Laser-induced plasma analysis of ammonia-oxygen and ammonia-oxygen-enriched-air flames at elevated pressures

Abstract

This study employed Laser-induced breakdown spectroscopy (LIBS) to measure the fuel-oxidizer ratio (FOR) of ammonia combustion with oxygen-enriched air and pure oxygen flames at elevated pressures (100 - 300 kPa). The correlations between the spectral line intensity ratios of nitrogen (N), hydrogen (H), oxygen (O), and equivalence ratio were used to quantify the FOR of flames at various pressures. The effect of pressure on the stability and precision of the calibration profiles for the elemental intensity ratios in flames was investigated. It was observed that the H/O correlation decreases with pressure increase for both ammonia flames. N/O correlations decrease with elevated pressure for the ammonia-oxygen flame. Furthermore, the nitrogen (NII) spectral emission lines at 568 nm and 595 nm were used to estimate the plasma temperature, while the hydrogen (Hα) line at 656 nm was used for electron number density measurements.