Effects of flame lifting and acoustic excitation on the reduction of NOx emissions

The effect of the combustion fluid dynamics of a lifted jet flame with and without acoustic excitation on the control of NOx formation in the flame is investigated. A partially premixed jet was used, operated under lift-off flame bifurcation condition in the hysteresis region. Results show that flame lifting and acoustic excitation are effective in reducing the emission index of NOx (EINOx). A lean premixed condition, achieved by the strong upstream mixing of the lifted flame base, can further reduce NOx formation without increasing CO emissions. The prompt NO mechanisms of super-equilibrium OH concentrations and three-body recombination through N₂O intermediates dominate in the initial region close to the flame base of the lean premixed, lifted flame. A lower initial prompt NOx and shorter flame length with reduced flame temperature, caused by the enhanced upstream mixing due to flame lifting and acoustic excitation, result in the low NOx and CO emissions in the present loan premixed jet flame.