Dynamics of ball-like flames in extremely low-speed counterflow field in near-lean limit low-Lewis number mixture

Abstract

For the understanding of ball-like flame behavior in counterflow field, transient three-dimensional computations with thermal-diffusion model were conducted for a low-Lewis number mixture near lean limit. Three types of flame behaviors were confirmed: stable spherical ball-like flame (spherical BLF) in A ≤ 0.010, stable non-spherical ball-like flame (non-spherical BLF) in 0.01 < A < 0.089 and splitting ball-like flame (splitting BLF) in A ≥ 0.089, where A is ordinary stretch rate normalized with laminar burning velocity SL and thermal diffusivity α. Analysis of flame structure for non-spherical BLF located its center at the stagnation point showed that the maximum temperature on the stagnation plane was higher than that on the counterflow axis because of the small difference between the flame curvatures on the stagnation plane and that on the counterflow axis. With the increase of stretch rate, the maximum temperature of the non-spherical BLF on the stagnation plane increased and the position of maximum temperature got away from the stagnation point. The maximum temperature on the counterflow axis decreased and the position of maximum temperature got closer to the stagnation point. Existence of unburned fuel was also confirmed near the stagnation point at A = 0.085. Thus, net fuel velocity was newly introduced to evaluate the effect of the unburned fuel diffusion. The profile of the net fuel velocity revealed two peaks in the case of A < 0.050 and four peaks in the case of A > 0.050. In the case A > 0.050, the inner two peaks were found to be due to the diffusion of unburned fuel to the outward direction. The analyses on the peak positions showed that the flame splitting occurs when the positions of the inner two peaks of the net fuel velocity are located outside of the reference flame ball radius.