Flame flickering frequency on a rotating Bunsen burner

The empirical correlation (St^*2/Ri=0.00028Re^*2/3: St^* is the reduced Strouhal number, Ri the Richardson number, Re^* the reduced Reynolds number) proposed by Kostiuk and Cheng [1995. The coupling of conical wrinkled laminar flames with gravity. Combustion and Flame 103, 27-40] for predicting buoyancy-induced flame flickering frequency, has been experimentally investigated under the swirling flow conditions produced by a rotating Bunsen burner, focusing on how the flame flickering frequency changes with increasing swirl number S. Under low swirling conditions up to S≈0.1, the flame flickering frequency f_t did not change and monotonically increased with increasing the bulk flow velocity from the burner tube U. The trend of the data is similar to those obtained by Durox et al. [1995. Some effects of gravity on the behavior of premixed flames. Combustion and Flame 82, 66-74] and Kostiuk and Cheng (1995) and could be fitted well with the empirical correlation Kostiuk and Cheng (1995). However, under high swirling  conditions (S>1), f_t significantly decreased up to ≈80% of that with non-swirling condition, and became insensitive to U. As a result, the data under S>1 could not be correlated by the empirical equation. From results obtained by laser doppler velocimetry (LDV), this inconsistency is due to flow divergence shown by the change in the velocity distribution between the burner exit and the flame tip with burner rotation. A minimum value of the centerline velocity u_j,min between the burner exit and the flame tip fulfills an important role in controlling the flame flickering frequency. The use of this parameter allows the empirical correlation to be extended to the high swirling case by means of a modified empirical correlation St^*2/Ri=0.00028 (Re^*exp(-0.64S^1.78))^2/3.