On the difficulty of interpreting NO-LIF measurements around 226 nm in confined ammonia flames

Abstract

This paper addresses how NH3 and NO absorption along the laser axis disrupts NO-LIF measurements made around 225 nm in NH3/O2/N2 premixed flames confined at low pressure (10 kPa). Measurements were performed in 3 flames (equivalence ratio ϕ = 0.87, 1.1 and 1.3) with 24 % N2 dilution. Together with the LIF measurements, single-pass absorption measurements were obtained in all flames as a function of the height above the burner (HAB) and of the wavelength. A significant NH3 absorption of the laser beam is found in the pre-heat zone and flame front of each flame, as well as in the burned gas in the rich flame. In the lean flame, whatever the HAB, an additional significant absorption is due to the NO present in the gas surrounding the flame. Whereas NH3 absorbance only occurs inside the flame under all conditions, NO absorbance is mainly due to the high concentration of NO produced in the lean flame and spread in the entire confined vessel around the flame. From this analysis, relative NO rotational population profiles were recorded by measuring the temporal peak of the LIF signals (prompt-LIF) which were corrected for absorption to account for the available energy at burner's center. NO-LIF thermometry was conducted to determine the temperature in the flames through collecting excitation LIF spectra and performing spectral fitting using a dedicated software (thermo NO-LIF, https://pc2a.univ-lille.fr/thermo-no-lif/). The procedure developed to collect the NO profiles could not be transposed to determine the temperature in the lean flame. Indeed, the attenuation of the laser beam depends on whether the laser is tuned on a “cold” transition (strongly impacted by absorption outside the flame) or on a “hot” transition of NO, little affected. Thus, two tubes (with N2 flushing) were installed inside the vessel along the laser axis and surrounding the flame to suppress the NO absorbance.