Simultaneous imaging of OH and temperature in lean premixed hydrogen/air flames: Which marker for thermodiffusive instability?

Abstract

This study investigates thermodiffusive (TD) instabilities in lean premixed hydrogen/air flames using simultaneous measurements of hydroxyl radical (OH) via planar laser-induced fluorescence (PLIF) and temperature via Rayleigh scattering. Correlations between flame front curvature and scalar (OH and temperature) gradients as surrogates for reaction rate were assessed in the presence of TD instabilities. Experimental 2D data were compared with corresponding 2D slices extracted from 3D direct numerical simulations (DNS). In both DNS and experiments, flame fronts defined by local maximum scalar gradients (gradient-based) leads to spurious results owing to very low gradients in highly negative curvature regions associated with near-extinction by TD instability. These discontinuous fronts exhibited weaker curvature–gradient correlations than the curvature–HRR (heat release rate) correlation, indicating that scalar gradients along gradient-based fronts are inappropriate surrogates for HRR. To address this limitation, two continuous flame fronts were evaluated: (1) laminar-based front, defined using temperature progress variable at local maximum gradients of scalar by laminar flame calculation, and (2) mode-based front, defined using the most probable temperature (mode value of PDF) at local maximum gradients of temperature. These fronts capture low gradient regions and exhibit stronger correlations between curvature and a surrogate for the reaction rate in both DNS and experiment. DNS analysis revealed that flame fronts based on temperatures (from both laminar-based and mode-based methods) and OH (from laminar-based method) exhibit strong correlation with HRR, with laminar-based flame front for OH showing the highest correlation. However, in experiments, laminar-based flame front for OH correlates poorly with HRR due to spatial misalignment between the temperature and OH fields. Flame fronts by temperature from both laminar-based and mode-based methods are determined to be the most reliable HRR surrogates in experiments. This study highlights that analyzing curvature–gradient correlations under TD instability requires a continuous flame front capturing negative curvature and low gradient regions.