Large eddy simulation of lean blow-off in swirl-stabilized flame with the subgrid dissipation concept

The lean blow-off mechanism of the premixed swirl flame is numerically investigated by large eddy simulation (LES) with the subgrid dissipation concept (SDC) combustion model. Three simulated cases cover stable, near blow-off, and transient conditions. Compared with the experiment, the LES-SDC approach captures the flow and combustion features for stable and near blow-off conditions. More importantly, the predictions of the blowoff procedure and duration agree satisfactorily with the experiment, indicating that the LES-SDC approach is a promising tool for predicting strong, unsteady turbulent combustion processes. Further, the numerical results are used to investigate the blow-off mechanism. Two stages in the blow-off procedure are specified. The first is the necking and extinction of the downstream flame surface, and the second is the shrinking of the upstream flame surface. The blow-off mechanism is well explained by the theory of stretched flame extinction. At the end of the recirculation zone, the large negative radial velocity pushes the flame to the central line. The combustion process here can be abstracted as the stretched counter-flame of the reactant-reactant configuration. The excessive flame stretch dominates the flame extinction and triggers the blow-off event. The upstream flame resists the intense stretch with the help of hot product recirculation, and the combustion here can be idealized as the counter-flame of the reactant-product configuration. The alignment of the temperature gradient and flow velocity, together with the excessive stretch, clearly indicates the tendency of flame local extinction. A Damk & ouml;hler number-based blow-off criterion is raised from the mean flow strain rate and laminar flame bulk extinction strain rate. Novelty and significance The current work examines the recently proposed subgrid dissipation concept (SDC) combustion model in near-blow-off and transient conditions for the first time. The remarkable accuracy in predicting the blow-off procedure and duration validates the LES-SDC approach in predicting strong, unsteady, turbulent combustion processes. The blow-off mechanism is well explained by the theory of stretched flame extinction. Both the reactant-reactant and reactant-product configurations of counter-flame are used to elucidate the stabilization and blow-off mechanism. The significant role of hot product recirculation in stabilizing the upstream excessively stretched flame is specified. A new indicator for local extinction tendency is proposed and verified by instantaneous numerical results. A global blow-off criterion is raised from the mean flow strain rate and laminar flame bulk extinction strain rate without any empirical constant.