Impact of the operating conditions on the OH* distribution and its correlation with the heat release rate in hydrogen–air flames

OH* chemiluminescence is widely used as heat release rate (HRR) marker in combustion experiments. Still, its suitability for hydrogen–air flames has not been extensively assessed for a wide range of operative conditions and flame archetypes. In the present work, correlations between OH* and HRR spatial distributions are first investigated in one-dimensional unstretched laminar premixed flames at variable pressure ([1; 20] atm), unburned gas temperature ([300; 900] K), and equivalence ratio ([0.3; 3.0]). At atmospheric pressure and unburned gas temperature, two main differences are observed: a characteristic shift between the OH* and HRR peak positions and, for equivalence ratios close to stoichiometry, the presence of a non-negligible concentration of OH* in the post-flame zone, where the HRR value is zero. When pressure and unburned gas temperature are increased, the peak shift is attenuated, while the OH* concentration in the burned gases is favored. For lean (ϕ=0.35) and stoichiometric mixtures, the effects of strain and curvature contributions of flame stretch are analyzed in one-dimensional counterflow flames and two-dimensional expanding flames. Higher strain rates slightly affect the peak shift, but sensibly enhance the production of OH* in the burned gas region. Stretch strongly impacts on expanding lean flames, for which the onset of intrinsic instabilities worsens the OH*-HRR correlation, in terms both of distribution shape and intensity. Finally, nonpremixed one-dimensional counterflow diffusion flames and a more complex two-dimensional triple flame are analyzed. In both configurations, a significant reduction of the peak shift is observed when the combustion occurs in diffusion-controlled regimes, sustaining the adequacy of OH* as HRR marker for hydrogen–air diffusion flames under various operating conditions. Novelty and significance statement This work presents a systematic investigation of the OH* distribution and its correlation with the heat release rate in several canonical premixed and nonpremixed laminar hydrogen–air flames under various operating conditions, extending the current literature on the subject. The chemical pathways leading to the peculiar behavior observed for stoichiometric flames are investigated, and the interaction of OH* with intrinsic thermodiffusive instabilities in lean flames is analyzed, showing how the correlation with the heat release rate is worsened. A coherent methodology to quantitatively assess heat release surrogates, which can be extended to other measurable quantities, is provided too. This knowledge is significant as it underlines how the parametric variation of operating conditions can differently affect the correlation between the OH* and the heat release rate distributions, highlighting the limits of OH*chemiluminescence as a combustion diagnostic technique to validate numerical simulations.