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 hbox {CH}_4/hbox {CO}_2 mixture and a two-step mechanism capturing the variation of the unstrained laminar flame speed with equivalence ratio and hbox {CO}_2 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 hbox {CO}_2 dilution. A successful ignition of hbox {CH}_4/hbox {CO}_2/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 hbox {CO}_2, although quantitative differences were observed.
Highlights
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.1 3 Vol.:(0123456789)Flow, Turbulence and Combustion (2021) 106:1437–1459the fossil fuel reserves in the world are finite which calls for its replacement with alternative renewable sources
The objectives of this work are to understand the effects of CO2 dilution and turbulence intensity on (i) the flame structure arising from the ignition of biogas mixtures and (ii) the edge flame speed statistics resulting from a successful ignition
Of particular interests are the onset of combustion and the subsequent flame propagation in partially-premixed mixtures for which previous studies have shown the presence of a triple flame structure
Summary
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.1 3 Vol.:(0123456789)Flow, Turbulence and Combustion (2021) 106:1437–1459the fossil fuel reserves in the world are finite which calls for its replacement with alternative renewable sources. 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 if the variations are large enough to affect the ignition, this may lead to adverse effects on the subsequent flame propagation (Lieuwen et al 2008). Experimental studies of forced ignition of biogas/air mixtures reported that the carbon dioxide content, that can be quite large (5–40% by volume), acts as a heat sink leading to higher energy requirement for ignition as well as a slower and cooler flame (Forsich et al 2004; Biet et al 2014; Galmiche et al 2011). Mulla et al (2016) reported similar conclusions for the laser ignition of methane/air mixtures diluted with CO2. The CO2 dilution can hinder the flame kernel formation and potentially lead to flame extinction for biogas/ air fueled gas turbines (Mordaunt and Pierce 2014). The CO2 dilution can hinder the flame kernel formation and potentially lead to flame extinction for biogas/ air fueled gas turbines (Mordaunt and Pierce 2014). Lafay et al (2007) showed a significant modification of the reaction zone structure and flame stability with the addition of CO2 in a gas turbine configuration
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