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Does Density Ratio Significantly Affect Turbulent Flame Speed?

A N Lipatnikov1, W Y Li2, L J Jiang2

  • 11Department of Applied Mechanics, Chalmers University of Technology, Gothenburg, Sweden.

Flow, Turbulence and Combustion
|August 3, 2018
PubMed
Summary
This summary is machine-generated.

The density ratio (σ) does not significantly impact flame displacement speed in turbulent combustion. This finding supports engine combustion models that do not account for density ratio effects on laminar flame speed (SL).

Keywords:
Darrieus-Landau instabilityDensity ratioExpanding spherical flameExperimentLow Turbulent IntensityThermal expansionTurbulent flame speed

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Area of Science:

  • Combustion science
  • Fluid dynamics
  • Chemical engineering

Background:

  • The density ratio (σ) is a critical parameter in combustion, influencing flame propagation.
  • Understanding its effect on flame displacement speed is crucial for optimizing combustion processes, especially in engines.
  • Previous studies have suggested a potential impact of σ on turbulent flame speeds.

Purpose of the Study:

  • To experimentally investigate the influence of the density ratio (σ) on flame displacement speed at low to moderate turbulent intensities.
  • To compare flame speeds using mixtures with significantly different σ but similar laminar flame speeds (SL).
  • To provide data for validating combustion models used in spark ignition engines.

Main Methods:

  • Designed two methane/oxygen/nitrogen mixtures with different σ (7.52 and 4.95) but identical SL (0.36 m/s) via preheating and nitrogen dilution.
  • Recorded flame propagation using direct imaging after spark ignition in a 3D cruciform burner.
  • Generated near-isotropic turbulence (rms turbulent velocity u' from 0.14 to 1.39 m/s) using counter-rotating fans and perforated plates.
  • Analyzed flame radius and displacement speed relative to unburned gas.

Main Results:

  • Experimental data revealed no significant effect of the density ratio (σ) on flame displacement speed (SL).
  • Flame displacement speeds at u'/SL = 0.4 were comparable to laminar flame speeds across all tested conditions.
  • The study confirmed that varying σ did not substantially alter flame propagation characteristics.

Conclusions:

  • The density ratio (σ) has a minor effect on flame displacement speed in turbulent combustion.
  • Results support the use of combustion models that do not incorporate density ratio effects on SL for engine simulations.
  • Model validation against room-condition data (higher σ) remains relevant for engine applications.