Search

The localised forced ignition and subsequent flame propagation have been analysed for stoichiometric C H 4 − C O 2 − air mixtures with different spatial distributions and mean levels of C O 2 dilution (i.e. mole fraction of C O 2 in C H 4 / C O 2 blend) for different flow conditions (e.g. quiescent laminar condition and different turbulence intensities) using three-dimensional Direct Numerical Simulations. The C H 4 + C O 2 mixture is taken to represent biogas, as C H 4 and C O 2 are its two principal constituents. A two-step chemical mechanism, which has been demonstrated to capture the effects of C O 2 dilution on the laminar burning velocity with sufficient accuracy, has been used for the purpose of a parametric analysis in terms of the mean value, standard deviation and integral length scale of the initial spatial Gaussian distributions of C O 2 dilution in the unburned gas. An increase of mean C O 2 dilution level was found to reduce the maximum values of temperature and the reaction rate magnitude of C H 4 . Moreover, an increase of mean C O 2 dilution acts to reduce the probability of finding large reaction rate magnitudes of C H 4 , which also leads to a decreasing trend of burned gas volume C O 2 irrespective of flow conditions. Furthermore, an increase in turbulence intensity acts to reduce the burned gas volume irrespective of mixture composition due to the enhancement of heat transfer from the hot gas kernel. However, the initial values of integral length scale and standard deviation of C O 2 dilution variation (i.e. l ψ and σ ψ ) have been found not to have significant influences on the burned gas volume for the parameter range considered here. Although a small value of l ψ promotes high rates of mixing of C O 2 within the unburned gas, the overwhelming probability of finding C O 2 dilution close to its mean value eclipses the effects of l ψ and σ ψ even under laminar conditions, and this trend strengthens further under high turbulence intensities due to enhanced mixing. [ABSTRACT FROM AUTHOR]

The effects of droplet diameter, overall (i.e. liquid and gaseous phases) equivalence ratio, and turbulence intensity on the variation of the Minimum Ignition Energy (MIE) for localized forced ignition of uniformly dispersed mono-sized n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on Direct Numerical Simulation (DNS) data. The MIE was evaluated just for (i) obtaining thermal runway irrespective of the fate of the resulting flame kernel, and (ii) also for a successful self-sustained flame propagation without the assistance of an external energy source following the energy deposition by the ignitor. It has been found that the MIE requirement increases with increasing turbulence intensity and this trend for the MIE increase is especially significant for large values of turbulence intensity. The MIE requirement increases with increasing initial droplet diameter and with decreasing overall equivalence ratio. The MIE requirements for droplet-laden mixtures have been found to be greater than the corresponding value for homogeneous mixture with the same nominal values of initial turbulence intensity and equivalence ratio for the parameter range considered here. This behavior arises due to the deposited energy being partially utilized to supply the latent heat of evaporation and also due to the predominantly fuel-lean composition of the gaseous flammable mixture. This tendency of obtaining fuel-lean mixture strengthens with increasing (decreasing) initial droplet diameter (overall equivalence ratio). It has also been demonstrated that combustion takes place predominantly in the fuel-lean premixed mode although there is a finite probability of having non-premixed combustion in all cases. Moreover, there is a small probability of fuel-rich combustion occurring for small droplets, especially under fuel-rich overall equivalence ratios. The stochastic nature of the ignition event has been demonstrated by considering different realizations of statistically similar turbulent flow fields. The conditions giving rise to a successful thermal runaway/self-sustained flame propagation have been identified by a detailed analysis of the energy budget. [ABSTRACT FROM AUTHOR]

The implications of the choice of reaction progress variable on the performances of the flame surface density (FSD) based mean reaction rate closure and the well-established sub-models of the FSD transport have been analysed in context of Reynolds Averaged Navier Stokes simulations. For this purpose, a detailed chemistry direct numerical simulation (DNS) database of freely-propagating statistically planar air flames (with an equivalence ratio of 0.7) spanning the corrugated flamelets (CF), thin reaction zones (TRZ) and broken reaction zones (BRZ) regimes of premixed turbulent combustion has been considered. The FSD and the unclosed terms of its transport equation have been analysed for reaction progress variables defined based on normalised and mass fractions and temperature. The performances of the closures for turbulent flux of FSD, and tangential strain rate term have been found to be mostly unaffected by the choice of reaction progress variable. However, the well-established existing models for the unresolved tangential strain rate term have been found not to perform well for the cases representing the CF and TRZ regimes of premixed combustion. The performance of a well-established existing model for the combined propagation and curvature terms has been found to be significantly dependent on the choice of reaction progress variable. Furthermore, the surface-averaged value of the density-weighted displacement speed cannot be approximated by the corresponding unstretched laminar flame value especially for the flames in the BRZ regime. Detailed explanations have been provided for the observed behaviours of the FSD based reaction rate closure and sub-models for the unclosed terms of the FSD transport equation in different combustion regimes for different choices of reaction progress variable. [ABSTRACT FROM AUTHOR]