Manifold resolution study of the FGM method for an igniting diesel spray

Spray H (baseline n-heptane) cases of Engine Combustion Network (ECN) are investigated with the Flamelet Generated Manifold (FGM) method. In the FGM method, all thermo-chemical properties are stored as a function of controlling variables, here the mixture fraction and the progress variable. Two approaches are used to construct the FGM tables, igniting counterflow diffusion flamelets (ICDF) and homogeneous reactors (HR). To capture ignition delay times in a reliable way, a sensitivity study is performed to determine the optimal FGM table properties. It is observed that using quadratic discretization in progress variable space, i.e. more points in early stages of chemistry, is essential to predict accurate ignition timings. Once quadratic spacing is applied, the ignition delay time results show good correlation with those of experiments. This is observed with both ICDF- and HR-based FGM tables. The similarity between ignition delay results of two approaches indicates that the effect of mixing on chemistry is low and the diffusion process plays a minor role at elevated ambient conditions such as in conventional diesel engines. Finally, the effect of including the mixture fraction variance is analyzed and the impact is found mainly on the lift-off length. The ignition delay is hardly affected.