Experimental investigation of thermo-diffusive instabilities in lean premixed hydrogen combustion

Climate change driven by the release of greenhouse gases from increasing fossil fuel consumption presents significant global challenges. Addressing these challenges necessitates a global transition toward renewable energy sources, which offer a sustainable and low-emission alternative to fossil fuels. Among the various candidates, hydrogen produced by low-emission methods is widely considered as a promising carbon-free alternative fuel for internal combustion engines, gas turbines and industrial burners. The integration of hydrogen into combustion systems presents several technical challenges, including flashback, flame instabilities, a wide flammability range, high burning velocities, and high nitrogen oxides emissions. To overcome these barriers, advanced hydrogen combustion technologies are essential. Among these, fuel-lean hydrogen combustion has attracted considerable research interest due to its potential to simultaneously reduce nitrogen oxides emissions and enhance thermal efficiency. One key challenge for fuel-lean hydrogen combustion is that it features intrinsic thermo-diffusive instabilities, which lead to cellular burning patterns. However, details of quantitative thermo-chemical states of cellular flames, and how these thermo-diffusive behaviors evolve with turbulence remain unclear. The main objective of this research is to advance the fundamental understanding in the effects of thermo-diffusive instabilities in fuel-lean premixed hydrogen combustion under laminar and turbulent flow conditions. To provide a comprehensive understanding of the thermo-diffusive instabilities effects, representative fuel-lean premixed hydrogen/methane/air and hydrogen/air flames with well-defined boundary conditions are studied. First, the effects of thermo-diffusive instabilities on the basic cellular structures are investigated with laminar fuel-lean premixed hydrogen/methane/air polyhedral flames (paper I and paper II). Furthermore, the effects of interactions between thermo-diffusive instabilities and turbulence on the flame structure are studied with turbulent fuel-lean premixed hydrogen/air flames over a wide range of turbulent intensities up to distributed burning regimes (paper III and paper IV). To quantify the flame topology and thermo-chemical states of fuel-lean premixed hydrogen/methane/air and hydrogen/air flames, advanced laser-based optical diagnostics measurements are employed. Two-dimensional measurements of planar laser-induced fluorescence of hydroxyl radicals are conducted to capture the macroscopic flame structure. Two-dimensional particle image velocimetry measurements are carried out to quantify the flow structure in turbulent flames. One-dimensional spontaneous Raman/Rayleigh spectroscopy is used to quantitatively measure the temperature and concentration of the major species. Two-dimensional Rayleigh thermometry is used to visualize the flame topology along with the one-dimensional Raman/Rayleigh measurements. The main conclusions of this dissertation are summarized as follows. In laminar flows, classic cellular burning patterns are observed in fuel-lean premixed hydrogen/methane/air polyhedral flames. The thermo-chemical states in positively and negatively curved flame segments show significant differences due to the focusing/defocusing of highly diffusive hydrogen by positive/negative curved flame surface. Specifically, flame regions with positive curvatures have a higher hydrogen mole fraction, local equivalence ratio and temperature compared to those with negative curvatures. The hydrogen mole fraction differences between positively and negatively curved flame cells are enlarged with decreasing flow velocity, and with increasing hydrogen content and equivalence ratio. In turbulent flows, interactions between turbulence and flame significantly modify the thermo-diffusive behaviors. At low turbulence intensities, locally intense burning characterized by elevated local equivalence ratio, high water mole fraction, and super-adiabatic flame temperature is mainly observed in post-flame regions surrounded by positively curved flame surfaces, where the highly diffusive hydrogen is locally enriched. This reveals that turbulence imposes synergistic effects with thermo-diffusive instabilities. As the turbulence intensity increases, the local burning enhancement near positively curved flame surfaces is weakened, even though the flame surface is more disturbed by turbulence, which indicates that both molecular diffusion and turbulent transport play significant roles in the combustion process. At high turbulence intensities, no intense burning regions are observed in the flame as turbulent mixing dominates over molecular mixing. However, increasing residence time leads to the occurrence of thermo-diffusive behaviors in positively curved cells formed by the fully developed initial turbulent eddies. The comprehensive multi-scalar data presented in this dissertation not only enhance the fundamental understanding of thermo-diffusive instabilities in fuel-lean premixed hydrogen combustion, but also provide crucial quantitative data for the development and validation of simulation models.