Effects of slope on fireline shape and heat transfer in surface fires

The shape of the fireline plays a critical role in determining the ignition of surrounding vegetation, yet the effects of slope on fireline morphology and associated heat transfer remain insufficiently understood. This study experimentally investigates surface fire behavior across slopes ranging from 0 degrees to 35 degrees, using pine needle fuel beds. Results show that airflow entrainment, modulated by slope, causes distinct flame front shapes: a rectangular front persists throughout early and late fire spread at low slopes, while high slopes induce a triangular flame front in the early stage that evolves into an inverted "V" shape later. As slope increases, fireline curvature, spread rate, radiative heat flux, and convective heat flux all rise, whereas flame inclination decreases. To account for stage-specific flame evolution, separate radiative heat transfer models are developed for early and late fire spread. In the early stage, steeper slopes concentrate radiative heat flux along the fuel bed centerline, promoting curved fireline propagation. In the late stage, simulated centerline radiative flux aligns well with measurements. Additionally, the contribution of convective heating to the preheating process is analyzed and discussed. These findings elucidate how slope-induced airflow patterns alter flame geometry and fireline dynamics. The staged modeling approach improves understanding of the coupled mechanisms between slope, flame shape, and heat transfer, providing new insights for predicting surface fire behavior in complex terrain.