A combustor section, comprising: a primary combustion system generating a stream of combustion products; and a secondary combustion system located downstream of the primary combustion system, characterized in that the secondary combustion system includes: at least one micro mixer injector to deliver a fuel/air mixture into the stream of combustion products, the at least one micromixer injector comprising: a fuel plenum to receive a supply of fuel; a plurality of premixing tubes extending through the fuel plenum, each tube having an air inlet at an intake end of the tube and an outlet at a discharge end of the tube, the air inlet of each tube being configured to receive a supply of air, each tube having a plurality of fuel holes formed therein to receive the supply of fuel for mixing with the air in the tube; and an upstream face having the inlet of each tube formed therein, and a downstream face having the outlet of each tube formed therein, wherein the air inlet of each tube has a first geometrical shape and the outlet of each tube has a second geometrical shape that is different from the first geometrical shape, and wherein an overall inlet area of the plurality of premixing tubes on the upstream face is larger than an overall outlet area of the plurality of premixing tubes on the downstream face such that the plurality of premixing tubes are relatively spaced-apart at the upstream face and more densely packed at the downstream face. a primary combustion system generating a stream of combustion products; and a secondary combustion system located downstream of the primary combustion system, characterized in that the secondary combustion system includes: at least one micro mixer injector to deliver a fuel/air mixture into the stream of combustion products, the at least one micromixer injector comprising: a fuel plenum to receive a supply of fuel; a plurality of premixing tubes extending through the fuel plenum, each tube having an air inlet at an intake end of the tube and an outlet at a discharge end of the tube, the air inlet of each tube being configured to receive a supply of air, each tube having a plurality of fuel holes formed therein to receive the supply of fuel for mixing with the air in the tube; and an upstream face having the inlet of each tube formed therein, and a downstream face having the outlet of each tube formed therein, wherein the air inlet of each tube has a first geometrical shape and the outlet of each tube has a second geometrical shape that is different from the first geometrical shape, and wherein an overall inlet area of the plurality of premixing tubes on the upstream face is larger than an overall outlet area of the plurality of premixing tubes on the downstream face such that the plurality of premixing tubes are relatively spaced-apart at the upstream face and more densely packed at the downstream face. a fuel plenum to receive a supply of fuel; a plurality of premixing tubes extending through the fuel plenum, each tube having an air inlet at an intake end of the tube and an outlet at a discharge end of the tube, the air inlet of each tube being configured to receive a supply of air, each tube having a plurality of fuel holes formed therein to receive the supply of fuel for mixing with the air in the tube; and an upstream face having the inlet of each tube formed therein, and a downstream face having the outlet of each tube formed therein, wherein the air inlet of each tube has a first geometrical shape and the outlet of each tube has a second geometrical shape that is different from the first geometrical shape, and wherein an overall inlet area of the plurality of premixing tubes on the upstream face is larger than an overall outlet area of the plurality of premixing tubes on the downstream face such that the plurality of premixing tubes are relatively spaced-apart at the upstream face and more densely packed at the downstream face.