Dynamical Mechanism And Behaviors Of Multiple Fires Burning

Group fires, generated by the burning of discrete combustible materials, can easily develop into urban or wildland mass fires in practice. Multiple fires burning is the fundamental process involved in group fires, in which the fires generally have significant influence on each other due to two major fire interaction effectsc the air entrainment restriction and radiation heat feedback enhancement. Due to the fire interaction effects, multiple fires burning may lead to extreme fire behaviors such as fire merging, fire whirl and spot fire, and especially, the data and theory for single fires cannot be directly extrapolated to multiple fires. For specific fire source conditions, the interaction effects among fires mainly depend on the fuel distribution conditions such as fire spacing and number of fire points.Different from previous quasi-stationary state burning experiments, we designed equidistant square fire array to simulate free burning group fires. This work aimed at proposing effective analysis methods to describe the free burning of multiple fires, in order to characterize the interaction effects among fires, whereby the theoretical descriptions upon the multiple fires burning could be developed for various fire spacing and fire array size conditions, and the dynamical mechanisms and behaviors involved in multiple fires burning could be revealed. The contents of this paper include two major parts:1) characterize the fire interactions by examining the burning of each fire point, and 2) understand the variation of global variation of burning rates and fire interaction mechanisms for fire arrays with different fire spacing and fire array size conditions.In view of the fact that previous test and analysis methods could not be used for large amount of fire points, we propose to use the burn-out time as a basic quantity to analyze the multiple fires burning behaviors. In physics, burn-out time characterizes the average burning rate in time scale. In tests, the data of burn-out time could be easily extracted from the test images, even for large amount of fire points.By analysis on the data of the average burning rate characterized by the burn-out time, some new fire merging criteria were developed to identify the conditions for fire merging and for identifying the fire points which are involved in fire merging. The criteria do not depend on the fire array size and the locations of fire points. Based on this, we developed a new theoretical analysis method to quantitatively characterize the interaction effects among fires. The results show that any fire point is mainly affected by its adjacent four fire points in the array. According to the quantitative analysis of shear flow effect, it was distinguished that the array can be divided into three regions which may expand or dwindle with fire point spacing, wind velocity and fire array size. The occurrences of fire whirls in the array were also examined, and it was found that the frequency and duration of fire whirls increase greatly when with strong turbulence flow induced by shear flow.Further, this paper proposed two new concepts of global average burning rate and fire area ratio for global analysis of the burning rate of fire array, whereby we developed a global model to describe the variation of global burning rates of fire array versus fire point spacing and fire array size. By global analysis, three regimes (higher order increasing regime, linear correlation regime and decaying regime) of the global average burning rate versus the fire area ratio for specified fire array size were definitely distinguished, each possessing specific fire interaction mechanisms. The global and local average burning rates were also compared, showing that both burning rates had the similar variation trend versus fire area ratio. Finally, the effects of fire array size expansion on the inner basic fire array were investigated, and it was indicated that for any specific fire spacing the burning rate of the inner fire array increases monotonically with the size of fire array, which agrees with the actual group fire.