Ignition Under Strained Conditions: A Comparison Between Instationary Counterflow and Non-premixed Flamelet Solutions

The transient evolution of counterflow diffusion flames can be described in physical space [i.e. by the model of Im et al. (Combust. Sci. Technol. 158:341–363, 2000)], and in composition space through flamelet equations. Both modeling approaches are employed to study the ignition of diluted hydrogen–air, methane–air and DME–air diffusion flames including detailed transport and chemistry modeling. Using the physical space solution as a reference, this work elucidates the capability of flamelet modeling to predict ignition characteristics in terms of ignition temperature and ignition delay time. Varying pressure and strain rate for the hydrogen–air configurations, the agreement between reference solution and flamelet results is shown to strongly depend on the ignition limits as characterized by Kreutz and Law (Combust. Flame 104:157–175, 1996). In limit 2 and at elevated temperatures, where the ignition kernel formation is governed by chemical reactions and less dependent on mass transport (high Damköhler numbers), the flamelet model yields accurate results. Close to the ignition limits 1 and 3 however, significant deviations can be observed. In these limits, the residence time of radicals during ignition kernel formation is strongly influenced by diffusive transport and Damköhler numbers are low. The analysis of the hydrocarbon flames shows that differences between the physical space model and the flamelet model are smaller. This is attributed to a smaller influence of differential diffusion on the ignition process for methane and DME as compared to hydrogen as fuel. This paper underlines that flamelet models can be used to describe ignition processes under strained conditions, but care should be taken if ignition takes place in certain parameter ranges, i.e. close to the ignition limits or at high strain rates.