  Although the evolution of premixed flames in turbulence has been frequently
studied, it is not well understood how small flames interact with large-scale
turbulent flow motion. Since this question is of practical importance for the
occurrence of cycle-to-cycle variations in spark ignition engines, the
objective of the present work is to fundamentally differentiate early flame
kernel development from well-established turbulent flame configurations. For
this purpose, a DNS database consisting of three flames propagating in
homogeneous isotropic turbulence (Falkenstein et al., Combust. Flame, 2019) is
considered. The flames feature different ratios of the initially laminar flame
diameter to the integral length scale. To quantify flame kernel development,
the time evolution of flame topology and flame front geometry are analysed in
detail. It is shown that some realizations of the early flame kernel are
substantially influenced by high compressive strain caused by large-scale
turbulent flow motion with characteristic length scales greater than the flame
kernel size. As a result, the initial spherical kernel topology may become
highly distorted, which is reflected in the stochastic occurrence of excessive
curvature variance. Two mechanisms of curvature production resulting from early
flame kernel/turbulence interactions are identified by analysis of the mean
curvature balance equation. Further, it is shown that the curvature
distribution of small flame kernels becomes strongly skewed towards positive
curvatures, which is contrary to developed turbulent flames. Hence, the
transition of ignition kernels to self-sustaining turbulent flames is very
different in nature compared with the development of a statistically planar
flame brush.
