Measurements and modeling of flamefront no formation and superequilibrium radical concentrations in laminar high-pressure premixed flames

Abstract

The first direct measurements and modeling of flamefront NO formation and superequilibrium OH radical concentrations in high-pressure laminar premixed flames are presented to better understand NO production at high pressures. Results are compared from five lean C2H6/O2/N2 flames at pressures from 1 to 9 atm with burnt gas temperatures less than 1800 K. The experimental and calculated NO profiles are fairly flat downstream of the reaction zone, confirming that NO is formed mainly in the flamefront of these flames. Measured and calculated peak OH concentrations in the flamefront show substantial OH superequilibrium at all pressures, with calculated peak O-atom concentrations 25–60 times larger than equilibrium. Measurements of temperature and NO concentrations are in good agreement with model results (within 50 K and a few ppm NO, respectively), confirming that the chemical mechanism is appropriate for these lean high-pressure flames. Model calculations show that the Zeldovich mechanism with equilibrium O-atom concentration contributes less than 1% of the NO formed in these low temperature flames. In the flamefront, NO is formed by the Zeldovich mechanism enhanced by superequilibrium O-atom concentrations, the N2O intermediate mechanism enhanced by O-atom superequilibrium, and the Fenimore mechanism. The relative contribution of each mechanism changes with pressure, equivalence ratio, and temperature. In particular, the N2O intermediate mechanism is found to be much more important than previously thought, contributing up to half of the total NO formed.