Calculations of the effect of nitrogen vibrational kinetics on laminar flame temperature profiles

Abstract

Calculations of several premixed and nonpremixed laminar flames have been performed using two chemical kinetics mechanisms: (1) a vibrational kinetics mechanism treating nitrogen as a number of distinct species corresponding to different vibrational energy levels, with reactions representing transitions between energy levels; and (2) a traditional mechanism that treats nitrogen as being in vibrational equilibrium. In the vibrational kinetics calculations, translational/vibrational and vibrational/vibrational energy transfers are included, as is the effect of collisions with CO 2 and H 2O on N 2 vibrational excitation. Vibrational temperatures are calculated from the populations of various N 2 species. For a stoichiometric, atmospheric-pressure premixed methane/air flame, the vibrational temperature is 40 K lower than the rotational/translational temperature in the region of high temperature gradient. The lag in filling upper vibrational energy levels of nitrogen also results in a lower effective heat capacity for the mixture in the vibrational kinetics case than in the vibrational equilibrium case. Rotational/translational temperatures exceed those calculated with the traditional mechanism by as much as 15 K in the region of steep temperature gradient. For diffusion flames over the range of strain rates investigated here, the effect of vibrational kinetics is much smaller. Sensitivity analysis indicates that, among the vibrational kinetics reactions, the initial vibrational excitation of N 2 by CO 2 and H 2O has the greatest impact on the temperature results.