Numerical Study of Critical Fire Merging Distances in Square Arrayed Multiple Fires
Merging of large-scale city fires induces rapid fire growth. Once a merging fire occurs, it becomes more difficult to suppress, with great potential damages. However, the details of interactions in multiple fires that cause fire merging have not been fully clarified. For the interactions in multiple fires, the space distance among fires greatly affects the merging phenomenon. The objective of this paper is to examine the detailed merging conditions, particularly the critical distance for merging, by numerical simulation of reduced scale fires using the software FDS. In the simulations, the temperature, radiative flux, wind velocity and heat release rate of fire arrays (each consisting of n × n fire points) are examined. In addition, large-scale merging fires in cities are simulated, based on the results of the reduced scale fire simulations. It is found that at the small space distance of s*=1, fires can merge to a remarkable extent into the center. At s*=4 and 5, fires merge slightly with short flame lengths. At s*=7 and 9, no merging is observed. At the array number of n=15, a steep heat release rate distribution model is introduced, which produces results similar to those of the reduced scale experiments, where the fire merging is highly affected by the space distance among fires. Thus, s*=5 is concluded to be the critical merging distance. Simulations of large-scale fires show that the merging tendencies are similar to results in the reduced scale fires and the flame length is about 150 m at Qtotal=30 GW.