuel to air ratio for a fuel and air mixing duct with a single fuel injection point and multiple air injection points are a few percent of the variation in the fuel to air ratio for a fuel and air mixing duct with a single fuel injection point and a single air injection point if the volume of the fuel and air mixing duct is such that the following equation is satisfied 
     
       
         
           LF/U&gt;X  
         
       
     
     Where L is the length of the fuel and air mixing duct, F is the frequency, U is the exit velocity of the fuel and air mixture and X is a number greater than 2. The greater the number X, the lower the variation in the fuel to air ratio. For example with X=2, the variation is about 7%, for X=3, the variation is about 4%, for X=4, the variation is about 3%. Preferably X is a number greater than 3, more preferably X is a number greater than 4 and more preferably X is a number greater than 5. 
     The progressive introduction of air along the length of the fuel and air mixing duct results in a number of physical mechanisms which contribute to the reduction, preferably elimination, of the pressure fluctuations, pressure waves or instabilities, in the combustion chamber. The physical mechanisms are the creation of a low velocity region, integration of the fuel to air ratio fluctuations, residence time distribution, damping of pressure waves and destruction of phase relationships. 
     The airflow in the vicinity of the fuel injector experiences fluctuations in its bulk velocity due to the pressure fluctuations in the fuel and air mixing duct. This creates a local fluctuation in fuel concentration, a local fuel to air ratio, which then flows downstream at the bulk velocity of the air in the fuel and air mixing duct. Due to the mixing of the fuel and air in the fuel and air mixing duct these fuel to air ratio fluctuations normally diffuse out, although the process is quite slow. However, if the local convective velocity is low and the local turbulent intensity is high, as in the present invention, any fuel to air ratio fluctuations are substantially dissipated by the time the fuel to air ratio fluctuations reach the combustion chamber. Hence, the combination of low velocity and high turbulence by the air injectors allows the mixing of the fuel and air to smooth out any fluctuations in the fuel concentration, fuel to air ratio, in the vicinity of the fuel injector. 
     Any fluctuation in the local fuel to air ratio in the vicinity of the fuel injector flows downstream and the progressive introduction of air along the length of the fuel and air mixing duct integrates out any fluctuations in the local fuel to air ratio due to the fuel injector. This is because the pressure of the air supplied from each of the air injectors fluctuates with time. If the average time of travel of a fluid particle from the vicinity of the fuel injector to the downstream end of the fuel and air mixing duct is longer than the time period of the pressure fluctuations, then the fluid particle originating from the vicinity of the fuel injector is subjected to a number of cycles of becoming leaner and richer that average out the initial fuel concentration fluctuation. This determines the spatial extent of the air injectors, i.e. the length D of the fuel and air mixing duct containing air injectors. This also determines the width, or cross-sectional area, of the fuel and air mixing duct as this affects the total residence time in the fuel and air mixing duct. 
     A clearly defined and dominant time delay between the fuel injector and the location of heat release in the combustion chamber is one mechanism for combustion instability. The presence of intense turbulent mixing in the fuel and air mixing duct, created by the longitudinally spaced air injectors, creates a large number of possible paths for a fuel particle to travel to the location of heat release. Associated with the large number of possible paths is an equally large number of possible residence times in the fuel and air mixing duct. The probability of the residence time in the fuel and air mixing duct follows an exponential distribution shifted by a certain delay time. This wide distribution of time delays, random in nature, makes it difficult for the system to maintain a coherent fuel to air ratio fluctuation of a large number of cycles and hence this makes resonant behaviour difficult to achieve. The residence time distribution is adjusted to prevent auto ignition of the fuel and air mixture in the fuel and air mixing duct. 
     The average air velocity is chosen so that the air injectors are sensitive to pressure fluctuations originating in the combustion chamber. As a pressure wave propagates from the downstream end of the fuel and air mixing duct towards the fuel injector it progressively loses amplitude because energy is used fluctuating the air pressure in the air injectors. This reduces the possibility of the pressure fluctuations producing a local fuel to air ratio fluctuation in the vicinity of the fuel injector. This also completely changes the coupling between the interior and exterior of the combustion chamber. 
     A consistent relationship is required between the pressure fluctuations inside the combustion chamber and the fluctuations in the chemical energy supplied to the combustion chamber in order for the occurrence of combustion instability. The chemical energy input to the combustion chamber is proportional to the strength of the fuel and air mixture supplied to the combustion chamber and the air velocity at the exit of the fuel and air mixing duct. The plurality of air injectors integrate out the pressure fluctuations and the fluctuations in the strength of the fuel and air mixture. Also any fuel to air ratio fluctuations present at the downstream end of the fuel and air mixing duct are uncorrelated with the pressure fluctuations that produced them. The possibility of positive reinforcement of pressure fluctuations or fuel to air ratio fluctuations is reduced. 
     The average bulk velocity increases along the length of the fuel and air mixing duct. Therefore it is necessary to progressively increase the cross-sectional area of the air injectors along the length of the fuel and air mixing duct to ensure sufficient penetration and mixing in the fuel and air mixing duct. 
     Another fuel and air mixing duct  120  according to the present invention is shown in FIGS. 5,  6  and  7 . A rectangular cross-section fuel and air mixing duct  120  comprises four sidewalls  122 ,  124 ,  126  and  128 . The walls  124  and  126  have a plurality of longitudinally and transversely spaced apertures  130  and  132  respectively which form an air intake to the fuel and air mixing duct  120 . The apertures  130  and  132  progressively increase in cross-sectional area between the upstream end  134  of the fuel and air mixing duct  120  and the downstream end  136  of the fuel and air mixing duct  120 . A single fuel injector  140  is provided to supply fuel into the upstream end  134  of the fuel and air mixing duct  120 . The fuel injector  140  is supplied with fuel from a fuel manifold  138 . 
     A further fuel and air mixing duct  150  according to the present invention is shown in FIGS. 8,  9  and  10 . A circular cross-section fuel and air mixing duct  150  comprises a tubular wall  152  which has a plurality of axially and circumferentially spaced apertures  154  which form an air intake to the fuel and air mixing duct  150 . The apertures  154  progressively increase in cross-sectional area between the upstream end  156  of the fuel and air mixing duct  120  and the downstream end  158  of the fuel and air mixing duct  150 . A single fuel injector  160  is provided to supply fuel into the upstream end  156  of the fuel and air mixing duct  150 . The fuel injector  160  is supplied with fuel from a fuel manifold. 
     Another primary fuel and air mixing duct  170  according to the present invention is shown in FIG.  13  and is similar to that shown in FIG.  3 . The primary fuel and air mixing duct  170  comprises walls  174  and  176  which are provided with a plurality of radially, and circumferentially spaced apertures  176  and  178  respectively which form a primary air intake to supply air into the primary fuel and air mixing duct  170 . The primary fuel and air mixing duct  170  also has a plurality of fuel injectors  172  positioned in the primary fuel and air mixing duct  170  upstream of the apertures  176  and  178 . Additionally a plurality of circumferentially spaced apertures  180  are provided to form part of the primary air intake upstream of the fuel injectors  172 . The apertures  180  supply up to 10% of the primary air flow upstream of the injectors  172 . The apertures  180  are provided to prevent the formation of a stagnant zone, a zone with no net velocity, at the upstream end of the primary fuel and air mixing duct  170 . The stagnant zone mainly consists of fuel and a small fraction of air, in operation, which results in long residence times for the fuel with an increased risk of auto ignition of the fuel in the primary fuel and air mixing duct  170 . The apertures  180  minimise the risk of auto ignition. The primary fuel and air mixing duct  170  also increases on cross-sectional area as shown in a downstream direction. The introduction of air upstream of the fuel injectors only has a minor effect on the fuel to air ratio as shown in FIG. 15, where line H indicates the fuel to air ratio in FIG.  3  and line I indicates the fuel to air ratio in FIG.  13 . 
     A further secondary fuel and air mixing duct  190  according the present invention is shown in FIG.  14  and is similar to that shown in FIG.  4 . The secondary fuel and air mixing duct  190  comprises inner annular wall  194  and outer annular wall  196 . The inner annular wall  192  is provided with a plurality of axially, and circumferentially, spaced apertures  198  which form a secondary air intake to supply air into the secondary fuel and air mixing duct  190 . The secondary fuel and air mixing duct  190  also has an annular fuel injector slot  192  positioned in the secondary fuel and air mixing duct  190  upstream of the apertures  198 . Additionally a plurality of circumferentially spaced apertures  200  are provided to form part of the secondary air intake upstream of the fuel injector slot  192 . The apertures  200  supply up to 10% of the secondary air flow. These apertures  200  also prevent the formation of a stagnant zone and auto ignition, at the upstream end of the secondary fuel and air mixing duct  190 . The secondary fuel and air mixing duct  190  also increases in cross-sectional area as shown in a downstream direction. A similar arrangement of additional apertures may be applied to the tertiary fuel and air mixing duct to prevent the formation of a stagnant zone and auto ignition. 
     The apertures in the walls of the fuel and air mixing duct may be circular, elongate for example slots, or any other suitable shape. The apertures in the walls of the fuel and air mixing duct may be arranged perpendicularly to the walls of the fuel and air mixing duct or at any other suitable angle. 
     The fuel supplied by the fuel injector may be a liquid fuel or a gaseous fuel. 
     The invention is also applicable to other fuel and air mixing ducts. For example the fuel and air mixing ducts may comprise any suitable shape, or cross-section, as long as there are a plurality of points of injection of air spaced apart longitudinally, in the direction of flow through the fuel and air mixing duct, into the fuel and air mixing duct. The apertures may be provided in any one or more of the walls defining the fuel and air mixing duct. 
     The invention is also applicable to other air injectors, for example hollow perforate members may be provided which extend into the fuel and air mixing duct to supply air into the fuel and air mixing duct. 
     The fuel and air mixing duct may have a swirler, alternatively it may not have a swirler. The fuel and air mixing duct may have two coaxial counter swirling swirlers. The swirler may be an axial flow swirler. 
     Although the invention has referred to an industrial gas turbine engine it is equally applicable to an aero gas turbine engine or a marine gas turbine engine.