Patent Publication Number: US-6209325-B1

Title: Combustor for gas- or liquid-fueled turbine

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a combustor for a gas- or liquid-fueled turbine. 
     A turbine engine typically includes an air compressor, at least one combustor and a turbine. The compressor supplies air under pressure to the combustor(s)—a proportion of the air is mixed with the fuel, while the remaining air supplied by the compressor is utilized to cool the hot surfaces of the combustor and/or the combustion gases, (i.e., the gases produced by the combustion process, and/or other components of the turbine plant). 
     With the aim of reducing the amount of pollutants produced by the combustion process (particularly No x ), lean burn combustors have been proposed. Such combustors involve the premixing of air and fuel, with a relatively low proportion of fuel being utilized. Combustion then occurs at relatively low temperatures, which reduces the amount of pollutants produced. However, in their basic form such lean burn combustors have a narrow operating range, i.e. they cannot work satisfactorily with large variations in the quantity of fuel being supplied, and are susceptible to flame blow-out or flash-back. 
     One known solution aimed to overcome difficulties inherent in this type of combustor is to stage the air and/or fuel supply relative to engine load, for example, so that optimum flow and mixture rates are achieved over the whole operating range. Stage combustors have, in the past, taken various designs, from those of fixed geometry which may have a number of burners and to which fuel is selectively directed depending on engine requirements, to those of a more complicated nature which may have movable parts to control the flow of combustion air. 
     The present invention seeks to provide a three stage combustor of relatively simple construction but which is nonetheless effective in minimizing the production of pollutants resulting from the combustion process and, in addition, operates with good combustion stability and an excellent turndown ratio whilst at the same time giving flashback-free combustion. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided a combustor for a gas- or liquid-fueled turbine comprising a main combustion chamber and a pre-chamber, a first injection means for supplying fuel or a fuel/air mixture to the pre-chamber, a second injection means for supplying air or a fuel/air mixture to the pre-chamber, a third injection means for supplying air or a fuel/air mixture to the main combustion chamber, the first, second and third injection means being operable progressively in sequence to provide fuel or a fuel/air mixture for combustion; and wherein the third injection means comprises at least one elongated passage means with an arrangement for introducing fuel into the passage means. 
     The combustion chamber and the pre-chamber are preferably defined by one or more cylindrical walls whereby the pre-chamber and the combustion chamber are each of cylindrical form, and with the cross-sectional area of the combustion chamber being greater than the cross-sectional area of the pre-chamber. Preferably, a transition region is defined between the pre-chamber and the combustion chamber. 
     The arrangement for introducing fuel into the passage means may comprise a spray bar. 
     Preferably at least part of the length of the passage means extends alongside the combustion chamber over at least part of the length of the combustion chamber. Further, at least part of the length of a passage for cooling air may extend alongside the combustion chamber over at least part of the length of the combustion chamber. 
     The elongated passage means may be of generally annular form having a radially inner wall and a radially outer wall, the radially inner wall being constituted at least partly by a wall defining the combustion chamber. 
     It is also envisaged that said elongated passage means and said passage for cooling air may both be of annular form with the passage for cooling air being situated radially outside the combustor chamber and the passage means being situated radially outside the passage for cooling air. 
     The axial direction of flow of fuel/air mixture in the elongated passage means may be counter to the axial direction of flow of cooling air in the passage therefor. 
     Alternatively the flow of fuel/air mixture in the elongated passage means may be in the same direction as the flow of cooling air in the passage therefor. 
     The passage means may include turbulence inducing means, which may comprise at least one tube extending between the walls defining the passage means. The or each tube may be open-ended and provide means for entry of cooling air from outside the combustor to the passage for cooling air. 
     The interior of the wall or walls defining the combustion chamber and the pre-chamber may have a thermal barrier coating applied thereto. 
     At least one of the walls defining the elongated passage means may be of corrugated section. 
     In a preferred arrangement the first injection means provides an air/fuel mixture with local fuel rich areas. 
     The second injection means may comprise a fuel spray bar, an air inlet means, and a chamber in which mixing of the fuel and air takes place. 
     When a passage for coolant air is provided it is envisaged that coolant air will pass from the passage into the interior of the combustor; at least a part of the coolant air may pass into the combustion chamber through at least one orifice adjacent the downstream region thereof, and/or at least a part of the coolant air may pass into the interior of the combustor through at least one orifice in a transition duct region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will be described, by way of example, with reference to the accompanying drawings in which: 
     FIGS. 1-5 show diagrammatic axial half-sections through five separate embodiments of “can-type” combustors according to the invention; and 
     FIGS. 6 and 7 show detailed views of a turbulence inducing means, for use with any of the embodiments of FIGS.  1 - 5 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION 
     The combustor may be embodied in any conventional turbine layout, e.g., tubular (single-can or multi-can), turboannular or annular. 
     Thus, the combustor  10  as illustrated in FIG. 1 is of generally circular cylindrical form with a central longitudinal axis marked by line “A” and as indicated above the combustor  10  may, for example, constitute one of a plurality of such combustors arranged in an annular array. The combustor has a pre-chamber  11  and a main combustion chamber  12 . The diameter of the major part of the main combustion chamber  12  is substantially greater than that of the pre chamber  11  with the transition region  100  between the chamber  11  and the chamber  12  being defined by a wall  101  of the combustor diverging in the downstream direction. At the upstream end of the combustor  10  is provided a first injection means  13  which is located co-axially of axis A. 
     The injection means  13  is provided with a supply of fuel (or a supply of fuel and air) as represented by the arrow  14 , which supply is discharged into the pre-chamber  11 . It is to be noted that the fuel may be gas or liquid. The injection means  13  which may be of dual fuel type provides a fuel/air mixture in the pre-chamber  11  which, although of overall lean constitution, nevertheless has local fuel-rich areas. This is achieved by the injection means  13  incorporating or having associated therewith appropriate mixing means. For example, if a fuel/air mixture is supplied to the injection means  13  at its upstream end the injection means may incorporate a swirl means to give the mixture the appropriate degree of mixing as delineated above—such swirl means may involve vanes and/or suitably angling of passage(s) through the means. If fuel alone is injected into the pre-chamber  11  by the injection means  13  then some means will be provided whereby air in the pre-chamber (see later) is mixed with the fuel to give the appropriate form of mixture. 
     The injection means  13  as diagrammatically represented comprises a circular cylindrical member formed with a plurality of passages therethrough. In one form a central passage  15  acts to supply fuel to pre-chamber  11  whilst an annular array of passages  16  supply (swirled) air to mix with the fuel in pre-chamber  11 . In use, injection means  13  acts as a first stage injection means or burner being supplied with fuel  14  (or fuel/air) for engine starting and being the only fuel source up to an engine load of approximately 25%. Because the otherwise lean mixture has local fuel rich areas, flame stability in the pre-chamber  11  is assured at these low power settings. 
     Mounted to extend generally radially outwardly from injection means  13  is a second stage injection means  17 . The second stage injection means  17  may extend orthogonally of injection means  13  or at an angle thereto. In this particular embodiment, the injection means  17  is designed as one of four mounted on the interior surface of an annular or frusto-conical wall extending from injection means  13 . Each injection means  17  comprises a fuel spray bar  18 , with a respective air inlet slot  19  extending therealongside: a respective mixing chamber  21  and a respective air/fuel outlet slot  20  are associated with the spray bar  18  and air inlet slot  19 . By suitable arrangement of the spray bar  18  and slots  19 ,  20 , the fuel and air are caused to contra-rotate in chamber  21  to give a mixture which is largely but not fully uniform in its air to fuel distribution. The injection means  17  thereby acts as a partial premix device. The direction of mixture issuing from the outlet slot  20  is arranged to be such that thorough mixing with the mixture supplied by the first injection means  13  is obtained but it must also be arranged that the velocity of the combined mixture is not reduced to the extent that flash-back might occur. 
     The second injection means  17  is operated to supply fuel for combustion between approximately 25% and 75% of engine local, which fuel is added to that which has already been supplied by the first injection means  13 . From approximately 75% to 100% engine load the fuel for combustion already supplied by the first injection means  13  and the second injection means  17  is supplemented by fuel supplied by a third injection means  30 . 
     The third injection means  30  is arranged to deliver fuel/air mixture into the upstream region of the main combustion chamber  12  optionally via the transition region  100 , such fuel/air mixture being fully pre-mixed, i.e., the fuel and air are substantially evenly distributed. 
     As shown, the third injection means  30  comprises an elongated passage  31  with an inlet  32  for air and including a fuel spray bar  33 , the air and fuel mixing as they pass along the passage as indicated by arrows  34  in an axial direction counter to the axial direction of flow of gases in the combustion chamber  12 . The passage  31  is formed radially outside the main combustion chamber  12 . The passage may be of annular form totally surrounding the combustion chamber  12  or there may be one or more separate cylindrical passages  31  running alongside the combustion chamber  12 . As shown the passage  31  is of annular form being formed between an annular sleeve  35  and the outer wall  36  of an annular passage  37  for cooling air surrounding the combustion chamber  12  and to be described in detail later. 
     As indicated above the passage  31  is relatively long which assists mixing of the air and fuel but in addition it may incorporate further means for creating turbulence to assist the mixing process. Such turbulence creating means may comprise vanes but, as shown, it comprises one or more open-ended tubes  40  extending across annular passage  31  between walls  35 ,  36 . Not only do these tubes  40  promote turbulence but they also act as entry conduits for cooling air. FIGS. 6,  7  show details of the form and positioning of these tubes and arrows  41  indicate the swirling motion of the fuel air mixture as promoted by tube  40 . 
     The walls  35 ,  36  are curved radially inwardly through a right angle as indicated at  50  so that the passage  31  is continued radially inwardly; this part of the passage includes one or more swirlers  51  immediately upstream of an outlet  52  which is arranged such that it directs the fully mixed air/fuel mixture axially into the combustion chamber  12  (optionally via transition region  100 ) at its upstream end. Once again, it has to be arranged that the mixture issuing from outlet  52  has a velocity sufficient to prevent flash-back. 
     As indicated above, the combustor involves cooling arrangements utilizing cooling air. The cooling air is supplied by the compressor of the gas turbine plant, with a certain percentage of air being supplied for combustion purposes and the remainder for cooling. 
     The flow of cooling air in the illustrated embodiment is indicated by arrows  61 . The combustion chamber is, in this embodiment, formed with a double wall whereof the radially outer wall  36  also constitutes the inner wall of the supply passage  31  and the radially inner wall  38  of passage  37  constitutes the axially extending wall of the combustion chamber  12 . The cooling air enters passage  37  via the open-ended tubes  40  and enters the combustion chamber  12  via orifices  62  in wall  38 . The wall  38  and its continuation  101 , which is attached to or integral with wall  38 , have a thermal barrier coating  63  on their interior surfaces as marked by dash lines. This barrier coating  63  restricts the heat passing through to the walls  38 ,  101  from where it is removed by the cooling air flow  61  flowing in passage  37  whereby the metal, of which walls  38 ,  101  are made, operates within its temperature limit. The spent and now heated cooling air enters the combustion chamber  12  (see arrow  63 ) in a dilution zone  70  downstream of the main combustion zone  71 . By such means heat taken out of the system at one point is usefully put back at another—such an arrangement is termed regenerative. 
     It should further be noted there is also transfer of heat from the cooling air flow  61  in passage  37  to the air/fuel mixture in passage  31 . This preheating of the mixture is useful in avoiding a quenching effect that might result if too cold a mixture is fed into the combustion chamber  12  (such quenching may result in the production of unwanted CO). Of course it must be ensured that not too much heat is transferred to passage  31 , otherwise there is a danger of mixture ignition in the passage  31  itself. 
     It should be noted that in the case of a single wall combustor where there is no annular passage  37  for flow of cooling air, the inner wall of passage  31  will be constituted by the single wall  38  of the combustor, and heat will be transferred straight from the combustion chamber  12  to the air/fuel mixture in passage  31 . 
     The embodiment of FIG. 2 differs from FIG. 1 inasmuch as the cooling air flow represented by arrows  261  enters passage  237  through an inlet  232  adjacent the downstream end of the combustor  210  and flows towards the upstream end of combustion chamber  12  where it enters the combustion chamber via a swirler  224 . In this arrangement, therefore, as compared with that of FIG. 1 there is no dilution air supplied to the combustion gases at the downstream end of the combustion chamber  12  but rather additional air is added to the fuel/air mixture. It is to be noted that in this embodiment the coolant air in passage  237  flows in the same axial direction as the fuel/air mixture represented by arrows  234  flowing in passage  231 . This means that there will be less heat transfer into the mixture  234 , than in the arrangement of FIG. 1, and less chance of ignition in passage  231 . 
     In the embodiment of FIG. 3, features of the embodiments of FIGS. 1 and 2 are effectively combined in that the cooling air enters passage  337  through open-ended tubes  340  that extend through passage  331  of the third injection means. Some of this air flows through passage  337  to enter the combustion chamber  12  at the downstream end thereof while the rest of the air flows into the upstream end of the combustor chamber  12  through a swirler  324 . 
     The embodiment of FIG. 4 is generally similar to that of FIG. 1 save that the dilution air enters a combustor/turbine transition duct region  480  downstream of the main combustion chamber  12 . This may result in better temperature profiling of the combustion gases in certain circumstances. 
     In the embodiment of FIG. 5, the cooling air represented by arrows  561  enters the annular passage  537  through impingement holes  590  provided in the transition duct region  580  and flows into the combustion chamber  12  through orifices  562  in the direction of arrow  563  to dilute the combustion gases and is also directed into the upstream end of the chamber  12  through orifices  591 .