CALCULATIONS OF THE EFFECT OF NITROGEN VIBRATIONAL KINETICS ON LAMINAR FLAME TEMPERATURE PROFILES

Calculations of several premixed and nonpremixed laminar flames have b een performed using two chemical kinetics mechanisms: (1) a vibrationa l 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 effec t of collisions with CO2 and H2O on N-2 vibrational excitation. Vibrat ional temperatures are calculated from the populations of various N-2 species. For a stoichiometric, atmospheric-pressure premixed methane/a ir flame, the vibrational temperature is 40 K lower than the rotationa l/ translational temperature in the region of high temperature gradien t. The fag in filling upper vibrational energy levels of nitrogen also results in a lower effective heat capacity for the mixture in the vib rational kinetics case than in the vibrational equilibrium case. Rotat ional/translational temperatures exceed those calculated with the trad itional mechanism by as much as 15 K in the region of steep temperatur e gradient. For diffusion flames over the range of strain rates invest igated here, the effect of vibrational kinetics is much smaller. Sensi tivity analysis indicates that, among the vibrational kinetics reactio ns, the initial vibrational excitation of N-2 by CO2 and H2O has the g reatest impact on the temperature results. Copyright (C) 1997 by The C ombustion Institute