← Back to Publications

Near-threshold soot formation in premixed flames at elevated pressure

Authors: Xiaotong Mi, Ahmad Saylam, Torsten Endres, Christof Schulz, and Thomas Dreier

Document type: Published journal article

Journal: Carbon

Publication details: Volume 181, pages 143–154, 2021

Publication date:

Access and rights note: This page provides bibliographic information and an independently written technical summary. No Elsevier Version of Record or other full-text copy is hosted on this website.

DOI: 10.1016/j.carbon.2021.05.014

Independent technical summary

The study investigates soot formation close to the lean sooting threshold in laminar premixed ethylene–air flames at pressures from 1 to 10 bar. Near-threshold conditions are useful for examining the onset and early development of measurable soot because soot loading is relatively low. They do not, however, provide a direct measurement of every molecular step involved in soot inception.

Soot-volume fraction was measured by laser extinction. Time-resolved laser-induced incandescence was used to infer primary particle diameters from the temporal decay of the incandescence signal following pulsed laser heating. Thermophoretic sampling and ex situ transmission electron microscopy provided an independent assessment of particle size and morphology.

For the investigated ethylene–air flames at an equivalence ratio of 2.1, the reported soot-volume fraction followed a pressure power law with fitted exponents of approximately 1.4–1.9. The dependence of soot-volume fraction on height above the burner became stronger over the studied pressure range. These exponents are empirical results for the tested configuration and should not be treated as universal pressure-scaling laws for soot formation.

Within the investigated operating range, measured soot-particle diameter increased with pressure and equivalence ratio. The sensitivity of particle size to mixture strength also increased with pressure in the reported cases, indicating a stronger observed particle-growth response to equivalence-ratio changes under those elevated-pressure conditions.

In the near-threshold range at equivalence ratios of approximately 1.90–1.95, the decay of the time-resolved LII signal varied only weakly with height above the burner and laser fluence. The authors interpret this behaviour as being consistent with an approximate balance between particle-surface growth and oxidation under the investigated conditions. This remains an interpretation of the diagnostic behaviour rather than a direct measurement of the two competing surface processes.

For a slightly sooting flame, particle sizes obtained from thermophoretically sampled TEM images agreed well with values derived from the time-resolved LII analysis. This cross-diagnostic comparison supports the selected LII evaluation method for particle sizing within the investigated near-threshold regime, while retaining the method-specific assumptions and uncertainties of both LII and TEM.

Technical contribution

The paper provides pressure-dependent measurements of soot loading and particle size in a regime close to the observable onset of soot formation. It combines in situ optical diagnostics with ex situ microscopy, enabling an independent cross-check of particle-sizing results within the investigated cases.

The reported results show stronger soot-related responses to equivalence ratio and downstream residence time as pressure increased within the investigated configuration. This is relevant to the design of high-pressure soot experiments and to validation planning for kinetic, particle-dynamics and optical-diagnostic models. The data do not by themselves validate a complete soot mechanism or all model subcomponents.

Experimental methods

  • laminar premixed ethylene–air flames;
  • pressure range from 1 to 10 bar;
  • operation near the lean sooting threshold;
  • laser extinction for soot-volume-fraction measurement;
  • time-resolved laser-induced incandescence for particle sizing;
  • thermophoretic particle sampling;
  • transmission electron microscopy for particle size and morphology.

Scope and application boundary

The reported pressure exponents, particle-size trends and diagnostic agreement apply to the investigated ethylene–air flames, burner configuration, equivalence ratios, pressures, heights above the burner and diagnostic assumptions.

Time-resolved LII particle sizing depends on the selected heat-transfer model, assumed soot optical and thermal properties, gas conditions, laser fluence, calibration strategy and signal-fitting procedure. Agreement with TEM in the investigated cases is a useful cross-check, but should not be treated as universal validation for all soot types, flame conditions or pressure ranges.

Thermophoretic sampling and TEM can also introduce sampling, deposition and image-analysis biases. Application to another flame, fuel or pressure range should therefore include independent diagnostic checks, uncertainty assessment and renewed evaluation of the diagnostic assumptions.

Near-threshold measurements characterize the onset and early development of measurable soot but do not by themselves resolve all molecular inception pathways or describe the complete evolution of mature soot in practical combustors, where turbulence, mixing, radiation, oxidation and long residence times may dominate.

Evidence interpretation

Laser extinction, time-resolved LII and TEM provide different observables and have different assumptions, spatial or temporal resolutions and uncertainty sources. Agreement among them strengthens confidence in the specific quantities compared, but the diagnostics should not be treated as interchangeable or as direct proof of a complete soot-inception mechanism.

Likewise, measured pressure trends support the reported behaviour of the investigated ethylene–air flames. They should not be converted directly into universal pressure exponents, transferable soot-kinetic laws or broad validation claims for unrelated burners, fuels or models.

Publisher access

The Version of Record is available from Elsevier through the DOI: access the publisher record . Access may require an institutional or individual subscription.

Citation

Mi, X., Saylam, A., Endres, T., Schulz, C., & Dreier, T. (2021). Near-threshold soot formation in premixed flames at elevated pressure. Carbon, 181, 143–154. https://doi.org/10.1016/j.carbon.2021.05.014