Flame wrinkling and self-disturbance in cellularly unstable hydrogen-air laminar flames

Particle image velocimetry (PIV) was employed to investigate the disturbance of unstable laminar hydrogen-air outwardly propagating spherical flames in a spherical explosion vessel. Both the flame speed and gas velocities exhibit acceleration due to the wrinkled flame surface. This acceleration occurs throughout the cycle of strong and weak acceleration phases, which is driven by the division and expansion of cells across the flame front. The flame speed acceleration exponent, α, and the gas velocity ahead of the flame acceleration exponent, αg, are nearly identical. This suggests a close interrelation between the self-acceleration of flame speed (Sn) and the gas velocity ahead of the flame front (ug). Stronger gas flow disturbances caused by more wrinkly flame surfaces are associated with smaller cell sizes. Furthermore, the power spectral density displayed by the flow ahead of the flame front shows remarkable similarity to the fluctuation observed in the flame front. This suggests that the driver for gas disturbance generated ahead of the flame front can be attributed to the wrinkled flame front, whether arising from flame instability. The Power Spectral Density (PSD) of gas velocities and flame wrinkling display a steeper slope (-2.7) compared to isotropic turbulence within the inertial sub-range, where energy spectra decay at -5/3, as measured in a previous study. This observation suggests that gas flow induced by flame instability differs from isotropic turbulence. The normalized gas flow disturbance intensity, (ugc′/ugc‾), has been proposed as an evaluation parameter for studying intrinsic disturbance intensity. The increased gas flow disturbance intensity is closely associated with both the wrinkled flame surface and a smaller cell size. Empirical correlations for gas flow disturbance intensity are explored in terms of the laminar critical Karlovitz number (Kcl), effective Lewis number (Leeff), and burnt gas Markstein number (Mab). There is no obvious causal link between self-acceleration and self-disturbance. Both phenomena are attributed to the wrinkled flame surface.