Patent Publication Number: US-6986255-B2

Title: Piloted airblast lean direct fuel injector with modified air splitter

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates generally to fuel injection assemblies for gas turbine engines. 
   2. Description of Related Art 
   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 utilized to power jet aircraft or generate electricity. Particularly, there is a continuing drive to reduce nitrous oxide (NO x ) emissions. 
   Advanced gas turbine combustors must meet these requirements for lower NO x  emissions under conditions in which the control of NO x  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 NO x  emissions and a 15 percent improvement in fuel efficiency compared to ICAO 1996 STANDARDS TECHNOLOGY. Realization of the fuel efficiency objective will require an overall cycle pressure ratio as high as 60 to 1 and a peak cycle temperature of 3000° F. or greater. The severe combustor pressure and temperature conditions required for improved fuel efficiency make the NO x  emissions goal much more difficult to achieve. 
   One approach to achieving low NO x  emissions is via a class of fuel injectors known as lean direct injectors (LDI), such as LDI injector  10  shown in FIG.  4 . Lean direct injection designs seek to rapidly mix the fuel and air to a lean stoichiometry after injection into the combustor. If the mixing occurs very rapidly, the opportunity for near stoichiometric burning is limited, resulting in low NO x  production. 
   Conventional fuel injectors that produce low NO x  emissions at high power conditions, such as LDI injector  10  shown in  FIG. 4 , have several disadvantages, including for example, the potential for excessive combustion dynamics or pressure fluctuations caused by combustion instability. Combustion instability occurs when the heat release couples with combustor acoustics such that random pressure perturbations in the combustor are amplified into large pressure oscillations. These large pressure oscillations, such as those pressure oscillations having amplitudes of about 1-5% of the combustor pressure, can have catastrophic consequences, and thus, must be reduced and/or eliminated. 
   SUMMARY OF THE INVENTION 
   This invention provides fuel injector systems that enable improved combustion efficiencies and reduced emissions of pollutants, particularly NO x  emissions and carbon monoxide (CO) emissions; 
   This invention also provides fuel injector systems for gas turbine engines which result in low emissions of pollutants, particularly low NO x  emissions and CO emissions at all power conditions; 
   This invention further provides fuel injector systems for gas turbine engines having superior lean blowout performance; 
   This invention still further provides fuel injector systems designed to operate at the high power conditions of advanced gas turbine engines without thermal damage to the fuel injector itself; and 
   In various other exemplary embodiments according to this invention, a fuel injector system that reduces and/or eliminates combustion instability includes a pilot fuel injector, a pilot swirler that swirls air past the pilot fuel injector, a main airblast fuel injector having an aft end, inner and outer main swirlers that swirl air past the main airblast fuel injector, and an air splitter located between the pilot swirler and the inner main swirler. The air splitter includes at least one aft end arm/cone angled radially outboard and axially positioned downstream of the main airblast fuel injector aft end. The air splitter divides a pilot air stream exiting the pilot swirler from an inner main air stream exiting the inner main swirler to create a bifurcated recirculation zone. 
   These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of the systems and methods of this invention described in detail below, with reference to the attached drawing figures, in which: 
       FIG. 1  is a cross-sectional schematic view of one exemplary embodiment of a piloted airblast fuel injector system with a modified air splitter according to this invention; 
       FIG. 2  is a detailed cross-sectional schematic view of the piloted airblast fuel injector with a modified air splitter of  FIG. 1 ; 
       FIG. 3  is a schematic illustration of an exemplary embodiment of a fuel flow control system utilized with this invention; and 
       FIG. 4  is a schematic illustration of a LDI fuel injector with a conventional air splitter. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   One of the mechanisms forcing the combustion instability is the modulation of equivalence ratio at the flamefront, caused by a modulation of the inner airstream as the combustor pressure fluctuates. This determination is based on numerical predictions in which the predicted instability is dampened when the airflow in the inner main airstream is held constant at the swirl vane exit. 
     FIG. 1  shows a cross-sectional schematic view of one exemplary embodiment of a piloted airblast fuel injector system  100  with a modified air splitter according to this invention.  FIG. 2  shows in more detail the modified air splitter region of the piloted airblast fuel injector system of FIG.  1 . The piloted airblast fuel injector system  100  includes three air passages and two fuel injectors. The piloted airblast fuel injector system  100  is mounted upon the dome wall  120  of a combustor  140  of a gas turbine engine. 
   As shown in  FIGS. 1 and 2 , in one exemplary embodiment, the piloted airblast fuel injector system  100  includes a pilot fuel injector  102  located on the centerline  101  of the piloted airblast fuel injector system  100 . A pilot swirler  104 , used to swirl air past the pilot fuel injector  102 , surrounds the pilot fuel injector  102 . The pilot swirler  104  shown in the exemplary embodiment is an axial type pilot swirler  104 . In general, the pilot swirler  104 , and any of the other swirlers, can be either radial or axial swirlers, and may be designed to have a vane-like configuration. 
   The piloted airblast fuel injector system  100  utilizes a pilot fuel injector  102  of the type commonly referred to as a simplex pressure atomizer fuel injector. As will be understood by those skilled in the art, the simplex pressure atomizer fuel injector  102  atomizes fuel based upon a pressure differential placed across the fuel, rather than atomizing fuel with a rapidly moving air stream as do airblast atomizers. 
   The piloted airblast fuel injector system  100  further includes a main airblast fuel injector  110  which is concentrically located about the simplex pressure atomizer pilot fuel injector  102 . Inner and outer main swirlers  108  and  112  are located concentrically inward and outward of the main airblast fuel injector  110 . The simplex pressure atomizer pilot fuel injector  102  and main fuel injector  110  may also be described as a primary fuel injector  102  and a secondary fuel injector  110 , respectively. 
   As it will be appreciated by those skilled in the art, the main airblast fuel injector  110  provides liquid fuel to an annular aft end  111  which allows the fuel to flow in an annular film. The annular film of liquid fuel is then entrained in the much more rapidly moving and swirling air streams passing through inner main swirler  108  and outer main swirler  112 , which air streams cause the annular film of liquid fuel to be atomized into small droplets which are schematically illustrated and designated by the numeral  113 . Preferably, the design of the airblast main fuel injector  110  is such that the main fuel is entrained approximately mid-stream between the air streams exiting the inner main swirler  108  and the outer main swirler  112 . 
   In the inner and outer main swirlers  108  and  112  have a vane configuration, the vane angles of the outer main swirler  112  may be either counter-swirl or co-swirl with reference to the vane angles of the inner main swirler  108 . 
   The fuel injection system  100  further includes a modified air splitter  106 , and a flared aft outlet wall  114 . The air splitter  106  is located between the pilot swirler  104  and the inner main swirler  108 . The geometry of and location of the air splitter  106  is such that the air splitter divides a pilot air stream exiting the pilot swirler  104  from a main air stream exiting the inner and outer main swirlers  108  and  112 , whereby a bifurcated recirculation zone  52  is created between the pilot air stream and the main air stream. 
   As shown in  FIGS. 1 and 2 , the air splitter  106  includes at least one aft end arm/cone  1062  angled radially outboard and axially positioned downstream of the main airblast fuel injector  110  aft end. The air splitter cone  1062  constricts the inner main air stream at a location  1063  close to or downstream of the location where the main fuel is injected. The inner main air constriction  1064  created by the air splitter cone  1062  reduces or prevents the inner main air stream from modulating with combustor pressure fluctuations. 
   In various exemplary embodiments, the air splitter cone  1062  is made to have a length  1065  as short as possible, as based on design constraints, manufacturing considerations and the like. Further, the air splitter cone  1062  is angled radially outboard relative to a wall  1066  of the air splitter  106 . In various exemplary embodiments, the air splitter cone  1062  is angled at an angle  1067  in a range of about 45° to about 75°. In an exemplary embodiment, the air splitter cone  1062  is angled at an angle  1067  of about 60° relative to the wall  1066  of the air splitter  106 . 
   In various exemplary embodiments, the air splitter  106  is manufactured of a high temperature metal. Because of the high temperature and/or high pressure environment in which it operates, the air splitter  106  may have thermal barrier coating layer, such as a ceramic layer, applied on its surface. 
   As shown in  FIG. 1 , the bifurcated recirculation zone is generally indicated in the area at  52 . It will be appreciated by those skilled in the art that the bifurcated recirculation zone  52  is a generally hollow conical aerodynamic structure which defines a volume in which there is some axially rearward flow. This bifurcated recirculation zone  52  separates the pilot airflow discharging from the injector  102  as designated by arrows  48  from the main airflow discharging from the injector  110  as designated by the arrows  50 . It is noted that there is no central recirculation zone, i.e. no reverse flow along the central axis  101  as would be found in conventional fuel injectors. 
   The creation of the bifurcated recirculation zone which aerodynamically isolates the pilot flame from the main flame benefits the lean blowout stability of the fuel injector. The pilot fuel stays nearer to the axial centerline and evaporates there, thus providing a richer burning zone for the pilot flame than is the case for the main flame. The fuel/air ratio for the pilot flame remains significantly richer than that for the main flame over a wide range of operating conditions. Most of the NOx formation occurs in this richer pilot flame, and even that can be further reduced by minimizing the proportion of total fuel going to the pilot flame. 
   The selection of design parameters to create the bifurcated recirculation zone  52  includes consideration of the diameter of the outlet  1070  of air splitter  106 , vanes  104  and the deflection angle of swirl  1069  (shown in  FIG. 2 ) imparted to the airflow flowing therethrough. As will be appreciated by those skilled in the art, the greater the angle of swirl, the greater the centrifugal effect, and thus increasing swirl angle will tend to throw the pilot airflow further radially outward. The tapered design of the air splitter  1069 , on the other hand, tends to direct the pilot airflow mixture radially inward. The combination of these two will determine whether the desired bifurcated recirculation zone is created. Also, the amount of pilot airflow through the fuel injector is controlled mainly by the diameter of the outlet  1070  and the angle of swirl through the outlet. If the percentage of pilot airflow is too low (less than two percent, for example), the main airflow will dominate and may produce a central recirculation zone. If the outlet opening  1070  is too small or if too great a swirl angle is provided to the pilot air flow, then the pilot airflow will be thrown too far radially outward so that it merges with the main fuel air flow, which will in turn create a conventional central recirculation rather than the desired bifurcated recirculation. In general, for designs like those illustrated, the swirl angle of the pilot air stream should be less than about 30 degrees. 
   To further describe the various flow regimes within the combustor  140 , the radial outer flow stream lines of the flow from the main airblast injector  110  are designated by arrows  50 . Also, there are corner recirculation zones in the forward corners of combustor  140  indicated by arrows  56 . 
   The outer flow streamlines of the fuel and air flowing from the main airblast injector  110  and inner and outer main swirlers  108  and  112  is further affected by the presence of an aft flared wall  114  downstream of the main airblast fuel injector  110 . The flare of aft flared wall  114  ends at an angle  60  to the longitudinal axis  101  which is preferably in the range of from about 45° to 70°. 
   The outwardly flared outer wall  114  has a length  1142  from the aft end of main airblast injector  110  to an aft end of the outer wall  114  sufficiently short to prevent autoignition of fuel within the outer wall  114 . The length  1142  may also be described as being sufficiently short to prevent fuel from the main fuel injector  110  from wetting the flared outer wall  114 . In a typical embodiment of the invention, the length  1142  will be on the order 0.2 to 0.3 inch. 
   The short residence time in the flared exit precludes autoignition within the nozzle. Significant evaporation and mixing does occur within the flared outlet, even for such a short residence time. The partial pre-mixing improves fuel/air distribution and reduces NOx. The extension combined with the flared exit also results in a larger stronger bifurcated recirculation zone  52 . 
   As noted, the swirlers  104 ,  108  and  112  schematically illustrated in  FIG. 1  each include axial swirl vanes which are straight. In alternative embodiments, swirlers  104 ,  108  and  112  may be provided with curved vanes. The curved axial swirl vanes are provided to reduce the Sauter Mean Diameter of the main fuel spray from the main airblast injector  110  as compared to the Sauter Mean Diameter that would be created when utilizing straight vanes. 
   It will be appreciated that in a typical fuel injection system  100 , all three swirlers  104 ,  108  and  112  are fed from a common air supply system, and the relative volumes of air which flow through each of the swirlers are dependent upon the sizing and geometry of the swirlers and their associated air passages, and the fluid flow restriction to flow through those passages which is provided by the swirlers and the associated geometry of the air passages. In one exemplary embodiment, the swirlers and passage heights are constructed such that from 5 to 20 percent of total swirler air flow is through the pilot swirler  104 , from 30 to 70 percent of total air flow is through the inner main swirler  108  and the balance of total air flow is through the outer main swirler  112 . 
   When utilizing the simplex pressure atomizer pilot fuel injector, the atomizer should be selected with a high spray angle to inject spray into the bifurcated recirculation zone, but not so high as to impinge onto the air splitter  106 . 
   In  FIG. 1 , a pilot fuel supply line  115  is shown providing fuel to the pilot fuel injector  102 , and a main fuel supply line  117  is shown providing fuel to the main airblast injector  110 . 
     FIG. 3  schematically illustrates a fuel supply control system  70  utilized with the fuel injector like the fuel injector system  100  of FIG.  1 . The fuel supply control system  70  includes control valves  72  and  74  disposed in the pilot and main fuel supply lines  115  and  117 , which supply lines lead from a fuel source  76 . A microprocessor based controller  78  sends control signals over communication lines  80  and  82  to the control valves  72  and  74  to control the flow of fuel to pilot fuel injector  102  and main fuel injector  110  in response to various inputs to the controller and to the pre-programmed instructions contained in the controller. In general, during low power operation of the gas turbine associated with the fuel injection system  100 , fuel will be directed only to the pilot fuel injector  102 , and at higher power operating conditions, fuel will be provided both to the pilot fuel injector  102  and the main airblast fuel injector  110 . 
   During low power operation of the fuel injector  100 , fuel is provided only to the pilot fuel injector  102  via the pilot fuel supply line  115 . The fuel is atomized into the small droplets. The swirling motion of the air streams from the pilot swirler  104  causes the pilot fuel droplets to be centrifuged radially outwardly so that many of them are entrained within the bifurcated recirculating flow zone  52 . This causes the pilot flame to be anchored within the bifurcated recirculation zone  52 . 
   At higher power operation of the fuel injector  100 , fuel is also injected into the main airblast injector  110  via the main fuel line  117 . The main fuel droplets  113  are entrained within the air flow between air stream lines of the outer and inner main swirlers  108  and  112 . 
   The air flow which flows through the swirlers  104 ,  108  and  112  preferably is divided in the proportions previously described. As this air flow flows past the air splitter  106 , the main air flow passing through main swirlers  108  and  112  is split away from the pilot air flow which flows through swirler  104  and which must flow through the air splitter  106  and exit the outlet  1070  thereof, thus creating the bifurcated recirculation zone  52  which separates the main air flow from the pilot air flow within the combustor  140 . 
     FIG. 1  also includes a schematic representation of the shape of both a pilot flame  116  and a main flame  118  at full power conditions and a 10/90 pilot/main fuel flow split. As previously noted, the pilot flame  116  is anchored by and generally contained within the bifurcated recirculation zone  52 . The pilot flame generally has a yellow color in its radial and axially aft extremities and a generally blue color in its axially forward axial portion. The main flame  118  is generally blue in color. In general, blue flames are fuel-lean flames, and are a necessary, but not sufficient, condition of low NOx emissions. This is because lean flames can still have local stoichiometry (fuel-to-air ratio) that approaches stoichiometric values and the hottest possible temperatures. The ideal situation (for lowest NOx emissions) would be for the main fuel to entirely prevaporize and premix with the main airflow before reaction occurs, thus producing a uniform stoichiometry and lowest possible flame temperatures. Although fuel/air uniformity is desired, many factors can influence how closely uniform stoichiometry is achieved in the real application, e.g. circumferential fuel uniformity, vane wakes from the swirlers, airfeed uniformity into the swirlers, etc. 
   Yellow flames are always indicative of fuel-rich flames, and stoichiometric flames somewhere in the flowfield. This type of flame is to be expected (and desired) for the pilot flame in order to minimize the fuel-to-air ratio of the fuel injector at lean blowout. Since only approximately 10 percent of the total fuelflow enters the pilot at full power conditions, the amount of NOx produced by the pilot flame is somewhat limited. If possible, the amount of pilot fuel should be reduced at full power conditions to minimize NOx emissions; however, at low pilot fuelflows, one must be concerned about carbon deposition within the pilot fuel circuit. For minimum full power NOx, pilot fuel flow can be eliminated if purging is performed. 
   As seen in  FIGS. 1 and 2 , the air splitter  106  may have small diameter holes  107 , in the range of 0.010 to 0.060 inch diameter placed around the tapered end portion, and spaced from 2 to 8 hole diameters apart, to improve durability of the splitter  106  and to eliminate carbon formation on the downstream face  109  of the splitter. 
   Although the invention has been described in detail, it will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the invention.