Sequential combustion with dilution gas

A method for operating a gas turbine having at least a compressor, and a sequential combustor arrangement including a first burner, a first combustion chamber, a mixer, a second burner, and a second combustion chamber arranged sequentially in a fluid flow connection, wherein the mixer is configured to guide combustion gases in a hot gas flow path extending between the first combustion chamber and the second burner with a duct having an inlet at an upstream end configured for connection to the first combustion chamber and an outlet at a downstream end configured for connection to the second burner, wherein the mixer includes a first group of first injection tubes pointing inwards from side walls of the mixer for admixing dilution gas to cool hot flue gases when leaving the first combustion chamber, a second group of second injection tubes pointing inwards from the side walls of the mixer for admixing the dilution gas to cool the hot flue gases when leaving the first combustion chamber, and a third group of third injection tubes pointing inwards from the side walls of the mixer for admixing the dilution gas to cool the hot flue gases when leaving the first combustion chamber, wherein:

• the first injection tubes of the first group are distributed along the side walls of the mixer, the second injection tubes of the second group are distributed along the side walls of the mixer, the third injection tubes of the third group are distributed along the side walls of the mixer;
• and
• wherein the first injection tubes of the first group have a first protrusion depth into the hot gas flow path, the second injection tubes of the second group have a second protrusion depth into the hot gas flow path, and the third injection tubes of the third group have a third protrusion depth into the hot gas flow path;
• the second protrusion depth being bigger than the third protrusion depth and the second protrusion depth being smaller than the first protrusion depth, and
• the side walls of the mixer having diffusion cooling holes, at least some of the diffusion cooling holes positioned in a first region that has a trapezoidal shape, the first region extending in a main flow direction of the hot gases from adjacent at least one of the first injection tubes to a position adjacent at least one of the third injection tubes, the first region being positioned downstream of a second region that is homogenously cooled, the first region also being upstream of the third injection tubes, the first region also being located in a space between immediately adjacent second injection tubes of the second injection tubes of the second group and downstream of the first injection tubes, the first region comprising a different hole spacing from a third region that is circumferentially adjacent to the trapezoidal shape;
• wherein the method comprising:
• guiding, in the mixer, combustion gases in the hot gas flow path extending between the first combustion chamber and the second burner;
• and
• admixing, in the mixer, the dilution gas to the hot gases leaving the first combustion chamber, wherein the dilution gases are admixed via the first injection tubes, the second injection tubes, and the third injection tubes of the mixer to cool the hot flue gases leaving the first combustion chamber, and the dilution gases are admixed in different regions of a cross section of the mixer via the first injection tubes, the second injection tubes, and the third injection tubes.