Wall effects on the propagation and extinction of steady, strained, laminar premixed flames

Abstract

A combined numerical and experimental investigation was conducted on the interaction between a chemically inert solid wall and steady, strained, laminar, premixed atmospheric methane/air flames. Experiments were conducted by using laser Doppler velocimetry for the determination of the axial velocity profiles along the centerline for both the opposed-jet and single jet-wall configurations. The numerical simulations were conducted by solving the conservation equations of mass, momentum, energy, and species along the stagnation streamline by using detailed description of chemical kinetics, molecular transport, and thermal radiation. The numerical solutions were also obtained for both the opposed-jet and single jet-wall configurations, and the effects of the no-slip condition, wall temperature, and H radical recombination at the wall on the flame response were assessed. The experimental data were compared with the numerical predictions obtained by using two independently developed recent C 2 kinetic mechanisms. Results indicate that when the flame is highly strained, it is stabilized close to the wall and the values of the strain rate distribution inside the flame are reduced compared to an opposed-jet flame which is characterized by the same strain rate in the hydrodynamic zone. Furthermore, close to extinction, the conductive heat loss to the wall leads to flame weakening, and a significant reduction of the extinction strain rate is observed compared to the adiabatic systems. It was also found that when heat loss is present, its effect on flame extinction is dominant and traditional arguments of the synergetic effect of stretch and preferential diffusion on flame extinction are not applicable. The studies on flame propagation showed that at low strain rates the flame speed is minimally affected by the wall, since the flame is stabilized closer to the nozzle and the downstream heat losses have a minor effect on the thermochemical processes taking place within the flame zone. This suggests that the single jet-wall configuration can be preferably used as an alternative technique for the determination of laminar flame speeds given that it has a number of advantages over the traditional opposed-jet technique in terms of experimental implementation.