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A combustor for a gas turbine engine includes a plurality of primary nozzles configured to diffuse or premix fuel into an air flow through the combustor; and a secondary nozzle configured to premix fuel with the air flow. Each premixing nozzle includes a center body, at least one vane, a burner tube

A combustor for a gas turbine engine includes a plurality of primary nozzles configured to diffuse or premix fuel into an air flow through the combustor; and a secondary nozzle configured to premix fuel with the air flow. Each premixing nozzle includes a center body, at least one vane, a burner tube provided around the center body, at least two cooling passages, a fuel cooling passage to cool surfaces of the center body and the at least one vane, and an air cooling passage to cool a wall of the burner tube. The cooling passages prevent the walls of the center body, the vane(s), and the burner tube from overheating during flame holding events.

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1. A combustor for a gas turbine engine, comprising: a plurality of primary nozzles configured to diffuse fuel into an air flowing through the combustor in a downstream direction; anda secondary nozzle configured to premix fuel with the air flow, the secondary nozzle comprising: a fuel passage exten

1. A combustor for a gas turbine engine, comprising: a plurality of primary nozzles configured to diffuse fuel into an air flowing through the combustor in a downstream direction; anda secondary nozzle configured to premix fuel with the air flow, the secondary nozzle comprising: a fuel passage extending downstream in the combustor and having a downstream end portion,a center body provided around the fuel passage,a burner tube provided around the center body and defining an annular air-fuel mixing passage between the center body and the burner tube, wherein the burner tube includes an inlet open to a volume of air flow;at least one vane assembly in the annular air-fuel mixing passage and upstream of the downstream end portion of the fuel passage, the at least one vane assembly including an internal chamber and swirl vanes wherein the internal chamber is upstream of the swirl vanes, andat least two cooling passages comprising a fuel cooling passage to cool surfaces of the center body and the at least one vane assembly, and an air cooling passage to cool a wall of the burner tube, wherein the fuel passage is configured to pass fuel in a downstream direction of the combustor and the fuel cooling passage includes an inlet to the fuel cooling passage proximate the downstream end of the fuel passage and an outlet of the fuel cooling passage open to the internal chamber of the at least one vane assembly and the air cooling passage is open to the volume of air flow providing air to the burner tube. 2. A combustor according to claim 1, wherein the fuel passage includes at least one hole configured to split fuel between impingement cooling a head end of center body and bypassing the reverse fuel passage. 3. A combustor according to claim 1, wherein the burner tube provided around the center body defines a fuel-air premixing passage and the burner tube wall is film-cooled by compressed air in the air cooling passage between the burner tube and an outer peripheral wall. 4. A combustor according to claim 3, wherein the internal chamber of the at least one vane assembly includes a cooling chamber configured to receive fuel from the fuel cooling passage, an outlet chamber configured to expel the fuel through at least one fuel injection port in the at least one vane assembly into the fuel-air premixing passage, and at least one divider provided between the cooling chamber and the outlet chamber to define a non-linear fuel path. 5. A combustor according to claim 4, wherein the at least one divider is provided with a by-pass hole configured to permit fuel flow directly from the cooling chamber to the outlet chamber. 6. A combustor according to claim 1, further comprising an inlet flow conditioner configured to angularly distribute the air flow. 7. A combustor according to claim 1, wherein the at least one vane assembly includes at least one spoke including at least one fuel injection hole configured to inject fuel into the air flowing the at least one vane. 8. A combustor according to claim 3, further comprising a plurality of circular rows of air cooling holes in the burner tube wall, each hole comprising an injection angle in the range of 0° to 45° with respect to a downstream wall surface, wherein a size of each hole, a number of holes in each circular row, and/or a distance between adjacent circular rows are arranged to achieve a desired wall temperature during flame holding events. 9. A combustor according to claim 1, wherein an air-fuel premixture is configured to produce a flame speed that is less than a velocity of the air flow. 10. A combustor according to claim 9, further comprising: a primary combustion chamber;a secondary combustion chamber; anda venturi between the primary combustion chamber and the secondary combustion chamber, wherein the air-fuel premixture is configured to produce a flame in the secondary combustion chamber that does not cross the venturi into the primary combustion chamber. 11. A method of operating a combustor of a gas turbine engine, the combustor comprising a plurality of primary nozzles provided in a primary combustion chamber and configured to diffuse fuel of a fuel supply to the combustor into an air flow through the combustor; and a secondary nozzle provided in a secondary combustion chamber and configured to premix fuel of the fuel supply with the air flow, the secondary nozzle comprising a a fuel passage, a center body provided around the fuel passage, a burner tube provided around the center body and defining an annular air-fuel mixing passage between the center body and the burner tube, at least one vane assembly in the annular air-fuel mixing passage including an internal chamber and swirl vanes downstream of the internal chamber and configured to swirl the air flow, and at least two cooling passages comprising a fuel cooling passage to cool surfaces of the center body and the at least one vane, and an air cooling passage to cool a wall of the burner tube, the method comprising: providing an air flow to the combustor;providing a fuel supply to at least one of the plurality of primary nozzles and the secondary nozzle;diffusing fuel supplied to the primary nozzles into the air flow;premixing fuel supplied to the secondary nozzle with the air flow, wherein the air flow enters the burner tube and mixes with fuel discharged from the vane assembly;cooling the center body and the at least one vane assembly with a portion of the fuel in the fuel cooling passage, wherein fuel flows through the fuel cooling passage in an upstream direction as compared to the downstream direction of the combustor and fuel from the fuel cooling passage passes through the internal chamber within the at least one vane, wherein the cooling of the vane assembly;discharging the fuel from the internal chamber through fuel injection apertures arranged on the vane assembly and upstream of the swirl vanes, andfilm cooling the burner tube with a portion of the air flow in the air cooling passage between the burner tube and an outer peripheral wall by providing film cooling holes in the burner tube. 12. A method according to claim 11, further comprising: passing fuel in a downstream direction of the combustor through a fuel passage; andpassing fuel in an upstream direction of the combustor through a reverse fuel passage defined by the center body provided around the fuel passage to cool the outer surface of the center body. 13. A method according to claim 12, further comprising: splitting fuel from the fuel passage to impinge cool the center body's head end and bypass the reverse fuel passage. 14. A method according to claim 11, further comprising determining an air-fuel premixture configured to produce a flame speed that is less than a velocity of the air flow. 15. A method according to claim 14, wherein a venturi is provided between the primary combustion chamber and the secondary combustion chamber, the method further comprising: producing a flame in the secondary combustion chamber that does not cross the venturi into the primary combustion chamber. 16. A method according to claim 11, wherein upon ignition of the combustor up to a first predetermined percentage of a load of the gas turbine engine, the method comprises: providing the entire fuel supply to the primary nozzles. 17. A method according to claim 16, wherein from the first predetermined percentage of the load to a second predetermined percentage of the load higher than the first predetermined percentage of the load, the method comprises: providing a first percentage of the fuel supply to the primary nozzles and a second percentage of the fuel supply to the secondary nozzle, the first percentage being larger than the second percentage. 18. A method according to claim 17, the method further comprising: providing a third percentage of the fuel supply to the primary nozzles and a fourth percentage of the fuel supply to the secondary nozzle from the second predetermined percentage of the load to 100% of the load of the gas turbine engine, wherein the third percentage of the fuel supply is higher than the first percentage of the fuel supply and the fourth percentage of the fuel supply is smaller than the second percentage of the fuel supply. 19. A method according to claim 18, wherein prior to providing the third percentage of the fuel supply to the primary nozzles and the fourth percentage of the fuel supply to the secondary nozzle, the method comprises: providing 100% of the fuel supply to the secondary nozzle. 20. A combustor for a gas turbine engine comprising: primary nozzles configured to diffuse fuel into an air flowing through the combustor in a downstream direction;an end cover having openings to receive a discharge end of each of the primary nozzles;a secondary nozzle configured to premix fuel with the air flow wherein the primary nozzles are arranged in an annular array and the secondary nozzle is aligned with a centerline of the array and the secondary nozzle extends through the end cover and downstream into the combustor in a direction of combustion gas flow, the secondary nozzle comprising; a tubular fuel passage extending downstream in the combustor and having a downstream end portion,a tubular center body provided around the fuel passage,a burner tube provided around the center body and defining an annular air-fuel mixing passage between the center body and the burner tube, wherein the burner tube includes an inlet upstream of the end cover and open to an airflow,a vane assembly in the annular air-fuel mixing passage and upstream of the downstream end portion of the fuel passage, the at least one vane assembly including an internal chamber and an annular array of swirl vanes;a reverse flow fuel cooling passage defined between the fuel passage and the center body, wherein the reverse flow fuel cooling passage includes a fuel inlet open to the fuel passage and proximate a downstream end of the fuel passage and an outlet upstream in a direction of combustion gas flow of the downstream end and aligned with the vane assembly, wherein the outlet of the reverse flow fuel cooling passage is in fluid communication with the internal chamber of the vane assembly, andan outer wall tube surrounding the burner tube and having an inlet end region connected to the end cover wherein the air flow enters the outer wall tube and flows through an annular cooling air passage between the burner tube and the outer wall tube. 21. The combustor of claim 20 wherein the internal chamber of the vane assembly includes an annular cooling chamber open to the outlet of the reverse fuel cooling passage, an annular outlet chamber separated by a dividing wall from the cooling chamber, wherein apertures adjacent the outlet chamber allow cooling fuel to flow towards the swirl vanes.