Experimental and Theoretical Approaches to Elucidate Fuel Bed Ignition Exposed to Firebrand Showers and Radiant Heat

Abstract

Firebrand showers are known to result in massive destruction in large outdoor fires. A key missing piece is how these ignition scenarios may be influenced by firebrand showers in conjunction with external radiant heat that would be generated by nearly combustibles. The combined effect of firebrand shower exposure and radiant heat exposure on fuel bed ignition under different firebrand arrival flux was investigated using two different approaches. The first approach considered the ignition based upon a unit area methodology. The second approach considered investigating each localized ignition. The time to ignition and the number of firebrands associated with each ignition process was measured under different experimental conditions. A theoretical treatment was devised to explain the experimental results. Findings show that the higher wind speeds and longer preheating times led to shorter times to ignition. The firebrand arrival flux became an important parameter on fuel bed ignition for low arrival flux values, resulting in intermittent heating of the fuel bed. In this case, fuel beds may lose heat due to convective cooling from the wind. The heat contribution from a firebrand was also calculated. Firebrands under higher wind speed contributed more to heat the fuel beds than firebrands under lower wind speed. The heat contribution from a firebrand calculated by the unit area approach decreased as the firebrand arrival flux decreased due to the intermittent arrival of the firebrands. The study remains the first work to show the importance of continuous arrival of firebrands for fuel bed ignition.