A laser-induced fluorescence and coherent anti-stokes Raman scattering study of NO formation in preheated, laminar, rich premixed, methane/air flames

Abstract

Measurements of temperature and NO mole fraction are reported in flat, laminar, stoichiometric and fuel-rich premixed methane/air flames subjected to varying degrees of preheating, up to 400 K. The flames are stabilized above a perforated ceramic tile burner. By varying the exist velocity of the preheated mixture, the flame temperature could be varied over a wide range. The temperature and NO mole fraction were measured by coherent anti-Stokes Raman scattering and laser-induced fluorescence, respectively. At φ =1, plotting the measured NO mole fraction as a function of flame temperature shows the exponential growth characteristic for the Zeldovich mechanism. At φ =1.3, the plot of NO versus flame temperature shows a region of more than 400 K in which the NO mole fraction grows only by a factor of 2, reflecting the behavior of the Fenimore mechanism. At temperatures above ∼2250 K, the NO formation initiated by O+N 2 becomes important, and the mole fraction begins to rise more steeply. At φ =1.5, the NO mole fraction is less than 10 ppm between 1750 and 1950 K, but increases by a factor of 9 within the next 200 K. Calculations using GRI-Mech 3.0 capture the essence of this behavior well, but show quantitative shortcomings for the fuel-rich flames. Comparison with the calculated equilibrium mole fractions as a function of temperature shows the observed NO mole fractions under fuel-rich conditions to be generally in excess of their equilibrium values. The experimental profiles and the GRI-Mech calculations indicate that relaxation to equilibrium is very slow, even at high temperatures. The results at φ =1.5 suggest that preheated fuel-rich combustion may be fruitful for practical use.