On the flame structure and stabilization characteristics of autoignited laminar lifted n-heptane jet flames in heated coflow air

Ki Sung Jung, Seung Ook Kim, Suk Ho Chung, Chun Sang Yoo

Research output: Contribution to journalArticlepeer-review

Abstract

The characteristics of the flame structure and stabilization of autoignited laminar lifted n-heptane jet flames in heated coflow air are investigated by performing 2-D numerical simulations with a 68-species skeletal chemical mechanism of n-heptane oxidation. The present simulations can reproduce a distinct transition of a lifted jet flame from a tribrachial edge flame mode to a moderate or intense low-oxygen dilution (MILD) combustion mode observed from a previous experimental study, featuring a significant variation in the liftoff height with the fuel jet velocity, U0. It is found that a lifted flame with the MILD combustion mode can exist further downstream of the stoichiometric mixture fraction isoline due to autoignition occurring upstream of the flamebase. The displacement speed and chemical explosive mode analyses reveal that the autoignition of lean mixtures plays a critical role in stabilizing lifted flames with the MILD combustion mode. It is further elucidated from additional numerical simulations that an autoignited laminar lifted n-heptane jet flames can be stabilized as one of the following forms depending on the inlet temperature, T0, and U0: a MILD combustion, a partially-premixed edge flame, a tribrachial edge flame, and a tetrabrachial edge flame. Based on the flame structures and stabilization mechanisms of the lifted flames, a flame regime diagram is constructed in the normalized U0 and Damköhler number space.
Original languageEnglish (US)
Pages (from-to)307-319
Number of pages13
JournalCombustion and Flame
Volume223
DOIs
StatePublished - Oct 19 2020

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