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.