This paper is devoted to some nonlinear propagation phenomena in periodic and more general domains, for reaction-diffusion equations with Kolmogorov-Petrovsky-Piskunov (KPP) type nonlinearities. The case of periodic domains with periodic underlying excitable media is a follow-up of the article [7]. It is proved that the minimal speed of pulsating fronts is given by a variational formula involving linear eigenvalue problems. Some consequences concerning the influence of the geometry of the domain, of the reaction, advection and diffusion coefficients are given. The last section deals with the notion of asymptotic spreading speed. The main properties of the spreading speed are given. Some of them are based on some new Liouville type results for nonlinear elliptic equations in unbounded domains.

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A weakly nonlinear parabolic equation pertinent to the flame front dynamics subject to the buoyancy effect, and to a statistical description of biological evolution is considered. In the context of combustion it is shown that the parabolic interface occurring in upward propagating flames in vertical channels may actually be merely quasi-equilibrium transient states which eventually collapse to a stable configuration in which the flame tip slides along the channel wall.