Macroscopic flame and flow structures in hydrogen and methane multi-regime combustion

The current experimental investigation focuses on the macroscopic structures of CH/air and H/air flames operated on the Darmstadt multi-regime burner adapted for hydrogen operation. Building upon previous research on lean-burn limits, this study utilizes simultaneous PIV and OH-PLIF measurements to examine notable differences in flame and flow structures. Six flame cases are studied, focusing on CH/air and H/air jet flames at equivalence ratios of 1.4, 2.2, and 3.5. It is observed that the hydrogen slot 2 flames exhibit unique behavior under ultra-lean conditions, demonstrating thermodiffusive effects that generate finger-like structures. Despite receiving less thermal support from the slot 2 flame, hydrogen jet flames burn faster and resist flame extinction in high-velocity regions. The extensive heat release from fuel-rich H jets maintains a stable location compared to CH jets at the same equivalence ratio, altering local flow dynamics. Additionally, the study identifies a reshaped primary inner recirculation zone (IRZ) and a secondary IRZ in H/air flame cases, which is absent in CH flames. The interplay between the jet flame and the primary IRZ results in visible flame enhancement in slot 2, indicating preheating and fuel enrichment effects. Overall, this research provides comprehensive insights into the distinct combustion behavior and flow structures of CH and H flames on a multi-regime burner.