A Numerical Study of Firebrands Scattering in Urban Fire Based on CFD and Firebrands Aerodynamics Measurements

Firebrands are found to be an important factor in the spread of large urban fire. In this study, in order to predict the scattering of firebrands in an urban fire, a computational fluid dynamics (CFD), turbulent combustion model, radiation model, and firebrand scattering model are coupled and it is validated by a fire wind tunnel experiment. The density, terminal velocity, and Stokes diameters of firebrands which are necessary for the firebrand scattering model are measured. The ratio of Stokes diameter to equivalent diameter is focused on 0.2—0.7. An urban fire simulation in a modeled urban area has been conducted using the coupled model and the firebrand experimental data. It is found that when the inflow wind velocity is comparatively slow, the size of the thermal plume becomes significant, and when the inflow wind velocity is high, the thermal plume is suppressed and greatly inclines to the leeward side, which increases the risk of the fire spreading to neighboring buildings. Firebrands can be scattered over 400 m. Firebrands are blown up by the thermal plume and the scattering distances are influenced by the background flow field, the magnitude of thermal plume, and the Stokes diameters of firebrands.