Response of Heat Release Rate to Flame Straining in Swirling Hydrogen-Air Premixed Flames

The main objective of this study is to clarify effects of flame straining on flame structures and heat release rate (HRR) of swirling flames. This is achieved by analyzing results of direct numerical simulations (DNS) of hydrogen—air turbulent swirling premixed flames considering two swirl number and two equivalence ratio cases. Statistical characteristics of HRR are investigated by examining the mean HRR conditioned on a reaction progress variable and the total HRR in the computational domain. Conditional means of the HRR show that the magnitude of the HRR in reaction zones is smaller for higher swirl number cases than that for lower swirl number cases. A direct comparison between strained laminar and swirling flames shows the influence of the strain rate on the flame structure and the progress of elementary reactions. As strain rate increases in a laminar flame, the peak of the HRR by an exothermic reaction H₂ + OH → H₂O + H shifts toward the burnt side, implying active production of H in the burnt side. The HRR of the above reaction also shows an increasing tendency in a laminar flame under the strain rates greater than 10⁶ s^− 1. The strain–flame interaction with this tendency affects the HRR on highly strained flame surfaces of the swirling flames. It is also clarified that the local HRR intensity is dominated not only by strain rate but also by diffusion of H from the burnt to unburnt side.