The present work aims at the detailed understanding of the local processes in premixed combustion of hydrogen, methane and propane flames at unsteady conditions. The methodology consists of the analysis of simulations of two-dimensional flame-vortex interactions as well as statistical data obtained from three-dimensional Direct Numerical Simulations (DNS) of the flame front interacting with a set of vortexes. Special attention is given to the relationship between the Lewis number (Le) of the fuel and the flame front stretch in terms of both curvature and strain rate. A large single vortex bends the flame front thus creating both positive and negative curvatures, which in turn enhance the heat release rate in some locations of the flame front and decrease it in others. The resulting effect is called "polarisation effect". The occurrence and the strength of the polarisation effect of curvature are tightly bound up with the Lewis number of the fuel. The polarisation effect is quantified by the ratio of maximum to minimum heat release rates along the flame front, which defines the Polarisation Effect Number (PEN). The more the Lewis number of a fuel deviates from unity, the stronger the polarisation effect is. Strong polarisation effects lead finally to local flame extinction. This is demonstrated for hydrogen flames with Le = 0.29 (lean) and Le = 2.2 (rich) as well as for artificially designed cases with Le = 0.1 and Le = 10.0. Therefore, flame extinction can occur for both thermodiffusively stable and unstable flames. It is shown that choosing an appropriate mixture of real fuels with different Lewis numbers, the homogeneity of the heat release rate along the flame front could be considerably enhanced. This relatively uniform heat release rate is not sensitive to curvature, which consequently decreases the occurrence of local extinction.
