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

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.

For the investigated configurations, the simulations show that lateral species diffusion and heat transfer exert their strongest influence under near-threshold sooting conditions and at atmospheric pressure. Their influence on the centreline temperature and species profiles was smaller at 10 bar for the cases examined. This pressure-dependent trend should not be generalized beyond the studied burner, fuel, shielding and operating ranges without renewed transport analysis.

Within the specific configurations evaluated, 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. These values are study-specific criteria rather than universal burner-design thresholds.

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, for the investigated configurations, when a premixed flat flame can reasonably be interpreted as quasi-one-dimensional at the centreline and when lateral boundary effects must be included. It links burner geometry and shielding strategy to the interpretation of measured and simulated centreline profiles under near-threshold soot-forming conditions.

The work is especially relevant to optical soot diagnostics, validation planning for detailed kinetic and soot models, selection of burner dimensions and comparison of atmospheric- and elevated-pressure flame experiments. Agreement of a model with these data supports only the observables and operating domain actually tested; it does not by itself validate all chemical, transport or soot submodels.

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 and can reveal lateral-transport sensitivity before soot-threshold data are interpreted.
• Numerical consistency and model assumptions should be checked before disagreements are attributed to new combustion or soot chemistry.

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 assessment. Any validation claim should be restricted to the measured quantities, uncertainty, operating range and model outputs that are actually compared.

Evidence interpretation

Experimental measurements and two-dimensional calculations provide complementary evidence in this study, but they are not interchangeable. The simulations help diagnose lateral transport and thermal effects; the measurements provide the physical comparison for defined observables. Agreement between them supports the specific quantities and conditions compared, not universal validation of the full kinetic or soot-modelling system.

Author Accepted Manuscript and supplemental material

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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

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