Abstract:
This invention relates to an apparatus and method for increasing the reactivity of a fuel/air mixture prior to homogenous combustion of the mixture. More specifically, this invention is a pilot for a gas turbine combustor which utilizes the heat of combustion within the pilot to increase the reactivity of a portion of the fuel/air mixture utilized by the pilot.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus and method for increasing the reactivity of a fuel/air mixture prior to homogenous combustion of the mixture. More specifically, the present invention is a pilot for a gas turbine combustor which utilizes the heat of combustion within the pilot to increase the reactivity of a portion of the fuel/air mixture utilized by the pilot. 
     2. Brief Description of the Related Art 
     Known dry low NOx combustion systems for gas turbines can achieve relatively low emissions levels; however, the use of continuous pilot systems, as distinguished from starter systems, is required to stabilize combustion over a wide range of gas turbine operational conditions and minimize emission levels. 
     U.S. Pat. No. 5,634,784 represents a state-of-the-art continuous pilot. The patent teaches a catalytic pilot that will make a portion of the fuel/air mixture destined for the pilot&#39;s combustion zone more reactive by passing it through a catalytic centerbody. The patent also teaches that by recirculating hot combustion gas products back on to the catalytic centerbody the catalytic centerbody can use the heat of combustion within the pilot to assure that the catalytic component of the centerbody is at a suitably high operating temperature. 
     The structure of the catalytic centerbody design previously taught has several shortcomings. In particular, no method is provided to limit the temperature of the centerbody, thus the surface temperature could reach the adiabatic flame temperature of the fuel/air mixture, generally above th e centerbody&#39;s material failure temperature. In addition, a catalyst is required. 
     It has now been found that a stabilizing pilot can be created without the use of a catalyst. By utilizing the fuel/air mixture passing through the centerbody more fully, a more versatile pilot can be created. The invention accomplishes this by increasing the channel length for the fuel/air mixture within the centerbody, and by utilizing the fuel/air mixture entering the centerbody for cooling the centerbody structure, increasing the temperature and overall combustibility of the fuel/air mixture, and allowing the centerbody to be exposed to greater temperatures, even temperatures above the material limit of the centerbody. 
     SUMMARY OF THE INVENTION 
     The Dry, Low NOx Pilot (hereinafter “pilot”) is a continuously operating pilot that stabilizes the combustion within a gas turbine combustor. The basic pilot utilizes two fuel/air flows. One fuel/air flow enters a centerbody, and by passing through the centerbody obtains a temperature rise by extracting heat from the centerbody. The second fuel/air flow passes through a flow condition er capable of creating a recirculation zone to provide heat to the centerbody. 
     The centerbody and flow conditioner are parts of an integrated assembly. In the pilot of the present invention, the combustion zone is maintained downstream of the centerbody. The flow conditioner has the dual functions of contacting the second fuel/air mixture with heated fuel/air exiting the centerbody, and creating a recirculation zone such that hot combustion gases, either through radiation or conduction, impart a temperature rise to the centerbody. The flow conditioner can be any structure capable of accomplishing these functions, such as a swirler, a bluff body, a dump, opposed flow jets, or a combination of any of the above. 
     The centerbody is attached to the flow conditioner. During operation, a portion of the centerbody is simultaneously exposed to the heat of the recirculation zone on one surface and the fuel/air mixture entering the centerbody on an opposite surface (or backside). The entering fuel/air mixture sufficiently interacts with this opposite surface to obtain a temperature rise thereby lowering the temperature of the centerbody. This backside cooling of the centerbody can allow the temperature of the recirculation gases to exceed the material limit of the centerbody. Backside cooling allows for an increased temperature rise to be imparted to the fuel/air mixture; as the temperature of the recirculation gases contacting the centerbody are increased the temperature rise imparted to the fuel/air mixture is increased for any given flow. 
     The centerbody can be of numerous three dimensional configurations, such as cylindrical or elliptical; symmetry is not required. 
     The centerbody should have a high thermal conductivity. Preferably, the thermal conductivity of the cap should allow for a uniform distribution of the heat imparted to the cap from the recirculating gases so the temperature of the cap is approximately uniform. 
     The centerbody can incorporate a fuel/air channel. The fuel/air channel is placed within the centerbody such that the fuel/air mixture enters the space between the centerbody and the fuel/air channel, forcing the fuel/air mixture to be turned by, and to interact with, the cap. The fuel/air channel is placed within the centerbody such that a minority of the exits from the centerbody are above the exit of the fuel/air channel. The term “above” refers to a direction parallel to (and in the same direction as) the flow of the fuel/air mixture in the fuel/air channel. For the present invention structures, the centerbody is attached to the fuel/air channel by securing the base of the centerbody directly to the fuel/air channel, thereby forming a baffle and forcing all the fuel/air entering the centerbody to enter through the fuel/air channel. 
     For the present invention two fuel/air mixture flows are required—a fuel/air mixture flow through the flow conditioner and a fuel/air mixture flow through the centerbody. The fuel/air mixture can either be a single flow split between the flow conditioner and the centerbody based on the flow characteristics of both, or separate fuel/air flows with different characteristics. If separate fuel/air flows are provided, flow conditions could vary significantly, such as different fuel/air mixture ratios (even to the degree that one is rich and the other lean), different flow velocities, or different fuels. It is also possible to split a single fuel/air mixture but provide additional fuel injection to one or both of the two resulting fuel/air streams, thereby varying flow conditions, fuel/air mixture ratios, or fuel composition. These design alternatives can be done by those skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section of a centerbody. 
     FIG. 2 is a cross-section of a centerbody incorporating a fuel/air channel. 
     FIG. 3 is a cross-section of the pilot using a centerbody and a swirler as a flow conditioner. 
     FIG. 4 is a cross-section of a pilot using a centerbody incorporating a fuel/air channel and a swirler as a flow conditioner. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 depicts a non-catalytic centerbody  30 . The centerbody  30  is comprised of cylinder, open at the end where the fuel/air enters and closed at the other end, with numerous intermediate exits  33 . 
     FIG. 2 is a non-catalytic centerbody  30  as shown in FIG. 1 with a fuel/air channel  31  and baffle  32 , which confines the fuel/air to entering the centerbody  30  though fuel/air channel  31 . The centerbody  30  is placed approximately concentrically over the fuel/air channel  31 . The sides of centerbody  30  extend to a point below the exit of fuel/air channel  20 . The term “below” refers to a direction opposite the flow direction of the fuel/air mixture in the fuel/air channel  20 . The fuel/air channel  31  is placed within centerbody  30  such that the exit of the fuel/air channel is below at least one exit  33 . The shape of centerbody  30  is based upon the design requirements of the centerbody, and FIGS. 1 and 2 should be considered illustrative rather than limiting. 
     FIG. 3 is a pilot employing a pilot wall  55 , a centerbody  30 , and a swirler  50 , as the flow conditioner structure. Swirler  50  is mounted approximately concentrically within the pilot wall  55 , and a centerbody  30  is mounted approximately concentrically within swirler  50 . A single fuel/air mixture  40  which is forced by pressure into the swirler  50  and the centerbody  30 , forming fuel/air mixtures  43  and  42  respectively. 
     Swirler  50  is selected such that the swirl of swirler  50  will cause a recirculation zone to form sufficient to cause the recirculating combustion gases to contact centerbody  30 , throughout a significant portion of the operating range of the pilot. 
     To create a proper recirculation zone, swirler  50  must cause vortex breakdown within the swirl zone sufficient to cause flow reversal and backmixing. Generally, swirl number greater than 0.5 is required to achieve this result. For this invention the swirl number is defined as the quotient which results when tangential momentum flux is divided by the product of swirler radius and axial momentum flux. Tangential momentum flux is defined as the product of density, axial velocity, tangential velocity, radius, and flow area. Axial momentum flux is defined as the product of the density, the square of axial velocity, and the flow area. The combination of a dump (sudden expansion in flow area) and swirl is particularly effective in creating a strong recirculation zone, especially at low swirl number (order of magnitude 0.5). In a preferred embodiment of the present invention, a dump is located downstream of the swirler to assist in stabilizing combustion and providing strong recirculation of hot combustion gases to contact the pilot centerbody. 
     The placement of the centerbody  30  within swirler  50  is based on the characteristics of the recirculation zone created by swirler  50 . Those skilled in the art will recognize that the strength and position of the recirculation created by swirler  50  will change over the operating range of the pilot. In the present invention, it is preferred that the recirculation created by swirler  50  be created in such a fashion that for a majority of the operating conditions of the pilot the recirculating gases  60  contact the top portion of centerbody  30 , thereby causing direct conduction heating of the centerbody  30 . While direct conduction heating of centerbody  30  has been described, the present invention is not limited in this regard. 
     As the fuel/air mixture  40  enters the pilot the fuel/air mixture flow is split naturally between the centerbody  30  and the swirler  50 . The minimum fuel/air flow entering the centerbody  30  is based upon the fuel/air mixture flow required to stabilize the combustion zone and provide cooling of the centerbody  30 , in the area where the centerbody is being heated by the recirculating gases  60 . Those skilled in the art will appreciate that the backside cooling provided by the fuel/air mixture flow maintains the temperature of centerbody  30  at an appropriate operating temperature, based upon the materials used to construct centerbody  30 . The fuel/air mixture  42  flow should be no less than approximately 1% of the total fuel/air mixture  40  flow and should not exceed approximately 25%. A preferred range is between 3% and 10%. 
     The required degree of interaction between the fuel/air mixture and the backside surface of the centerbody is determined by the desired temperature rise in fuel/air mixture  42 ; a temperature rise of the fuel/air mixture  42  is preferably at least 25 degrees Celsius. Allowable residence time is limited by either the auto-ignition delay time of the fuel/air mixture  42 , or the requirement for maintaining sufficient velocity within the passage to prevent flashback of the flame. A nominal residence time is approximately 1 msec, but it could range from 0.1 to 10 msec. 
     FIG. 4 is a pilot employing a non-catalytic centerbody with a fuel/air channel and a swirler  50  as the flow conditioner. The use of a fuel/air channel enhances the ability of the furl/air mixture  42  to backside cool centerbody  30 .