Analysis of premixed flame kernel/turbulence interactions under engine conditions based on direct numerical simulation data

Abstract

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 cycleto-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.

Development of a Large-Eddy Simulation Methodology for the Analysis of Cycle-to-Cycle Combustion Variability of a Lean Burn Engine

“…In the last decade several research groups have conducted modeling studies and have assessed the capacity of LES to predict complex phenomena occurring in ICE and more particularly for the study of knock prediction [2][3][4] or cycle to cycle variations in spark-ignition engines [5][6][7][8][9][10][11]. More recently, attention has been paid to the description of the early flame kernel development and its interaction with turbulence which is of crucial importance because cycleto-cycle variations (CCV) mostly find their origins during the early stage of ignition as explained in [12]. Such DNS analysis have helped to understand the intrinsic mechanisms of flame/turbulence interactions occurring in engine conditions.…”

“…At high air-fuel equivalence ratio conditions, laminar flame speeds are indeed very low and the influence of the flame curvature on the flame propagation needs to be considered in particular at the onset of ignition where curvatures are at their highest levels. While the former is usually taken into account in the solvers used in industry, the latter is a phenomenon that has been largely neglected up-to-now except in a few fundamental studies on flame ignition [9,12]. The effect of the spark plug orientation on early flame kernel development and cycle-to-cycle variability has been studied in [16] in stoichiometric to rich conditions.…”