Thermal regulation of forest surface fires by pre-applied foam barriers under slope conditions

Firefighting foam is widely used for fire suppression due to its cooling and smothering capabilities; however, its effectiveness in blocking upslope forest surface fire spread remains insufficiently understood. In this study, pine-needle fuel beds were used to simulate forest surface fuels, and pre-applied foam experiments were conducted under different slope conditions to investigate the influence of foam mass and slope on fire spread behavior. The results show that foam application alters the structure of the fuel bed and foam layer. With increasing slope, foam penetration decreases while downslope sliding becomes more pronounced, reducing the stability of the foam firebreak. When the slope is below 30°, increasing foam mass significantly decreases fuel-bed temperature and fire spread rate, and fire propagation transitions from flaming spread to smoldering spread and eventually termination. A critical foam dosage required to completely block fire spread is identified, and this threshold increases with slope angle. A heat conduction-radiation analysis further reveals that foam penetration reduces the effective thermal conductivity of the fuel bed, while the surface foam layer attenuates radiative and convective heat transfer from the flame. These findings provide insight into the thermal regulation mechanisms of foam and offer guidance for the engineering design of foam-based firebreaks in sloped forest environments.