Distortion of expanding n-heptane flames at high unburned-gas temperatures behind reflected shocks

Simultaneous, dual-perspective, high-speed imaging is used to provide new insight into the morphology of flames generated at high unburned-gas temperatures behind reflected shock waves. Single-perspective, end-wall imaging left previous applications of the shock-tube flame speed method reliant on an assumption of axial symmetry when interpreting experimental results. Here, we report qualitative and quantitative observations from the first application of simultaneous side- and end-wall emission imaging to characterize the three-dimensional morphology of these flames. Side-wall imaging reveals that, while the expected flame symmetry is observed under static and relatively low temperature post-reflected-shock conditions, symmetry can break down at higher temperatures. These results reveal that the concentric regions of emission previously observed in flame experiments in the negative-temperature-coefficient ignition regime can be explained as the axial integration of emission through a distorted flame. Several physical mechanisms are evaluated towards the goal of identifying the underlying cause of flame distortion. The post-reflected-shock flow field and pressure-wave-flame interactions are both found insufficient to explain the results. Presently, not enough is known about the recently identified local-doubleflame structure to assess the likelihood of its relevance in the present experiments. As such, while the dual-perspective imaging results provide important new insight into the morphology of high-temperature flames and inform the correct interpretation of previous experimental observations, the specific mechanism leading to the observed distortion remains a topic requiring further attention. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.