Effects of Biogas Composition on the Edge Flame Propagation in Igniting Turbulent Mixing Layers
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Effects of Biogas Composition on the Edge Flame Propagation in Igniting Turbulent Mixing Layers C. Turquand d’Auzay1,2 · V. Papapostolou1 · N. Chakraborty1 Received: 7 December 2019 / Accepted: 14 August 2020 © The Author(s) 2020
Abstract Three-dimensional compressible Direct Numerical Simulations have been used to investigate the localised forced ignition of statistically planar biogas/air mixing layers for different levels of turbulence intensity and biogas composition. The biogas is represented by a CH4 /CO2 mixture and a two-step mechanism capturing the variation of the unstrained laminar flame speed with equivalence ratio and CO2 dilution was used. The mixture composition was found to significantly affect the flame kernel development which was reflected in the diminished growth rate of the burned gas volume with increasing CO2 dilution. A successful ignition of CH4/CO2/air mixing layer gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilised on the stoichiometric mixture fraction iso-surface. The most probable edge flame speed decreases in time and converges to a value that is at most equal to its laminar theoretical limit, and can even locally become negative for large values of the dilution and/or turbulence intensity. The decomposition of the edge flame speed showed a negligible or negative contribution of the mixture fraction surface displacement speed, while the displacement speed of the fuel mass fraction surface appeared as the dominant contributor. Finally, the edge flame speed dependences to the fuel mass fraction and mixture fraction gradients, fuel mass fraction iso-surface curvature and tangential strain rate have been analysed and found, within the dilution values considered, qualitatively similar to those of undiluted mixtures regardless of the amount of CO2 , although quantitative differences were observed. Keywords Direct numerical simulation · Edge flame · Biogas · Localised forced ignition · Inhomogeneous mixture
1 Introduction Fossil fuel combustion provides most of the primary energy required for power generation and transportation, and will likely remain predominant in the foreseeable future, especially for engineering applications requiring high energy density such as aircraft gas turbines. * C. Turquand d’Auzay [email protected] 1
School of Engineering, Newcastle University, Newcastle‑Upon‑Tyne NE1 7RU, UK
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Ricardo Ltd., Shoreham Technical Centre, Old Shoreham Rd., Shoreham‑by‑Sea BN43 5FG, UK
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Vol.:(0123456789)
Flow, Turbulence and Combustion
However, the fossil fuel reserves in the world are finite which calls for its replacement with alternative renewable sources. Biogas is now a widely accepted sustainable fuel and can be used either as a complement or a replacement, as well as used in similar applications as natural gas. Its major components are methane and CO2 , but it is difficult to produce with a fixed composition when generated from biological sources (Vasavan et al. 2018), and i
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