Analysis of NO formation in high temperature diluted air combustion in a coaxial jet flame using an unsteady flamelet model

The high temperature diluted air combustion, which improves the flame stability while lowers the NO emission level, has been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the laminar flamelet concept with the standard k–ε turbulence model. The NO formation is estimated by solving the Eulerian particle transport equations in a postprocessing mode. Calculations are performed for a coflowing jet flame for various conditions of inlet air temperature and oxygen concentration. A production rate analysis of elementary reactions reveals major paths for NO formation. When the oxygen concentration is high, the reaction zone is formed near the fuel nozzle and the NO formation by the thermal mechanism becomes dominant, due to the increase in flame temperature. On the other hand, when the oxygen concentration is low, the reaction is spread out more uniformly in the furnace. The NO formation by the prompt route is dominant compared to other routes especially when the air is diluted with nitrogen.