Effects of Fuel Lewis Number on Localised Forced Ignition of Globally Stoichiometric Stratified Mixtures: a Numerical Investigation

Abstract

The influences of fuel Lewis number Le F on localised forced ignition of globally stoichiometric stratified mixtures have been analysed using three-dimensional compressible Direct Numerical Simulations (DNS) for cases with Le F ranging from 0.8 to 1.2. The globally stoichiometric stratified mixtures with different values of root-mean-square (rms) equivalence ratio fluctuation (i.e. φ = 0.2, 0.4 and 0.6) and the Taylor micro-scale l φ of equivalence ratio φ variation (i.e. l φ /l f = 2.1, 5.5 and 8.3 with l f being the Zel'dovich flame thickness of the stoichiometric laminar premixed flame) have been considered for different initial rms values of turbulent velocity u . A pseudo-spectral method is used to initialise the equivalence ratio variation following a presumed bi-modal distribution for prescribed values of φ and l φ /l f for global mean equivalence ratio φ = 1.0. The localised ignition is accounted for by a source term in the energy transport equation that deposits energy for a stipulated time interval. It has been observed that the maximum values of temperature and the fuel reaction rate magnitude increase with decreasing Le F during the period of external energy deposition. The initial values of Le F , u /S b(φ=1)

Effects of spatial distribution of CO2 dilution on localised forced ignition of stoichiometric biogas-air mixtures

“…Figure 7 demonstrates that V c�0:9 =V sp decreases with increasing ψ f mean (solid to dashed lines) for a given u 0 =s 0 l , which is consistent with previous experimental (Galmiche et al 2011;Larsson, Berg, Bonaldo 2013) andcomputational (Turquand d'Auzay et al 2019a) findings. An increase in initial turbulence intensity acts to increase the turbulent eddy diffusivity D t~u 'l t and thus enhances the heat transfer rate from the hot gas kernel, and acts to reduce the burned gas volume, which is consistent with several previous analyses (Chakraborty, Cant, Mastorakos 2007;Chakraborty and Mastorakos 2008;Chakraborty, Hesse, Mastorakos 2010;Wandel, Chakraborty, Mastorakos 2009;Patel and Chakraborty 2014, 2016aTurquand d'Auzay et al 2019a,b). The growth of the hot gas kernel depends on the competition between the heat release rate and heat transfer rate from the hot gas kernel.…”

“…The species included are CH 4 , CO, CO 2 , N 2 , H 2 O and O 2 whose Lewis numbers are bounded by 0:8 � Le � 1:39 and thus, the unity Lewis number assumption does not significantly introduce any inaccuracy in a qualitative sense. This was also confirmed in previous analytical (He 2000;Sibulkin and Siskind 1987) and DNS (Chakraborty, Hesse, Mastorakos 2010;Patel and Chakraborty 2016a) studies. The ratio of specific heats (γ ¼ C gaseous specific heats at constant pressure and volume, respectively) and Prandtl number (Pr ¼ μC g p =λ ¼ 0:7, where μ is the dynamic viscosity and λ is the thermal conductivity of the gaseous phase) take standard values, respectively.…”

“…A well-known DNS code SENGA+ (Chakraborty, Cant, Mastorakos 2007;Chakraborty and Mastorakos 2008;Chakraborty, Hesse, Mastorakos 2010;Wandel, Chakraborty, Mastorakos 2009;Patel and Chakraborty 2014, 2016aTurquand d'Auzay et al 2019a,b) was used for all the simulations. A cubic computational domain of size 66δ 0 z � 66δ 0 z � 66δ 0 z was used, and discretized using a Cartesian grid of 356 � 356 � 356, with a uniform grid spacing Δx: This discretization ensures η k > Δx, where η k is the Kolmogorov length scale (e.g.…”

“…Pera, Chevillard, and Reveillon (2013) demonstrated that the mixture inhomogeneity induced by residual burnt gas has significant influence on early flame propagation and cyclic variability in the case of forced ignition using DNS. Further numerical investigations were carried out by Patel and Chakraborty (2014;, 2016a to analyze the effects of turbulence intensity on forced ignition of stratified mixtures. It was found that an increase of turbulence intensity with a fixed length scale has a detrimental effect on successful ignition.…”

“…Existing computational investigations (Chakraborty, Cant, Mastorakos 2007;Chakraborty and Mastorakos 2008;Patel and Chakraborty 2014, 2016aTurquand d'Auzay et al 2019b) focused primarily on single fuels such as methane or hydrogen but not compound fuels such as biogas. To be more specific, the effects arising from the spatial distribution of the diluent of choice (i.e CO 2 for biogas) on the localized forced ignition and subsequent flame propagation have not yet been investigated either experimentally or computationally to the best of the authors' knowledge.…”