Thermal-diffusive instabilities in unstretched, planar diffusion flames

The recent development of a novel research burner at EPFL has opened the way for experimental investigations of essentially unstretched planar diffusion flames. In particular, it has become feasible to experimentally validate theoretical models for thermal-diffusive instabilities in idealized one-dimensional diffusion flames. In this paper, the instabilities observed close to the lean extinction limit are mapped in parameter space, notably as function of the two reactant Lewis numbers. Cellular and pulsating instabilities are found at low and high Lewis numbers, respectively, as predicted by linear stability analyses. The detailed investigation of these two types of instabilities reveals the dependence of cell size and pulsation frequency on the transport properties of the reactants and on flow conditions. The experimental scaling of the cell size is found in good agreement with linear stability. The comparison between experimental and theoretical pulsation frequencies, on the other hand, was hampered by the impossibility of experimentally reproducing the parameters of the stability calculations. Hence, a heuristic correlation between pulsation frequency and flow parameters, transport properties, in particular the Damkِhler number, and oscillation amplitude has been developed and awaits theoretical interpretation.