Experimental And Theoretical Research On Upslope Surface Fire Spread

This paper presents experimental and theoretical researches on upslope surface fire spread. The aim is to investigate the spreading behaviors of flame fronts and the mechanisms of fuel preheating in the fire spread process. The research scope covers the general upslope fire, canyon fire, eruptive fire and jump fire. All the experiments were performed on an upslope fuel bed bench. The burning behaviors, flame geometrical characteristics, heat flux distribution and flow velocities are examined. This work tries to reveal the governing physical mechanism for the evolution of fire spread under different conditions.Upslope fire tests (slope angles:0°～35°) for line ignition were carried out, by using pine needles as the fuels. Experimental results indicate that when slope angles were less than20°, the fire line contour remained nearly linear in the tests. When slope angles were over25°, the fire line contour underwent a transition from linear shape to V-shape, with a steady fire line angle. The rate of fire spread remained steady in global sense for the two types of fire lines.In this paper, the impact of fire line contour on the burning behaviors and fuel preheating is investigated by experiments on pine needle fuel bed and computational analyses. Numerical results show that the radiation heat flux in the centerline has minor dependence on fire line angle within certain ranges, and is mainly attributable to the central fire line segment of a certain size. The radiation heat flux depends on the fire tilt angle and flame length in the whole range. The fire line angle is extracted from experimental results, while the flame tilt angle and flame length are evaluated respectively by the empirical equations combined with experimental data. When the fire line changes from linear shape to V-shape with increasing slope angle, the radiation heat transfer along the centerline undergoes a reverse decrease. For the calculation of the heat radiation distribution on the unburnt fuel bed area, the maximum heat flux appears on the centerline of the fuel bed for a linear fire line, while the maximum value appears at a certain lateral position for a V-shape fire line. Further, analyses on the calculated radiant heat flux distribution, the measured velocity data, and the energy conservation equation justify the convective heating in V-shape fire lines. It is revealed that radiant heating is the dominant mechanism for a linear fire line spreading over pine needle fuel beds, while the fire spread for a V-shape fire line is dominated by the combined effects of radiant and convective heating.In canyon topography, central slope angle and lateral slope angle are the two key parameters. This paper investigates the effect of canyon slope on the fire line evolution by a geometrical model and experimental data. For zero lateral slope angle, which corresponds to normal upslope fires, both line ignition and point ignition were used. Fire head spreads along the maximum slope angle line. For canyon fire tests with non-zero lateral slope angles, dead pine needles were used as the fuels, and the fires were all initiated by a point ignition. It is found that in canyon fire, the trace of fire head deviates from the line of the maximum slope and approaches the centerline of the fuel bed. When the central slope angle is over a certain value (30°in this work), the fire head would spread just along the centerline of the fuel bed. For a fixed lateral slope angle, different central slope angles may cause distinct fire line contours. For higher central slope angles, the significant interaction between the two lateral fire lines enhances both the heat radiation from the two lateral fires and the forward hot gas flow, leading to significant enhancement in convective heating. Due to the positive feedback mechanism of heat convection, both the fire spread rate and the heat release rate increase sharply, which may cause an eruptive fire.Eruptive fires usually occur in special terrains of canyon, long and steep slope, or trench. Eruptive fires are characterized by the sudden increase of fire spread rate and the heat release rate. This paper presents an overview on the current status and challenges for the research of eruptive fires. However, there is obvious lack of experimental data and theoretical models for interpretation of the physical mechanisms and dynamics of eruptive fires. One eruptive fire accident in Sichuan grassland fire is analyzed. The theory of fire acceleration is introduced to interpret the generation of eruptive fire in upslope fires or canyon fires. The flame attachment is a key factor for eruptive fire in upslope fire. The positive feedback mechanism of heat convection in canyon fire is suggested to be the potential mechanism for eruptive fire.When two fire lines intersect with each other, the crossing point spreads very quickly under the enhanced convective and radiative heat transfer mechanism, which could induce jump fire. This paper presents an elementary analysis on the difference and similarity between upslope fire and jump fire, by a series of experiments performed in laboratory. The rate of fire spread, fire line angle and non-dimensional radiant heat flux for the two kinds of phenomena are investigated. For upslope fire with point ignition, the initially generated fire line angle remains steady. For jump fire, the fire line angle increases with time. For upslope fire, it is found that ROS remains almost steady for line ignition, while for point ignition, the ROS increases with time to a peak value and then decreases. For jump fire, the ROS first increases sharply and then decreases gradually. For jump fire, both the fire line angle and fire spread rate depend on the slope angle more significantly than the initial fire line angle. Non-dimensional radiant heat flux for fuel preheating is calculated which effectively explains the ROS development in upslope fire with line ignition and that in jump fire.