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Turbulent mixing and flame stability in a dual-swirler ammonia/methane co-flame burner : Reynolds number effects on NOx emissions

Xu, Leilei LU ; Sjögren, Carl Otto Olsson LU orcid ; Zhou, Yuchen LU ; Chen, Fang ; Hassan, Zubayr O. ; Alsuhaibani, Abdulrahman S. ; Solami, Bandar H. ; Jamal, Aqil ; Siddiqui, Osamah and Roberts, William L. , et al. (2026) In Combustion and Flame 287.
Abstract

An innovative coaxial dual-swirl combustor was developed to address the challenges of combustion instability and NOx emissions inherent to ammonia combustion. The combustor employs an inner swirler supplying a premixed ammonia/air stream and an outer swirler generating a premixed methane/air flame that stabilizes the inner ammonia flame. Combined large-eddy simulations (LES) and planar laser-induced fluorescence (PLIF) measurements were conducted to systematically examine the effects of the inner-stream equivalence ratio and outer-stream Reynolds number on NO emissions. The results demonstrate a marked reduction in NO emissions for stoichiometric to fuel-rich ammonia/air flames, while lean flames near the blowout limit exhibit strong... (More)

An innovative coaxial dual-swirl combustor was developed to address the challenges of combustion instability and NOx emissions inherent to ammonia combustion. The combustor employs an inner swirler supplying a premixed ammonia/air stream and an outer swirler generating a premixed methane/air flame that stabilizes the inner ammonia flame. Combined large-eddy simulations (LES) and planar laser-induced fluorescence (PLIF) measurements were conducted to systematically examine the effects of the inner-stream equivalence ratio and outer-stream Reynolds number on NO emissions. The results demonstrate a marked reduction in NO emissions for stoichiometric to fuel-rich ammonia/air flames, while lean flames near the blowout limit exhibit strong sensitivity of NO emissions to the outer-stream Reynolds number. LES and PLIF analyses reveal that flame–flame interactions in the shear layer between the two flames govern this behavior. Depending on the inner equivalence ratio, merged single reaction zone, distinct twin reaction zones or triple-flame structures form, altering radical concentrations and NO formation pathways. The central recirculation zone (CRZ), originating from vortex breakdown, also plays a key role in stabilizing the flames and oxidizing residual fuel. Hot gases from the outer flame recirculate into the inner ammonia stream, promoting complete combustion similar to the Rich–Quench–Lean (RQL) concept. Overall, NO emissions are primarily governed by the intensified flame–flame interactions. At higher outer-stream Reynolds numbers, lean flames ((Formula presented) ) exhibit enhanced NO formation via the HNO pathway, driven by local flame extinction, partial mixing of methane and ammonia through extinction holes, and subsequent downstream oxidation. Near-stoichiometric flames show reduced NO emissions due to dilution and radical pool modification, without evidence of local extinction. In contrast, fuel-rich flames ((Formula presented) ) exhibit effective de-NOx reduction with only moderate sensitivity to the Reynolds number, owing to the robust triple-flame structure. This study provides critical insights into flame–flame interactions and NOx formation in ammonia/methane dual-swirl flames, offering a pathway to more stable, low-emission ammonia combustion technologies and advancing the practical deployment of ammonia as a carbon-free fuel.

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type
Contribution to journal
publication status
published
subject
keywords
Ammonia combustion, Decarbonization, Dual-swirl flames, Flame-flame interaction, Low-NOx combustion
in
Combustion and Flame
volume
287
article number
114883
publisher
Elsevier
external identifiers
  • scopus:105034193927
ISSN
0010-2180
DOI
10.1016/j.combustflame.2026.114883
language
English
LU publication?
yes
id
a8011689-0287-4923-b55e-e69206c71c8f
date added to LUP
2026-04-24 14:51:15
date last changed
2026-04-24 14:52:22
@article{a8011689-0287-4923-b55e-e69206c71c8f,
  abstract     = {{<p>An innovative coaxial dual-swirl combustor was developed to address the challenges of combustion instability and NOx emissions inherent to ammonia combustion. The combustor employs an inner swirler supplying a premixed ammonia/air stream and an outer swirler generating a premixed methane/air flame that stabilizes the inner ammonia flame. Combined large-eddy simulations (LES) and planar laser-induced fluorescence (PLIF) measurements were conducted to systematically examine the effects of the inner-stream equivalence ratio and outer-stream Reynolds number on NO emissions. The results demonstrate a marked reduction in NO emissions for stoichiometric to fuel-rich ammonia/air flames, while lean flames near the blowout limit exhibit strong sensitivity of NO emissions to the outer-stream Reynolds number. LES and PLIF analyses reveal that flame–flame interactions in the shear layer between the two flames govern this behavior. Depending on the inner equivalence ratio, merged single reaction zone, distinct twin reaction zones or triple-flame structures form, altering radical concentrations and NO formation pathways. The central recirculation zone (CRZ), originating from vortex breakdown, also plays a key role in stabilizing the flames and oxidizing residual fuel. Hot gases from the outer flame recirculate into the inner ammonia stream, promoting complete combustion similar to the Rich–Quench–Lean (RQL) concept. Overall, NO emissions are primarily governed by the intensified flame–flame interactions. At higher outer-stream Reynolds numbers, lean flames ((Formula presented) ) exhibit enhanced NO formation via the HNO pathway, driven by local flame extinction, partial mixing of methane and ammonia through extinction holes, and subsequent downstream oxidation. Near-stoichiometric flames show reduced NO emissions due to dilution and radical pool modification, without evidence of local extinction. In contrast, fuel-rich flames ((Formula presented) ) exhibit effective de-NOx reduction with only moderate sensitivity to the Reynolds number, owing to the robust triple-flame structure. This study provides critical insights into flame–flame interactions and NOx formation in ammonia/methane dual-swirl flames, offering a pathway to more stable, low-emission ammonia combustion technologies and advancing the practical deployment of ammonia as a carbon-free fuel.</p>}},
  author       = {{Xu, Leilei and Sjögren, Carl Otto Olsson and Zhou, Yuchen and Chen, Fang and Hassan, Zubayr O. and Alsuhaibani, Abdulrahman S. and Solami, Bandar H. and Jamal, Aqil and Siddiqui, Osamah and Roberts, William L. and Bai, Xue Song and Elbaz, Ayman M.}},
  issn         = {{0010-2180}},
  keywords     = {{Ammonia combustion; Decarbonization; Dual-swirl flames; Flame-flame interaction; Low-NOx combustion}},
  language     = {{eng}},
  publisher    = {{Elsevier}},
  series       = {{Combustion and Flame}},
  title        = {{Turbulent mixing and flame stability in a dual-swirler ammonia/methane co-flame burner : Reynolds number effects on NOx emissions}},
  url          = {{http://dx.doi.org/10.1016/j.combustflame.2026.114883}},
  doi          = {{10.1016/j.combustflame.2026.114883}},
  volume       = {{287}},
  year         = {{2026}},
}