A Review of interactive smoldering-flaming fires at the wildland-urban interface

Abstract

Wildland–urban interface (WUI) fires show complex interactions among multi-fuel burning, the built environment, and extreme atmospheric conditions, posing a growing global hazard. Although flaming has traditionally dominated the combustion research for wildfires and their mitigation frameworks, increasing evidence indicates that smoldering combustion plays a critical and often underrecognized role in fire ignition, spread, and persistence. Smoldering of WUI fuels often coexists with flaming, and their dynamical interactions through different ignition and extinction processes control the spread of WUI fires. This work first synthesizes distinctive WUI fire phenomena, including firebrand showers, cascading vegetation-structure interactions, and structure-to-structure fire spread. The fundamentals of smoldering and flaming fire behaviors are reviewed to highlight their similarities and differences in the initiation and development of large outdoor fires. Then, we present a detailed analysis of bi-directional smoldering-flaming transition and symbiotic co-existence as unifying mechanisms linking diverse fire behaviors. Finally, we highlight challenges and future research directions in understanding WUI fire dynamics, including innovative modeling methods, data-driven and AI-assisted fire forecasting, characterization of coupled and derived fire hazards, and the integration of combustion fundamentals into performance-based community fire protection codes and mitigation frameworks.