Quantitative laser-saturated fluorescence measurements of nitric oxide in a heptane-fueled lean direct-injection spray flame at varying global equivalence ratios

Abstract

We report the influence of equivalence ratio on nitric oxide (NO) concentration profiles in an unconfined heptane spray flame at atmospheric pressure. The associated burner is based on the lean direct-injection design and incorporates a pressure-atomized, hollow-cone spray nozzle. Helical vanes in the air passage coupled with a divergent exit and preheated air produce a strongly swirling, clean, blue flame. NO concentration profiles are measured using laser-saturated fluorescence (LSF) at four axial heights above the burner. Because of the liquid droplet environment, an extension of LSF utilizing a Mie-scattering back-ground subtraction technique is required to make quantitative measurements of NO concentration. The NO profiles are measured in three flames with global equivalence ratios of 1.0, 0.9, and 0.8. Probe measurements of CO 2 and O 2 mole fractions are used to determine representative effective equivalence ratios, which are lower than the global equivalence ratios because of significant entrainment of ambient air Through an analysis of area-averaged NO concentrations at various axial heights, the downstream [NO] is found to be mainly redistributed as the flame spreads owing to the intense recirculation of combustion products. Moreover, comparisons of [NO] profiles as a function of equivalence ratio demonstrate that dilution air is a large influence in the reduction of NO concentration at equivalence ratios leaner than stoichiometric.