Dynamical and statistical properties of high-temperature self-propagating fronts: an experimental study

We present a detailed experimental study of high-temperature self-propagating fronts using image processing techniques. The intrinsic features of the wave propagation are investigated as a function of the combustion temperature TC for a model system made of titanium and silicon powders. Different front behavior is realized by changing the molar ratio x of the mixture Ti+xSi. Outside the range x=[0.3,1.5], no thermal front is propagating while inside, three regimes are observed: steady-state combustion which is characterized by a flat front propagating at constant velocity and two unsteady regimes. The combustion temperature (or the corresponding ratio x) is thus playing the role of bifurcation parameter leading from stationary state to complex behavior. In the titanium-rich mixture, the position of the front oscillates and hot spots propagate along the external border of the sample. At lower amounts of Ti, localized bright regions appear randomly and deform the front profile. The associated dynamical behavior is a relay-race mechanism which becomes more pronounced close to the combustion limit. Methods are developed to characterize the structural and dynamical properties of thermal waves near instabilities, with a special emphasis on the statistical aspects. It is clearly demonstrated that the mesoscopic scale phenomena interfere significantly with the macroscopic behavior. The experiments reveal front behaviors that cannot be described using the usual macroscopic theories.