Effects of Turbulence Intensity and Biogas Composition on the Localized Forced Ignition of Turbulent Mixing Layers

ABSTRACT

Three-dimensional compressible Direct Numerical Simulations (DNS) have been used to investigate the localized forced ignition of statistically planar mixing layers of biogas/air mixtures for different levels of turbulence intensity and biogas composition. The biogas is represented by a CH${ }_4$/CO${ }_2$ mixture and a two-step mechanism involving incomplete oxidation of CH${ }_4$ to CO and H${ }_2$O, and an equilibrium between the CO oxidation and the CO${ }_2$ dissociation has been used. This two-step mechanism captures the variation of the unstrained laminar flame speed with equivalence ratio and CO${ }_2$ dilution with sufficient accuracy when compared with detailed chemistry results. A successful ignition of CH${ }_4$/CO${ }_2$/air mixing layer initially gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilized on the stoichiometric mixture fraction iso-surface. Long after the energy deposition has ended, the lean branch may merge with the diffusion flame, but edge flame propagation along the stoichiometric mixture fraction iso-surface is observed at all stages. The highest heat release rate (HRR) is obtained on the rich premixed branch irrespective of the turbulence intensity and biogas composition, but its magnitude decreases with increasing CO${ }_2$ dilution. The most probable edge flame displacement speed decreases in time and converges to its theoretical value for laminar cases. As the turbulence level increases, the edge flame displacement speed assumes a larger range of values, although its mean is consistently lower than the corresponding laminar one. Furthermore, in the turbulent cases, the probability of finding negative values of the edge flame displacement speed has been found to be non-zero. This probability is larger than the one of finding positive values for the cases failing to reach a self-sustained flame propagation without the energy deposition assistance. This becomes more probable as the turbulence intensity and CO${ }_2$ dilution increase. This is reflected in the diminished burning extent, quantified in terms of burned gas volume, with increasing turbulence intensity and CO${ }_2$ dilution.

KEYWORDS:

• Localized forced ignition
• inhomogeneous mixtures
• edge flame
• turbulence intensity
• biogas

Acknowledgments

The authors are grateful to the British Council for financial support and to EPSRC (ARCHER, Cirrus) and Newcastle University (Rocket) for computational support.

Additional information

Funding

This work was supported by the British Council [Newton Grant] and by the EPSRC [EP/K025163/1].