The main driving forces for the propagation of urban fires are the thermal radiation from the fire and the leaping flames due to the drifting firebrands. In the present work, drifting firebrands in an urban fire are numerically simulated by means of CFD with a turbulence model. Firebrands are treated as airborne particles subject to sedimentation due to the gravity. Characteristics of brand drifting are investigated under various wind conditions, sedimentation speed and locations of a brand source. 1) When the approaching flow is relatively slow, the vertical thermal plume is raised by the fire and reaches up to around 100m height. In comparison, when the approaching flow is fast, the incoming flow acts to suppress the thermal plume. 2) The sedimentation fluxes over the downstream building roofs increase under high wind conditions, so the risk of the fire spreading increases on the leeward side of the fire. 3) Drifting patterns measured with the sedimentation fluxes over the downstream building roofs and so on, depend much on the effective diameter of brands. 4) When the upward plane of the building catches fire, the circulation flow in front of the burning surface suppresses thermal diffusion in the horizontal direction. In contrast, when the leeward plane of the building catches fire, the leeward circulation behind the building promotes horizontal thermal diffusion. The latter flow results in the reduction in the velocity of thermal plume. 5) When the roof or the leeward plane of the building catches fire, it developed into hot ascending flow patterns which promote the drifting of the firebrand.
