Edge flame propagation statistics in igniting monodisperse droplet-laden mixtures

Abstract

The effects of droplet diameter, overall (i.e., liquid+gaseous phases) equivalence ratio, and turbulence intensity on the edge flame propagation statistics for localized forced ignition of uniformly dispersed n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on direct numerical simulations data. It has been found that the edge flame structure becomes increasingly prominent for large overall equivalence ratios and droplet diameters. Although the mean edge flame speed has been found to be positive and its most probable value remains comparable to the theoretical value for laminar edge flames in purely gaseous mixtures, the mean values have been found to decrease and the probabilities of finding locally negative edge flame speeds have been found to increase with increasing turbulence intensity. The marginal probability density function and curvature and strain rate dependences of the edge flame speed have been found to be principally governed by the displacement speed of the fuel mass fraction isosurface intersecting the stoichiometric mixture fraction isosurface. The displacement speed of the stoichiometric mixture fraction isosurface has also been found to influence the local scalar gradient dependences of the edge flame speed in this configuration, especially for large droplets. The displacement speed of the fuel mass fraction isosurface Sd has been found to be principally governed by leading order contributions of the reaction and molecular diffusion components and the evaporation contribution remains weak in comparison to these leading order contributors. The local edge flame speed exhibits nonlinear curvature and strain rate dependences and its variation with the magnitudes of both fuel mass fraction and mixture fraction gradients has been found to be nonmonotonic for all cases considered here. The correlations of the edge flame speed with curvature, strain rate, and scalar gradient have been found to be qualitatively similar to the corresponding statistics reported in the existing literature for edge flames in purely gaseous mixtures. Additionally, the curvature and tangential strain rate dependences of the edge flame speed have been found to be dependent on the droplet size and overall equivalence ratio, and these dependences become weak for cases with large droplets.

Effects of the spatial distribution of CO2 dilution on localised forced ignition of stoichiometric CH4 - CO2 - air mixtures

“…Recently, it has been demonstrated by the authors (Papapostolou et al, 2020) that the MIE variation with turbulence intensity for the stoichiometric homogeneous mixed biogas-air mixture using this chemical mechanism provides excellent qualitative and quantitative agreements between DNS and experimental (Cardin et al, 2013a,b) results. Moreover, the flame structure resulting from localised forced ignition of turbulent gaseous mixing layer and droplet-laden mixtures using single step Arrhenius type chemistry (Chakraborty and Mastorakos, 2006, Papapostolou et al, 2019 are qualitatively similar to those obtained from detailed chemistry simulations (Ray et al, 20021;Neophytou et al, 2010). Turquand to the results obtained from detailed chemistry DNS studies (Echekki and Chen, 1998;Chen, 1999,2001).…”

“…This, in turn, means that the errors introduced by the assumption of equal specific heat are small. In this analysis, the specific heat at constant pressure 𝐶 𝑝 , dynamic viscosity 𝜇, thermal conductivity 𝜆 and the density-weighted mass diffusivity 𝜌𝐷 are considered to be constant for the sake of simplicity following previous analyses (Chakraborty and Mastorakos, 2006;Hesse et al, 2009;Wandel et al, 2009;Hesse et al, 2012;Papapostolou et al, 2019Papapostolou et al, ,2021Turquand d'Auzay et al, 2019, which ensures that the Lewis number does not change within the flame. The constant thermophysical properties do not alter the mixing statistics in the unburned gas.…”

A direct numerical simulation analysis of localised forced ignition in turbulent slot jets of CH4/CO2 blends

“…Recently, it has been demonstrated by Papapostolou et al 12 that the minimum ignition energy variation with turbulence intensity for the stoichiometric homogeneous mixed biogas-air mixture using this chemical mechanism provides excellent qualitative and quantitative agreements between DNS 46,12 and experimental 53,54 results. Moreover, the flame structure resulting from localised forced ignition of turbulent gaseous mixing layer and droplet-laden mixtures using single-step Arrhenius-type chemistry 24,55,56 are qualitatively similar to those obtained from detailed chemistry simulations. 57,58 It was demonstrated by Turquand d'Auzay et al 31 that the edge flame propagation statistics for forced ignition of turbulent mixing layers for the chemical and transport models used in this article have been found to be qualitatively similar to the detailed chemistry DNS studies.…”

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

“…This, in turn, means that the errors introduced by the assumption of equal specific heat are small. In this analysis, the specific heat at constant pressure C p , dynamic viscosity μ, thermal conductivity λ and the densityweighted mass diffusivity ρD are considered to be constant for the sake of simplicity following previous analyses (Chakraborty and Mastorakos 2006;Hesse, Chakraborty, Mastorakos 2009;Wandel, Chakraborty, Mastorakos 2009;Hesse, Malkeson, Chakraborty 2012;Papapostolou et al 2019Papapostolou et al , 2021Turquand d'Auzay et al 2019a;Turquand d'Auzay, Papapostolou, Chakraborty 2021), which ensures that the Lewis number does not change within the flame. The constant thermophysical properties do not alter the mixing statistics in the unburned gas.…”

“…Recently, it has been demonstrated by the authors (Papapostolou, Turquand d'Auzay, Chakraborty 2020) that the MIE variation with turbulence intensity for the stoichiometric homogeneous mixed biogas-air mixture using this chemical mechanism provides excellent qualitative and quantitative agreements between DNS and experimental (Cardin et al 2013a(Cardin et al , 2013b results. Moreover, the flame structure resulting from localized forced ignition of turbulent gaseous mixing layer and droplet-laden mixtures using single step Arrhenius type chemistry Mastorakos 2006, 2008;Papapostolou et al 2019) are qualitatively similar to those obtained from detailed chemistry simulations (Ray et al, 2001;Neophytou, Mastorakos, Cant 2010). Turquand d'Auzay, Papapostolou, and Chakraborty (2021) demonstrated that the edge flame propagation statistics for forced ignition of turbulent mixing layers for the chemical and transport models used in this paper are qualitatively similar to the results obtained from detailed chemistry DNS studies (Echekki and Chen 1998;Chen 1999, 2001).…”