Numerical Study of Cellular Instability in Burner Stabilized Adiabatic Laminar Premixed Flames
Abstract: The thesis deals with experiments and numerical simulations of adiabatic laminar premixed flames anchored to a heat-flux burner using different mixtures under various operating conditions. The aim is to obtain a better understanding of cellular instability in heat-flux burner anchored flames. Numerical and experimental studies were carried out to characterize the onset and the evolution of cellular flames in CH4/O2/CO2 (oxy-fuel) and CH4/air mixtures under various standoff distances between the flame and the burner. The numerical simulations were based on a high-order numerical scheme for low Mach number reacting flows coupled with detailed chemical kinetic mechanisms and transport properties. A critical standoff distance was found, above which cellular flame instability was observed. It was shown that the critical standoff distance is closely correlated with the density ratio and with the laminar flame thickness in each flame studied. The observed onset of cellular flames is dictated by the hydrodynamic instability mechanism, which is generally suppressed by the burner when the flame is very close to the exit plate of the burner. Diffusive-thermal effects play a lesser role in these methane/oxidizer mixtures. Additional studies carried out on adiabatic H2/O2/N2 premixed flames anchored to the heat-flux burner showed that H2 flames with an effective Lewis number of less than unity are subjected not only to hydrodynamic instability but also to strong diffusive-thermal instability, these flames thus very easily becoming cellular. In general, when the flames were very close to the burner, the burner suppressed the flame instability. At a moderate standoff distance, the burner was able to suppress the instability often found with large wavenumbers but not with certain small wavenumbers. In the case of lean H2 flames, when these became cellular, the mean burning velocity could be 2-3 times higher than that of a corresponding planar flame, local extinction occurring due to differential diffusion. Numerical studies showed that the nonlinear evolution of cellular flames can lead to rather chaotic flame shapes and that the evolution process itself can be very sensitive both to the initial conditions and to the numerical schemes and the grid resolutions involved. After a sufficiently long temporal evolution, whatever the initial conditions were, large cells were found to become the dominant mode in the cellular flame front. The onset of large cells was governed by the degree of hydrodynamic instability that was found.
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