Initiation and propagation of one-dimensional planar flames in mixtures with variable reaction progress

Abstract

In light of the recent high temperature flame experiment conducted in a shock tube, the main objective of the present study is to understand flame initiation regimes and propagation features with various ignition energy and various extents of unburnt mixture reaction progress. Flame initiation and propagation are numerically investigated for a partially reactive, near stoichiometric n-heptane/air/He mixtures under elevated temperature 700 K in a one-dimensional planar domain under nearly isobaric condition. Emphasis on flame initiation is placed on the effects of ignition energy and initial reaction progress. For ignition timing at tig = 0 s, while regular hot flames are directly initiated with higher ignition energy as expected, a class of autoignition-assisted cool flames can be directly triggered with relatively low ignition energy. By postponing ignition at later times, more pronounced chemical reaction progress is associated with the unburnt mixture such that autoignition-assisted hot flame can be achieved. For such flames, a notable flame speed increment is observed after the first-stage ignition occurs in the upstream unburnt mixture. It is found that both autoignition-assisted cool and hot flames prior to the first-stage ignition strongly depend on the ignition energy and ignition timing, and hence are case-sensitive. Extra caution needs to be taken to evaluate and compare these data. Hot flames in the post first-stage ignition mixture nevertheless exhibit much less variation to the ignition energy and initial reaction progress. For steady-state simulation, it is emphasized that a sufficiently short upstream and sufficiently long downstream domain are needed to obtain desired flame solution in the classical diffusion-reaction limit. A good agreement between the steady and unsteady simulations demonstrates the unsteady flame propagates in a quasi-steady manner with the classical flame structure, as long as τchem/τflame>>1. Sensitivity analysis and reaction network analysis have further suggested the similar key reactions on flame speed compared with regular conditions and demonstrated indirect effects from the low temperature ignition chemistry. This work provides useful insights into flame initiation regimes and propagation features under elevated temperature.