Abstract:
To meet emissions standards, many gas turbine engines use some form of lean, pre-mixed combustion systems. These systems may lead to combustion oscillations or other instabilities. Jet combustion techniques provide a stable alternative lean, pre-mixed combustion system. The present invention presents a jet combustion system that includes a first cylinder having a first array of orifices. A second cylinder is positioned coaxially with the first cylinder. The second cylinder has a second array of orifices offset from the first array of orifices.

Description:
TECHNICAL FIELD 
   The present invention is directed to an apparatus and method for burning a mixture of fuel and air. More particularly, the present invention is directed to an apparatus and method for burning a mixture of fuel and air in a gas turbine engine. 
   BACKGROUND 
   Producers of gas turbine engines have made great strides in reducing regulated emissions such as NOx through a number of methods including lean, pre-mixed combustion systems (discussed in U.S. Pat. No. 5,660,045 issued to Ito et al. on 26 Aug. 1997) wherein a mass of fuel and a mass of air mix prior to ignition. The mass of air in such a system substantially exceeds the stoichiometric mass of air needed to chemically react with the mass of fuel. Further increasing the mass of air flowing through such a system may increase the NOx reduction by further reducing a primary combustion zone temperature. NOx emissions generally form when an excess of oxygen reacts with nitrogen at elevated temperatures. However, increasing the mass of air may also lead to instabilities in combustion. 
   Adding additional air also assumes availability of additional air. Lean, pre-mixed combustion may still require at least a portion of a mass of air exiting a compressor of the gas turbine for use in cooling a combustion liner surrounding the primary combustion zone. This requirement limits the mass of air available for pre-mixing. Alternative cooling schemes may allow the mass of air to be used both for cooling the combustion liner and for pre-mixing with fuel (U.S. Pat. No. 6,314,716 issued to Abreu et al. on 13 Nov. 2001). These systems may require additional control mechanisms necessary to maintain a desired distribution of air for cooling and pre-mixing. 
   An alternative combustion design suggests using a radiant surface burner (U.S. Pat. No. 6,330,791 issued to Kendall et al. on 18 Dec. 2001). Radiant burners provide a compact design that allows pre-mixing of fuel and air similar to lean, pre-mixed combustion systems. Past radiant burners have used a porous ceramic structure or fibrous mat made of either ceramic or metallic material. These materials have a tendency to accumulate various particles eventually leading to partial blocking of portions of the porosities. 
   The apparatus and method of the present invention solves one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   The present application discloses a fuel burner for a gas turbine engine including a first cylinder having a first array of orifices. A second cylinder having a second array of orifices is coaxial with the first cylinder. The first array of orifices is offset from the second array of orifices. 
   In addition, the application describes a method of burning a fuel in a gas turbine engine. The method includes supplying a mixture of fuel and air to a first cylinder, flowing said mixture of fuel and air through a first array of orifices in said first cylinder, reducing a pressure of said mixture of fuel and air, impinging a second cylinder with said mixture of fuel and air, transferring heat from said second cylinder to said mixture of fuel and air during said impinging, mixing further said mixture of fuel and air after said transferring, passing said mixture of fuel and air through a second array of orifices in said second cylinder, and igniting said mixture of fuel and air. 
   In addition, the application also describes a method of cooling a fuel burner for a gas turbine engine. The method includes supplying a mixture of fuel and air to a first cylinder, circulating said mixture of fuel and air with a circulating device in said first cylinder, flowing said mixture of fuel and air through at least one orifice of said circulating device, flowing said mixture of fuel and air through a first array of orifices in said first cylinder, impinging a second cylinder with said mixture of fuel and air; and transferring heat from said second cylinder to said mixture of fuel and air during said impinging. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a gas turbine engine including an exemplary embodiment of the present invention; 
       FIG. 2  is a diagrammatic view of a combustion system for the gas turbine engine with an exemplary embodiment of the present invention; 
       FIG. 3  is a plan view of a portion of fuel burner in the combustion system through FIG.  3 — 3 ; 
       FIG. 4  is a sectional view of the combustion system through line  4 — 4  of  FIG. 2 ; and 
       FIG. 5  is a sectional view of the fuel mixing combustion system through line  5 — 5  of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a gas turbine engine  10  including a compressor section  12 , a combustion system  14 , and a turbine section  16 . The compressor section  12  fluidly connects with the combustion system  14  to supply a compressed mass of air (not shown) to the combustion system  14 . The turbine section  16  fluidly connects with the combustion system  14  and receives a mass of exhaust gas (not shown) from the combustion system  14 . The mass of exhaust gas expands through the turbine section  16 . The compressor section  12  and turbine section  16  connect through a force transmitting means  18  between the turbine section  16  and compressor section  12 . In the present embodiment, the force transmitting means  18  is shown as a shaft  20 . Other conventional methods for transmitting a force may include a hydraulic accumulator/motor, electric motor/generator, and gear systems. 
   The combustion system  14  as shown in  FIG. 1  may include a combustor liner  22 , a fuel burner  26 , a fuel supply line  28 , a dome  30 , and a mixing conduit  32 . The combustion system  14  while shown in an annular configuration may be of any conventional configurations such as can-annular or can. The combustor liner  22  has a hot side  34 , a cold side  36 , a first portion  38 , and a second portion  40 . The hot side  34  defines a combustion zone  24 . The cold side  36  along with a casing  42  defines an air channel  44 . The dome  30  may attach to the hot side  34  proximate the first portion  38 . A grommet  45  may separate the fuel burner  26  from the dome  30 . The mixing conduit  32  is in fluid communication with the fuel burner  26 . The fuel supply line  28  may introduce a fuel (not shown) into the mixing conduit  32 . A second fuel supply line  43  may provide a second fuel (not shown) into the mixing conduit  32 . The combustor liner  22  may include any conventional manner of augmentation cooling devices (not shown) on the cold side  36  such as trip strips, dimples, or designs allowing for impingement cooling. 
   As shown in  FIGS. 2 and 3 , the fuel burner  26  includes a first cylinder  46  about a central axis  48 . The first cylinder  46  has a first end portion  50  and a second end portion  52 . The first end portion  50  may attach to the grommet  45 . A first array of orifices  54  extend between the first end portion  50  and second end portion  52 . The first array of orifices may extend a length L or only some portion thereof. Any conventional forming method may create the first cylinder  46  and associated orifices  54  such as machining, casting, or forging. The first array of orifices  54  may include orifices of different diameters such as increasing or decreasing the orifice diameters as a function of an axial position along the central axis  48 . In the present application, the term cylinder means a vessel having a volume that may at least partially bound a fluid and may have an irregular shaped profile other than a rectangle. A circulating device  55  may be positioned within the first cylinder  46  for circulating the mixture of fuel and air. As shown, the circulating device is a perforated cone positioned proximate the second end portion  52  of the first cylinder  46 , however, it should be appreciated to one of ordinary skill in the art that any similar structure for circulating the mixture of fuel and air adjacent the second end portion  52  may be used. The circulating device  55  may include an orifice  57  extending through the circulating device  55 . However, it should be appreciated to one skilled in the art that a plurality of orifices associated with the circulating device may be used. 
   Similarly, a second cylinder  56  is concentric with and displaced radially from the first cylinder  46 . The second cylinder  56  generally has a larger circumference than the first cylinder  46  at any given point along the central axis  48 . The second cylinder  56  has a first end portion  58  connecting with the grommet  45  and a second end portion  60  adjacent the second end portion  52  of the first cylinder  46 . The second cylinder includes a solid portion  59  defining a second array of orifices  62  extending between the first end portion  58  and the second end portion  60 . Any conventional method may be used to create the second cylinder  56  including machining, casting, or forging. In addition, the second cylinder  56  may be made of any material able to withstand temperatures exhibited in a combustion environment such as a ceramic, nickel alloys, or materials coated with a thermal barrier coating. Like the first cylinder  46 , the second array of orifices  62  may have hole diameters that vary as a function of location along the central axis  48 . The diameter of the second cylinder  56  may also vary along the central axis  48 . 
   The first array of orifices  54  and second array of orifices  62  are offset from one another. In the present application offset means that any single orifice from the first array of orifices  54  will not overlap any single orifice from the second array of orifices  62  as shown specifically in  FIG. 3 . The offset may be accomplished by offsetting the arrays axially about the central axis, tangentially about the central axis, or both as shown in  FIG. 3 . Orifice diameters in the second array of orifices  62  will generally be larger than orifice diameters in the first array of orifices  54 . A gap or annular region  61  is defined by some radial distance (not shown) between the first cylinder  46  and second cylinder  56 . 
   The mixing conduit  32 , as best shown in  FIGS. 2 and 4 , may attach to the grommet  45  opposite the first cylinder  46  and second cylinder  56 . The mixing conduit  32  may include a fluid mixing means  63  for mixing the fuel with the mass of compressed air. In the present embodiment the fluid mixing means may include a plurality of vortex generator tabs  64 . The fluid mixing means  63  also may include introducing a portion of the mass of compressed air through an array of mixing orifices  66  positioned along the mixing conduit  32  upstream of the dome  30 . The array of mixing orifices  66  may include offset mixing orifices  68  positioned upstream of the vortex generator tabs  64  and vortex energizing slots  70  downstream of the vortex generator tabs  64 . The offset mixing orifices  68  may introduce the portion of compressed air with a tangential component of velocity (not shown). A mixing nozzle  72  may be positioned downstream from the vortex generator tabs  64  and upstream from the first cylinder  46 . 
   The fuel supply line  28  feeds fuel to a fuel nozzle  73  positioned at an entrance portion  74  of the mixing conduit  32 . The fuel supply  28  line also feeds a pilot mass of fuel (not shown) to a fuel gallery  76 . As shown in  FIG. 5 , the pilot mass of fuel may exit the fuel gallery  76  through an array of pilot fuel supply orifices  78  positioned in a slot  80  concentric with and radially disposed from the second cylinder  56 . The pilot fuel supply orifices  78  may be offset from a plane (not shown) perpendicular to the central axis  48  wherein the offset creates a rotational component of velocity (not shown) for the mass of pilot fuel exiting the slot  80  with respect to the central axis  48 . The fuel nozzle  73  may include a conventional liquid fuel nozzle  84 , a conventional gaseous fuel nozzle/spoke  86 , or both. 
   INDUSTRIAL APPLICABILITY 
   The present combustion system  14  provides a low-cost method of achieving jet combustion to reduce emissions of NOx and other regulated emissions. The jet combustion also provides lower incidents of combustion oscillation found in current lean, pre-mixed combustion systems. By using the first array of orifices  54  and second array of orifices  62 , small particles are not as likely to block the mixture of fuel and air from flowing through the fuel burner  26 . 
   Operation of the combustion system  14  involves introducing the fuel through the fuel nozzle  73  into the mixing conduit  32 . The liquid fuel nozzle  84  may atomize the fuel using one of numerous techniques such as air blast atomization. As the fuel moves through the mixing conduit  32 , compressed air from the compressor section  12  is introduced through the array of mixing orifices  66  creating a swirling motion (not shown). Fuel becomes entrained in the swirling compressed air creating a mixture of fuel and air. The vortex generator tabs  64  and vortex energizing slots  70  may further increase homogeneity of the mixture of fuel and air. Increasing the homogeneity reduces localized hot spots when the mixture of fuel and air combusts. These hot spots often result in formation of NOx emissions. The mixing nozzle  72  accelerates the mixture of fuel and air as it passes into the fuel burner  26 . During the acceleration, the fuel and air mixture also experiences a drop in pressure. 
   Upon entering the fuel burner  26 , the mixture of fuel and air passes through the first array of orifices  54  of the first cylinder  46 . The circulating device  55  circulates the mixture of fuel and air that passes through the first cylinder  46  to the second end portion  52  of the first cylinder  46 . The orifice  57  of the circulating device  55  allows a portion of the mixture of fuel and air to pass through and cool the second end portion  60  of the second cylinder  56 . The circulating device  55  may also reduce stagnation of the mixture of fuel and air near the second end portion  52  of the first cylinder  46 . The mixture of fuel and air may lose a majority of its pressure in relation to an initial pressure at the entrance portion  74  of the mixing conduit  32 . 
   Making the first array of orifices  54  offset from the second array of orifices  62  allows all of the mixture of fuel and air passing through the first array of orifices  54  to perpendicularly impact the inside of the second cylinder  56  with turbulent jets of the mixture of fuel and air. The gap  61  between the first cylinder  46  and second cylinder  56  sets up turbulence to allow substantially all of the mixture of fuel and air to further mix. The gap  61  may be sized such that after impacting the second cylinder  56  the mixture of fuel and air attain sufficient temperature and energy to both maintain stable combustion as the mixture enters the combustion zone  24 . Furthermore, the turbulent jets impacting the inside of the second cylinder  56  and the turbulence in the gap  61  of the mixture of fuel and air increase the cooling of the second cylinder. 
   Pressure in the gap  61  eventually drives the mixture of fuel and air through the second array of orifices  62 . The mixture of fuel and air exits the gap  61  as highly mixed, pre-heated jets or plumes of the mixture of fuel and air. The pilot fuel issuing from the slot  80  forms a rich combustion mixture for maintaining flame stability. The rotational component of velocity may further enhance stability by creating a toroidal, diffusion flame (not shown) about the first end portion  58  of the second cylinder  56 . In addition, the solid portion  59  between the second array of orifices  62  creates multiple independent ignition points and multiple recirculation regions (not shown) both of which promote stable combustion throughout the combustion zone  24 . 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed fuel burner for a gas turbine engine without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only.