This work experimentally investigates the fire spread of discrete fuels by using fuel beds of laser-cut cardboards in a wind tunnel. Two distinct particle ignition modes are identified: under lower wind speed and packing ratio, a part of the tilted flame front inside the fuel bed touched and ignited the fuel elements; under a higher wind speed or packing ratio, the flame trough intermittently impinged on and ignited the fuels. When the wind speed is lower than a critical value (1.5 m s−1 in this work), the rate of spread (ROS), varying with the packing ratio, has a weak dependence on the wind speed. When the wind speed exceeds the critical value, ROS decreases with the packing ratio, and the dimensionless ROS almost linearly increases with the wind speed. The optimal packing ratio decreases with the wind speed. The characteristic length (the distance the flame front spreads in the shallow fuel layer) increases with the wind speed for lower packing ratios (0.018–0.032 in this work), while it first fluctuates and then increases with the wind speed under higher packing ratios (0.054 in this work). The temporal surface and internal flow velocities strongly depend on whether a stable peak-and-valley structure is sustained, affected by the wind speed and packing ratio. Nevertheless, the internal flow velocity near the flame front increases under all conditions, enhancing the internal convective heating on the unburned fuel.
He Q, Liu N, Xie X, Zhang L, Lei J, Zhang Y, and Wu D. 2022. Experimental study on fire spread over discrete fuel bed-Part II: Combined effects of wind and packing ratio. Fire Safety Journal 128: 103520. https://doi.org/10.1016/j.firesaf.2021.103520