Activation energy effect on flame propagation in large-scale vortical flows

The propagation of a premixed flame through a large-scale vortical flow is studied numerically employing a conventional reaction-diffusion-advection model. It is shown that the response of the flame speed to the flow intensity is strongly influenced by the form of the reaction-rate expression that describes the chemical kinetics, in particular the activation energy. For high-activation- energy kinetics typical of gaseous flames this response is characterized by a peculiar non-monotonicity, thereby reflecting the flow-induced changes within the flame front structure and, hence, deviation from the classical Huygens propagation. At low activation energies, however, the non-monotonicity vanishes, which also helps to explain its absence in the isothermal autocatalytic reaction waves spreading through strongly stirred liquid solutions where the amplification factor of propagation speed may reach extremely high values compared to gaseous flames. Additionally, it is shown that the transition from Huygens to non-Huygens propagation occurs at nearly the same Karlovitz number for all activation energies, thereby showing the utility of this parameter for characterizing flame propagating in non-uniform flows when appropriately defined.