2. Background of the Related Art Gas turbines are employed in a variety of applications including electric power generation, military and commercial aviation, pipeline transmission and marine transportation. In a gas turbine engine, fuel and air are provided to a burner chamber where they are mixed and ignited by a flame, thereby initiating combustion. Several major technical problems are associated with the combustion process in gas turbine engines. These problems include, for example, thermal efficiency of the burner/combustor, proper mixing of the fuel and air, flame stabilization, the elimination of pulsations and noise, and the control of polluting emissions, especially nitrogen oxides (NOx). Flame stabilization refers to fixing the position and intensity of the flame within the burner so as to, among other things, eliminate pulsations and reduce noise.

Stable combustion in gas turbine engines requires a cyclic process of <BR> combustion producing products, i. e. , heat and free radicals, which are transported back upstream to the flame initiation point to facilitate the combustion process.

It is presently known to provide swirled air to the fuel-air mixture or to impart a swirl to the fuel-air mixture in order to improve flame stabilization and thereby stabilize the combustion process. Swirl stabilized combustion flows facilitate combustion by developing reverse flow about the centerline of the burner, which returns heat and free radicals back upstream to the un-burnt fuel-air mixture.

U. S. Patent Nos. 5,131, 334; 5,365, 865; and 5,415, 114 to Monroe et al. each disclose coal fired burners that include a flame stabilizer for imparting a swirl to the fuel-air mixture. The disclosed flame stabilizers include a plurality of radially spaced- apart vane elements mounted on a ring member which is positioned over a central fuel supply tube. The vanes are shaped and oriented to provide swirled air to the downstream end of the fuel supply tube.

U. S. Patent No. 5,477, 685 to Samuelson, which is herein incorporated by reference in its entirety, discloses a swirl stabilized, lean burn injector for a gas turbine combustor. In an illustrative embodiment, a fuel-air mixture exits a centrally positioned nozzle through a plurality of radially-oriented exit ports. An air swirler and a quarl device are attached to the downstream end of the Samuelson injector for facilitating the re-circulating flow. The fuel-air mixture exiting radially from the nozzle is met by air traveling axially through the injector in a helical path due to the air swirler. A quarl is a device, which is used in industrial boilers and furnaces to strengthen and modify the shape of the recirculating hot combustion products.

Conventional burners, which utilize swirl stabilized combustion, such as those disclosed above, must have a swirl strength that is sufficient to allow the recirculation about the centerline to develop as shown in Fig. 1. As noted above, in swirl-stabilized combustion, the combustion is stabilized when the heat and free radicals produced by the process are transported back upstream in the recirculation zone to mix with and initiate combustion of the un-reacted fuel-air mixture. Stable combustion is very dependant upon the recirculation of these hot combustion products back upstream.

Still further, when the velocity of the recirculated combustion products is increased, the flux of hot and chemically active combustion products upstream increases and the combustion process tends to become more stable over a wider range of operating conditions.

The swirl strength strongly influences the size, shape and strength of the recirculation zone of hot combustion products. The swirl strength is measured by a nondimensional number defined as the ratio of axial flux of angular momentum to the axial flux of axial momentum. In general, a recirculation zone is not created when the swirl number is less than 0.4,. When the swirl number increases, this causes the total pressure at the forward stagnation point to decrease. Shown in Fig. 1, the forward stagnation point is the point where the upstream flow of combustion products along the centerline meets the downstream axial flow of the air from the burner, at this point all velocities are zero. Typically, a swirl number greater than approximately 0.6 creates a low pressure region at the forward stagnation point. This low pressure region causes the combustion products to flow from the downstream end of the burner where the pressure within the burner is higher, upstream to the forward stagnation point where the pressure is reduced. This is the mechanism that results in the formation of the main recirculation zone (see Fig. 1).

Increasing the swirl number (Sn) tends to decrease the pressure at the forward stagnation and increases the upstream recirculation velocity near the centerline.

This increased upstream flow of combustion products increases the flux of hot gas and chemically active species to the forward stagnation point where strong combustion can be initiated. When the swirl number is low (i. e. , 0.4 < Sn < 0.6), the pressure at the forward stagnation point is only slightly lower that the pressure at the aft stagnation point of the recirculation zone. As a result, the flux of the hot and chemically reactive combustion products that are transported upstream is low and the combustion is less stable, especially when the combustion is lean.

The swirl number has other effects on the recirculation zone. For example, increasing Sn reduces the low pressure at the forward stagnation point and pulls the aft stagnation point upstream, making the recirculation zone shorter. Additionally, the circumferential forces that increase as Sn increases, result in the diameter of the recirculation zone increasing as well.

A quarl is a device, which is used in industrial boilers and furnaces to desensitize the length and diameter of the recirculation zone from the magnitude of the swirl number. The quarl also allows the diameter of the recirculation zone to be expanded to the diameter of the exit of the Quarl without having to increase Sn. Still further, when a quarl is used, the length of the recirculation zone is less sensitive to the swirl number and assumes a length of about 2 to about 2.5 times the quarl exit diameter.

A quarl allows for a high Sn to be used without producing a large diameter recirculation zone. However, in burners which use a quarl, when the swirl strength is strong, the flame has a tendency to move upstream, deep inside the burner causing damage to the burner components. Additionally, when the combustion is initially on the lean side of stoichiometric, making the mixture richer increases the flame speed. This increase in flame speed causes the flame to travel further upstream into the burner. In addition to damaging the burner hardware, the uncontrolled movement of flame deep into the burner can result in high NOx emissions.

Moreover, stability issues can become magnified when changes are made to the fuel/air ratio. When lean premixed combustion becomes very lean, the flame speed becomes very sensitive to changes in the fuel/air ratio. The continually varying flame speed often results in a shifting flame position, causing combustion pressure oscillations and noise.

Combustions instability can also occur when the flame moves inside the burner causing the fuel/air ratio to become richer, which causes the flame to move deeper into the burner. The richer fuel/air ratio is typically counteracted by decreasing the swirl strength. However, this will result in a cyclic process of the flame moving in and out of the burner. This common instability problem can result in very high-pressure pulsations and an increase in NOx emissions. This instability is typically a low frequency instability, generally ranging between 80 to 150 Hz. The amplitude of the pressure pulsations can exceed 0.1 bar and are destructive to the gas turbine engine. Furthermore, during the part of the instability cycle where the combustion is rich, significant amounts of NOx can be produced.

In view of the foregoing, a need exists for an improved burner, which improves flame stabilization, reduces pressure pulsations, noise and NOx emissions.

SUMMARY OF THE INVENTION The subject application is directed to a burner for a gas turbine combustor that uses a central bluff body flame holder and a quarl to stabilize the combustion process. The burner includes, among other elements, a cylindrical main body and a flame holder.

The main burner body includes axially opposed upstream and downstream end portions and has at least one fuel inlet passage and at least one air inlet passage formed therein. The fuel and air inlet passages are adapted to supply fuel and air respectively to a mixing chamber that is formed in the downstream end portion of the main body. The mixing chamber has a plurality of circumferentially spaced-apart surfaces formed on an interior thereof for swirling and mixing the fuel and air supplied to the mixing chamber.

The flame holder is disposed within the mixing chamber and includes a base portion and an elongated bluff body. The base portion engages with the main body of the burner in a supporting manner and the elongated bluff body extends in an axially downstream direction from the base portion through the internal mixing chamber so as to position a combustion ignition point downstream of the internal mixing chamber.

The burner further includes a quarl device disposed adjacent to the downstream end portion of the main body. The quarl device defines an interior recirculation chamber and a burner exit. The interior recirculation chamber is adapted for receiving precombustion gases from the mixing chamber and for recirculating a portion of the combustion products gases in an upstream direction so as to aid in stabilizing combustion.

It is presently envisioned that the bluff body of the flame holder is centered within the mixing chamber and has a tapered upstream section and a substantially cylindrical tip region. Ideally, the flame holder has an axial length which is adapted for achieving an Sn of greater than about 0.6. The swirl number being a ratio of the tangential momentum to axially momentum that defines how much of the combustion air going through the burner is rotating versus how much of the combustion air exiting the burner is in an axial flow condition. A mathematical definition of the swirl number can be found in U. S. Patent No. 5,365, 865 to Monroe, which is herein incorporated by reference in its entirety.

In an exemplary embodiment, the at least one air inlet passage is formed in a substantially radially inward direction and the fuel enters the mixing chamber of the main body in a substantially axial direction. Ideally, the air enters in a tangential and radially inward direction of the air inlet imparts swirl on the air passing through the burner, which is adapted for achieving an Sn of greater than about 0.6.

Those skilled in the art will readily appreciate that the inventive aspects of this disclosure can be applied to any type of combustor or burner, such as a solid fuel burner or furnace.

BRIEF DESCRIPTION OF THE DRAWINGS So that those having ordinary skill in the art to which the present application appertains will more readily understand how to make and use the same, reference may be had to the drawings wherein: Fig. 1 is a perspective view in cross-section of a prior art swirl stabilized burner ; Fig. 2 is a perspective view in cross-section of the swirl stabilized burner of the subject invention which includes a bluff body flame holder ; Fig. 3 is a cross-sectional view of the burner of Fig. 2 illustrating the swirl flow within the burner and the anchoring of the forward stagnation point of a main recirculation zone and the flame front by the center bluff body flame holder ; Fig. 4a is a cross-sectional view of a burner constructed in accordance with a preferred embodiment of the present invention which illustrates the flame stabilized on the center bluff body flame holder ; Fig. 4b is a cross-sectional view of a prior art burner without a center bluff body flame holder illustrating the flame in the flash back position; Fig. 4c is a cross-sectional view of the burner of Fig. 4b illustrating the flame positioned in the downstream end portion of the burner, near the exit ; and Fig. 4d is a cross-sectional view of the burner of Fig. 4b illustrating the flame positioned outside the burner exit.

These and other features of the burner of the present application will become more readily apparent to those having ordinary skill in the art form the following detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings wherein like reference numerals identify similar structural aspects of the subject invention, there is illustrated in Fig. 2 a burner for a gas turbine combustor designated generally as reference numeral 100. Burner 100 uses a central bluff body flame holder 20 and a quarl device 80 to stabilize the combustion process. The burner 100 includes, among other elements, a cylindrical main body 50, a flame holder 20 and a quarl device 80. The main body 50 and the flame holder 20 may be attached to one another in a conventional manner, or held together by an interference fit, or mechanically interlocked.

The burner main body 50 includes axially opposed upstream and downstream end portions, 52 and 54, respectively. A plurality of axially-oriented fuel inlet passages 56 and a plurality of radially-oriented air inlet passages 58 are formed in main body 50. Those skilled in the art would readily appreciate that the location, quantity and orientation of the fuel inlet passages 56 and air inlet passages 58 can vary without departing from the inventive aspects of this disclosure and the configuration depicted herein is for illustrative purposes only.

The fuel and air inlet passages 56 and 58 are adapted to supply fuel and air respectively to a mixing chamber 60 that is formed in the downstream end portion 54 of the main body 50. The mixing chamber 60 has a plurality of circumferentially spaced- apart surfaces 62 or vanes formed on an interior thereof for imparting a swirling motion to and mixing the fuel and air supplied to the mixing chamber 60.

The flame holder 20 is disposed within the mixing chamber 60 and includes a base portion 22 and an elongated bluff body 24. The base portion 22 engages with the main body 50 of the burner 100 in a supporting manner and the elongated bluff body 24 extends in an axially downstream direction from the base portion 22 through the internal mixing chamber 60 so as to position a combustion ignition point or forward stagnation point 75 (see Fig. 3) downstream of the internal mixing chamber 60. The elongated bluff body 24 has a plurality of axially-extending flutes 27 formed in its outer surface to define the scale of turbulence within burner 100.

Quarl device 80 disposed adjacent to the downstream end portion 54 of the burner main body 50. The quarl device 80 defines an interior recirculation chamber 82 and a burner exit 84. The interior recirculation chamber 82 which is defined by interior surface 82a is adapted for receiving precombustion gases from the mixing chamber 60 and for recirculating a portion of the combustion product gases in an upstream direction, so as to aid in stabilizing combustion. In the embodiment disclosed herein, interior recirculation chamber 82 is shaped in a classical venturi shape. However, other shapes that accomplish the pressure gradient separation of the mixing chamber and the recirculation chamber, are contemplated by the invention herein.

The bluff body portion 24 of the flame holder 20 is centered within the mixing chamber 60 and has a tapered upstream section 26 and a downstream neck region 28 that has a radially enlarged head. The shape of the neck region can be adapted to further improve the recirculation of the combustion products and flame stability. The length of the flame holder 20 is chosen so as to anchoring a main recirculation witha a swirl number of greater than about 0.6, but not larger than about 2.0. As stated previously, the swirl number is defined as the ratio of the amount of rotating combustion air going through the burner versus the amount of the combustion air exiting the burner is in an axial flow condition.

Burner 100 is adapted for making the cyclic combustion process more stable and significantly reduces the tendency for a gas turbine engine using a lean premixed combustion to flame out or to produce pressure pulsations resulting from unstable combustion. The center body flame holder 20 and the quarl 80 have two key effects: 1) the point where combustion is initiated is fixed in space, and 2) higher swirl velocities can be obtained without combustion to flash back into the mixing chamber 50 of the burner 100. The anchoring of the flame using the bluff body flame holder 20 on the central axis allows for natural fluctuations of the fuel/air ratio and variations in the swirling velocity to occur without a change in the flame position. The ability to increase swirl strength without causing a flash back and the fixing the combustion initiation point, both make the combustion process more stable. Therefore, the use of a quarl 80 and the bluff body flame holder 20, fundamentally changes the stability of swirl-stabilized combustion, as compared to prior art burners The flame holder 20 physically prevents the flame from flashing back up the centerline of the burner 100 into the mixing chamber 50. By preventing flash back up the centerline into the mixing chamber 50 the fuel-air mixture can have a higher tangential swirl component. Increasing the swirl strength without flash back makes the quarl more efficient in strengthening the recirculation of hot gas upstream, making the entire combustion process more stable. Increasing the amount of heat re-circulated upstream allows stable combustion of leaner fuel-air mixtures. This provides for greater flexibility and robustness in engine operation, while maintaining low engine emissions.

Quarl device 80 is used to make the recirculation zone smaller than what would be produced as a result of the influence of the swirl number alone. The quarl device 80 allows for a high swirl number while maintaining a small diameter and short length of the recirculation zone. The high swirl number results in a large difference in pressure between the forward and aft stagnation points. This high pressure gradient results in high velocity and high flux of hot chemically active combustion products to flow about the centerline to the forward stagnation region where combustion is initiated.

A high flux of hot chemically active combustion products at the location where combustion is to be initiated allows for stable combustion of lean fuel and air mixtures.

Stable combustion of lean fuel and air mixtures is important to produce low nitrous oxide, NO and N02, emissions in gas turbine engines.

Keeping the recirculation zone small helps to preserve the chemical activity of the hot combustion gases, allowing for more rapid and stable initiation of combustion, especially at low combustion temperatures, such as those that often occur below 1700 K in low NOx (NO and N02) engines. This low residence time in the recirculation for the chemically reactive products of combustion becomes more important as combustion pressures rise and combustion temperatures decrease. At high pressure chemically reactive species, also known as free radicals that are useful to initiate rapid combustion, relax to equilibrium level quickly under the effect of high pressure. The life of free radicals that are above equilibrium levels become shorter as pressure is increased.

Effective use of these high non-equilibrium levels of free radical becomes more important when the combustion temperature is low, such as in low NOx engines, because the equilibrium levels of free radicals are low at low temperatures.

Referring now to Fig. 3, there is illustrated a cross-section of the swirl stabilized burner 100 that depicts the recirculation of combustion products upstream so as to sustain the combustion process. The upstream and downstream ends of burner 100 are identified by reference character"U"and"D", respectively. As shown in this figure, the flow of combustion products is separated into distinct zones, namely a main recirculation zone 90 and an outer recirculation zone 92.

As stated earlier, the process of swirling the fuel-air mixture so as to cause the combustion products to travel upstream is commonly used to stabilize combustion. In the disclosed burner 100, the bluff body flame holder 20 anchors the main re-circulation zone 90 into a fixed position. The flame front or combustion initiation point 94 of the premixed flow occurs along the outer surface of the main re-circulation zone 90 where the heat and free radicals mix and initiate combustion of the unreacted premixed fuel and air. The flame starts at the end of the flame holder 20 and expands in a conic pattern downstream.

Burner 100 maintains the flame position fixed to the tip 24 of the flame holder 20, even when significant changes occur in the fuel/air ratio. When lean premixed combustion becomes very lean the flame speed becomes very sensitive to the fuel/air ratio. This change in flame speed often results in shifts in flame position, which may result in combustion pressure oscillations. By anchoring the flame with the center bluff body flame holder 20 and preventing the flame from moving, pressure oscillations can also be prevented.

Fig. 4a provide a cross-sectional view of burner 100 that illustrate conical flame 98 anchored to flame holder 20. Figs. 4b-4d depict a burner 200 that does not include a center body flame holder. In burner 200, when the swirl strength is strong, or the premixed fuel/air ratio is rich, the flame 298 has a tendency to move deep inside the burner, as shown in Fig. 4b. When the combustion is on the lean side of stoichiometric, making the mixture richer increases the flame speed. Increased flame speed makes it possible for the flame to travel further upstream. Increasing the swirl strength will also produce the same tendency to move the flame further upstream. Generally, it is not desirable to have the flame 298 move into the burner mixing region 260, as shown in Fig.

4b. The uncontrolled movement of flame deep into burner 200 can result in damage to hardware and result in high NOx ernissions. The addition of the center bluff body flame holder 20 to the quarl modified burner anchors the forward stagnation point 96 of the main recirculation zone 90 to the end of the flame holder 20 preventing the main recirculation zone 90 and flame from traveling into the mixing chamber 60. The center bluff body flame holder anchors the forward stagnation point 96 (see Fig. 3) to the end of the flame holder 20 for swirl strengths that would have otherwise driven the forward stagnation 96 point deep inside the burner 100, or toward the exit 84, or even outside, the burner 100. The center bluff body flame holder 20 anchors the forward stagnation point 96 and flame 198 to a single fixed location, instead of moving continuously as the swirl strength varies.

There is an optimum location for the center bluff body flame holder 20 where the swirl number can be equally increased or decreased and the forward stagnation point 96 and flame 198 stays attached to the flame holder 20. If the swirl strength is continually decreased, the flame 198 will stay attached to the flame holder until finally the flame jumps off the flame holder and stabilizes a significant distant downstream, or outside the burner's exit 84. Starting from the same optimum swirl number and center bluff body flame holder position, increasing the swirl strength will not effect the flame position, until at some critical swirl strength, the flame position will jump upstream engulfing the end of the flame holder 20 inside the main re-circulation zone. As long as the operating conditions stay within reasonable range of swirl strength and fuel/air ratio, the flame position will stay fixed even as engine conditions change. These ranges have been shown to be very wide, which is a positive attribute of burner 100.

The movement of flame position is a significant problem for combustion systems that operate very lean. The flame 198 shown in Figs. 4c and 4d. is produced at the leanest fuel/air ratios and/or the lowest swirl strength. When the combustion becomes very lean, swirl stabilized combustion can become very unstable. However, the method shown to be most successful in reducing NOx emissions is to make the combustion so lean that the temperature of the flame is reduced below the temperature at which diatomic Nitrogen and Oxygen (N2 and O2) disassociate and recombine into NO and NO2 When nearly twice the amount of air is mixed with the fuel before the fuel and air mixture combust, the excess air acts as inert matter that is heated by the combustion process. The amount of energy released by the combustion process is determined only by the amount of fuel burnt, as long as sufficient or greater amount of air is supplied to the combustion process. The air in excess of the amount necessary for combustion does not effect the amount of energy released by the combustion process, but because the combined mass of fuel and air is increased while the energy released is constant, the flame temperature and the temperature of the combustion products is reduced. This reduction of flame temperature reduces the formation of NOx (NO and N02). This is the principle upon which virtually all-low NOx emissions gas turbine engines are currently based.

As stated above, the addition of the center bluff body flame holder 20 to the burner 100 allows the swirl strength to be increased without the flame flashing back into the mixing chamber 50. The ability to increase the swirl strength increases the reverse flow of hot combustion products back upstream. The increased flow of hot combustion products provides more heat and free radicals, which makes the combustion more robust and less susceptible to instabilities.

If the flame front starts external to the burner, the burner will have a maximum airflow rate for a fixed pressure drop across the burner. If some perturbation allow the flame to jump inside the burner, the mass flow rate of air through the burner will decrease, because the heat from the combustion process will cause the air to expand increasing the volumetric flow rate through the exit of the burner. This increase is volumetric flow rate for a fixed pressure drop results in a decrease in the mass flow rate of air through the burner. For most gas turbine engines, six to 100 burners would be used depending upon the power rating of the engine. If the flame jumps into some burners, but not all burners, the burners that have flame inside will burn richer. This is because the same fuel will be supplied to all burners equally through a common fuel manifold. The burners with the flame inside are richer because the mass flow rate of air decreases due to the increased volumetric flow rate as a result of the combustion inside the burner exit.

The result of the richer combustion when the combustion is initially lean is to increase the flame speed. The increase in flame speed allows the flame to move deeper into the burner. This will increase the volumetric airflow rate and decrease additionally the mass flow rate of air making the combustion even richer. Once inside it is possible for the flame to stay inside a few of the burners, while staying external to the rest of the burners.

When this occurs high NOx results and hot spots occur entering the turbine inlet corresponding to the richer burners.

The combustion processes inside the burner will also affect the swirl characteristics, which can lead to the reversal of the previous process pulling the flame inside the burner. When the flame pulls inside the burner the mass flow rate of air decreases. The density of air passing through the swirler does not change resulting in lower velocity and a decrease in swirl strength. The decrease in swirl strength will tend to cause the forward stagnation point of the main re-circulation to move downstream.

The combustion process itself will also tend to decrease the swirl strength, because the combustion process expands the flow uniformly in all directions.

Instability can occur when the flame moves inside the burner causing the fuel/air ratio to become richer, which causes the flame to move deeper into the burner. Counteracting the richer fuel/air ratio that produces higher flame speed is the decay of swirl strength. This will result in a cyclic process of the flame moving in and out of the burner. This common instability can result in very high-pressure pulsations and an increase in NOx emissions.

This instability is a common low frequency instability generally of 80 to 150 Hz. The amplitude of the pressure pulsations can exceed 0.1 bar pressure oscillations and be destructive to the gas turbine engine. During the part of the cycle where the combustion is rich, significant amounts of NOx can be produced. The invention of the center bluff body flame holder applied to the quarl based burner makes the position of the flame insensitive to changes in the swirl strength and fuel/air ratios allowing the flame to stabilize in a fixed location at the end of the flame holder. This eliminates the pressure oscillations and the elevated NOx emissions that would have resulted from the movement of the flame.

While the invention has been described with respect to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications can be made to the invention without departing from the spirit or scope of the invention as defined by the appended claims.

The swirl number being a ratio of the tangential momentum to axially momentum that defines how much of the combustion air going through the burner is rotating versus how much of the combustion air exiting the burner is in an axial flow condition.