In this work, a series of experiments across a porous fuel bed of pine excelsior with two sidewalls were conducted under different slope and fuel bed width conditions. It was observed that the fire line stayed straight during fire spread after the initial line ignition for all tests. Flame length and rate of spread (ROS) increased with increasing slope angle or fuel bed width. A dimensionless correlation of ROS was developed that integrated the effects of slope and fuel bed width. For a lower slope angle (≤ 20° in this work), the reverse airflow that denoted natural convective cooling for unburnt fuel was identified from the measured flow velocity data, for which flame radiation was the dominant preheating mechanism. For a higher slope angle (30° in this work), it was identified that the forward airflow from the burning area induced convective heating ahead of the flame front, and thus the fuel preheating was controlled by both flame radiation and convective heating. The effect of convective heating was more significant under larger fuel bed widths.