On Unraveling Community Ignition Processes: Joint Influences of Firebrand Showers and Radiant Heat Applied to Fuel Beds

Abstract

ABSTRACT Devastating large outdoor fires have been responsible for the destruction of vast amounts of infrastructure and the loss of human life. Wildland fires that spread into urban areas, known as wildland-urban interface (WUI) fires, are capable of enormous destruction. WUI fires are distinct from wildland fires; WUI fires include the combustion of both vegetative and human-made fuels and occur where population centers exist, whereas wildland fires include the combustion of vegetative fuels and occur in uninhabited areas. The rise of densely populated urban areas has also seen the development of large urban fires. In addition, the rise of informal settlement communities in Southeast Asia and Africa continues to result in large outdoor fires capable of great destruction. As a wildland fire reaches an urban area, structure-to-structure fire spread processes will occur via the same mechanisms as those in informal settlement fires and urban fires: radiant heat, direct flame contact, and firebrands. Firebrands are in fact the main culprit to destroy structures in large outdoor fires. An additional complication is related to the showers of firebrands produced in large outdoor fires from vegetative and structural fuel combustion, and in the presence of the combustion of these fuel types, there is also radiant heat produced that may only enhance ignition processes. These combined ignition processes have been overlooked and are the subject of this experimental investigation. To this end, a custom experimental apparatus that produces firebrand showers, in the presence of applied radiant heat, was installed inside a wind facility. The firebrand generator was tuned to produce firebrands typical of those quantified from the combustion of structures and the coupled influences of firebrand showers and radiant heat, under various wind speeds was determined. The total firebrand mass for smoldering ignition (SI) decreased as the wind speed and preheating time increased. In all experiments, SI was observed to transition to flaming ignition (FI). To better understand the experimental observations, a simple model was developed and is presented.