Experimental And Theoretical Study On Surface Fire Spread Under Effect Of Wind And Slope

In wildland fires, a lot of attention addresses surface fire spread, which is significantly influenced by ambient wind, moisture content of fuel and terrain or slope. In previous studies, most experiments were conducted under slope ranges lower than30°, and the effect of slope were considered very simply in many fire spread models. In this work, wind-driven (0-5.5m/s) fire spread experiments were conducted in combustion wind tunnel, using grasses (Hulunbeir Grassland) with various moisture content (4%-32%) as fuels, and a series of fire spread experiments with a linear flame front were performed on a6m long combustion platform with slopes from zero until32°, using pine needles of Pinus sylvestris (Jilin city, Jilin Province) as fuels. The temperature above the fuel bed was measured using K type thermocouples in wind-driven and upslope fires. Additionally, the fuel mass consumption in flaming fire spread, the velocities of the inflow around the flame, and the heat fluxes (total and radiant) near the end of the fuel bed were also measured in upslope fires.The rate of fire spread (ROS) is calculated using the characteristic ignition time of fuel extracted from temperature curve. The overall ROS calculated in global scale of fuel bed shows that the fire spread can be regarded as a steady process. However, the local ROS calculated in the region between adjacent thermocouples shows higher fluctuations as compared to the overall ROS. It is suggested that the fuel bed should be long enough to achieve a stable fire spread.In wind-driven fires, ROS is obviously enhanced by ambient wind, and increases linearly with windspeed under4.6m/s. ROS is also remarkably influenced by moisture content of fuels, but varies in a reverse way as wind. According to experimental data, a semi-empirical model including windspeed and moisture content is developed, which agrees well with the experimental results.By experimental observations and data analysis, three regions of slope are identified in upslope fires:a) low slope region of0°-20°with smaller ROS; b) moderate slope region of25°-29°with gradual increase of ROS; c) high slope region of30°-32°for rapid fire spread. It is revealed that the role of slope in ROS involves two types of mechanisms. First, the slope of fuel bed naturally reduces the angle between flame front and fuel bed. Especially this is the governing mechanism in lower slopes. Second, the slope induces upslope wind which is dominant in higher slopes.By using pitot tubes data, it is revealed that the upslope wind promotes the burning especially in high slopes. The velocity data shows that weak reverse inflow exists in front of the flame, and at the same time the upslope wind induced by the flame itself exists behind the flame front. The significant differences between the velocities of the reverse inflow and the upslope wind are suggested to be the major cause for the forward tilting of the flame front. The mass loss rate measured by load cells varies with slope in a similar way as ROS. However, the fuel consumption efficiency varies in a reverse way, from nearly unity at zero slope to0.1at slope of32°. This implies that the assumption of thermally thin fuel layer in modeling study is effective in low slope region, while it fails in higher slopes.The experimental data of heat flux meters and reverse inflow velocities by pitot tubes are used to further interpret the transfer mechanisms in fuel preheating of fire spread with a linear flame front. The results indicate that besides radiation heating, both natural convection cooling and flame-induced convection cooling exert impacts, but with different spatial influence ranges. Natural convection takes effect in a region from far field to near field (close to the flame), while flame-induced convection cooling (induced by reverse inflow) takes effect within the region close to the flame. With increasing slope angles, the convection cooling is dominated by natural convection under lower slope angles, and is jointly influenced by natural and flame-induced convections (mixed convection) under higher slope angles, in which flame-induced convection plays a dominant role.A theoretical fire spread model for a linear flame front based on energy conservation and heat transfer analysis is developed, in which radiation, convection and radiation loss are considered in details. A natural cooling convection and the fuel consumption efficiency are involved in the model. The new model is found to fit well with the experimental data of ROS, and its reliability especially for higher slopes is verified by comparisons with other models.