Safety zones and convective heat: numerical simulation of potential burn injury from heat sources influenced by slopes and winds

Abstract

Although slope and winds are common factors in most wildland fires, current guidelines for safety zone dimensions used in the United States assume flat ground and do not consider wind speed as a factor. Similarly, while convective heat transfer is an essential part of fire behavior and is often highly significant to firefighter safety, it has not been considered in past work establishing safety zone criteria. In recent years, 3D, dynamic, physics-based dynamic fire models have been developed which can help us to understand fire behavior and firefighter safety. Here, we used a 3D dynamic fire model, WFDS, to explore different factors influencing potential burn injuries that could arise from both radiative and convective heat transfer over a range of heat sources, slopes and wind speeds. In the present study, we considered a fixed (non-moving) heat source on an inclined plane. Above this heat source, at regular intervals along the slope, synthetic “sensors” tracked wind velocities, temperature, radiative and total heat fluxes, facilitating analysis of potential burn injury as a function of distance from the heat source, analogous to the radius of a safety zone. Our primary finding was that convective heat could result in burn injuries at distances several times what would result from radiation alone. We also found that, while all factors were important, the nature of the heat source (heat release rate per unit area, flaming zone depth and residence time) had more pronounced effects on potential burn injury than slope or wind speed. Both of these findings have significant implications for how we think about firefighter safety, both in terms of how big safety zones might need to be to protect firefighters from convective heat, and in terms of characterizing the fuel as a heat source. This is a new arena of research investigation, and our work is just an early step; more work is needed to fully understand the implications of convective heat for firefighter safety and decision support.