Heat transfer during fire spread through fine fuels under momentum vs. buoyancy-driven flow.

Understanding and modeling wildfires is essential for protecting the lives of firefighters and mitigating damage to communities located at the wildland-urban interface (WUI). Wildfires become particularly destructive in extreme weather conditions, including low humidity and high wind speeds. Wind-driven flame spread is simulated in a wind tunnel at the US Forest Service Missoula Fire Sciences Laboratory using a bed of fine pine needles as fuel. Pushed by wind speeds of 0.5 m/s and 1 m/s, flames spread along a fuel bed with up to a fuel break where the flame will either jump the gap or extinguish. Wind speed, fuel moisture content (3%-15%), and gap size (0-60cm) are varied in the experiment. In addition to infrared and go-pro videos, high-frequency (1 kHz) total and radiative heat flux data is collected before and after fuel breaks to understand the relationship between radiative and convective heating under different wind, moisture, and fuel break conditions. These experiments show that the flame acts similarly to a buoyancy-driven flow at lower wind speeds while at higher wind speeds it is momentum-driven, resulting in changing profiles of downstream heat transfer. These experiments seek to understand this behavior and its impact on the heating and ignition of small-diameter fuels.