Raman spectroscopy for quantitative measurements of temperature and major species in high-pressure non-premixed NH3/H2/N2 counterflow flames

Ammonia has shown great potential as a carbon-free fuel, in particular for marine transportation and energy production. Its low laminar flame speed, and the tradeoff between ammonia slip and NOx emission, pose challenges for industrial applications, and a more in-depth understanding of the combustion of ammonia is therefore needed. Raman spectroscopy is a powerful diagnostic often employed to investigate turbulence chemistry interactions and resolve the thermo-chemical structure of hydrogen and hydrocarbon-air flames. This work extends Raman spectroscopy to the instantaneous and spatially resolved measurement of major species concentrations and temperature in ammonia flames. The lack of detailed ammonia spectra at high temperatures, the strong flame luminosity, and fluorescence interference are the major obstacles to the implementation of Raman spectroscopy to ammonia flames. This paper introduces a novel approach to estimate the temperature dependence of the Raman signal and of fluorescence interference contributions from a series of counterflow diffusion flames. This approach was first validated in high-pressure hydrogen flame, and then extended this approach to high-pressure ammonia flames to obtain the Raman response of ammonia and its crosstalk. A high-contrast optical shutter is sufficient to suppress flame luminosity. Finally, species concentrations and temperature profiles from measurements are shown, and their accuracy and precision are discussed.