Abstract:
A method for providing cooling air to the venturi and the combustion chamber in a low NOx emission combustor as used in a gas turbine engine that includes the steps of providing an annular air passage surrounding said combustion chamber and venturi where said cooling air under pressure enters the combustion chamber/venturi near the aft portion of the combustion chamber, passing the air along the combustion chamber, past the venturi where the air exits near the front portion of the convergent area of the venturi. The method prevents any channel/passage cooling air from being received into the combustion chamber, and at the same time, introduces the outlet of the cooling air, after the air has passed over the combustion chamber of the venturi and has been heated, back into the premix chamber thereby improving the efficiency of the combustor while reducing and lowering NOx emission in the combustion process. In an alternate embodiment, a combustion system is disclosed incorporating a shaped deflector to direct cooling air into a cooling passageway of a venturi. The deflector is configured to provide a cooling system with increased total pressure to increase overall cooling effectiveness.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 10/064,248, filed Jun. 25, 2002 now U.S. Pat. No. 6,484,509 and assigned to the same assignee hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for cooling the combustion chamber and venturi used in a gas turbine engine for reducing nitric oxide emissions and to a structure for improved cooling of a venturi cooling passageway. Specifically a method is disclosed for cooling the combustion chamber/venturi to lower nitric oxide (NOx) emissions by introducing preheated cooling air into the premix chamber for use in the combustion process. 
     2. Description of Related Art 
     The present invention is used in a dry, low NOx gas turbine engine typically used to drive electrical generators. Each combustor includes an upstream premix fuel/air chamber and a downstream combustion chamber separated by a venturi having a narrow throat constriction that acts as a flame retarder. The invention is concerned with improving the cooling of the combustion chamber which includes the venturi walls while at the same time reducing nitric oxide emissions. 
     U.S. Pat. No. 4,292,801 describes a gas turbine combustor that includes upstream premix of fuel and air and a downstream combustion chamber. 
     U.S. Pat. No. 5,117,636 and U.S. Pat. No. 5,285,631 deal with cooling the combustion chamber wall and the venturi walls. The patents state that there is a problem with allowing the cooling air passage to dump into the combustion chamber if the passage exit is too close to the venturi throat. The venturi creates a separation zone downstream of the divergent portion which causes a pressure difference thereby attracting cooling air which can cause combustion instabilities. However, it is also essential that the venturi walls and combustion chamber wall be adequately cooled because of the high temperatures developed in the combustion chamber. 
     The present invention eliminates the problem discussed in the prior art because the cooling circuit for the venturi has been adjusted such that the cooling air no longer dumps axially aft and downstream of the venturi throat into the combustion zone. In fact, cooling air flows in the opposite direction so that the air used for cooling the combustion chamber and the venturi is forced into the premix chamber upstream of the venturi, improving the efficiency of the overall combustion process while eliminating any type of cooling air recirculation separation zone aft of the venturi as discussed in the U.S. Pat. No. 5,117,636. 
     Recent government emission regulations have become of great concern to both manufacturers and operators of gas turbine combustors. Of specific concern is nitric oxide (NOx) due to its contribution to air pollution. 
     It is well known that NOx formation is a function of flame temperature, residence time, and equivalence ratio. In the past, it has been shown that nitric oxide can be reduced by lowering flame temperature, as well as the time that the flame remains at the higher temperature. Nitric Oxide has also been found to be a function of equivalence ratio and fuel to air (f/a) stoichiometry. That is, extremely low f/a ratio is required to lower NOx emissions. Lowering f/a ratios do not come without penalty, primarily the possibility of “blow-out”. “Blow-Out” is a situation when the flame, due to its instability, can no longer be maintained. This situation is common as fuel-air stoichiometry is decreased just above the lean flammability limit. By preheating the premix air, the “blow-out” flame temperature is reduced, thus allowing stable combustion at lower temperatures and consequently lower NOx emissions. Therefore, introducing the preheated air is the ideal situation to drive f/a ratio to an extremely lean limit to reduce NOx, while maintaining a stable flame. 
     In a dual-stage, dual-mode gas turbine system, the secondary combustor includes a venturi configuration to stabilize the combustion flame. Fuel (natural gas or liquid) and air are premixed in the combustor premix chamber upstream of the venturi and the air/fuel mixture is fired or combusted downstream of the venturi throat. The venturi configuration accelerates the air/ fuel flow through the throat and ideally keeps the flame from flashing back into the premix region. The flame holding region beyond the throat in the venturi is necessary for continuous and stable fuel burning. The combustion chamber wall and the venturi walls before and after the narrow throat region are heated by the combustion flame and therefore must be cooled. In the past, this has been accomplished with back side impingement cooling which flows along the back side of the combustion wall and the venturi walls where the cooling air exits and is dumped into combustion chamber downstream of the venturi. 
     The present invention overcomes the problems provided by this type of air cooling passage by completely eliminating the dumping of the cooling air into the combustion zone downstream of the venturi. The present invention does not permit any airflow of the venturi cooling air into the downstream combustion chamber whatsoever. At the same time the present invention takes the cooling air, which flows through an air passageway along the combustion chamber wall and the venturi walls and becomes preheated and feeds the cooling air upstream of the venturi (converging wall) into the premixing chamber. This in turn improves the overall low emission NOx efficiency. 
     BRIEF SUMMARY OF THE INVENTION 
     An improved method for cooling a combustion chamber wall having a flame retarding venturi used in low nitric oxide emission gas turbine engines that includes a gas turbine combustor having a premixing chamber and a secondary combustion chamber and a venturi, a cooling air passageway concentrically surrounding said venturi walls and said combustion chamber wall. A plurality of cooling air inlet openings into said cooling air passageway are disposed near the end of the combustion chamber. 
     The combustion chamber wall itself is substantially cylindrical and includes the plurality of raised ribs on the outside surface which provide additional surface area for interaction with the flow of cooling air over the combustion cylinder liner. The venturi walls are also united with the combustion chamber and include a pair of convergent/divergent walls intricately formed with the combustion chamber liner that includes a restricted throat portion. The cooling air passes around not only the cylindrical combustion chamber wall but both walls that form the venturi providing cooling air to the entire combustor chamber and venturi. As the cooling air travels upstream toward the throat, its temperature rises. 
     The cooling air passageway is formed from an additional cylindrical wall separated from the combustion chamber wall that is concentrically mounted about the combustion chamber wall and a pair of conical walls that are concentrically disposed around the venturi walls in a similar configuration to form a complete annular passageway for air to flow around the entire combustion chamber and the entire venturi. The downstream end of the combustion chamber and the inlet opening of the cooling air passageway are separated by a ring barrier so that none of the cooling air in the passageway can flow downstream into the combustion chamber, be introduced downstream of the combustion chamber, or possibly travel into the separated region of the venturi. In fact the cooling air outlet is located upstream of the venturi and the cooling air flows opposite relative to the combustion gas flow, first passing the combustion chamber wall and then the venturi walls. The preheated cooling air is ultimately introduced into the premix chamber, adding to the efficiency of the system and reducing nitric oxide emissions with a stable flame. 
     The source of the cooling air is the turbine compressor that forces high pressure air around the entire combustor body in a direction that is upstream relative to the combustion process. Air under high pressure is forced around the combustor body and through a plurality of air inlet holes in the cooling air passageway near the downstream end of the combustion chamber, forcing the cooling air to flow along the combustor outer wall toward the venturi, passing the throat of the venturi, passing the leading edge of the venturi wall where there exists an outlet air passageway and a receiving channel that directs air in through another series of inlet holes into the premix chamber upstream of the venturi throat. With this flow pattern, it is impossible for cooling air to interfere with the combustion process taking place in the secondary combustion chamber since there is no exit or aperture interacting with the secondary combustion chamber itself. Also as the cooling air is heated in the passageway as it flows towards the venturi and is introduced into the inlet premix chamber upstream of the venturi, the heated air aides in combustor efficiency to reduce pollutant emissions. 
     The outer combustor housing includes an annular outer band that receives the cooling air through outlet apertures upstream of the venturi. The air is then directed further upstream through a plurality of inlet air holes leading into the premix chamber allowing the preheated cooling air to flow from the air passageway at the leading venturi wall into the premix area. 
     The combustion chamber wall includes a plurality of raised rings to increase the efficiency of heat transfer from the combustion wall to the air, giving the wall more surface area for air contact. Although a separate concentric wall is used to form the air cooling passageway around the combustion chamber and the venturi, it is possible in an alternative embodiment that the outer wall of the combustor itself could provide that function. 
     In an alternate embodiment of the present invention, a combustion system is disclosed that includes a deflector configured to direct cooling air into a venturi cooling passageway. The deflector contains a radial profile for capturing and directing the necessary amount of air into the passageway while minimizing pressure loss, thereby resulting in improved cooling effectiveness and lower operating temperatures. 
     It is an object of the present invention to reduce nitric oxide (NOx) emissions in a gas turbine combustor system while maintaining a stable flame in a desired operating condition while providing air cooling of the combustor chamber and venturi. 
     It is another object of this invention to provide a low emission combustor system that utilizes a venturi for providing multiple uses of cooling air for the combustor chamber and venturi. 
     And yet another object of this invention is to lower the “blow-out” flame temperature of the combustor by utilizing preheated air in the premixing process that results from cooling the combustion chamber and venturi. 
     And yet a further object of this invention is to provide a gas turbine combustion system utilizing a venturi having increased cooling flow with increased total pressure of the cooling flow, thereby resulting in improved cooling in a venturi cooling passageway. 
     In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side elevational view in cross-section of a gas turbine combustion system that represents the prior art, which shows an air cooling passage that empties into and around the combustion chamber. 
     FIG. 2 shows a gas turbine combustion system in a perspective view in accordance with the present invention. 
     FIG. 3 shows a side elevational view in cross-section of a gas turbine combustor system in accordance with the present invention. 
     FIG. 4 shows a cut away version in cross section of the combustion chamber and venturi and portions of the premix chamber as utilized in the present invention. 
     FIG. 5 shows a cross-sectional view, partially cut away of the cooling air passageway at the upstream end of the venturi in the annular bellyband chamber for receiving cooling air for introducing the air into the premix chamber. 
     FIG. 6 is a cut away and enlarged view of the aft end of the combustion chamber wall in cross-section. 
     FIG. 7 is a cross section view of a portion of a combustion system of the prior art. 
     FIG. 8 is a cross section view of a combustion system incorporating the alternate embodiment of the present invention. 
     FIG. 9 is an enlarged cross section view of a portion of the combustion system of the alternate embodiment of the present invention. 
     FIG. 10 is an enlarged cross section view of the aft end of the venturi and combustion liner in accordance with the alternate embodiment of the present invention. 
     FIG. 11 is a perspective view of the aft portion of the combustion system in accordance with the alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an existing gas turbine combustor well known in the prior art  110  is shown. The combustor  110  includes a venturi  111 , a premixing chamber  112  for premixing air and fuel, a combustor chamber  113  and a combustion cap  115 . As shown in this prior art combustor, cooling air represented by arrows flows under pressure along the external wall of the venturi  111 . The cooling air enters the system through multiple locations along the liner  110 . A portion of the air enters through holes  120  while the remainder runs along the outer shell. The cooling air, which is forced under pressure, with the turbine compressor as the source, enters the system through a plurality of holes  121 . As seen in FIG. 1 the cooling air impinges and cools the convergent/divergent walls  127  of the venturi  111 , which are conically shaped and travel downstream through the cylindrical passage  114  cooling the walls of combustion cylinder chamber  113 . The cooling air exits along the combustion chamber wall through annular discharge opening  125 . This air is then dumped to the downstream combustion process. A portion of the cooling air also enters the premixing zone through holes  126 . The remaining cooling air proceeds to the front end of the liner where it enters through holes  123  and the combustion cap  115 . The portion of the cooling air that does not enter through holes  123  enters and mixes the gas and fuel through area  124 . U.S. Pat. No. 5,117,636 discusses the prior art configuration of the venturi shown in FIG.  1 . Problems are discussed regarding the cooling air exiting adjacent the venturi  111  through passage exit  125  which interferes with the combustion process and mixture based on what the &#39;636 Patent states as a separation zone. 
     The present invention completely alleviates any of the problems raised in the &#39;636 Patent. 
     Referring now to FIGS. 2 and 3, the present invention is shown as gas turbine combustor  10  including a venturi  11 . 
     The venturi  11  includes a cylindrical portion which forms the combustor chamber  13  and unitarily formed venturi walls which converge and diverge in the downstream direction forming an annular or circular restricted throat  11   a . The purpose of the venturi and the restricted throat  11   a  is to prevent flash back of the flame from combustion chamber  13 . 
     Chamber  12  is the premix chamber where air and fuel are mixed and forced under pressure downstream through the venturi throat  11   a  into the combustor chamber  13 . 
     A concentric, partial cylindrical wall  11   b  surrounds the venturi  11  including the converging and diverging venturi walls to form an air passageway  14  between the venturi  11  and the concentric wall  11   b  that allows the cooling air to pass along the outer surface of the venturi  11  for cooling. 
     The outside of the combustor  10  is surrounded by a housing (not shown) and contains air under pressure that moves upstream towards the premix zone  12 , the air being received from the compressor of the turbine. This is very high pressure air. The cooling air passageway  14  has air inlet apertures  27  which permit the high pressure air surrounding the combustor to enter through the apertures  27  and to be received in the first portion  45  of passageway  14  that surrounds the venturi  11 . The cooling air passes along the venturi  11  passing the venturi converging and diverging walls and venturi throat  11   a . Preheated cooling air exits through outlet apertures  28  which exit into an annular bellyband chamber  16  that defines a second portion  46  (FIG. 4) of the passageway  14 . The combustor utilizes the cooling air that has been heated and allowed to enter into premix chamber  12  through apertures  29  and  22 . Details are shown in FIGS. 5 and 6. Note that this is heated air that has been used for cooling that is now being introduced in the premix chamber, upstream of the convergent wall of the venturi and upstream of venturi throat  11   a . Using preheated air drives the f/a ratio to a lean limit to reduce NOx while maintaining a stable flame. 
     Referring now to FIG. 4, the cooling air passageway  14  includes a first portion  45  having a plurality of spacers  14   a  that separate venturi  11  from wall  11   b . The bellyband wall  16  defines a radially outer boundary of the second portion  46  of the passageway  14  and provides a substantially annular chamber that allows the outside pressure air and the exiting cooling air to be received into the premix chamber  12 . At the downstream end of the combustion chamber  13 , defined by the annular aft end of venturi  11 , there is disposed an annular air blocking ring  40  which prevents any cooling air from leaking downstream into the combustion chamber. This alleviates any combustion problems caused by the cooling air as delineated in the prior art discussed above. 
     Referring now to FIG. 5 the air passageway  14  is shown along the venturi section having the convergent and divergent walls and the throat  11   a  with cooling air passing through and exiting through apertures  28  that go into the air chamber formed by bellyband wall  16 . Additional air under a higher pressure enters through apertures  32  and forces air including the now heated cooling air in passageway  14  to be forced through apertures  22  and  29  into the premix chamber  12 . 
     FIG. 6 shows the aft end portion of the combustion chamber  13  and the end of venturi  11  that includes the blocking ring  40  that is annular and disposed and attached in a sealing manner around the entire aft portion of the venturi  11 . The cooling air that enters into passageway  14  cannot escape or be allowed to pass into any portions of the combustion chamber  13 . Note that some air is permitted into the combustor  10  well beyond combustion chamber  13  through apertures  30  to  31  which are disposed around the outside of the combustor  10  and for cooling the aft end of the combustor. 
     The invention includes the method of improved cooling of a combustion chamber and venturi which allows the air used for cooling to increase the efficiency of the combustion process itself to reduce NOx emissions. With regard to the air flow, the cooling air enters the venturi outer passageway  14  through multiple apertures  27 . A predetermined amount of air is directed into the passageway  14  by element  17 . The cooling air is forced upstream by blocking ring  40  which expands to contact the combustor  10  under thermal loading conditions. The cooling air travels upstream through the convergent/divergent sections of the first portion  45  of passageway  14  where it exits into the second portion  46  of passageway  14  through apertures  28  in the venturi  11  and the combustor  10 . The cooling air then fills a chamber created by a full ring bellyband  16 . Due to the pressure drop and increase in temperature that has occurred throughout the cooling path, supply air which is at an increased pressure is introduced into the bellyband chamber  16  through multiple holes  32 . See FIGS. 4 and 5. The cooling air passes around multiple elements  18  which are located throughout the bellyband chamber  16  for support of the bellyband under pressure. The cooling air is then introduced to the premix chamber through holes  22  and slots  29  in the combustor  10 . Undesired leakage does not occur between the cooling passageway  14  and the premixing chamber  12  because of the forward support  19  which is fixed to the combustor  10  and venturi  11 . The remainder of the cooling air not introduced to passageway  14  through apertures  27  passes over the element  17  and travels upstream to be introduced into the combustor  10  or cap  15 . This air is introduced through multiple locations forward of the bellyband cavity  16 . 
     It is through this process, rerouting air that was used for cooling and supplying it for combustion, that lowers the fuel to air ratio such that NOx is reduced without creating an unstable flame. 
     Referring to back to FIGS. 6 and 7, alternate venturis are shown that utilize the improved cooling concept disclosed in the preferred embodiment. Cooling air enters the passageway  14  and  220  through first apertures  27  and  223 , respectively. In the venturi configuration shown in FIG. 7, which is a venturi of the prior art, cooling air is drawn into passageway  220  through first apertures  223  due to the lower operating pressure within passageway  220  when compared to the pressure outside liner  201 . It was determined that utilizing the pressure difference as the sole means for drawing cooling air into the passageway was not sufficient to provide the desired cooling to the passageway. Inadequate cooling of venturi  212  can result in increased operating temperatures, accelerated component degradation, and shorter component life. As a result, an air direction element  17  was added, as shown in FIG. 6, to liner  10  in order to increase the quantity of cooling air into passageway  14 . While this device helped to increase the supply of cooling air to passageway  14 , air pressure loss was still a concern requiring further improvements to be made to further increase the cooling air supply volume and raise cooling air supply pressure. A further increase in cooling air supply volume and total air pressure will result in lower venturi operating temperatures due to the greater capability to cool the hot walls of the venturi region. Lower metal temperatures within the venturi will result in a greater durability, longer component life, and hence lower operating costs. 
     Referring now to FIGS. 8-11, an alternate embodiment of the present invention is shown in detail. In this alternate embodiment, improvements have been made in the region surrounding the venturi cooling passageway to enhance cooling effectiveness. As with the preferred embodiment, and shown in FIGS. 8 and 9, a venturi  60  is positioned within liner  61  having a first generally annular wall  62  and outer surface  62 A. Liner  61  contains a premix chamber  63  for mixing fuel and air and a combustion chamber  64  proximate venturi  60  such that premixing chamber  63  is in fluid communication with combustion chamber  64 . First generally annular wall  62  contains at least one first aperture  65  and at least one second aperture  66 , radially outward of premix chamber  63 . It is preferable that both first aperture  65  and second aperture  66  comprise a plurality of first and second apertures spaced circumferentially about first generally annular wall  62 . Liner  61  also contains an improved air direction element or deflector  85  which is fixed to outer surface  62 A of first generally annular wall  62  proximate at least one first aperture  65  by a means such as brazing or welding. Deflector  85  is shown in greater detail in FIGS. 10 and 11 and comprises a generally annular ring having a forward end  86  and an aft end  87  in spaced relation to forward end  86  thereby defining a first length  88 . Deflector  85  also contains an inner ring surface  89  and an outer ring surface  90  radially outward from inner ring surface  89  thereby defining a first height  91 . Furthermore, deflector  85  includes a forward face  92  and an aft face  93 , each of faces  92  and  93  extend from inner ring surface  89  to outer ring surface  90  and forward face  92  is spaced in relation to aft face  93 . Aft face  93  also contains a first region of curvature  94  with a first radius R 1 . First length  88 , first height  91 , and first radius R 1  vary in size depending on the size of the combustor and the amount of cooling air required to cool passageway  14 . Typically first length  88  is at least 0.100 inches, first height  91  is at least 0.100 inches, and first radius R 1  is at least 0.250 inches. Furthermore, in the preferred embodiment of deflector  85 , forward face  92  further comprises a first member  95  which is generally perpendicular to inner ring surface  89  and a second member  96  which extends from first member  95  to outer ring surface  90  and is oriented at α pitch angle a relative to outer ring surface  90 . In the preferred embodiment, pitch angle α is at least 5 degrees. Having a second member  96  with a pitch angle α such that second member  96  is directed towards first generally annular wall  62  of liner  61  encourages cooling air not entering passageway  14  and passing along outer ring surface  90  to “reattach” to the liner surface thereby increasing cooling along first generally annular wall  62  of liner  61 . 
     Referring back to FIGS. 8 and 9, venturi  60  includes a second generally annular wall  67  having a first converging wall  68  abutting a first diverging wall  69  at a first plane  70  that is generally perpendicular to first generally annular wall  62 . Venturi  60  further contains a throat portion  11 A at first plane  70  such that throat portion  11 A is positioned between premix chamber  63  and combustion chamber  64 . Second generally annular wall  67  is positioned radially inward from first generally annular wall  62  and has an aft end  71  adjacent to at least one first aperture  65 . Venturi  60  further includes a third generally annular wall  72  radially outward of second generally annular wall  67  and radially inward of first generally annular wall  62 . Third generally annular wall  72  contains a second converging wall  73  and a second diverging wall  74  connected at a first region of curvature  75  proximate first plane  70  and having a second radius R 2 . 
     Venturi  60  also contains a passageway  14  for flowing air to cool second generally annular wall  67 . Passageway  14  extends from at least one first aperture  65  to at least one second aperture  66  in liner  61 . Passageway  14  includes a first portion  45  located radially inward from third generally annular wall  72  and radially outward of second generally annular wall  67  as well as a second portion  46  radially outward of first portion  45  where second portion  46  extends from first portion  45  to at least one second aperture  66 , as shown in FIG. 8. A substantially annular bellyband wall  80  is located radially outward from first generally annular wall  62  thereby defining the radially outer boundary of second portion  46  of passageway  14 . At least one third aperture  81  is located in first generally annular wall  62  and communicates with second portion  46 . It is preferable that at least one third aperture  81  comprises a plurality of third apertures which are spaced circumferentially about first generally annular wall  62  and radially outward of venturi  60  for communicating cooling flow from first portion  45  with second portion  46 . Further characteristics of passageway first portion  45 , which are shown in FIGS. 9 and 10, include at least one first aperture  65  located radially outward of first portion  45  and first portion  45  having a third region of curvature  76  with radius R 3  proximate throat region  11 A. In the preferred configuration of this alternate embodiment second radius R 2  is smaller than third radius R 3  with third radius R 3  being at least 0.150 inches. 
     Extending from aft end  71  is a blocking ring  40  that is in sealing contact with first generally annular wall  62 . Blocking ring  40  is utilized to prevent cooling air that is in first portion  45  of passageway  14  from flowing directly into combustion chamber  64  without first flowing through second portion  46  of passageway  14  and into premix chamber  63 . 
     Through utilizing this venturi structure, not only are emissions reduced by improving overall combustion efficiency through introducing cooling air from passage  14  into the combustion process, but cooling effectiveness within passageway  14  is improved due to an improved air deflector design directing additional cooling air with a greater total air pressure into first apertures  65 . 
     While the invention is been described and is known as presently the preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, it is intended to cover various modifications and equivalent arrangements within the scope of the following claims.