Patent Publication Number: US-2019195497-A1

Title: Fuel injector

Description:
This is a Continuation of U.S. application Ser. No. 15/179,495 filed Jun. 10, 2016, which claims the benefit of British Application No. 1512000.9 filed Jul. 9, 2015. The disclosures of the prior applications are hereby incorporated by reference therein in their entireties. 
    
    
     FIELD OF THE INVENTION 
     This invention concerns fuel injector assemblies for gas turbine engines. 
     BACKGROUND TO THE INVENTION 
     There is a continuing need, driven by environmental concerns and governmental regulations, for improving the efficiency of and decreasing the emissions from gas turbine engines of the type utilised to power jet aircraft, marine vessels or generate electricity. Particularly there is a continuing drive to reduce oxides of nitrogen (NOx) emissions. 
     Advanced gas turbine combustors must meet these requirements for lower NOx emissions under conditions in which the control of NOx generation is very challenging. For example, the goal for the Ultra Efficient Engine Technology (UEET) gas turbine combustor research being done by’ NASA is a 70 percent reduction in NOx emissions and a 15 percent improvement in fuel efficiency compared to ICAO 1996 standards technology. Realisation of the fuel efficiency objectives will require an overall cycle pressure ratio as high as 60 to 1 and a peak cycle temperature of 1600 degrees centigrade or greater. The severe combustor pressure and temperature conditions required for improved fuel efficiency make the NOx emissions goal much more difficult to achieve. 
     Typical staged low NOx fuel injectors that seek to address this issue have concentrically arranged pilot and main injectors with the main injector surrounding the pilot injector. However, typical staged low NOx injector arrangements have several operational disadvantages, including for example, flame stability and re-light characteristics, the potential for excessive combustor dynamics or pressure fluctuations caused by combustor instability. Combustion instability occurs when the heat release couples with combustor acoustics such that random pressure perturbations in the combustor are amplified into larger pressure oscillations. 
     These large pressure oscillations, having amplitudes of about 1-5 percent of the combustor pressure, can have catastrophic consequences and thus must be reduced or eliminated. The phenomenon is sometimes referred to as “rumble”. 
     There are many suspected causes for rumble but one understood cause is that a pressure wave generated by acoustic resonance in the combustor coherently interacts with an oncoming fuel-air mixture (reactants) from the fuel injector. The reactants feed the combustion flame with a heat release frequency which is coherent with the pressure wave and this can result in a positive growth rate or resonance of the pressure wave and consequently greater noise. In addition to the noise issue, the nature of the interaction of the incident acoustic wave with the reactants is such as to impair characteristics of the reactants and consequently the quality of the combustion. 
     STATEMENT OF THE INVENTION 
     In accordance with present invention there is provided a fuel injection system for a gas turbine comprising; a main airblast fuel injector section, the main airblast fuel injector section having an aft end facing a combustion chamber and wherein a surface exposed to air flow through the injection system is non-axisymmetric, or, non-planar in a reference circumferential plane, and configured to generate acoustic impedance at or adjacent the aft end where, in use, the airflow collides with an oncoming acoustic wave. 
     The fuel injection system may further comprise a pilot fuel injector section and the surface may be a surface of the pilot fuel injector section. 
     Characteristics of the reactants which may affect interaction with an oncoming acoustic wave include; the fuel flux (i.e. the mass flow of the mixture) to the flame, the stoichiometry (i.e. air to fuel ratio), mixture properties such as drop size distribution and droplet dispersion, or the velocity vector of the reactants (fuel placement). Fuel injectors are conventionally designed for a given air and fuel pressure drop but without contemplation of acoustic impedance. The fuel injection system of the present invention addresses the issue of acoustic impedance. 
     It will be appreciated that reactants will exit the fuel injection system and encounter oncoming pressure waves from the combustion chamber at the aft end of the main air blast fuel injector system. The provision of a non-axisymmetric surface as described results in a spatially and/or temporarily varied circumferential reaction to the oncoming wave thereby reducing the coherence of the response relative to an axisymmetric/planar design and hence reducing the incidence of acoustic amplification in the chamber. 
     Typically, a fuel injector section (which can be a pilot fuel or a main blast fuel injector section) comprises one or more fuel injectors having associated air swirlers which generate swirling, fast moving air for mixing with fuel injected from the fuel injector(s) prior to delivery to a combustion chamber. In accordance with the invention, a surface of an air swirler is made non-axisymmetric or non-planar resulting in a non-axisymmetric distribution of the reactants at the aft end of the main blast fuel injector section where they collide with an oncoming acoustic wave. 
     A conventional air swirler comprises at least a pair of coaxially aligned circumferential walls defining an annular channel therebetween. Vanes extend across the channel, optionally engaging with both walls. The walls are conventionally uniformly round with a flat aft end in a plane orthogonal to the axis, the annular channel of uniform width and the vanes are conventionally arranged in radially symmetrical arrays. The vanes may be separated into pre-swirl vanes and swirl vanes, the latter sitting downstream of the former. Pre-swirl vanes in general are bent or curved so as to turn an incoming axial flow tangentially. Swirl vanes are angled relative to an incoming non-axial airstream (having passed through the pre-swirl vanes) and shaped more gently to guide the airstream in a circumferential direction. In embodiments of the present invention, any one or more of these components of the air swirler is adapted to create non-uniformity/a lack of radial symmetry/a non-planar axially facing surface. In some embodiments of the invention, the transmission of an acoustic wave is affected by adapting an array of swirl vanes in the air swirler. The adapted vane arrangements serve to change the transmission of an acoustic wave by varying the open area between blades and thereby damping the wave. 
     In a first embodiment, swirl vanes are arranged in an axially stepped pattern, one or more vanes being arranged along a circumference which is axially displaced from a circumference along which one or more other vanes are arranged. In this embodiment, the vanes may be equally circumferentially spaced and have the same pitch and chord. 
     In other embodiments, vanes are arranged along a common circumference but may be unequally spaced. In these embodiments, for a given spacing (pitch) between vanes, the vane size, angle and/or shape may be adapted to preserve the required swirl characteristics for achieving flow structure for good flame anchoring. For example, an array of vanes might comprise a first plurality of vanes with a first length, pitch and thickness and a second plurality of vanes having a second length, pitch and thickness, the first length being shorter than the second length, the first pitch being smaller than the second pitch and the first thickness being greater than the second thickness, or a combination thereof. The first plurality and second plurality may be sub divided into groups, groups of the first plurality being interspersed between groups of the second plurality around the common circumference. 
     In an alternative arrangement, groups of similarly configured vanes are arranged with varying pitches around a common circumference. Desirably in these arrangements, the vanes have an elongated chord configured to conserve swirl. 
     In yet another embodiment, pre-swirl vanes are elongated whereby to increase inertia in the air stream. The pre swirl vanes may be grouped into a small plurality of pre-swirl vanes having gradually decreasing lengths at an axially downstream end of the vanes. The inertia in passages between adjacent longer vanes is greater than between adjacent shorter length vanes and so results in a varied circumferential flow pattern exiting the fuel injection system and consequent impedance of an oncoming acoustic wave. A similar effect can be achieved by arranging small groups of pre-swirl vanes of similar configuration at an angle to a circumferential reference plane. An incoming air stream will travel further to the most axially distant pre-swirl vanes than to the more proximal vanes and emerge at different times creating a non-axisymmetric flow pattern dynamic response downstream. 
     In yet other embodiments, the walls of the air swirler can be adapted. For example, the aft end could be axially stepped rather than flat, thereby impeding a circumferentially uniform oncoming acoustic wave. In another alternative, the radius of one or both walls could be varied adjusting the radial height of the channel around a circumferential reference plane resulting in a non-axisymmetric flow pattern downstream. 
     An annular fuel injection section may comprise an annular prefilming surface onto which fuel is dispensed prior to atomisation at the axial tip of the prefilmer by air exiting an adjacent air swirler. An axially downstream facing end of the prefilmer (which defines the tip) is conventionally axisymmetric. In some embodiments of the invention, the tip has a varying axial length. For example, the tip may present axially with an undulating, a crenelated or a serrated profile. The tip profile may have a uniform pattern about the circumference. 
     Alternatively, the tip is provided with a non-uniform pattern of axial notches. 
     In another alternative, a radial face of the prefilming surface can be profiled in an axial direction, for example, the surface is undulated concave or convex in cross section. 
     In yet another alternative, surfaces of the air swirler walls or prefilmer can be textured to introduce small radial or axially facing variations in the surface which are sufficient to generate an acoustic impedance that locally alters the fuel mixture delivery. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a sectional side view of a gas turbine engine; 
         FIG. 2  is a section through a fuel injection system as is known in the prior art; 
         FIG. 3  is a schematic plan view of a wall of an air swirler in the fuel injection system of  FIG. 2 . 
         FIG. 3  is a schematic plan view of a wall of an air swirler adapted in accordance with an embodiment of the invention; 
         FIG. 4  is a schematic plan view of a wall of an air swirler adapted in accordance with another embodiment of the invention; 
         FIG. 5  is a schematic plan view of a wall of an air swirler adapted in accordance with another embodiment of the invention; 
         FIG. 6  is a schematic plan view of a wall of an air swirler adapted in accordance with an embodiment of the invention; 
         FIG. 7  is a schematic plan view of a wall of an air swirler adapted in accordance with another embodiment of the invention; 
         FIG. 8  is a schematic plan view of a wall of an air swirler in the fuel injection system of  FIG. 2  showing the pre-swirler vane geometry; 
         FIG. 9  is a schematic plan view of a wall of an air swirler adapted in accordance with another embodiment of the invention; 
         FIG. 10  is a schematic plan view of a wall of an air swirler adapted in accordance with another embodiment of the invention; 
         FIG. 11  is schematic end view of a section of an axial tip of the prefilmer adapted in accordance with another embodiment of the invention; 
         FIG. 12  is schematic end view of a section of an axial tip of the prefilmer adapted in accordance with another embodiment of the invention; 
         FIG. 13  is schematic end view of a section of an axial tip of the prefilmer adapted in accordance with another embodiment of the invention; 
         FIG. 14  is a schematic view showing examples of profiling of a prefilming surface in an axial direction in accordance with embodiments of the invention; and 
         FIG. 15  is a schematic end view showing an annular array of main airblast fuel injectors arranged in a combustor of a gas turbine engine. 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS AND EMBODIMENTS 
     With reference to  FIG. 1 , a gas turbine engine is generally indicated at  10 , having a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , a low-pressure turbine  17  and an exhaust nozzle  18 . A nacelle  20  generally surrounds the engine  10  and defines the intake  12 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first air flow into the high-pressure compressor  14  and a second air flow which passes through a bypass duct  21  to provide propulsive thrust. The high-pressure compressor  14  compresses the air flow directed into it before delivering that air to the combustion equipment  15 . 
     In the combustion equipment  15  the air flow is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high and low-pressure turbines  16 ,  17  before being exhausted through the nozzle  18  to provide additional propulsive thrust. The high 16 and low  17  pressure turbines drive respectively the high pressure compressor  14  and the fan  13 , each by suitable interconnecting shaft. 
     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. three) 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. 
       FIG. 2  shows in more detail a fuel injection system of a type known to be used in the combustion equipment  15  of a gas turbine engine  10 . As can be seen a pilot fuel injector  22  is arranged on an axis with annular components coaxially arranged around it. Radially adjacent the pilot fuel injector  22  is a pilot air swirler  23 . A separator  24  bounds the pilot air swirler and further serves as a radially inner wall to a radially outboard main fuel air swirler having radially outer wall  25  and vanes  26  which extend between walls  24  and  25 . Radially outboard of the main fuel air swirler is an annular main airblast fuel injector  27 . Fuel injected from the main airblast fuel injector  27  spreads along prefilmer surface  28  and is atomised at the axial tip of the prefilmer  29  through interaction with air exiting the main fuel air swirler. The atomised air/fuel mixture is delivered to the combustion chamber  30 . 
       FIG. 3  shows a schematic view of a circumferential wall  25  of the main fuel air swirler and illustrates the arrangement of the vanes thereon. 
       FIG. 4  shows an air swirler passage wall in accordance with a first embodiment of the invention. In comparison to the air swirler of  FIGS. 2 and 3 , the wall  35  is made relatively longer in an axial dimension by a length d for part of the circumference. The vanes  36  are substantially the same and similarly arranged as the vanes of  FIG. 3  to provide nominally similar levels of swirl but different local impedances. 
       FIG. 5  shows an air swirler wall in accordance with a second embodiment of the invention. In comparison to the air swirler of  FIGS. 2 and 3 , the passage wall  45  is made relatively longer in an axial dimension by a length d along the entire circumference. The vanes  46  are substantially the same configuration as those of  FIG. 3  but are arranged in an axially stepped configuration to provide nominally similar levels of swirl but different local impedances. 
       FIG. 6  shows an air swirler passage wall in accordance with a third embodiment of the invention. In comparison to the air swirler of  FIGS. 2 and 3 , the wall  55  is made relatively longer in an axial dimension by a length d along the entire circumference. There are two groups of vanes  56  and  57 . The vanes  56  are individually substantially the same configuration as those of  FIG. 3  and are similarly arranged. Vanes  57  of the second group are longer and spaced further apart to provide nominally similar levels of swirl but different local impedances. 
       FIG. 7  shows an air swirler wall in accordance with a fourth embodiment of the invention. In comparison to the air swirler of  FIGS. 2 and 3 , the wall  65  is made relatively longer in an axial dimension along the entire circumference. The vanes  66  are consistent in length but are arranged at varying separations e, f, g around the circumference. 
       FIG. 8  shows an air swirler wall known to be used in a fuel injection system such as that in  FIG. 1 . This figure shows in more detail the shape of pre-swirl vanes  70  which would sit axially upstream of the vanes already discussed. The figure provides a comparison for a fifth embodiment of the invention also shown in  FIG. 8 . In the embodiment, vanes  76  individually have a similar configuration to those of the prior art arrangement but are grouped and each group is inclined at an angle ΔX to a reference circumferential plane  77 . 
       FIG. 9  shows a sixth embodiment of the invention which is broadly similar to the fifth embodiment of  FIG. 8 . However, in this embodiment the vanes  86  are extended back to the reference circumferential plane  87 . It will be appreciated that greater inertia will be effected on the air flow in passage  88  than in passage  89 . 
       FIG. 10  shows a seventh embodiment of the invention wherein the circumferential wall  95  of an air swirler with a radial dimension r is radially stepped to provide two regions  97  and  98  of different radial dimensions R 1  and R 2 . 
       FIGS. 11, 12 and 13  show eighth, ninth and tenth embodiments of the invention. In each of these embodiments, the axially downstream facing end of the prefilmer (tip)  29  has been profiled. In  FIG. 11 , axially extending undulations  101  are provided. In  FIG. 12 , axially extending serrations are provided. In  FIG. 13 , an axially extending notch  103  is provided.  FIG. 14  shows cross section views of the prefilmer surface of four pre-filmers which are suited to use in the present invention. Prefilmer surface  141  has a continuous radius along its length. Prefilmer surface  142  has a scarfed surface with a radius gradually increasing towards the downstream end. Prefilmer surface  143  has a concave surface, the radius varying across the surface. Prefilmer surface  144  has an undulating surface, the radius varying across the surface. A combination of the described features can be implemented around the annulus to reduce coherence and the associated acoustic waves. 
     In some embodiments of injection fuel systems in accordance with the present invention, multiple main airblast fuel injector sections are arranged in an annular array as shown in  FIG. 15 . Alternate main airblast fuel injector sections  150  are adapted in accordance with one or more of the previously described embodiments. Other main airblast fuel injector sections have a prior art configuration. 
     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.