Reaction zone thickness and formation of nitric oxide in turbulent diffusion flames

Abstract

The Gibson-Libby [1] formula for the thickness of the reaction zone in a diffusion controlled-reaction is revised to account properly for the concentration gradients that occur in fully turbulent flow. The result is that the reaction zone is thickened considerably. When used to calculate reaction zone thickness in turbulent diffusion flames these are now found to be of the same order as the Kolmogoroff length scale. The presence of a relatively diffuse reaction zone allows the oxygen atom concentration to be above equilibrium over a significant proportion of the flow. For a hydrogen-air flame, O atom concentrations can be calculated from the assumption of equilibrium for the fast radical exchange reactions, with the hydrogen and oxygen molecule concentrations computed from the Kolmogoroff concentration fluctuation scale. With these assumptions nitric oxide formation rates show the Reynolds number dependence and “rich shift” found by Kent and Bilger [2] and Bilger and Beck [3].