Measurements of NO Distributions and Fluorescence Lifetimes in a Nonpremixed Counterflow CH4 /Air Flame Using Picosecond Time-Resolved Laser-Induced Fluorescence

Abstract

We have applied picosecond laser-induced fluorescence (ps-LIF), excited in the linear regime, to obtainspatial profiles of NO concentrations and NO A2_+ effective fluorescence lifetimes in a non-premixed, counterflow CH4/air flame at atmospheric pressure. We used an excitation pulse much shorter than the LIF lifetime and recorded the temporal evolution of the LIF decays. Analysis of the decays with corrections for instrumental broadening yielded NO concentrations nearly free of quenching uncertainties. This study is the first application, to our knowledge, of ps-LIF to measure naturally occurring NO concentrations in flames. Compared to saturated fluorescence measurements using nanosecond pulses (ns-LSF), linear ps-LIF was found to be less susceptible to interferences in fuel-rich regions of the flame and less affected by errors resulting from rotational energy transfer (RET) refilling. Analysis based on a simple four-level, density-matrix model for the ps-LIF excitation motivated the use of non-saturating excitation. Experimental NO concentrations compared favorably with flame model results within uncertainties of current prompt-NO reaction mechanisms. Experimental NO LIF lifetimes were substantially lower in fuel-rich regions than predictions based on quenching cross-section models and flame species calculations. Such overpredictions could explain discrepancies in fuel-rich regions between ns-LSF and linear, quenching-corrected ns-LIF measurements of NO.