LES/PDF modeling of swirl-stabilized non-premixed methane/air flames with local extinction and re-ignition

Abstract
Turbulent non-premixed flames with local extinction and re-ignition exhibit multiple combustion modes including ignition waves, diffusion flames, partially premixed flames, and ignition-assisted partially premixed flames. The mechanisms of local extinction and re-ignition are not well understood and numerical modeling of multi-mode combustion is a challenging task. In this work, a specially designed swirl-burner was used to study local extinction and re-ignition of non-premixed turbulent methane/air flames. High speed Particle Image Velocimetry (PIV) and laser induced fluorescence of OH radicals (OH-PLIF) measurements along with Large Eddy Simulation (LES) were carried out to investigate the mechanisms of extinction and re-ignition processes in the burner. LES is based on a transported probability density function model within the framework of Eulerian Stochastic Fields (PDF-ESF). It is found that local extinction occurs when the scalar dissipation rate around the stoichiometric mixture fraction is high. The characteristic time scale for local extinction and re-ignition in the present flames is an order of magnitude longer than the characteristic time scale of diffusion/extinction of laminar flamelets. There are two mechanisms for flame hole re-ignition in the present flames. First, under low degree of local extinction conditions (i.e., for small flame holes surrounded by flames) the flame hole re-ignition is due to the mechanism of turbulent flame folding. Second, under high degree of extinction conditions (i.e., with large regions of extinction and lifted flames), re-ignition of the locally extinguished flame is due to the mechanism of ignition assisted partially premixed flame propagation. The results show that the PDF-ESF model is capable of simulating the quenching and re-ignition process found in the experiments.

Citation
Yu, S., Liu, X., Bai, X. S., Elbaz, A. M., & Roberts, W. L. (2020). LES/PDF modeling of swirl-stabilized non-premixed methane/air flames with local extinction and re-ignition. Combustion and Flame, 219, 102–119. doi:10.1016/j.combustflame.2020.05.018

Acknowledgements
The experimental work was supported by competitive research funding from King Abdullah University of Science and Technology (KAUST), Saudi Arabia. The LES work was partly sponsored by Swedish Research Council (VR), and the Swedish Energy Agency (STEM) through the national center for combustion science and technologies (CeCOST) and Lund University Competence Center for Combustion Process (KC-FP). S. Yu and X. Liu were sponsored by China Scholarship Council (CSC). The computations were performed using the computer facilities provided by High Performance Computing Center North (HPC2N), and Center for Parallel Computers (PDC).

Publisher
Elsevier BV

Journal
Combustion and Flame

DOI
10.1016/j.combustflame.2020.05.018

Additional Links
https://linkinghub.elsevier.com/retrieve/pii/S0010218020302017

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