Unsteady phenomena affecting the propagation of surface fires

Abstract

The unsteadiness coming from thermo-convective and shear instabilities or from the time variations of the external conditions affecting the fire (wind gusts for example), affecting the propagation of a surface fire, has been studied numerically using a multiphase approach. Two regimes of propagation (plume dominated and plume driven) have been identified, governed by two forces: the buoyancy resulting from the density gradient inside and outside the plume and the inertia of the wind. The degree of non-linearity associated with these two physical mechanisms, can explain the magnitude of unsteadiness of the fire behaviour. It participates also of the impact (sometimes linear and sometimes strongly non-linear) of the impact of wind conditions upon the fire spread. For weak wind conditions (exhibiting potentially a more non linear behaviour), a sinusoidal time variation of the wind speed has been tested with five frequencies (0.25, 0.5, 1, 2 and 3 Hz) nearly equal to the frequency (1.4 Hz) characterizing the thermo-convective instability (in assimilating the fire front as a pool fire) and to the shear instability (0.26 Hz).