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Influence of lateral species diffusion and heat transfer on the evaluation of near-threshold sooting flames

Authors: Ahmad Saylam, Torsten Endres, and Christof Schulz

Document type: Published journal article

Journal: Combustion and Flame

Publication details: Volume 253, Article 112775, July 2023

Hosted version: Author Accepted Manuscript with author-prepared supplemental material

Access and rights note: This website hosts the Author Accepted Manuscript and author-prepared supplemental material, not the final Elsevier Version of Record. The hosted manuscript incorporates changes resulting from peer review but does not include publisher copy-editing, typesetting, CrossMark or other publisher-added features. Please cite the final published article.

DOI: 10.1016/j.combustflame.2023.112775

Independent technical summary

Premixed flat flames are widely used to investigate soot formation because they can provide well-defined axial temperature and composition histories for optical diagnostics and model validation. Near the soot-formation threshold, however, lateral transport between the target flame and its surroundings can disturb the nominally one-dimensional centreline state.

This study combines two-dimensional simulations with measurements of temperature, species-concentration and soot-volume-fraction profiles to evaluate the effects of burner diameter and shielding strategy. Methane–air and ethylene–air flames were examined at atmospheric pressure and 10 bar. The target flame was surrounded either by a nitrogen coflow or by a non-sooting fuel-rich methane–air flame.

The simulations show that lateral species diffusion and heat transfer have their strongest influence for near-threshold sooting conditions and at atmospheric pressure. Their influence on the centreline temperature and species profiles decreases at 10 bar.

The study reports quasi-unchanged axial temperature and soot-volume-fraction profiles for burner diameters of at least 40 mm when the target flame is shielded by a non-sooting flame, and at least 60 mm when it is shielded by nitrogen.

For ethylene–air flames with an equivalence ratio of 2.1, measured soot-volume-fraction profiles for different burner diameters and shielding flows differed little at heights of 14 mm or less above the burner. Differences became more significant farther downstream, especially for the 20 mm flame shielded by nitrogen.

Technical contribution

The paper quantifies when a premixed flat flame can reasonably be interpreted as quasi-one-dimensional and when lateral boundary effects must be included. It links burner geometry and shielding design directly to the reliability of measured and simulated centreline profiles under near-threshold soot-forming conditions.

The work is especially relevant to optical soot diagnostics, validation of detailed kinetic and soot models, selection of burner dimensions and comparison of atmospheric- and elevated-pressure flame experiments.

Experimental and modelling implications

  • Burner diameter should be treated as part of the experimental boundary condition rather than as a purely mechanical detail.
  • Nitrogen shielding and reactive-flame shielding are not interchangeable because they impose different lateral species and thermal boundary conditions.
  • Agreement at low heights above the burner does not guarantee agreement farther downstream, where lateral transport has had more time to alter the centreline state.
  • One-dimensional calculations should be used cautiously when the target flame is narrow, weakly sooting or strongly affected by its shielding environment.
  • Two-dimensional simulations can support selection of burner diameter and measurement height before interpreting soot-threshold data.

Scope and application boundary

The reported diameter criteria and transport effects apply to the investigated fuels, equivalence ratios, pressures, burner configurations, shielding conditions and measurement range. They should not be transferred unchanged to other burners or fuels without renewed transport analysis.

A quasi-one-dimensional centreline profile does not imply that the full flame is one-dimensional. Radial gradients, edge chemistry and shielding interactions remain present even when their centreline influence is acceptably small.

Application to another system should evaluate burner diameter, target-flame reactivity, pressure, residence time, shielding-gas composition, thermal boundary conditions and the height range used for diagnostics or model validation.

Author Accepted Manuscript and supplemental material

View the Author Accepted Manuscript and supplemental material

This is an accepted-manuscript version of an article published in Combustion and Flame, volume 253, Article 112775, July 2023. The final Version of Record is available through the DOI.

Licence and reuse

The hosted manuscript states that it is made available under the Creative Commons Attribution–NonCommercial–NoDerivatives 4.0 International licence .

The licence permits sharing with attribution for non-commercial purposes, provided the material is distributed unchanged. It does not permit distribution of modified versions under that licence.

Publisher access

The final Version of Record is available from Elsevier through the DOI: access the publisher record . Access conditions are determined by the publisher.

Citation

Saylam, A., Endres, T., & Schulz, C. (2023). Influence of lateral species diffusion and heat transfer on the evaluation of near-threshold sooting flames. Combustion and Flame, 253, 112775. https://doi.org/10.1016/j.combustflame.2023.112775