A combustion system includes a head end comprising a liquid fuel cartridge. The liquid fuel cartridge has liquid fuel injection ports and is configured to produce combustion products via a diffusion flame. A liner is configured to deliver the combustion products from the head end to an aft frame, and an injector having an outlet is located along the liner between the head end and the aft frame. The injector outlet delivers a stream of oxidant inwardly into the liner, such that a mixedness and a velocity of the combustion products are increased prior to the combustion products reaching the aft frame. A method of producing combustion products having characteristics of a premixed flame in a liquid fuel combustion system is also provided herein.