This study develops a physical model of city fire spreading. First, we developed a fundamental physical model of a city fire. Then we made a simulation program of a city fire based on the fundamental physical model. Using this model of city fire, risk of fire in urban area can be evaluated in detail.
Factors that cause spreading of urban fires among buildings are flame contact, radiant heat, convective heat, and firebrands. There are many cases in which firespread is caused by leaping flames resulting from many firebrands in places that are distant from the fire outbreak site, especially under strong wind conditions. Firebrands and leaping flames caused by them are important factors to clarify the mechanism of urban fires. Nevertheless, their systematic engineering study has not been greatly implemented because of limited availability of experimental facilities. Therefore, the authors performed a Fire Wind Tunnel experiment using a real-scale fire preventive wooden house to investigate firebrands. This study is aimed at quantitatively and qualitatively elucidating the chronological relationship of fire evolution and firebrand generation.
Firebrands are found to be an important factor in the spread of large urban fire. In this study, the scattering of firebrands in an urban fire was numerically simulated by means of CFD (Computational Fluid Dynamics). Firstly, the thermal plume of the urban fire is predicted using a modified compressible k-ε turbulent model. Then a Lagrangian trajectory model that incorporated forces due to the drag, pressure, and gravity force of the firebrands is applied to predict the trajectory of the firebrands. The influences of the inflow wind velocity, diameter, generating site, and initial generating velocity were investigated. It is found that when the inflow wind velocity is comparatively low, the thermal plume is significant, and when the inflow wind is strong, the thermal plume is suppressed. It is shown that the lighter the mass of the firebrand is, the farther the firebrand may be scattered. The stronger the fire's thermal plume is, the higher it may be scattered. More firebrands can be scattered out of firing building by increasing the initial generating velocity.
In this paper, simulating a group fire in a densely inhabited area with weak small wooden buildings, we performed reduced scale model experiments to investigate flame merging. To study this phenomenon, a lot of experiments were performed using crib and liquid fuel. In this work, however two or more square propane porous burners are used, and the flame height, heat flux, and temperature distribution on the center axis of fires are measured. Consequently, the influence of the heat release rate, the number of fire sources and the distance between fire sources upon flame merging has been investigated. It is found that each of those parameters affected flame merging, although the number of fire sources seemed to be the most important parameter.