Patent Application
20130061594
The invention relates to fuel combustion in a gas turbine, and particularly relates to guiding compressed air to a combustion zone in a combustor.
A gas turbine combustor mixes large quantities of fuel and compressed air, and burns the resulting air and fuel mixture. Conventional combustors for industrial gas turbines typically include an annular array of cylindrical combustion “cans” in which air and fuel are mixed and combustion occurs. Compressed air from an axial compressor flows into the combustor. Fuel is injected through fuel nozzle assemblies that extend into each can. The mixture of fuel and air burns in a combustion chamber of each can. The combustion gases discharge from each can into a duct that leads to the turbine.
Pressurized air from the compressor enters a combustion can at the back end of the can, which is the same end from which hot combustion gases flow from the can to the turbine. The compressed air flows through an annular duct formed between a cylindrical wall of the can and an inner cylindrical combustion liner. The relatively cool compressed air cools the wall of the liner as the hot combustion gas flows through the interior of the liner. The hot combustion gas flows in a generally opposite direction to the flow of the compressed air through the duct.
As the compressed air reaches the head-end of the combustor can, the air is turned 180 degrees to enter one of the fuel nozzles. To enter the outer fuel nozzles the compressor air makes a tight and quick reversal of flow direction. This abrupt turn can create low velocity flow zones in the air while other zones of the air flow are at significantly higher velocities. The occurrence of low velocity flows is most acute as the air enters the outer fuel nozzles which are closest to the double walled flow path in the combustion chamber for compressed air.
Uniform flow velocities through a fuel nozzle are desired to provide uniform mixing of the air and fuel, and uniform combustion. Zones of low velocity airflow in the fuel nozzle also pose a flame holding risk inside the nozzle as low velocity zones provide an area for a flame to anchor inside the fuel nozzle. A flame in the fuel nozzle can destroy the hardware of the nozzle. In addition, low velocity air flows can cause localized variations in the air and fuel mixture. These variations can include regions where the fuel and air mixture is too rich resulting in too high combustion temperatures and excessive generation of nitrous oxides (NOx). There is a long felt desire to hold a steady flame in a combustor can, reduce NOx emissions from combustion in a gas turbine and maintain uniform airflow velocities through the fuel nozzles.
A fuel nozzle assembly has been conceived for a gas turbine, the assembly including: a cylindrical center body; a cylindrical shroud coaxial with and extending around the center body, and a turning guide having an downstream edge extending into the inlet of a passage between the center body and the shroud, wherein the turning guide extends only partially around the center body.
The turning guide may be a thin sheet shaped to conform to an inlet region of the shroud. The turning guide may have a wide mouth curved inlet region and a generally straight outlet region. The turning guide may be mounted to the shroud or center body by a rib or post. The turning guide may extend in an arc around the fuel nozzle, and the arc may be in a range of 200 degrees to 35 degrees. The turning guide may be on a side of the shroud adjacent an outer doubled-walled annular flow duct through which compressor air passes and is turned radially inward towards the assembly.
A combustion chamber has been conceived for a gas turbine comprising: an annular flow duct through which pressurized air flows in a direction opposite to a flow of combustion gases formed in the chamber; an end cover assembly having an inside surface; a radially inward turn in the flow duct proximate to the inside surface of the end cover assembly; at least one fuel nozzle assembly including a cylindrical center body, a cylindrical shroud coaxial with and extending around the center body, and a turning guide having an downstream edge extending towards a passage between the center body and the shroud, wherein the turning guide extends only partially around the center body, and the turning guide is aligned and proximate to an outlet of the annular flow duct such that the turning guide directs air from the annular flow duct into the passage between the center body and the shroud. The turning guide may be on a side of the shroud adjacent the annular flow duct.
A method has been conceived to direct pressurized air into an air flow duct of a fuel nozzle assembly in a combustion chamber, the method comprising: moving pressurized air in a first direction through an annular duct in the combustion chamber and turning the air radially inward from the duct towards the fuel nozzle; the turned pressurized air flowing into a passage between a cylindrical shroud and a center body of the fuel nozzle assembly; as the turned pressurized air flows into the passage, the air is directed by a turning guide having an inlet edge aligned with the turned air flowing from the annular duct and an outlet edge aligned with the passage, wherein the turning guide extends only partially around the center body.
The turning guide may be adjacent the outlet of the annular duct and directs air entering the passage at a location on a side of the center body opposite to the annular duct. The turning guide may be proximate to the inlet to the shroud and the directed air is air flowing near the inlet to the shroud. The turning guide may increase the velocity of air flowing into a radially outward portion of the passage. The turning guide may direct the turned air into a narrow gap between the turning guide and an inlet portion of the shroud, wherein the inlet portion has a wide mouth and the turning guide directs the turned air into the narrow gap between the turning guide and the wide mouth of the shroud.
Pressurized air 10 enters an end of the combustion chamber 6 and flows (see arrow 32) through an annular duct 34 formed between a cylindrical sleeve 22 and an inner cylindrical liner 36 of the chamber 6. The pressurized air 32 flows through the duct 34 towards the end cover assembly 26 in a flow direction opposite to the flow of combustion gases formed in the chamber. The pressurized air is turned by an annular portion of the duct 34 which may be U-shaped 38 in cross-section.
To assist in the turning of the air flow, a turning guide 42 is positioned on each of the fuel nozzle assemblies 20 and near the outlet of the U-shaped portion 38 of the air duct 34. The turning guide 42 may be mounted to be proximate to a rear collar 44 of the fuel nozzle.
The rear collar 44 connects the fuel nozzle assembly to a flange 27 which is attached to the end cover assembly 26. The collar may be brazed or welded to a flange 27. The flange 27 may be bolted to the end cover 26.
The turning guide may 42 have a cross-sectional shape conforming to the end of the U-shaped portion 38 of the annular duct. The turning guide 42 may extend in an arc partially around the circumference of the collar 44, such as 180 degrees around the collar. The arc of the turning guide may be in a range of 35 to 200 degrees. The upstream end of the turning guide 42 may extend, at least partially, into the U-shaped portion 38 of the flow duct. The downstream end of the turning guide may be aligned with the inlet of the annular duct 52 between the cylindrical shroud 46 and center body 50. The turning guide may extend partially into the annular duct 52. The downstream end of the turning guide may be radially inward of the shroud 46 such that a gap 53 exits between the shroud and the downstream end of the turning guide. The gap is at the radially outer region of the annular duct 52. Air flowing on the radially outer surface of the turning guide moves into the gap to ensure an air velocity at the radially outer region of the annular duct.
The turning guide 42 assists in providing a uniform flow of the pressurized air being turned into the fuel nozzle assemblies and cylindrical liner 36. The turning guide forms a flow path that increases the velocity of the pressurize air flow near the radially outer part of the shroud 46. The increase in the air velocity due to the turning guide suppresses the tendency of relatively low velocity air flows forming at the outer portion of the shroud. Using the turning guide to increase the flow velocity at the radially outer portion of the annular duct 52 creates a more uniform flow velocity through the entire fuel nozzle.
Air flow having a uniform velocity in the fuel nozzle promotes uniform fuel air mixing and promotes flame holding resistance in the fuel nozzle.
The air flowing through the annular duct 52 mixes with fuel entering the duct from the swirl vanes 54. The air-fuel mixture passing through the annular duct 52 is swirled by swirl vanes 54. The swirl vanes may be a generally cylindrical device mounted between the center body and shroud. The spiral flow induced by the swirl vanes promotes mixing of air and fuel in the duct 52. The mixture of fuel and air flows from the end of the duct 52 to the combustion zone 55 of the combustion chamber. The mixture of fuel and compressed air combust in the combustion zone and the combustion gases flow (see combustion flow arrow 14 in
The turning guide 42 may include an inlet portion 68 in the outlet region that is curved radially outward to conform to a desired flow path of air coming from the U-turn 38 shown in
The turning guide may extend partially around the wide mouth inlet 56 as an arc, half-circle or other portion of circle. As illustrated in
The turning guide 70 may be formed of a ceramic or metal, and may be an integral component. The turning guide 70 may have an inlet section 66 that curves radially inwardly to the axis of the center body, and a cylindrical outlet section 68 that is straight along the axis.
The turning guide 70 may be attached to the shroud 72 by ribs 74 and posts 76 extending from the wide mount shroud inlet 56, through the gap 53 and to the curved inlet 66 of the turning guide. The rib may be aligned to be parallel to the axis of the center body to reduce air flow resistance through the gap 53. The rib 74 may be at the center of the turning guide and the posts 76 may be near the sides of the turning guide.
The turning guide 70 may be shaped to conform to the wide mouth inlet 56. The gap 64 formed between the turning guide 70 and the wide mouth inlet 56 may have a uniform width and be proximate to the radially outer region of the duct between the turning guide and wide mouth. The inlet to the gap may extend generally radially inward and turn axial at the discharge of the gap. The gap is the guided flow passage for a portion of the pressurized air entering the annular air passage between the shroud and the collar and center body.
An air velocity profile 106 illustrates the generally uniform velocity of the air flow through the duct when a turning guide is at the inlet to the duct. The air velocity profile 108 shows the large variation in air velocity when a turning guide is not present. In particular, the air near the shroud 50 moves substantially slower than the air near the center body 78. As shown in
The more uniform air velocity through the duct 52 resulting from the turning guide may provide advantages such as reduced NOx emissions from the combustion chamber, and an increase in steady flame performance of the chamber.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
