Abstract: [Objective] In the present work we studied the characteristics of the interception of the radiant heat flux by two typical wildland combustibles and the associated ignition phenomena during the spread of a wildland fire through theoretical calculation, based on the principle of radiant heat transfer and fire behavior model.[Method] Thermal radiation was sourced from the burning zone in a forest fire, which was considered as an equivalent rectangular emissive surface. The length of the emissive surface corresponded to the flame length, whereas the width was determined by the fireline size; The rectangular emissive surface had a tilted angle varying with the environmental wind speed. Two typical combustibles around the edge of a forest, i.e. the dead standing timbers and the dead fallen timbers, were selected as the thermal attack targets with the receiver elements set at 1.5 m high above the ground and on the ground, respectively. By taking into account of the radiation heat exchange during the fire spread and the fire extinction process of the fire at the edge of a forest, a mathematical model was developed to quantify the transient rate of heat exchange between the flame zone and a target at the forest edge and then the accumulated amount of radiation heat intercepted by the target during a fire. Meanwhile, in combination with the existing understanding on forest fire behavior, the formulas were also derived to correlate the parameters of the fire behavior presented during fire spread and the fire extinction process, which enabled the proceeding of the thermal radiation transfer calculations and the evaluation of ignition conditions for various fire scenarios. In light of the principle of fire safety assessment with effectiveness, the worst fire scenario was considered during evaluation, where the flame had a temperature of 1 200 K and an emissivity of 1.0, and the atmospheric transmittance for thermal radiation was set at 1.0 as well. Furthermore, the flame width was evaluated at 50 m, and the flame length was ranged from 5 m to 40 m, whereas the wind speed varied from 0 m·s^-1 to 10 m·s^-1 to simulate drastic change of the environmental conditions.[Result] The calculations for specific fire situations were performed and then compared with the available field test results. It was observed that the calculated radiant heat flux coincides with the field measurements, which confirmed the reliability of the model for determining the radiant heat transfer in forest fire. The examination of various practical fire scenarios indicated that a dead standing timber near the forest edge takes the advantage of both intercepting radiant heat flux and accumulation of radiation heat in comparison with a dead fallen timber as a rule, although this advantage is weaken with an increase in the environmental wind speed. The radiant heat flux received by individual targets reaches the maximum just after a fire source arrives at the forest edge, and the energy accumulation by a target at this stage plays a major role in the ignition of the target.[Conclusion] With the increase in the distance between the set target and the forest edge, the levels of energy accumulation by the target decreased rapidly; meanwhile, the ignition of a target was altered by the flame length and the local wind speed owing to their roles in the change in the duration of thermal radiation and the radiant heat transfer efficiency between fire source and the target. A large amount of calculation results for the dead standing timber near the forest edge suggested that, a non-fuel interval with a width of above 30 m is sufficient to avoid the ignition triggered by thermal radiation from a forest fire with the flame length up to 40 m. However, in case of construction of a biological fire-prevention belt, the separation distance can be reduced to a large extent. This work provides a specific theoretical guideline for the design of forest fire barriers in forest areas as well as the protection of structures and facilities in wildland fire-prone areas.

Key words: forest fire, flame radiation, ignition by radiation, separation distance, forest fire blocking system