Method for premixing air with a gaseous fuel and burner arrangement for conducting said method

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;
• and
• mixing 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 and
• comprising 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;
• and
• further 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.