Session chaired by Pr. Christine Rousselle
Localised forced ignition (spark, laser) of flammable mixture is a topic of fundamental importance in combustion science. A thorough knowledge of ignition is needed for safety standards as well as in the design of efficient and reliable Spark ignition and Direct Injection engines, in which misfire causes inef fective combustion, but also for the relight of gas turbine at altitude. The ignition of turbulent flammable mixtures is not only influenced by the minimum ignition energy or the critical flame radius, but it also is strongly dependent on the local properties of the energy deposition region (turbulence characteristics, scalar gradients, mixture composition, etc.) and of the overall flow field [1–3].
Furthermore, the flammable mixture resulting from the dilution of methane with carbon dioxide is generally referred to as ”biogas” and has been identified as a carbon neutral fuel when originating from anaerobic digestion of organic matter by living organisms . It is also widely accepted as a sustainable fuel that can be used either as a complement or a replacement in applications such as power generation or within the transport sector [5, 6]. However, to date, limited effort has been directed to the understanding of its ignition, and the uncertain combustion behaviours arising from the various amount of methane and CO2 on the ignition process are yet to be analysed in detail. In particular, the ignition of biogas in inhomogeneous mixtures and in the presence of shear has not been studied.
The ignition of a simple canonical turbulent round methane jet in ambient air has been extensively studied by both experimental and numerical means [3, 7–10]. Building on previous numerical studies of methane/air ignition [1, 3], three-dimensional compressible Direct Numerical Simulations (DNS) have been carried out to investigate the ignition of a biogas planar jet. A two-step mechanism involving incomplete oxidation of CH4 to CO and H2O and an equilibrium between the CO oxidation and the CO2 dissociation has been used . This two-step mechanism captures the variation of the unstrained laminar flame speed with equivalence ratio and CO2 dilution with sufficient accuracy when compared with detailed chemistry results. The study focuses on the three stages of flame evolution, i.e., (i) flame kernel growth, (ii) downstream flame expansion and radial propagation, and (iii) potential upstream flame propagation which relies on edge flame propagation. The flame expansion spans different combustion modes, from premixed to non-premixed in the presence of edge flames. The addition of CO2 in the fuel stream affects significantly the flame kernel development and the subsequent flame behaviour by reducing the unstrained laminar flame speed and the heat release rate.
The effect of CO2 dilution on the mixture fraction field will be discussed and it will be shown that an increase in CO2 content may lead to more favourable regions for the flame development closer to the jet nozzle. The different flame development stages reported experimentally have also been captured irrespective of the CO2 dilution level, starting from the initial growth and downstream convection of the kernel to the final upstream propagation and stabilisation of the flame through the radial expansion and downstream propagation stage. The flame structure arising from the kernel growth appears tribrachial in which a triple point propagates along the stoichiometric mixture fraction iso-surface. The stabilisation of the flame was found to primarily rely on the propensity of the triple point to locally propagate faster than the streamwise flow velocity, allowing the lifted height of the flame to decrease slowly in time until an equilibrium with the streamwise velocity is found. The CO2 dilution was found to alter this height relatively long after the energy deposition has ended, when the edge flame speed becomes the main parameter driving the flame movement, as this speed decreases with an increase in dilution.  C. Turquand d’Auzay, V. Papapostolou, S. F. Ahmed, N. Chakraborty, Combust. Flame 201 (2019) 104–117.
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