Effects of Turbulence on Self-sustained Combustion in Premixed Flame Kernels: A Direct Numerical Simulation (DNS) Study

Abstract

The effects of mean flame radius and turbulence on self-sustained combustion of turbulent premixed spherical flames in decaying turbulence have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius chemistry. Several flame kernels with different initial radius or initial turbulent field have been studied for identical conditions of thermo-chemistry. It has been found that for very small kernel radius the mean displacement speed may become negative leading ultimately to extinction of the flame kernel. A mean negative displacement speed is shown to signify a physical situation where heat transfer from the kernel overcomes the heat release due to combustion. This mechanism is further enhanced by turbulent transport and, based on simulations with different initial turbulent velocity fields, it has been found that self-sustained combustion is adversely affected by higher turbulent velocity fluctuation magnitude and integral length scale. A scaling analysis is performed to estimate the critical radius for self-sustained combustion in premixed flame kernels in a turbulent environment. The scaling analysis is found to be in good agreement with the results of the simulations.

Effects of Mixture Distribution on Localized Forced Ignition of Stratified Mixtures: A Direct Numerical Simulation Study

“…The probability of findings high values of c decreases with increasing u 0 =S b ϕ¼1 ð Þ due to enhanced heat transfer rate from hot gas kernel for both types of initial mixture distributions. The smaller extent of burning rate for higher values of u 0 =S b ϕ¼1 ð Þ and the adverse effect of u 0 =S b ϕ¼1 ð Þ on the extent of burning are consistent with previous findings (Ballal and Lefebvre, 1977a;Chakraborty et al, 2007Chakraborty and Mastorakos, 2008;Huang et al, 2007;Klein et al, 2008;Patel and Chakraborty, 2014;Poinsot et al, 1995). Furthermore, Figures 8 and 9 show that an increase in ϕ 0 leads to a decrease of the mean value of m b c !…”

“…Ballal and Lefebvre (1980) showed that an increase in turbulent velocity fluctuation has detrimental effects on successful ignition and early stages of combustion of inhomogeneous mixtures. Similar effects have been reported for premixed combustion based on both experimental (Ballal and Lefebvre, 1977a;Huang et al, 2007) and numerical (Chakraborty et al, 2007;Klein et al, 2008;Poinsot et al, 1995) analyses. A number of DNS-based analyses (Chakraborty et al, 2007Chakraborty and Mastorakos, 2008) also demonstrated that an increase in turbulent velocity fluctuation for a given value of integral length scale of turbulence has a detrimental effect on the success of localized forced ignition of single-phase gaseous inhomogeneous mixtures.…”

Effects of Energy Deposition Characteristics on Localised Forced Ignition of Homogeneous Mixtures

“…The localised ignition and subsequent flame propagation in homogeneous mixtures have been studied from analytical [1][2][3][4], experimental [5][6][7][8][9][10] and numerical [10][11][12][13][14][15][16][17] viewpoints by several authors. Interested readers are referred to Ref.…”

“…[18] and references therein for an extensive review on localised forced ignition of homogeneous mixtures. In recent times, Direct Numerical Simulation (DNS) has become an important tool for fundamental understanding and subsequent modelling of complex combustion phenomena including localised forced ignition of homogeneous [15][16][17] and inhomogeneous [19][20][21] gaseous and droplet-laden [22][23][24] mixtures. An extensive review of experimental evaluation of minimum ignition energy, optimum energy deposition time for homogeneous mixture can be found in Lefebvre [5], which indicated that the spark kernel needs to exceed a critical dimension (d q ) for the resulting flame to survive.…”

“…The adverse effects of turbulent velocity fluctuation on ignition and early stages of flame propagation in homogeneous mixtures were reported and analysed from various viewpoints in Refs. [5][6][7][8][9][10][11][12][13][14][15][16][17]. In a recent analysis Wandel [23] demonstrated that the absence of a substantial region containing hot products with small values of scalar dissipation rate could be detrimental to the ignition of partiallypremixed gases resulting from droplet-laden mixtures, and such an extinction predictor can also be relevant to the ignition of gaseous homogeneous mixtures.…”