SUMMARYNumerical study with detailed chemistry has been conducted to investigate the e!ect of CO addition on #ame structure and NO V formation in CH }air counter#ow di!usion #ame. Radiation e!ect is found to be dominant especially at low strain rates. The addition of CO makes radiation e!ect more remarkable even at high-strain rates. It is, as a result, seen that #ame structure is determined by the competition between the radiation and strain rate e!ects. The important role of CO addition is addressed to thermal and chemical reaction e!ects, which can be precisely speci"ed through the introduction of an imaginary species. Thermal e!ect contributes to the changes in #ame structure and NO formation mainly, but the e!ect of chemical reaction cannot be neglected. It is noted that #ame structure is changed considerably due to the addition of CO , in such a manner, that the path of methane oxidation prefers to take CH PCH PC H PC H instead of CH PCH PCH PCH.

Large eddy simulation was performed to visualize the three-dimensional vortical structures interacting with a turbulent premixed in a lean premixed swirl combustor with varied equivalence ratio. It was found that the fluctuation of unsteady heat release due to the deformation of flame surface was significantly decreased as the equivalence ratio increased because of the change in interaction between inner vortical structures and flames. This phenomenon was another evidence of the amplification mechanism in the combustion instabilities due to the strong flame-vortex interactions under lean premixed conditions.