Effects of fuel Lewis number on and Damkohler number scaling of nitric oxide emission level of Burke-Schumann type flames

Results are reported on the thermo-diffusive effects in laminar diffusion flames. The flames studied are those of the Burke-Schumann type, with the exceptions that the axial velocity is uniform in the inlet instead of having uniform mass flux, and that the fuel Lewis number is not equal to unity. The fuel Lewis number was varied using argon and helium dilution of the fuel. Dramatic Lewis number effects were experimentally observed. In some cases a higher degree dilution may cause the measured flame temperature (radiation corrected) to be higher than that with lesser dilution if increasing dilution reduces the Lewis number. The NOx emission level of the flame follows similar trend, consistent with the diffusive-thermal effects. A scaling analysis of NOx emission recently proposed for turbulent jet diffusion dames was examined for the present flames. It was found that as the Damkohler number decreases the normalized NOx emission increases. This further confirms the super-equilibrium effects for NOx emission previously found in turbulent jet diffusion flames. The fuel Lewis number was not found to qualitatively affect the Damkohler number scaling of NOx emission, nor did it quantitatively correlate the value of the exponent of the Damkohler number.