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Although experimental evidence for the correlation between early flame kerneldevelopment and cycle-to-cycle variations (CCV) in spark ignition (SI) engineswas provided long ago, there is still a lack of fundamental understanding ofearly flame/turbulence interactions, and accurate models for full enginesimulations do not exist. Since the flame kernel is initiated with small size,i.e. with large positive curvature, differential diffusion is expected toseverely alter early flame growth in non-unity-Lewis-number(${\mathrm{Le}\neq1}$) mixtures as typically used in engines. In this work, aDNS database of developing iso-octane/air flame kernels and planar flames hasbeen established with flame conditions representative for stoichiometric enginepart-load operation. Differential diffusion effects on the global heat releaserate are analyzed by relating the present findings to equivalent flamescomputed in the ${\mathrm{Le}=1}$ limit. It is shown that in the early kerneldevelopment phase, the normal propagation velocity is significantly reducedwith detrimental consequences on the global burning rate of the flame kernel.Besides this impact on the overall mass burning rate, the initial production offlame surface area by the normal propagation term in the flame area balanceequation is noticeably reduced. By using the optimal estimator concept, it isshown that strong fluctuations in local heat release rate inherent to${\mathrm{Le}\neq1}$ flames in the thin reaction zones regime are mainlycontained in the parameters local equivalence ratio, enthalpy, and H-radicalmass fraction. Differential diffusion couples the evolution of these parametersto the unsteady flame geometry and structure, which is analyzed in Part II ofthe present study (Falkenstein et al., Combust. Flame, 2019).

combustion and flame

The Role of Differential Diffusion during Early Flame Kernel Development  under Engine Conditions -- Part I: Analysis of the Heat-Release-Rate Response