Mechanism of Urban Fire Spread by Firebrands using Fire Wind Tunnel Experiments and CFD Analysis

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1.Purpose of the ResearchThis study was intended to perform fire wind tunnel experiments to elucidate the properties and behavior of firebrands produced from burning houses under a wind load and to develop a firebrand-based fire-spreading simulation model by combining the experiment results with the Computational Fluid Dynamics(CFD) technique.2.Findings of the Research(1)With regard to firebrand generationA fire resistant house was constructed inside the fire wind tunnel facility at the Building Research Institute, Japan. Time-series measurements were conducted for quantity, shape, size, and other aspects of firebrands generated during a fire, systematically varying the speed of inflow wind and other parameters. Based on those results, the quantity of firebrands generated from a burning house under a wind load and other factors were generalized.(2)With regard to the scattering of firebrandsA numerical simulation model was developed through modification of the Fire Dynamics Simulator(FD … More S) of the National Institute of Standards and Technologies(NIST) to predict the firebrand scattering area. It was designed to deal with rapid thermal and density changes in a fire, as well as to take into account the drag and lift forces working on scattering particles. The model was subjected to full validation based on the results of a fact-finding survey of the firebrand scattering area in a fire that broke out at the Shirahama Spa, Wakayama prefecture. Cone calorimeter tests were also conducted using wood chips resembling firebrands to clarify the thermal behavior of scattering firebrands in terms of the heat generation rate and mass changes.(3)With regard to the thermal damage potential of falling firebrandsIn a post-earthquake situation in which roof tiles have usually fallen off of a structure, the spot fire risks become high because of the exposure of wooden materials used for roof structures. Given that, based on aerial photographs and field surveys performed of the city of Kobe after the Great Hanshin-Awaji Earthquake, the number of wooden buildings with fallen roof tiles was counted, classifying the level of structural damage. In addition, a general expression was proposed for predicting the shedding of roof tiles immediately after an earthquake. To clarify the spot fire behavior of roofs, fire wind tunnel experiments were performed using the shedding status of roof tiles and wind speed as parameters.(4)Development of firebrand fire spread simulation modelA firebrand-based fire-spreading simulation model was developed by integrating the findings described in (1),(2) and (3) above. Case studies of actual built-up areas were also conducted. Less