Wildland surface fire spread: Mechanism transformation and behavior transition

Abstract

Surface fire is the most prevalent type of wildland fire and dominates the cost of wildland firefighting. The high-intensity burning behaviors in surface fires stem from the significant interaction of combustion with heat transfer and fluid flow under complicated fuel, meteorology, and topography conditions. Over the past several decades, research has resulted in incrementally enhanced insights into the mechanism transformations and behavior transitions related to surface fires, with theories and models developed and validated. These advances have improved the fundamental understanding of surface fire evolution significantly. However, the current knowledge is still inadequate for dealing with practical surface fire problems. In this paper, we specifically address two complexities associated with fire growth. First, the fuel preheating mechanisms involving various forms of flame heat transfer may change under different conditions. Second, a surface fire may transition to other intensely burning behaviors by interacting with environmental conditions. For the first complexity, we initially discuss the fuel preheating mechanism, flame propagation across discrete fuels, and flame geometry transformation in wind-driven fire spread. Then, we examine the firelines interaction and its induced junction fire. Followed are the fireline transformations under different topography conditions, focusing on the flame attachment in upslope terrain and the fireline behaviors in canyon terrain. For the second complexity, we highlight the transitions between surface fire and other behaviors with distinct mechanisms, including smoldering, crown fire, fire whirl, and spot fire. Despite significant advances, more understanding of the mechanisms underlying fuel preheating processes and fireline behaviors is needed. Furthermore, we need more exploration into the scaling law of combustion dynamics of surface fires and the non-steady transitions to other intense burning behaviors. All these challenges have proven an elusive goal for future work to enhance surface fire predictive capabilities.