Forced swirl tubular flame under different equivalence ratios and oxygen mole fractions

The swirl tubular flame has been widely used in combustors owing to its unique merits of high combustion efficiency, uniform flame temperature and inherent safety to avoid flashback in the rapidly mixed (RM) mode. However, it also suffers from combustion instability, which would deteriorate the system performance. To explore the mechanism behind, the influences of acoustic forcing disturbances on both the premixed (PM) and rapidly mixed flame characteristics are experimentally investigated, in which the equivalence ratio (Φ) and the oxygen mole fraction (β) are also adjusted to examine the flame responses. Firstly, the measured combustion regimes show that there is an additional flame liftoff region for the RM flames in the mid-acoustic frequency range compared with the PM flames. In addition, the forced RM swirl tubular flame present similarly low-pass filtering with the PM flame, but has a 10 Hz smaller critical frequency for the flame extinction. Finally, the forced flame is found to be lifted off when Φ or β is low. By increasing Φ or β, the lifted flame evolves to the anchored flame, and the transition process is studied via the Proper Orthogonal Decomposition analysis and the 1D opposed-flow flame analysis. The results show that, for both of PM and RM tangential tubular swirl flames, the lifted flame would become anchored when the energy fraction of the axial mode is smaller than 5% of total energy and the extinction strain rate is larger than 1000 s⁻¹.