Simultaneous measurements of NH2 and major species and temperature with a novel excitation scheme in ammonia combustion at atmospheric pressure

Abstract

This work reports the first 1D, simultaneous, laser-induced fluorescence (LIF) measurements of the amidogen (NH2) radical concentration and Raman/Rayleigh measurements of major species and temperature in laminar counterflow non-premixed and premixed NH3/H2/N2–air flames. The instrument presents a novel excitation scheme for NH2, which add no complexity to the existing Raman/Rayleigh instrument. The NH2 LIF is excited by three Nd:YAG lasers, temporally stretched to 500 ns using a pulse stretcher, resulting in total energy of ∼ 1 J at 532 nm. LIF spectra collected in NH2 produced from photolysis of NH3, in an oxygen-free environment, match those obtained in the counterflow flames. The NH2-LIF signal is sampled in the 620–630 nm, together with the Raman signal, using the same collection hardware. The weak absorption cross-section at 532 nm is compensated by the high laser energy available, and the long pulse allows repeated excitation and emission from the same molecule, leading to the partial saturation region. A simplified model which neglects quenching, and contributions from other species, but includes Raman crosstalk from H2, N2, and NH3 is proposed and calibrated by comparing measurements and 1D Chemkin simulation of a counterflow flame. Finally, this simplified model was validated in more than 23 flames with varying fuel compositions, strain rates, and non-premixed and premixed flames to assess accuracy and precision. The overall agreement between the measured and simulated NH2 using the same mechanism is within 5%, and the single-shot detection limit is below 100 ppm. NO2-LIF is one of the main interferences to NH2-LIF, and its effects can be negligible on the fuel side, but care only need to be taken on the air side with T < 800 K, in which the maximum interference ∼33 ppm was observed. The accuracy of chemical kinetic models, with an uncertainty of ∼ 20%, limits the accuracy of the proposed NH2 measurements. The technique is best suited to investigate the turbulence-chemistry interaction in turbulent NH3 flames combined with major species and temperature from Raman/Rayleigh scattering, and it can provide useful validation data for the numerical model when the same chemical model is used.