Behavior of Premixed Sooting Flame in a High-Pressure Burner

Abstract

The study numerically investigates how burner optical ports, edge inter-matrices and pressure influence soot formation and the behaviour of premixed sooting flames in a high-pressure burner.

Three-dimensional CFD simulations of premixed C₂H₄/air flames at 1.01 and 10 bar were first performed using a one-step chemistry approach. Their role was geometric and hydrodynamic screening: to assess whether an axisymmetric two-dimensional representation could capture the burner-flow features relevant to the later analysis. These calculations were not intended to resolve detailed soot chemistry.

The resulting two-dimensional analysis indicates that edge inter-matrices can generate axial vorticity. At elevated pressure, thermal expansion of the burned gases can strengthen or generate multiple axial vortical structures in the model. The associated multiple sooting regions were reported to correspond qualitatively with luminous soot streaks observed experimentally; this qualitative correspondence should not be interpreted as full quantitative validation of the local flow or soot field.

In the model, relatively low-temperature regions below approximately 1800 K, together with enhanced mixing around and above the edge inter-matrix location, were associated with conditions favourable to soot formation. This is a model-based interpretation of coupled transport, temperature and chemistry rather than a universal soot-formation threshold. The calculated influence on centreline soot-volume fraction remained limited: below about 3% for the atmospheric-pressure case and below about 10% for the 10 bar case.

Under the investigated conditions, cases with higher modelled reactivity—obtained by reducing the rich equivalence ratio or increasing pressure—showed earlier soot-precursor formation and an upstream shift of the sooting region. This trend is conditional on the selected fuel, chemistry, pressure range and burner configuration.

Technical contribution

The work links specific high-pressure-burner design features to local flow structures and spatial soot behaviour. It shows that optical access and internal support structures can influence the flame even when their effect on the centreline soot-volume fraction is comparatively modest.

The study also demonstrates a staged modelling workflow in which three-dimensional simulations are used to test whether important geometric effects can be represented adequately before adopting a computationally less demanding axisymmetric model for more detailed analysis. That dimensional reduction is a case-specific modelling decision, not a general rule for high-pressure burner simulations.

Principal findings

• Burner optical ports and edge inter-matrices can disturb the local flow field and should not automatically be treated as negligible.
• Edge inter-matrices can generate axial vortical structures and locally enhance species and thermal mixing.
• Within the investigated cases, elevated pressure increased modelled flame reactivity and strengthened thermally driven flow structures.
• Multiple local sooting zones can arise even when the centreline soot response remains relatively limited.
• Within the simulated domain, more reactive conditions shifted predicted soot-precursor formation and the sooting region closer to the burner.

Scope and application boundary

The conclusions apply to the investigated burner geometry, premixed ethylene–air flames, pressures, equivalence ratios, boundary conditions and modelling assumptions. They should not be transferred unchanged to a different burner or fuel.

The three-dimensional screening calculations used one-step chemistry. Such chemistry can support geometric and flow-field assessment but does not resolve detailed fuel breakdown, aromatic growth, soot-precursor chemistry, particle inception, surface growth, agglomeration or oxidation.

Axisymmetric modelling can represent only geometric and flow features compatible with rotational symmetry. Fully three-dimensional optical ports, manufacturing asymmetries and non-axisymmetric instabilities may require explicit three-dimensional treatment.

Quantitative application to another facility should include geometry-specific meshing, grid-independence assessment, heat-loss and radiation treatment appropriate to the facility, detailed chemistry and soot modelling, uncertainty in inlet and wall boundary conditions, and comparison with measured temperature, velocity, species and soot fields. Agreement in centreline soot alone would not be sufficient to validate the complete flow–chemistry–soot model.

Evidence interpretation

The study combines several evidence levels: three-dimensional geometric screening, two-dimensional CFD analysis, soot modelling and qualitative comparison with observed luminous structures. These evidence types support different conclusions and should not be treated as interchangeable.

A qualitative match between simulated sooting structures and luminous streaks supports the proposed flow–soot interpretation, but it does not independently establish local temperature, velocity, species or soot-volume-fraction accuracy. Quantitative validation requires spatially resolved measurements of the variables used to support the corresponding claim.

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Licence and reuse

The article is distributed under the Creative Commons Attribution 4.0 International licence .

The licence permits sharing and adaptation, including commercial reuse, provided appropriate attribution is given, a link to the licence is supplied and any changes are indicated.

Citation

Saylam, A. (2023). Behavior of Premixed Sooting Flame in a High-Pressure Burner. Reactions, 4(1), 155–170. https://doi.org/10.3390/reactions4010009

Relation to diagnostics and kinetic-intelligence work

This paper focuses on burner geometry, flow structure and soot-related CFD behaviour. The separate high-pressure-flame-facility diagnostics work addresses experimental characterization of the facility, while later kinetic-intelligence work addresses reaction-network analysis, reduction and adaptive chemistry. These are complementary but distinct evidence layers and should not be presented as if one validates the others automatically.

Related technical resources

• Near-Threshold Soot Formation in Premixed Flames at Elevated Pressure
• Influence of Lateral Species Diffusion and Heat Transfer on the Evaluation of Near-Threshold Sooting Flames
• Characterization of a High-Pressure Flame Facility Using High-Speed Chemiluminescence and OH LIF Imaging
• A Transient Nano-Dense Molecular State in Nanoparticle Inception
• Kinetic Intelligence — reduction, network analysis and adaptive chemistry