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A method for premixing air with a gaseous fuel for being burned in a combustion chamber includes: guiding the air in an air stream along a burner axis through a coaxial air tube into a combustion chamber arranged at an end of said air tube; andimpressing a swirl on the air stream by passing it throu

A method for premixing air with a gaseous fuel for being burned in a combustion chamber includes: guiding the air in an air stream along a burner axis through a coaxial air tube into a combustion chamber arranged at an end of said air tube; andimpressing a swirl on the air stream by passing it through a first swirl device concentrically arranged within the air tube and comprising a plurality of radially oriented first blades. The method further includes:injecting gaseous fuel into the air stream at the first swirl device; andmixing said air in said air stream with the injected gaseous fuel in a first mixing zone arranged just after said first swirl device.

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1. A method for premixing air with a gaseous fuel for being burned in a combustion chamber, said method comprising: guiding said air in an air stream along a burner axis through a coaxial air tube into a combustion chamber arranged at an end of said air tube;impressing a swirl on said air stream by

1. A method for premixing air with a gaseous fuel for being burned in a combustion chamber, said method comprising: guiding said air in an air stream along a burner axis through a coaxial air tube into a combustion chamber arranged at an end of said air tube;impressing a swirl on said air stream by passing it through a first swirl device concentrically arranged within said air tube and comprising a plurality of radially oriented first blades;injecting gaseous fuel into said air stream at said first swirl device; andmixing said air in said air stream with the injected gaseous fuel in a first mixing zone arranged just after said first swirl device;sending the mixed fuel/air stream leaving said first mixing zone through a second swirl device concentrically arranged within said air tube andcomprising a plurality of radially oriented second blades to reduce the swirl of the mixed fuel/air stream;wherein the camber line of a blade of the second blades is aligned at a leading edge of the blade of the second blades with the camber line of a blade of the first blades at a trailing edge of the blade of the first blades to avoid flow separation,injecting gaseous fuel into said mixed fuel/air stream at said second swirl device; andfurther mixing said mixed fuel/air stream with the injected gaseous fuel in a second mixing zone arranged just between said second swirl device and said combustion chamber. 2. The method as claimed in claim 1, wherein the gaseous fuel is injected at said first and second swirl device by means of gas holes provided on the suction sides and/or pressure sides of said first and second blades. 3. The method as claimed in claim 2, wherein said gas holes are arranged in rows oriented perpendicular to the burner axis. 4. The method as claimed in claim 1, wherein said first swirl device has a first swirl number, said second swirl device has a second swirl number, and said second swirl number is smaller than said first swirl number. 5. The method as claimed in claim 1, wherein each of said first and second swirl devices has a number of first and second blades between 6 and 10, respectively. 6. The method as claimed in claim 1, wherein the cylindrical cross section of the blades of the first and second swirl device has the shape of an airfoil in order to reduce a pressure drop. 7. The method as claimed in claim 6, wherein each of the blades of the first and second swirl device has a leading edge and a trailing edge, whereby the leading edge of the blades of the second swirl device is aligned in terms of inflow angle with outflow angle of the trailing edge of the blades of the first swirl device. 8. The method as claimed in claim 6, wherein said airfoils of said swirl devices are designed to produce a certain exit flow angle α of the air/fuel flow, whereby said exit flow angle α has a predetermined dependence α(r) of the radius r with respect to the burner axis. 9. The method as claimed in claim 7, wherein tan α(r)=H·r+K with constants H and K. 10. The method as claimed in claim 7, wherein tan α(r) is proportional to 1/r. 11. The method as claimed in claim 7, wherein tan α(r)=const. 12. The method as claimed in claim 1, wherein the air is guided through the cylindrical coaxial air tube having an inner air tube radius, in an annular space between said coaxial air tube and a concentric central bluff body having an outer bluff body radius, whereby the ratio between said outer bluff body radius and said inner air tube radius is between 0.3 and 0.8. 13. The method as claimed in claim 2, wherein the fuel is supplied to the blades of the first and second swirl device via respective cavities within said first and second blades by means of a fuel distribution system, which allows to control the fuel supply to each swirl device, each blade within said first and second swirl device, and each of said suction and pressure sides of each blade of the first and second blades, and that combustion instabilities within said combustion chamber are controlled by means of said fuel distribution system via fuel staging between the first and second swirl devices and/or different sides of each blade of the first and second blades. 14. The method as claimed in claim 1, wherein said first and second swirl devices have an outer radius R and said first mixing zone has an axial length L, and said ratio L/R is between 0.5 and 4. 15. A burner arrangement for conducting a method according to claim 1, comprising an air tube extending along a burner axis and opening at one end into a combustion chamber, a first coaxial swirl device arranged concentrically within said air tube at a first distance from said combustion chamber, said first swirl device comprising a plurality of radially oriented first blades and first means for injecting fuel into an air stream passing said first swirl device, characterized in that a second swirl device is arranged within said air tube downstream of said first swirl device, thereby defining a first mixing section between said first and second swirl device, whereby said second swirl device comprises a plurality of radially oriented second blades and second means for injecting fuel into a fuel/air stream passing said second swirl device. 16. The burner arrangement as claimed in claim 15, wherein said second swirl device is arranged at a second distance from said combustion chamber, thereby defining a second mixing section. 17. The burner arrangement as claimed in claim 15, wherein said first and second fuel injection means comprises a plurality of gas holes provided on the suction sides and/or pressure sides of said first and second blades. 18. The burner arrangement as claimed in claim 17, wherein said gas holes are arranged in rows oriented perpendicular to the burner axis. 19. The burner arrangement as claimed in claim 15, wherein said first swirl device has a first swirl number, said second swirl device has a second swirl number, and said second swirl number is smaller than said first swirl number. 20. The burner arrangement as claimed in claim 15, wherein each of said first and second swirl devices has a number of blades between 6 and 10. 21. The burner arrangement as claimed in claim 15, wherein the cylindrical cross section of the blades of the first and second swirl device has the shape of an airfoil. 22. The burner arrangement as claimed in claim 21, wherein each of the blades of the first and second swirl device has a leading edge and a trailing edge, whereby the leading edge of the blades of the second swirl device is aligned in terms of inflow angle with outflow angle of the trailing edge of the blades of the first swirl device. 23. The burner arrangement as claimed in claim 21, wherein said airfoils of said swirl devices are designed to produce a certain exit flow angle α of the air/fuel flow, whereby said exit flow angle α has a predetermined dependence α(r) of the radius r with respect to the burner axis. 24. The burner arrangement as claimed in claim 23, wherein tan α(r)=H·r+K with constants H and K. 25. The burner arrangement as claimed in claim 23, wherein tan α(r) is proportional to 1/r. 26. The burner arrangement as claimed in claim 23, wherein tan α(r)=const. 27. The burner arrangement as claimed in claim 15, wherein the air tube is cylindrical in shape having an inner air tube radius, that a concentric central bluff body is arranged within said air tube having an outer bluff body radius, and that the ratio between said outer bluff body radius and said inner air tube radius is between 0.3 and 0.8. 28. The burner arrangement as claimed in claim 17, wherein the fuel is supplied to the blades of the first and second swirl device via respective cavities within said blades by means of a fuel distribution system, which allows to control the fuel supply to each swirl device of the first and second swirl devices, each blade of the first and second swirl devices, and each of said suction and pressure sides of each blade of the first and second swirl devices. 29. The burner arrangement as claimed in claim 23, wherein said first and second swirl devices have an outer radius R and said first mixing zone has an axial length L, and said ratio L/R is between 0.5 and 4.