Numerical study on flames with repetitive extinction and ignition interacting with cool and blue (warm) flames

Abstract

Recent experiments in a micro flow reactor with a controlled temperature profile (MFR) have realized the simultaneous observations of cool flames and flames with repetitive extinction and ignition (FREI) for an n-heptane/air mixture under atmospheric pressure. The primary objective of this study is to numerically reproduce the experimental observations and to investigate the effects of the cool flame on the FREI dynamics through one-dimensional transient reactive flow simulations with various reactor diameters and inlet velocities. The simulation results show that the reaction front speed of FREI decreases monotonically in the case of non-interaction with the cool flame at an inner diameter of 1 mm. Conversely, the reaction front speed temporarily increases due to thermal–chemical effects from stabilized cool flame for 2 mm diameter. Previous experimental and numerical studies in MFR have revealed strong pressure dependence of the three-stage oxidation reactions, comprising cool flame (CF), blue flame (BF) and hot flame (HF) in steady weak flame regime at low inflow velocity conditions. The second objective is to extend pressure-dependent flame response diagram up to FREI regime observed at higher inflow velocity conditions. Simulations demonstrate that there are pressure-dependent four distinct flame regimes: HF-dominant weak flame, HF-driven FREI, BF-dominant weak flame, and BF-driven FREI emerge with increasing pressure. In addition, a notable difference in ignition modes between HF-driven and BF-driven FREIs is observed. The HF-driven FREI exhibits a single reaction front propagating upstream, whereas the BF-driven FREI exhibits bifurcation in ignition, leading to the reaction fronts that propagate both upstream and downstream. Overall, the findings in this study provide a comprehensive understanding of the interactions between flame dynamics and low to intermediate temperature ignition chemistry in the thermally stratified flow fields.