A new review published in Engineering offers systematic experimental insights into thermodiffusive instabilities (TDI) in lean hydrogen combustion and their interactions with turbulence, supporting the design of low‑emission hydrogen‑fueled combustion systems. As a carbon‑free energy carrier, hydrogen is widely used in gas turbines, aero engines, internal combustion engines, and industrial burners, yet its high diffusivity and reactivity introduce TDI that distorts flame structures, modifies local equivalence ratios and temperatures, and influences NOₓ formation under lean‑burn conditions.

TDI occurs in lean hydrogen flames when the Lewis number is less than unity, creating an imbalance between molecular and thermal diffusion that distorts flame fronts into positively curved troughs and negatively curved cusps. Troughs show mass and heat focusing with elevated local equivalence ratios, temperatures, and burning speeds, while cusps exhibit defocusing and reduced reactivity, reinforcing flame curvature and instability. Practical combustors rely on turbulent flows to increase fuel consumption, yet the interplay between turbulence and TDI has lacked sufficient quantitative experimental data, especially across realistic technical conditions including elevated pressures.

Researchers from Technische Universität Darmstadt reviewed recent experiments using three increasingly complex setups: laminar Bunsen flames, piloted turbulent jet flames, and optically accessible internal combustion engines, measured by advanced laser diagnostics including 1D Raman/Rayleigh spectroscopy, 2D Rayleigh thermometry, OH‑PLIF, and high‑speed SO₂‑PLIF. The work addresses how TDI affects flame thermochemical states, how turbulence modulates TDI, and how these interactions can be quantified under high pressure where optical access is limited.

Results show that TDI dominates at low Karlovitz numbers, creating strong cellular structures, local fuel enrichment, and superadiabatic temperatures. As turbulence intensity and Karlovitz number rise, turbulent transport increasingly suppresses differential diffusion effects, making local thermochemical states more uniform and reducing TDI‑driven distortions. In high‑pressure engine conditions, increasing engine speed raises turbulence intensity and weakens TDI, as indicated by the correlation between fluorescence intensity gradients and flame front curvature used as a TDI marker. Across all configurations, the ratio of diffusive to convective time scales plays a critical role in determining the significance of thermodiffusive instabilities.

The review consolidates quantitative experimental evidence that can validate numerical models for turbulent hydrogen combustion, supporting improved predictions of flame structure, stability, and emissions in low‑carbon energy conversion systems.

The paper “Experimental Insights into Thermodiffusive Instabilities in Lean Hydrogen Combustion,” is authored by Tao Li, Benjamin Böhm, Andreas Dreizler. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.09.016. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.