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

Abstract

ABSTRACT The localized forced ignition and subsequent flame propagation have been analyzed for stoichiometric biogas-air mixtures ( blend) with different spatial distributions (uniform, Gaussian and bimodal) and mean levels of dilution (i.e. mole fraction of ) for different flow conditions (e.g. quiescent laminar condition and different turbulence intensities) using three-dimensional Direct Numerical Simulations. A two-step chemical mechanism with sufficient accuracy is used to capture the effects of dilution on the laminar burning velocity of the . A parametric analysis in terms of the mean, standard deviation and integral length scale of the initial Gaussian and bimodal distributions of spatial dilution in the unburned gas has been conducted. Qualitatively similar behavior has been observed for all three initial spatial distributions of . A departure from uniform conditions was found to increase the range of reaction rate magnitudes of , which impacts the burned gas volume and, in turn, increases the variability of the outcomes of the ignition event (successful thermal runaway and subsequent self-sustained propagation or misfire). Turbulence intensity and the mean level of dilution were found to have significant impacts on the outcome of the localized forced ignition, and an increase of either quantity acts to reduce the burned gas volume irrespective of mixture composition due to the enhancement of heat transfer from the hot gas kernel, and the heat sink effects of , respectively. An increase of the integral length scale for the spatial distribution of dilution increased the probability of a successful outcome of the ignition event. A uniform initial spatial distribution was found to be optimal, and in the case of a departure from non-uniform conditions, larger variability in the outcome are obtained for a Gaussian initial distribution of dilution than a bimodal one. The variance of the dilution distribution has been found to have a negligible impact on the outcomes observed, as it was dominated by the effects arising from turbulence intensity, nature of initial distribution and integral length scale of dilution.