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.

逆流扩散火焰的瞬态演化可以在物理空间中描述[即通过Im等人的模型。（Combust。Sci。Technol。158：341–363，2000）]，并通过火焰小方程在合成空间中。两种建模方法都用于研究稀释的氢气-空气，甲烷-空气和DME-空气扩散火焰的点火，包括详细的运输和化学建模。这项工作以物理空间解决方案为参考，阐明了小火焰建模能够根据点火温度和点火延迟时间预测点火特性的能力。氢-空气配置的压力和应变率各不相同，参考溶液和小火焰结果之间的一致性在很大程度上取决于Kreutz和Law（Combust。Flame 104：157-175，1996）表征的着火极限。在极限2和高温下，点火核的形成受化学反应支配，而对质量传递的依赖性较小（高Damköhler数），小火焰模型可得出准确的结果。但是，接近点火极限1和3，可以观察到明显的偏差。在这些限制下，自由基在点火核形成过程中的停留时间受到扩散传输的强烈影响，而Damköhler数很低。对碳氢化合物火焰的分析表明，物理空间模型和小火焰模型之间的差异较小。与氢作为燃料相比，这归因于差异扩散对甲烷和DME点火过程的影响较小。本文着重指出，小火焰模型可用于描述应变条件下的点火过程，