Direct numerical simulations of non-premixed ethylene–air flames: Local flame extinction criterion

Abstract

a b s t r a c tDirect Numerical Simulations (DNS) of ethylene/air diffusion flame extinctions in decaying twodimensional turbulence were performed. A Damköhler-number-based flame extinction criterion as provided by classical large activation energy asymptotic (AEA) theory is assessed for its validity in predicting flame extinction and compared to one based on Chemical Explosive Mode Analysis (CEMA) of the detailed chemistry. The DNS code solves compressible flow conservation equations using high order finite difference and explicit time integration schemes. The ethylene/air chemistry is simulated with a reduced mechanism that is generated based on the directed relation graph (DRG) based methods along with stiffness removal. The numerical configuration is an ethylene fuel strip embedded in ambient air and exposed to a prescribed decaying turbulent flow field. The emphasis of this study is on the several flame extinction events observed in contrived parametric simulations. A modified viscosity and changing pressure (MVCP) scheme was adopted in order to artificially manipulate the probability of flame extinction. Using MVCP, pressure was changed from the baseline case of 1 atm to 0.1 and 10 atm. In the high pressure MVCP case, the simulated flame is extinction-free, whereas in the low pressure MVCP case, the simulated flame features frequent extinction events and is close to global extinction. Results show that, despite its relative simplicity and provided that the global flame activation temperature is correctly calibrated, the AEA-based flame extinction criterion can accurately predict the simulated flame extinction events. It is also found that the AEA-based criterion provides predictions of flame extinction that are consistent with those provided by a CEMA-based criterion. This study supports the validity of a simple Damköhler-number-based criterion to predict flame extinction in engineering-level CFD models.

Chemical Explosive Mode Analysis for Local Reignition Scenarios in H2/N2 Turbulent Diffusion Flames

“…As mentioned above, the local Damköhler number calculated by CEMA is applied as a diagnosis strategy. This method has been used and verified by Lecoustre et al, and a comparison to the extinction diagnosis through the critical temperature ( T c, Z = Z st < 1000 K) is shown here for qualitative analysis. Figure shows the scatter plots of the temperature for two simulated flames.…”

“…The existence of chemical explosive mode means that the local mixture tends to autoignite if it is put in an isolated environment (adiabatic and constant volume) . The transition of a CEM between explosive and non-explosive modes is strongly correlated with limit flame phenomena, such as ignition, extinction, and premixed flame front locations. , To quantify the normalized contribution of each variable (species and temperature) of the CEM, the vector of explosion index (EI) can be defined by where a e and b e are the right and left eigenvectors of the eigenmode related to λ e , respectively. ⊗ and | | denote element-wise multiplication and their absolute values, respectively.…”

Modeling flame extinction and reignition in large eddy simulations with fast chemistry

“…These works have successfully identified the primary physical mechanisms for extinction (thermal, aerodynamic, and kinetic quenching) [1–9], while others have made progress toward developing simple formulations to model flame extinction in cases applicable to realistic fire scenarios [10–13]. Additional studies have highlighted the primary features of flame reignition events, which may follow localized extinction in large-scale turbulent flames [14–19].…”

Development of High Fidelity Soot Aerosol Dynamics Models using Method of Moments with Interpolative Closure

“…Details of the test configuration has been reported in (Arias et al 2011b;Lecoustre et al 2013). The initial flame profile is that of a fuel core in an initially isotropic, homogeneous, two-dimensional turbulent flow field generated with a PassotPouquet energy spectrum (Passot and Pouquet 1987).…”

“…The physical configuration of the test is similar to a fuel core (i.e., centrally placed infinitely long strip of fuel) surrounded by air on either side. Details of the configuration can be found in (Arias et al 2011b;Lecoustre et al 2013). A steady-state solution for an opposed-flow steady laminar diffusion flame (Lutz et al 1996) with scalar dissipation rate of χ st = 7s −1 was mapped to the DNS domain based on the mixture fraction Z using the following relationship:…”