← 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 especially useful for examining soot inception and early particle growth because soot loading remains low enough to limit the influence of later-stage agglomeration and oxidation.

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.

At an equivalence ratio of 2.1, the reported soot-volume fraction followed a pressure power law with exponents of approximately 1.4–1.9, even close to the onset of soot formation. The dependence of soot-volume fraction on height above the burner became stronger as pressure increased.

Measured soot-particle diameter increased with pressure and equivalence ratio. The sensitivity of particle size to mixture strength also increased with pressure, indicating that relatively small changes in equivalence ratio can produce larger particle-growth responses under 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 indicating an approximate balance between particle-surface growth and oxidation under the investigated conditions.

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 comparison supports the use of the selected LII evaluation method for particle sizing within the investigated near-threshold regime.

Technical contribution

The paper provides pressure-dependent measurements of soot loading and particle size in a regime close to soot inception. It combines in situ optical diagnostics with ex situ microscopy, enabling cross-validation of particle-sizing results.

The results demonstrate that elevated pressure amplifies the response of soot formation to equivalence ratio and residence time. This is relevant to the design of high-pressure soot experiments and to the validation of kinetic, particle-dynamics and optical-diagnostic models.

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, soot optical and thermal properties, gas conditions, laser fluence and signal-fitting procedure. Agreement with TEM in the investigated cases should not be treated as universal validation for all soot types 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 and uncertainty assessment.

Near-threshold measurements characterize soot inception and early growth but do not by themselves describe the complete evolution of mature soot in practical combustors, where turbulence, mixing, radiation, oxidation and long residence times may dominate.

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