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

Type
Article

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

KAUST Department
Combustion and Laser Diagnostics Laboratory
Mechanical Engineering Program
Physical Science and Engineering (PSE) Division

Online Publication Date
2020-10-19

Print Publication Date
2021-01

Date
2020-10-19

Submitted Date
2020-06-04

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.

Citation
Jung, K. S., Kim, S. O., Chung, S. H., & Yoo, C. S. (2021). On the flame structure and stabilization characteristics of autoignited laminar lifted n-heptane jet flames in heated coflow air. Combustion and Flame, 223, 307–319. doi:10.1016/j.combustflame.2020.10.008

Acknowledgements
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2018R1A2A2A05018901). SHC was supported by KAUST. This research used the resources of the KAUST Supercomputing Laboratory and UNIST Supercomputing Center.

Publisher
Elsevier BV

Journal
Combustion and Flame

DOI
10.1016/j.combustflame.2020.10.008

Additional Links
https://linkinghub.elsevier.com/retrieve/pii/S0010218020304284

Permanent link to this record