Laser induced grating spectroscopy measurements of temperature and water vapour in turbulent lean premixed methane-hydrogen-air flames at pressure

Abstract

This study explores the application of Laser-Induced Grating Spectroscopy (LIGS) for non-intrusive measurements of temperature and water vapour concentration in premixed flames of air with blends of hydrogen and methane under high-pressure conditions. Employing a swirl-stabilized burner, the research demonstrates the capabilities of tracer-free LIGS, using thermal and electrostrictive gratings generated by a 1064 nm Nd:YAG laser, to measure local temperatures and water molar fractions. The study also includes an extension of the thermoacoustic model for characteristics LIGS signals, built in order to extract the relative concentration of water vapour from the ratio of second (thermal) to first (electrostrictive) peak magnitudes. Experiments are conducted in non-sooting flames, leveraging a high-pressure optical chamber (HPOC) with pressures up to 3 bar. The study evaluates mixtures of methane and hydrogen (100%, 70%, and 40% CH\(_{4}\)) and identifies key relationships between LIGS signal characteristics, frequency distributions, and combustion dynamics. Results show excellent agreement between measured temperatures and adiabatic flame temperatures, alongside measurements of water vapour molar fractions. The spatial temperature and water vapour maps are related to the complex mixing and recirculation patterns associated with the flame’s shear and recirculation zones. The results establish LIGS as a robust diagnostic tool for combustion analysis, with potential applications in advancing hydrogen-rich energy systems.