Analysis of local thermochemical states in turbulent H2-air multi-mode flames by Raman/Rayleigh spectroscopy

The effects of turbulent mixing and mixture inhomogeneity on the flame structures of turbulent hydrogen–air multi-mode flames stabilized on a modified Darmstadt multi-regime burner are investigated in this study. Near-simultaneous one-dimensional Raman/Rayleigh and two-dimensional Rayleigh scattering measurements are used to quantify the internal flame structures and flame topologies, whereas simultaneous two-dimensional laser-induced fluorescence of hydroxyl radicals and particle image velocimetry are performed to characterize the macroscopic flame structures and flow fields. Quantitative multi-scalar data, including temperature and major species mole fractions, combined with two-dimensional flame topologies, enable characterization of the local thermochemical states. The examined hydrogen–air multi-mode flames consist of a lifted central jet reaction zone, a primary recirculation zone, a secondary recirculation zone, and an outer reaction zone. Quantitative multi-scalar results reveal an intense jet flame reaction zone characterized by local temperature and water mole fraction peaks. These burning behaviors, which differ from previously investigated methane–air multi-mode flames, are attributed to the higher reactivity and the wider flammability range of hydrogen. Global thermochemical state data indicate significant different reaction trajectories in flames with either different turbulent mixing levels or mixture inhomogeneities. Instantaneous thermochemical states conditioned on the central jet flame front demonstrate the variety of reaction trajectories spanning a wide range of equivalence ratios in the flame with the highest initial jet equivalence ratio. Combined with the heat release rate results derived from one-dimensional simulations, the example single-shot data suggest premixed and stratified combustion modes near the jet flame stabilization position. Local thermochemical state results indicate that an increasing air flow velocity from a surrounding slot increases the jet flame lift-off height and modifies the local equivalence ratio distribution, while a higher jet equivalence ratio promotes a broader diversity of reaction trajectories. Novelty and significance statement Comprehensive experimental investigations of hydrogen–air flames featuring multi-mode characteristics, which are relevant to practical rich-quench-lean operating conditions in gas turbines, have been scarcely reported in the literature. In this work, the effects of turbulent mixing and mixture inhomogeneity on the global flame structures and the local thermochemical states of multi-mode hydrogen–air flames are experimentally investigated using laser-based optical diagnostics. Quantitative multi-scalar results resolve the internal flame structures and provide insight into the various local reaction trajectories in the central reaction zone. To the best of the authors’ knowledge, this work presents the first such set of quantitative scalar measurements for hydrogen–air multi-mode flames, which is of significance for developing and validating simulation models.