Experimental investigation of flame surface topology and temperature in lean premixed H2/air flames with 2D Rayleigh thermometry

Two-dimensional (2D) Rayleigh scattering measurements are employed to investigate flame surface topology and temperature in lean premixed hydrogen/air turbulent jet flames as the Karlovitz number increases toward the distributed combustion regime. Quantitative temperature fields obtained from 2D Rayleigh thermometry are validated against one-dimensional (1D) temperature profiles measured via quasi-simultaneous 1D Raman/Rayleigh spectroscopy. The 2D temperature data are used to extract flame front locations and to establish direct correlations between local flame surface curvature and temperature characteristics through joint probability density functions (JPDFs). Results highlight the thermo-diffusive nature of the flame, with higher local temperatures frequently observed in regions of positive curvature. The JPDFs further reveal that non-uniform and asymmetric temperature distributions on the flame surface result from the combined effects of turbulence and differential diffusion. As the Karlovitz number increases, these asymmetries diminish, and temperature distributions shift, signaling a transition toward the distributed combustion regime. Based on these trends, potential temperature-based markers for thermo-diffusive instabilities are proposed, offering new insights into turbulence–chemistry interactions and their role in the onset of distributed burning in lean premixed hydrogen/air flames.