Effect of cross-wind on firebrand flame: An experimental study and scaling analysis

Spot fire is an important ignition pathway in the rapid spread of wildland and wildland-urban interface (WUI) fires. Firebrand combustion regime (flaming or glowing) and flame characteristics (flame tilt angle, flame length, and flame duration) are critical in predicting firebrand burning rate, spotting distance, and ignition hazard. Experiments were conducted to study firebrand combustion with various firebrand diameters ( = 10 to 20 mm) and wind speeds ( = 0 to 6 m s). The effects of the cross-wind on the pyrolysis rate, flame tilt angle (), flame length, and flame duration are investigated before the transition from flaming to glowing combustion ( 5 m s in this work). The pyrolysis rate is derived, showing it is proportional to ( is pyrolysis diameter). Then, the tan, flame length, and flame duration models are proposed based on the pyrolysis rate model. The tan is determined by wind speed and flame uprising speed (a speed controlled by the pyrolysis rate). The wind speed has a noticeable effect on the flame tilt angle. The derived flame length positively correlates with the volatile combustion rate, which is related to the pyrolysis rate. The flame length is proportional to and inversely proportional to for < 90°. The flame duration model is also derived, showing that the flame duration is related to the reduction rate of and is thus also affected by the pyrolysis rate. The flame duration is proportional to and inversely proportional to . The model predictions in tan, flame length, and flame duration agree well with the experimental data. Finally, a correlation equation of flame extinction ( 5 m s in this work) is related to critical wind speed and firebrand diameter.