火旋风环境中飞火颗粒的上升行为研究

Study on the ascending behavior of firebrands in fire whirling

将火旋风流场分为火焰区和羽流区,采用数值模拟方法建立了球形飞火颗粒在火旋风火焰区和羽流区的上升轨迹模型。模型结合颗粒动力学方程和燃烧模型,模拟了飞火颗粒在火旋风环境中的上升行为,提出了飞火颗粒在火旋风流场中上升的临界条件,建立了飞火颗粒在竖直方向上升轨迹的计算方法。结果表明,在火旋风中能够上升的飞火颗粒的直径和密度之积应小于临界值,燃烧对于颗粒的上升行为有很大影响。相对于不燃烧颗粒,燃烧颗粒上升更高,速度更快。

In this paper, we are inclined to present an ascending model for a sphere-shaped firebrand in fire whirl flying field for the purpose of fire spotting simulation. In the model, we prefer to divide the fire whirling fling field into two distinct regions： the flame and the plume （RegionsⅠandⅡ）, based on our research results. In Region, the centerline velocity is found to increase with the fire height whereas in RegionⅡ, the centerline velocity tends to decline with the fire height. Then, when we describe the ascending behavior of the firebrand, it can be found that both the equations of the fire whirling flow field and those of the firebrand momentum conservation and the firebrand combustion model are in perfect correspondence with each other. Besides, the model proposed by us is mainly focused on the trajectory of firebrand ascended from the centerline of fire whirl because in the centerline of fire whirl, the firebrand ascended faster and higher than the whirling fling field, and so the firebrand tends to ascend from the centerline of fire whirl more violently. And, when an approximate form of the centerline velocity of fire whirl has been applied and the diameter regression combustion mode was conducted to determine the burning characteristics of firebrand, we have studied the trajectory of firebrand ascending in fire whirl flow and proposed the critical condition of firebrands ascending in fire whirl. Thus, the results of investigation indicate that there exists a critical value for the product of the diameter and density of the loftable spherical firebrand particle. With given the particle density, the maximum loftable diameter dmax of a particle can be obtained on the condition that the initial particle height z0 was equal to the maximum fire flame height H. As compared with the non-burning particles, the trajectory of burning particles seem to be more complex. The burning particles could ascend higher and the particle velocity relative to the ground tends to increase faster. Thus, it can be seen that the model proposed by us is potential of great help for us to understand and control the long-distance fire spot.