Presentation given at the 13th US National Combustion Meeting in College Station TX on March 20 2023
X-ray diagnostics are widely used in materials science and structural biology, but not in gas-phase reacting flows where the densities are 10,000 times lower. However, recent advances in synchrotron sources and x-ray optics have made combustion applications increasingly feasible. In this work we use x-ray techniques to measure temperature and mixture fraction in-situ in methane/air and ethylene/air coflow nonpremixed flames, then compare the results to detailed flame simulations. More specifically, krypton atoms are seeded into the reactants and krypton number density is measured throughout the flame with x-ray fluorescence (XRF). Since krypton is an inert, if it is added to both reactant streams, then its mole fraction is nearly constant throughout the flame, and its number density can be converted to temperature via the ideal gas law. If krypton is only added to the fuel, then its number density is directly related to mixture fraction. The krypton XRF technique offers several benefits compared to conventional diagnostics with UV or visible lasers. First, the data analysis is simple since the x-ray absorption and emission physics are independent of temperature or collisional environment; no corrections are required for Boltzmann factors or collisional quenching. Second, the spatial resolution is very high: the excitation wavelength is only 0.08 nm (15 keV x-rays), so the beam is focused to a 6 μm spot size. Third, our measurements confirm that there are negligible interferences from reflections off the burner surface or from soot scattering, even in the ethylene flames where the soot volume fraction is as high as 2 ppm. The measured temperatures and mixture fractions provide a stringent test for the simulations, but very good agreement is observed.
X-ray diagnostics are widely used in materials science and structural biology, but not in gas-phase reacting flows where the densities are 10,000 times lower. However, recent advances in synchrotron sources and x-ray optics have made combustion applications increasingly feasible. In this work we use x-ray techniques to measure temperature and mixture fraction in-situ in methane/air and ethylene/air coflow nonpremixed flames, then compare the results to detailed flame simulations. More specifically, krypton atoms are seeded into the reactants and krypton number density is measured throughout the flame with x-ray fluorescence (XRF). Since krypton is an inert, if it is added to both reactant streams, then its mole fraction is nearly constant throughout the flame, and its number density can be converted to temperature via the ideal gas law. If krypton is only added to the fuel, then its number density is directly related to mixture fraction. The krypton XRF technique offers several benefits compared to conventional diagnostics with UV or visible lasers. First, the data analysis is simple since the x-ray absorption and emission physics are independent of temperature or collisional environment; no corrections are required for Boltzmann factors or collisional quenching. Second, the spatial resolution is very high: the excitation wavelength is only 0.08 nm (15 keV x-rays), so the beam is focused to a 6 μm spot size. Third, our measurements confirm that there are negligible interferences from reflections off the burner surface or from soot scattering, even in the ethylene flames where the soot volume fraction is as high as 2 ppm. The measured temperatures and mixture fractions provide a stringent test for the simulations, but very good agreement is observed.