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Despite the high technical relevance of early flame kernel development forthe reduction of cycle-to-cycle variations in spark ignition engines, there isstill a need for a better fundamental understanding of the governingin-cylinder phenomena in order to enable resilient early flame growth. Toisolate the effects of small- and large-scale turbulent flow motion on theyoung flame kernel, a three-dimensional DNS database has been designed to berepresentative for engine part load conditions. The analysis is focussed onflame displacement speed and flame area in order to investigate effects offlame structure and flame geometry on the global burning rate evolution. It isshown that despite a Karlovitz number of up to 13, which is at the upper rangeof conventional engine operation, thickening of the averaged flame structure bysmall-scale turbulent mixing is not observed. After ignition effects havedecayed, the flame normal displacement speed recovers the behavior of a laminarunstretched premixed flame under the considered unity-Lewis-number conditions.Run-to-run variations in the global heat release rate are shown to be primarilycaused by flame kernel area dynamics. The analysis of the flame area balanceequation shows that turbulence causes stochastic flame kernel area growth byaffecting the curvature evolution, rather than by inducing variations in totalflame area production by strain. Further, it is shown that in local segments ofa fully-developed planar flame with similar surface area as the investigatedflame kernels, temporal variations in flame area rate-of-change occur.Contrasting to early flame kernels, these effects can be exclusively attributedto curvature variations in negatively curved flame regions.

DNS Study of the Global Heat Release Rate During Early Flame Kernel Development under Engine Conditions