The present work investigates and compares between three models developed to simulate the flow and combustion in a typical gas turbine reversed combustor. The first model relies on conventional combustion which uses preheated air using a recuperative heat exchanger. The second and third models investigate the Mild and Intense Low oxygen Dilution "MILD" or Flameless oxidation (High Temperature Air combustion (HiTAC)). HiTAC is based on very efficient preheating of the combustion air by means of regenerators. The regenerators are heated by the exhaust flue gases. The Combustion is carried out using internal Flue gas recirculation coupled with high temperature air under diluted oxidant conditions. The three models use methane as fuel. The models rely on the computational code "FLUENT 6.2.3" which was used to solve the conservation equations of mass, momentum, energy, and transport equations of species concentrations. Turbulence, combustion and radiation modeling in addition to NOx modeling equations were solved together to represent finally temperature and NOx distribution inside the burner. All the models are considered 2D axisymmetric. The combustion process has been simulated as non-premixed and non-adiabatic. The effects of excess air, preheated air, diluted air and diluted fuel on the combustion process are also discussed